Features of determining the thermal conductivity of building materials. Measuring thermal conductivity. TERMODUCTETHETRIC SENSOR SENDERS TURNING MAPERS TEMPERATURE SENSORS

Federal Agency for Technical Regulation and Metrology

NATIONAL

STANDARD

Russian

Federation

Composites

Official edition

STSHDFTTFTSM

GOST R 57967-2017

Preface

1 prepared by the Federal State Unitary Enterprise "All-Russian Research Institute of Aviation Materials" together with the autonomous non-profit organization "Center for the rationing, standardization and classification of composites" with the participation of the association of legal entities "Union of manufacturers of composites" based on official translation into Russian language of the English-language version of the specified Paragraph 4 of the Standard, which was completed by TC 497

2 Submitted by the Technical Committee on Standardization of TC 497 "Composites, Designs and Products of them"

3 approved and enacted by order of the Federal Agency for Technical Regulation and Metrology of November 21, 2017 No. 1785-st

4 This standard is modified in relation to the ASTM E1225-13 standard "Standard test method for determining the thermal conductivity of solids by a comparative longitudinal-fenced heat flux method" (ASTM E122S-13 "Standard Test Method for Thermal Conductivity of Solids using the Guard ED-Comparative -LONGITUDINAL HEAT Flow Technique », Mod) by changing its structure to bring in accordance with the rules set in GOST 1.5-2001 (subsections 4.2 and 4.3).

This standard does not include paragraphs 5. 12. Subparagraphs 1.2, 1.3 of the applied ASTM standard. which are inappropriate to apply in Russian national standardization due to their redundancy.

These items and subparagraphs not included in the main part of this Standard are given in the additional appendix yes.

The name of this standard has been changed relative to the name of the specified ASTM standard to bring in accordance with GOST R 1.5-2012 (subsection 3.5).

Comparison of the structure of this Standard with the structure of the specified ASTM standard is given in the additional application of dB.

Information on the compliance of the reference National Standard Standard ASTM. Used as a reference in the applied ASTM standard. shown in the additional application of DV

5 introduced for the first time

The rules for applying this standard are established in Article 26 of the Federal Law of June 29, 2015 N9 162-FZ "On Standardization in the Russian Federation". Information about the changes to this standard is published by the E-annual (as of January 1), the information indicators "National Standards", and the official text of the changes and the floor of the launch of the monthly information indicator "National Standards". In case of revision (replacement) or the cancellation of this standard, the appropriate notification will be published in the nearest issue of the monthly information indicator "National Standards". Relevant information. Notification and texts are also posted in the public information system - on the official website of the Federal Agency for Technical Regulation and Metrology on the Internet ()

This standard cannot be fully or partially reproduced, is replicated and distributed as an official publication without the permission of the Federal Agency for Technical Regulation and Metrology

GOST R 57967-2017

1 area of \u200b\u200buse............................................... ..................one

3 Terms, definitions and designations ............................................ .......one

4 Essence of the method ............................................... ..................... 2.

5 Equipment and materials ................................................ .............four

6 Preparation for testing ............................................. .......eleven

7 Testing ............................................................. ...............12

8 Processing test results ................................................ .......13

9 Test Protocol ................................................. ..................13

Appendix Yes (Reference) Original text of not included structural elements

aSTM standard applied ........................................... 15

Application dB (reference) comparison of the structure of this standard with the structure

aSTM standard applied in it ...................................... 18

Appendix DV (reference) information on the compliance of the reference national standard ASTM standard. Used as a reference in the applied ASTM standard ............................................ .............nineteen

GOST R 57967-2017

National Standard of the Russian Federation

Composites

Determination of thermal conductivity of solid bodies by stationary one-dimensional heat flux with a security heater

Composites. Determination Of Thermal Conductivity of Sohds by Stationary One-Dimensional Heat Flow

with a Guard Heater Technique

Date of introduction - 2018-06-01

1 area of \u200b\u200buse

1.1 This standard establishes the determination of the thermal conductivity of homogeneous opaque solid polymer, ceramic and metal composites using a stationary one-dimensional heat flux with a security heater.

1.2 This standard is intended for use in testing materials having affective thermal conductivity in the range from 0.2 to 200 W / (M-K) in the temperature range from 90 to 1300 K.

1.3 This standard can also be applied when testing materials having efficient thermal conductivity outside of the specified ranges with lower accuracy.

2 Regulatory references

This standard uses regulatory references to the following standards:

GOST 2769 Surface roughness. Parameters and characteristics

GOST R 8.585 State system for ensuring unity of measurements. Thermocouples. Nominal static conversion characteristics

Note - When using this standard, it is advisable to check the action of reference standards in the public information system - on the official website of the Federal Agency for Technical Regulation and Metrology on the Internet or on the National Standards Annual Information Signal, which is published as of January 1 of the current year, and on the issues of the monthly information pointer "National Standards" for the current year. If the reference standard is replaced, to which the undated link is given, it is recommended to use the current version of this standard, taking into account all changes made to this version. If the reference standard is replaced by a dated reference, it is recommended to use the version of this standard with the above-mentioned approval (adoption). If, after approval of this standard in the reference standard, which is given dated dated, the change is made, affecting the provider to which the link is given, this provision is recommended to be applied without taking into account this change. If the reference standard is canceled without replacement, the position in which the reference is given to it is recommended to be applied in a portion that does not affect this link.

3 Terms, Definitions and Designations

3.1 This standard applies the following terms with the corresponding definitions:

3.1.1 Thermal conductivity / .. W / (M K): the ratio of the density of the heat flux under stationary conditions through the unit of the area to the unit of temperature gradient e direction perpendicular to the surface.

Official edition

GOST R 57967-2017

3.1.2 Conducting thermal conductivity: if there are other ways to transfer heat through the Mate * Rial, except thermal conductivity, the results of measurements made under the present method of testing. represent seeming or efficient thermal conductivity.

3.2 8 This standard applies the following notation:

3.2.1 x M (t), W / (M K) - thermal conductivity of reference samples depending on temperature.

3.2.2 ECI, W / (M K) is the thermal conductivity of the upper reference sample.

3.2.3 xjj '. 8T / (M K) is the thermal conductivity of the lower reference sample.

3.2.4 EDT), W / (M K) - the thermal conductivity of the test sample adjusted to the heat exchange in non * crazy.

3.2.5 x "$ (T), W / (M K) - the thermal conductivity of the test sample, calculated without taking into account the amendment for heat exchange.

3.2.6\u003e U (7), W / (M K) - thermal conductivity of insulation depending on temperature.

3.2.7 g, K - absolute temperature.

3.2.8 z, M - distance measured from the upper end of the package.

3.2.9 /, M - length of the test sample.

3.2.10 g (, K - temperature at z r

3.2.11 q ", W / m 2 - thermal flow per unit area.

3.2.12 SKH, etc. - deviations of X. G. DR.

3.2.13 g a, m - radius of the test sample.

3.2.14 g, M - the inner radius of the security shell.

3.2.15 F 9 (z), K is the temperature of the security shell, depending on the distance Z.

4 Essence of the method

4.1 General scheme of a stationary one-dimensional heat flux using OH * An early heater is shown in Figure 1. Test sample with an unknown thermal conductivity X s. having an estimated specific thermal conductivity x s // s. We are installed under the load between two reference samples with thermal conductivity of X M, which have the same cross-sectional area and the specific thermal conductivity X ^ // ^. The design is a package consisting of a disc heater with a test sample and reference samples on each side between the heater and the heat sink. In the test packet, a temperature gradient is created, heat losses are minimized by using a longitudinal security heater having an approximation of the same temperature gradient. After each sample, approximately half of the energy flows. 8 equilibrium condition The thermal conductivity coefficient is determined based on the measured grades of the temperature of the test sample and the corresponding reference samples and the thermal conductivity of reference materials.

4.2 apply power to the package to ensure good contact between the samples. The package is surrounded by insulating material with thermal conductivity insulation enclosed in a security unit with a radius of g 8, which is at temperatures T d (2). Set the temperature gradient in the package by maintaining the upper part at temperatures t t and the bottom at the temperature T in. Temperature T 9 (Z) is usually a linear temperature gradient approximately appropriate gradient installed in the test package. Can an isothermal security heater with a temperature t? (Z). equal to the average temperature of the test sample. It is not recommended to use the design of the measuring cell of the device without security heaters due to possible large thermal losses, especially at elevated temperatures. In the stationary state, temperature gradients along the plots are calculated based on the measured temperatures along two reference samples and the test sample. The value of x "s Excluding amendments to the heat exchange is calculated by the formula (the symbols are shown in Figure 2).

T 4 -G 3 2 U 2 -z, z e -z 5

where r, temperature at Z ,. K T 2 - temperature at z 2, K g 3 - temperature at z 3. TO

GOST R 57967-2017

G 4 - temperature at z 4. TO;

G 5 - temperature at z s. TO:

G - temperature at z e. TO:

Z, - coordinate of the 1st temperature sensor, m;

Zj - coordinate of the 2nd temperature sensor, m;

Z 3 - coordinate of the 3rd temperature sensor, m;

Z 4 - coordinate of the 4th temperature sensor, m;

Z 5 - coordinate of the 5th temperature sensor, m;

Z E - coordinate 6\u003e th temperature sensor, m.

Such a scheme is idealized, as it does not take into account the heat exchange between the packet and insulation at each point and the uniform heat transfer on each boundary of the separation samples and the test sample. The errors caused by these two assumptions can change much. Due to these two factors, restrictions on this test method should be provided. If required to achieve the necessary accuracy.

1 - temperature gradient in the security shell: 2 - temperature gradient in the package; 3 - thermocouple: 4 - clamp.

S - top heater. B - upper reference sample: 7 - lower reference sample, B - lower heater: B - refrigerator. 10 - Upper Secure Natreahel: And - Inzia Wild Heater

Figure 1 - Scheme of a typical tested package and security shell, showing the compliance of temperature gradients

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7 b.

Refrigerated нг.

Olya Oimshpram

Insulation; 2 - security heater. E - Metal or ceramic security shell: 4 - heater. S is a reference sample, b - test sample, x - approximate location of the thermocouple

Figure 2 - diagram of the method of one-dimensional stationary heat flux using a security heater indicating possible locations of temperature sensors

5 Equipment and materials

5.1 Reference samples

5.1.1 For reference samples, reference materials or standard materials with known thermal conductivity values \u200b\u200bshould be used. Table 1 shows some of the generally accepted reference materials. Figure 3 shows an approximate change\u003e. m with tempera * tour.

GOST R 57967-2017

Typlofoeodoost, IML ^ M-K)

Figure 3 - Reference values \u200b\u200bof thermal conductivity of reference materials

Note - Selected for reference samples The material must have the thermal conductivity closest to the thermal conductivity of the measured material.

5.1.2 Table 1 is not exhaustive, and other materials can be used as reference. The reference material and the source of the values \u200b\u200bof the X M must be specified in the test protocol.

Table 1 - Reference data Characteristics of reference materials

GOST R 57967-2017

End of Table 1.

Table 2 - thermal conductivity of electrolytic iron

Temperature. TO	Thermal conductivity. W / (mk)

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Table 3 - Tungsten thermal conductivity

Temperature, K.	Thermal conductivity. 6T / (MK)

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Table 4 - thermal conductivity of austenitic steel

Temperature. TO	Thermal conductivity, W / (m k)

GOST R 57967-2017

End of Table 4.

5.1.3 Requirements for any reference materials include the stability of properties in the entire operating temperature range, compatibility with other components of the measuring cell of the device, the ease of fastening the temperature sensor and the exactly known thermal conductivity. Since errors due to heat loss for a particular increase of K are proportional to the change in K and JK S, the reference material C) should be used for reference samples. m. The closest to\u003e. s.

5.1.4 If the thermal conductivity of the test sample K s is between the thermal conductivity values \u200b\u200bof the two reference materials, the reference material with a higher thermal conductivity to and is to use. To reduce the total temperature drop along the package.

5.2 Insulating materials

As insulating materials, powder, dispersed and fibrous materials are used to reduce the radial heat flux into the ring space and heat loss along the package. It is necessary to take into account several factors when choosing insulation:

Insulation should be stable in the expected temperature range, have a low thermal conductivity value to and be easy to use;

Isolation should not pollute the components of the measuring cell of the device, such as temperature sensors, it should have low toxicity and not a bill of electrical out.

Usually use powders and solid particles, as they are easy to ravibly. Low density fibrous mats can be used.

5.3 Temperature sensors

5.3.1 On each reference sample, at least two temperature sensors and two on the test sample must be installed. If possible, reference samples and the test sample must contain three temperature sensors in each. Additional sensors are required to confirm the layer of temperature distribution along the package or detection of an error due to the non-therapist of the temperature sensor.

5.3.2 Type of temperature sensor depends on the size of the measuring cell of the device, the temperature range and the environment in the measuring cell of the device, determined by insulation, reference samples, the test specimen and gas. To measure the temperature, any sensor that has sufficient accuracy can be used, and the measuring cell of the device must be quite large so that the perturbation of the heat flux from the temperature sensors was insignificant. Typically used thermocouples. Their small sizes and ease of fastening make up explicit advantages.

5.3.3 Thermocouples should be made of wire with a diameter of no more than 0.1 mm. For all cold spa, a constant temperature should be provided. This temperature is supported by a cooled suspension, a thermostat or electronic reference point compensation. All thermocouples should be made either from the calibrated wire or from wire, which was certified by the supplier to ensure the limits of the error indicated in GOST R 8.585.

5.3.4 The thermocouple methods are shown in Figure 4. Internal contacts can be obtained in metals and alloys by welding individual thermoelements to surfaces (Figure 4A). Spi the thermocouple, welded or with a kolkom can be rigidly attached with forging, cementing or welding in narrow grooves or small holes (Figures 4P. 4C and 4

5.3.5 In Figure 46, the thermocouple is located in the radial groove, and in Figure 4c, the thermocouple is pulled through the radial hole in the material. 8 case of using thermocouples in a protective shell or thermocouple, both thermoelement which is located in an electric insulator with two

GOST R 57967-2017

holes, the thermocouple fastening can be used, shown in Figure 4D. In the last three cases, the thermocouple should be thermally connected to the solid surface with a suitable glue or high-temperature reader. 8se four procedures shown in Figure 4. should include hardening wires on surfaces, wire turns in isothermal zones, heat grounding of wires on a security casing or a combination of all three.

5.3.6 Since the inaccuracy of the temperature sensor leads to large errors. Special attention should be paid to the definition of the correct distance between the sensors and the calculation of a possible error as a result of any inaccuracy.

b - internal cheese with separated thermoelements, welded to the test sample or reference samples so that the signal passes through the material. 6 - radial groove on a flat surface of the attachment of a bare wire or a thermocouple sensor with ceramic insulation; C is a small radial hole drilled through a test sample or reference samples, and uninsulated (it is allowed if the material is an electric insulator) or an insulated thermocouple, stretched through the hole: D - a small radial hole, drilled ■ test sample or reference samples, and thermocouple placed about the hole

Figure 4 - Fastening the thermocouple

Note - In all cases, thermoelements should be thermally hardened or thermally grounded to the security shell to minimize the measurement error due to the heat flux to or from hot spa.

5.4 Loading system

5.4.1 Test method requires uniform heat transfer across the boundary of the section of reference samples and the test sample, when the temperature sensors are located at a distance in the limits of the part of the partition. To do this, it is necessary to ensure a uniform contact

GOST R 57967-2017

the tyal of the adjacent zones of reference samples and the test sample, which can be created by applying the axial load in combination with the conductive medium on the interface. It is not recommended to carry out measurements in a vacuum if it does not require a DDI protective goals.

5.4.2 When testing materials with low thermal conductivity, thin test samples are used, so temperature sensors must be installed close to the surface. In such cases, a very thin layer of high thermal conducting fluid, paste, soft metal foil or screen should be introduced at the interfaces.

5.4.3 In the design of the measuring instrument, items should be provided for overlapping and permanent loading a package to minimize interfacial resistance at the boundaries of the reference samples and the test sample. The load can be applied pneumatically, hydraulically, the action of the spring or the location of the cargo. The above load application mechanisms are constant when changing the package temperature. In some cases, the strength to compress the test sample may be so low that the applied force must be limited to the weight of the upper reference sample. In this case, special attention should be paid to the errors that can be caused by bad contact, for which the temperature sensors need to be located away from any perturbation of the heat flux at the interfaces.

5.5 Security shell

5.5.1 A package consisting of a test sample and reference samples must be enclosed in a protective shell with proper circular symmetry. The security shell can be metallic or ceramic, and its inner radius should be such that the ratio of g ^ g a was in the range from 2.0 to 3.5. The security shell must contain at least one security heater for adjusting the temperature profile of the shell.

5.5.2 The security shell must be designed and function in such a way that its surface temperature is either isothermal and approximately equal to the average temperature of the test sample, or have an approximate linear profile, coordinated at the upper and lower ends of the security shell with the corresponding positions of the Oder package. In each case, at least three temperature sensors should be installed on a security shell in pre-corrodinous points (see Figure 2) for measuring the temperature profile.

5.6 Measuring equipment

5.6.1 The combination of the temperature sensor and the measuring instrument used to measure the output signal of the sensor must be adequate to ensure the accuracy of temperature measurement ± 0.04 K and absolute error less than ± 0.5%.

5.6.2 The measuring equipment of the DDA of this method should maintain the desired temperature and measurement of all respective output voltages with an accuracy proportionate to the accuracy of temperature measurement temperature sensors.

6 Preparation for testing

6.1 Requirements for test samples

6.1.1 The test samples under investigators under this method are not limited to the candy geometry. Most preferably, the use of cylindrical or prismatic samples. The conductivity areas of the test sample and reference samples should be the same with an accuracy of 1% and any difference in the area should be taken into account when calculating the result. For a cylindrical configuration, the radii of the test sample and reference samples should be coordinated with an accuracy of ± 1%. And the radius of the test sample G A should be such that R B FR A is from 2.0 to 3.5. Each flat surface under test and the reference samples should be flat with a surface roughness not more than R a 32 in accordance with GOST 2789. And normal to each surface should be parallel to the axis of the sample with an accuracy of up to ± 10 min.

Apply N and E - In some cases, this requirement is not necessary. For example, some devices can consist of reference samples and test samples with high values\u003e. M and\u003e. s. Where mistakes due to heat loss are insignificant for long sections. Such sections may have sufficient length, allow

GOST R 57967-2017

it is to secure the temperature sensors at a sufficient distance from the contact places, thereby ensuring the uniformity of the heat flux. The length of the test sample must be selected on the basis of information about the radius and thermal conductivity. When). and higher than the thermal conductivity of stainless steel, long test samples can be used with a length of 0g A "1. Such long test samples can use long distances between temperature sensors, and this reduces the error obtained due to inaccuracies in the location of the sensor. When). m below than the thermal conductivity of stainless steel, the length of the test sample must be reduced, since the measurement error due to heat loss becomes too large.

6.1.2 Unless otherwise established in the regulatory document or technical documentation for the material. For testing use one test sample.

6.2 Equipment Setup

6.2.1 Calibration and equipment calibration is performed in the following cases:

After assembling the equipment:

If the ratio of x m to x s is less than 0.3. or more than 3. and choose the thermal conductivity values \u200b\u200bis not possible;

If the shape of the test sample is a complex or test sample small:

If changes have been made to the geometrical parameters of the measuring cell of the device;

If it was decided to use the materials of reference samples or isolation other than those shown in sections 6.3 and 6.4:

If the equipment has previously functioned to a sufficiently high temperature at which the properties of the components may change, such as. For example, the sensitivity of the thermocouple.

6.2.2 The specified checks should be carried out by comparing at least two reference materials as follows:

Select the reference material, the thermal conductivity of which is closest to the intended thermal conductivity of the test sample:

The thermal conductivity of the X test sample made from the reference material is measured using reference samples made from another reference material, which is X. The closest to the value of the test sample. For example, check can be carried out on the satal sample. Using reference samples made of stainless steel. If the measured thermal conductivity of the sample is not consistent with the value from Table 1 after applying the heat exchange amendment, it is necessary to determine the sources of errors.

7 Testing

7.1 Choose reference samples so that their thermal conductivity is the same order of magnitude that is expected for the test sample. After equipping the necessary reference samples with temperature sensors and their installations in the measuring cell, the test sample is equipped with similar means. The test sample is inserted into the package so that it is placed between the reference samples and in contact with the adjacent reference samples at least 99% of the area of \u200b\u200beach surface. To reduce surface resistance, a soft foil or other contact medium can be used. If the measuring cell must be protected from oxidation during the test, or if the measurement requires a certain gas or gas pressure to control X / T, the measuring cell is filled and is purged by a mounted pressure gas. To load the package, the power must be applied to reduce the effects of uneven thermal resistance at the border of the phase partition.

7.2 Includes the upper and lower heaters at both ends of the package and adjust until then. While the temperature difference between points 2, and Zj. Z3 and Z 4. And also z s and 2 ^ will not be greater than the 200-fold error of the temperature sensor, but not more than 30 K. and the test sample will not be at the average temperature required for measurement. Despite. that the exact temperature profile along the security shell is not required for 3. The power of the security heaters is adjusted to those LOR, while the temperature profile is along the shell T G (thermal conductivity of the best heat conductor - silver) to X. About 10 _6 (thermal conductivity of the least conductive gases).

The thermal conductivity of gases increases greatly with increasing temperature. For some gases (GH 4: NH 3), the relative thermal conductivity with increasing temperature increases sharply, and for some (NE) it decreases. According to the kinetic theory, the thermal conductivity of gases should not depend on pressure. However, various causes lead to the fact that, with an increase in pressure, thermal conductivity increases slightly. In the pressure range from atmospheric up to several Millibars, thermal conductivity does not depend on pressure, since the average magnitude of the fluid mileage of molecules increases with a decrease in the number of molecules per unit volume. With a pressure -20 mbar, the free path of molecules corresponds to the size of the measuring chamber.

The thermal conductivity measurement is the oldest physical method of gas analysis. It was described in 1840, in particular, in the works of A. Shleyermah (1888-1889) and since 1928 it is used in industry. In 1913, Siemens developed a hydrogen concentration meter for airship. After that, for decades, devices based on measuring thermal conductivity were developed with great success and widely used in a rapidly growing chemical industry. Naturally, only binary gas mixtures were first analyzed. The best results are obtained with a large difference in thermal conductivity of gases. Among the gases the largest thermal conductivity has hydrogen. In practice, the concentration of CO S in flue gases was also justified, since the thermal conductivity of oxygen, nitrogen and carbon monoxide is very close to each other, which allows the mixture of these four components to be considered as a quasi-primary.

Temperature coefficients of thermal conductivity of different gases of unequal, so you can find the temperature in which the thermal conductivity of different gases coincide (for example, 490 ° C - for carbon dioxide and oxygen, 70 ° C - for ammonia and air, 75 ° C - for carbon dioxide and argon) . When solving a certain analytical problem, these coincidences can be used by taking a triple gas mixture for quasi-bar.

In gas analysis we can assume that the thermal conductivity is an additive property. Measuring the thermal conductivity of the mixture and knowing the thermal conductivity of the pure components of the binary mixture, it is possible to calculate their concentrations. However, this simple addiction cannot be applied to any binary mixture. For example, the mixtures of air - water vapor, air - ammonia, carbon monoxide - ammonia and air - acetylene at a certain ratio of the components have the maximum thermal conductivity. Therefore, the applicability of the thermal conductivity method is limited to a specific area of \u200b\u200bconcentrations. For many mixtures there is a non-linear dependence of thermal conductivity and composition. Therefore, it is necessary to remove the calibration curve, according to which the scale of the registering device should be made.

Heat conduction sensors (Thermoconductometric sensors) consist of four small-filled small volume chambers with placed in them isolated from the body with thin platinum conductors of the same sizes and with the same electrical resistance. Through the conductors proceeds the same permanent current of a stable value and heats them. Conductors - heating elements - surrounded by gas. Two chambers contain a measured gas, the other two is a comparative gas. All heating elements are included in the depths of the CEMTEON, with which the measurement of temperature difference of about 0.01 ° C does not represent difficulties. Such high sensitivity requires accurate equality of temperature of measuring chambers, therefore the entire measuring system is placed in a thermostat or to the measuring diagonal of the bridge, include resistance for temperature compensation. As long as the heat dissipation from the heating elements in the measuring and comparative chambers is the same, the bridge is in equilibrium. When the gas is applied to the measuring chambers with another thermal conductivity, this equilibrium is broken, the temperature of the sensitive elements changes and together with this resistance. The resulting current in the measuring diagonal is proportional to the concentration of the measured gas. To increase the sensitivity, the operating temperature of sensitive elements should be increased, however, it is necessary to ensure that a sufficiently large difference in gas thermal conductivity is preserved. So, for various gas mixtures there is optimal heat conduction and sensitivity temperature. Often the difference between the temperature of the sensitive elements and the temperature of the walls of the chambers is selected from 100 to 150 ° C.

Measuring cells of industrial thermoconductometric analyzers consist, as a rule, from a massive metal case, in which the measuring chambers are drilled. This ensures a uniform temperature distribution and good gradation stability. Since the gas flow rate is influenced by the speed of the gas stream, the gas input to the measuring chambers is carried out through the bypass channel. Solutions of various designers to ensure the required gas exchange are shown below. In principle, they proceed from the fact that the main gas flow is connected by connecting channels with measuring chambers through which the gas flows under a small difference. At the same time, diffusion and thermal convection have a decisive effect on gas renewal in measuring chambers. The volume of measuring chambers can be very small (several cubic millimeters), which ensures a slight effect of convective heat transfer to the measurement result. To reduce the catalytic effect of platinum conductors, they are plugged into thin-walled glass capillaries. To provide the durability of the measuring chamber to corrosion, all gas pipes are covered with glass. This allows you to measure the thermal conductivity of mixtures containing chlorine, hydrogen chloride and other aggressive gases. Thermoconduetheometric analyzers with closed comparative chambers are distributed mainly in the chemical industry. The selection of the appropriate comparative gas simplifies the calibration of the device. In addition, you can get a scale with an depressed zero. To reduce the zero drift, good tightness of comparative cameras should be provided. In special cases, for example, with strong oscillations of the composition of the gas mixture, you can work with flowing comparative chambers. At the same time, with the help of a special reagent from the measured gas mixture, one of the components is removed (for example, with a solution of caustic potassium solution), and then a gas mixture is sent to comparative chambers. The measuring and comparative branches differ in this case only by the absence of one of the components. This method often makes it possible to analyze complex gas mixtures.

Recently, instead of metal conductors, semiconductor thermistors are sometimes used as sensitive elements. The advantage of thermistors is 10 times higher than the temperature coefficient of resistance in comparison with metal thermal resistance. This achieves a sharp increase in sensitivity. However, at the same time, much higher requirements for stabilizing the current of the bridge and the temperature of the walls of the cameras are presented.

Previously, other and most widely thermoconductometric instruments began to be used to analyze the exhaust gases of the heat furnaces. Due to high sensitivity, high speed, ease of maintenance and reliability of the design, as well as its low cost analyzers of this type later quickly implemented in the industry.

Thermoconduetheometric analyzers are best suited for measuring hydrogen concentration in mixtures. When choosing comparative gases, a mixture of various gases should also be considered. As an example of the minimum measurement ranges for various gases, you can use the following data (Table 6.1).

Table 6.1

Minimum measurement ranges for various gases,

% To volume

The maximum measurement range is most often the range of 0-100%, while 90 or even 99% can be suppressed. In special cases, the thermoconductometric analyzer makes it possible to have several different measurement ranges on one device. This is used, for example, when controlling the processes of filling and emptying by hydrogen cooled turbogenerators on thermal power plants. Due to the danger of explosions, the filling of the generator housing is carried out not by air, but first, carbon dioxide is introduced as purge gas and then hydrogen is already injected. Similarly produce gas production from the generator. With a sufficiently high reproducibility on one analyzer, the following measurement ranges can be obtained: 0-100% (Volume.) CO (in air for purged gas purged), 100-0% H 2 V Co (for filling with hydrogen) and 100-80% H 2 (in the air to control the purity of hydrogen during the operation of the generator). This is a cheap measurement method.

To determine the content of hydrogen in the electrolysis of chloride chloride chlorine with a thermoconductometric analyzer, it is possible to work with a sealed comparative gas (S0 2, AG) and with a flowable comparative gas. In the latter case, a mixture of hydrogen and chlorine is first sent to the measuring chamber, and then in the afterburn furnace with a temperature\u003e 200 ° C. Hydrogen burns with excess chlorine and forms hydrogen chloride. The resulting mixture of NA and C1 2 is supplied to the comparative chamber. At the same time, hydrogen concentration is determined by the difference in thermal conductivity. This method significantly reduces the effect of impurities of small air.

To reduce the error arising from the analysis of wet gas, the gas must dry out that they are carried out either using the moisture absorber or decrease in the gas temperature below the dew point. There is another possibility to compensate for the influence of humidity, which is applicable only during measurement according to a flow chart with a flow of comparative gas.

To work with explosive gases, a number of firms make instruments in explosion-proof performance. In this case, the chambers of the thermal conductivity meters are calculated on high pressure, fireprocessors are installed at the inlet and at the outlet of the chambers, and the output signal is limited by intrinsically safe level. However, such devices cannot be used to analyze mixtures of explosive gases with oxygen or hydrogen with chlorine.

Santimeter - gram - second - a system of units of measure, which was widely used before the adoption of the international system of units (C).

GOST 7076-99

UDC 691: 536.2.08: 006.354 Group G19

Interstate standard

Construction materials and products

Method for determining thermal conductivity and thermal resistance

in stationary thermal mode

BUILDING MATERIALS AND PRODUCTS

Method of Determination Of Steady-State Thermal

cONDUCTIVITY AND THERMAL RESISTANCE

Date of introduction 2000-04-01

Preface

1 Developed by the Research Institute of Construction Physics (Niizf) of the Russian Federation

Made by Gosstroke Russia

2 Adopted by the Interstate Scientific and Technical Commission for Standardization, Technical Registration and Certification in Construction (MNTKS) May 20, 1999

Name of state	Name of state authority construction management
Republic of Armenia	Ministry of Urban Planning of the Republic of Armenia
The Republic of Kazakhstan	Committee on Construction Affairs of the Ministry of Energy, Industry and Trade of the Republic of Kazakhstan
Republic of Kyrgyzstan	State Inspectorate for Architecture and Construction under the Government of the Kyrgyz Republic
The Republic of Moldova	Ministry of Development of Territories, Construction and Communal Services of the Republic of Moldova
the Russian Federation	Gosstroy Russia
The Republic of Tajikistan	Committee on Architecture and Construction of the Republic of Tajikistan
The Republic of Uzbekistan	State Committee for Architecture and Construction of the Republic of Uzbekistan
	State Committee for Construction, Architecture and Housing Policy of Ukraine

3 instead of GOST 7076-87

4 enacted from April 1, 2000 as the State Standard of the Russian Federation by the Resolution of the Gosstroy of Russia of December 24, 1999 No. 89

Introduction

This standard is harmonized with ISO 7345: 1987 and ISO 9251: 1987 in terms of terminology and complies with the basic provisions of ISO 8301: 1991, ISO 8302: 1991, establishing methods for determining thermal resistance and efficient thermal conductivity using a device equipped with a heather and a device with hot Security zone.

In accordance with ISO standards, this standard establishes requirements for samples, the device and its graduation, two main test schemes are adopted: asymmetric (with one heat meter) and symmetrical (with two heat meters).

1 area of \u200b\u200buse

This standard applies to building materials and products, as well as on materials and products intended for thermal insulation of industrial equipment and pipelines, and establishes a method for determining their effective thermal conductivity and thermal resistance at an average sample temperature from minus 40 to + 200 ° C.

Standard does not apply to materials and products with thermal conductivity of more than 1.5 W / (m × K).

GOST 166-89 caliper. Technical conditions

GOST 427-75 Metal measuring rules. Technical conditions

GOST 24104-88 Laboratory laboratory scales and exemplary. General technical conditions

3 Definitions and Designations

3.1 This standard uses the following terms with appropriate definitions.

Heat flow - The amount of heat passing through the sample per unit of time.

The density of thermal flux - Thermal flow passing through the unit area.

Stationary thermal mode - The mode in which all the thermal treatment parameters under consideration do not change over time.

Thermal sample resistance - The ratio of the difference in the temperature of the faceplate of the sample to the density of the heat flux in the conditions of the stationary thermal regime.

The average temperature of the sample - The average temperature of the temperatures measured on the facial sample faces.

Effective thermal conductivityl. EFF material (corresponds to the term "coefficient of thermal conductivity", adopted in existing standards for construction heat engineering) - the ratio of the thickness of the test sample material d. to Its thermal resistance R.

3.2 Designations of quantities and units of measurement are shown in Table 1.

Table 1

Designation	Value	unit of measurement
l EFF.	Effective thermal conductivity	W / (m × K)
	Thermal resistance	m 2. × k / w
	Sample thickness before testing
	Thermal resistances of standard samples	m 2. × k / w
D T 1, D. T. 2	The difference of temperature faces of standard samples
e 1 e. 2	Output signals of the heat meter of the device with its graduation with standard samples
f 1, f. 2	Conditioning coefficients of the device heat meter with its graduation using standard samples	W / (MV × m 2)
	Sample thickness during testing
	Thermal resistance of the test sample	m 2. × k / w
	Relative change in the mass of the sample after drying
	Relative change in the mass of the sample in the process of testing
	Mass sample when it received from the manufacturer
	Mass sample after drying
	Sample weight after testing
D T U.	The difference of temperature faces of the test sample
	The average temperature of the test sample
	The temperature of the hot facial edge of the test sample
	The temperature of the cold facial edge of the test sample
	The value of the graduation coefficient of the heatherers of the device corresponding to the value of the heat flux flowing through the test sample after establishing stationary thermal regime (with an asymmetric test diagram)	W / (MV × m 2)
	Output signal of the device heat meter after establishing a stationary heat flux through a test sample (with an asymmetric test diagram)
	Thermal resistance between the face of the sample and the working surface of the device plate
l EFFU.	Effective thermal conductivity of the material of the test sample	W / (m × K)
	Thermal resistance of the sheet material from which the bottom and the lid of the drawer for the sample of the bulk material are made	m 2. × k / w
f. ¢ U. , F.² U.	The values \u200b\u200bof the graduate coefficient of the first and second heat meters of the device corresponding to the value of the heat flux flowing through the test sample after establishing a stationary thermal mode (with a symmetrical test diagram)	W / (MV × m 2)
e. ¢ U. E.² U.	The output signal of the first and second heat meters after establishing a stationary heat flux through a test sample (with a symmetric test diagram)
	The density of the stationary heat flux passing through the test sample
	Square zone measurement
	Electrical power supplied to the heater zone measuring hot plate device

4 General

4.1 The essence of the method is to create a stationary heat flux passing through a flat sample of a certain thickness and directed perpendicular to the front (greatest) patterns of the sample, measuring the density of this heat flux, the temperature of the opposite facial faces and the thickness of the sample.

4.2 The number of samples needed to determine the effective thermal conductivity or thermal resistance, and the sampling procedure must be specified in the standard for a particular material or product. If the standard for a particular material or product does not specify the number of samples to be tested, efficient thermal conductivity or thermal resistance are determined on five samples.

4.3 Temperature and relative humidity of the room of the room in which tests must be carried out (295 ± 5) to and (50 ± 10)%.

5 Means of measurement

For testing apply:

the device for measuring efficient thermal conductivity and thermal resistance, certified in the prescribed manner and satisfying the requirements given in Appendix A;

device for determining the density of fibrous materials according to GOST 17177;

the device for determining the thickness of flat fibrous products according to GOST 17177;

electric cabinet drying, the upper limit of heating of which is at least 383 K, the limit of the permissible error of the task and automatic temperature control - 5 K;

schortencyircle according to GOST 166:

To measure the outer and internal dimensions with a measurement range of 0-125 mm, the countdown value by nonius is 0.05 mm, the limit of the permissible error is 0.05 mm;

To measure the outer dimensions with a range of measuring 0-500 mm, the countdown value of nonius is 0.1 mm, the limit of the permissible error is -0.1 mm;

metal measuring line according to GOST 427 with an upper measurement limit of 1000 mm, the limit of the allowed deviation from the nominal values \u200b\u200bof the length of the scale and distances between any stroke and beginning or the end of the scale - 0.2 mm;

laboratory laboratory scales according to GOST 24104:

With the highest weighing limit of 5 kg, the price of division is 100 mg, the average quadratic deviation of the scales - no more than 50.0 mg, the error from the barberhood of the rocker - no more than 250.0 mg, the limit of the permissible error is 375 mg;

With the highest weighing limit of 20 kg, the price of division - 500 mg, the average quadratic deviation of the testimony of weights - not more than 150.0 mg, the error from the risk inequality is not more than 750.0 mg, the limit of the permissible error is 1500 mg.

It is allowed to use other measurement tools with metrological characteristics and equipment with technical specifications not worse than those specified in this standard.

6 Test preparation

6.1 Made a sample in the form of a rectangular parallelepiped, the largest (facial) faces of which have the shape of a square with a side equal to the side of the operating surfaces of the device plates. If the working surfaces of the plates of the instrument have the form of a circle, then the greatest face of the sample should also have a circle form, the diameter of which is equal to the diameter of the operating surfaces of the device plates (Appendix A, P. A. 2.1).

6.2 The thickness of the test sample should be less than the length of the edge edge or diameter at least five times.

6.3 The facet of the sample, in contact with the working surfaces of the instrument slabs, should be flat and parallel. The deviation of the facial edges of the rigid sample from parallelism should not be more than 0.5 mm.

Rigid samples having multipleness and deviations from flatness are grinding.

6.4 The thickness of the parallelepiped sample is measured with a calipercule with an error of not more than 0.1 mm in four angles at a distance (50.0 ± 5.0) mm from the top of the angle and in the middle of each side.

The sample thickness is measured by a caliper with an error of no more than 0.1 mm by forming, located in four mutually perpendicular planes passing through the vertical axis.

Over the thickness of the sample, the average-parent value of the results of all measurements takes.

6.5 The length and width of the sample in the plan is measured by a ruler with an error of no more than 0.5 mm.

6.6 The correctness of the geometric shape and the sample size of the heat-insulating material is determined according to GOST 17177.

6.7 The average size of the inclusions (filler granules, large pores, etc.), differ in its thermophysical indicators from the main sample, should be not more than 0.1 sample thickness.

A test of a sample having inhomogeneous inclusions is allowed, the average size of which exceeds 0.1 of its thickness. In the test protocol, the average inclusion size must be specified.

6.8 Determine the mass of the sample M. 1 When it receives from the manufacturer.

6.9 The sample is dried to a constant mass at a temperature indicated in the regulatory document on the material or product. The sample is considered dried to constant mass, if the loss of its mass after the next drying for 0.5 h does not exceed 0.1%. At the end of the drying, the mass of the sample is determined M. 2 and its density r. U.After which the sample is immediately placed either into the device to determine its thermal resistance or in a sealed vessel.

A wet sample test is allowed at a cold face facial temperature of more than 273 K and a temperature drop of no more than 2 to 1 cm of the sample thickness.

6.10 The sample of the dried bulk material should be placed in the box, the bottom and the cover of which are made of thin sheet material. The length and width of the box must be equal to the appropriate size of the operating surfaces of the device plates, the depth is the thickness of the test sample. The thickness of the sample of the bulk material should be at least 10 times the average size of the granules, grains and scales, of which this material consists.

The relative hemispherical radiative ability of the bottom surfaces and the lid of the box should be more than 0.8 at the temperatures that these surfaces have during the test.

Thermal resistance R L. The sheet material from which the bottom and the box cover makes it should be known.

6.11 The sample of the bulk material is divided into four equal parts, which alternately pumped into the box, sealing each part so that it takes the corresponding part of it internal drawer. The box is closed with a lid. The lid is attached to the side walls of the drawer.

6.12 Weigh the box with a sample of bulk material. By a certain value of the mass of the drawer with the sample and the predetermined values \u200b\u200bof the internal volume and the mass of the empty box, the density of the sample of the bulk material is calculated.

6.13 The error of the mass definition and sizes should not be more than 0.5%.

7 Testing

7.1 Tests should be carried out at a pre-graded device. The order and frequency of graduation are shown in Appendix B.

7.2 The test sample is placed in the device. Sample location is horizontal or vertical. With the horizontal arrangement of the sample, the direction of the heat flux from top to bottom.

In the process of testing the difference in the temperatures of the facelights of the sample D. T U. Must be 10-30 K. The average sample temperature during the test should be specified in the regulatory document on a specific type of material or product.

7.3 Set the setpoint values \u200b\u200bof the operating surfaces of the device plates and sequentially every 300 s are measured:

signals heat meters e U. and sensors of the facial faces of the sample, if the density of the heat flux through the test sample is measured using a heatheraper;

power supplied to the heater of measuring the hot plate of the device, and signal sensor signals of the sensor of the sample of the sample, if the density of the heat flux through the test sample is determined by measuring the electrical power supplied to the heater of the zone measuring the hot plate of the device.

7.4 The heat flux through the test sample is considered to be established (stationary) if the thermal resistance values \u200b\u200bof the sample calculated from the results of five consecutive measurements of the temperature sensors and the density of the heat flux differ from each other in less than 1%, while these values \u200b\u200bdo not increase and not decrease monotonously.

7.5 After reaching the stationary thermal mode, the thickness of the sample placed in the device is measured d U. Schunzirkul with an error of not more than 0.5%.

7.6 After the end of the test determine the mass of the sample M. 3 .

8 Test Results Processing

8.1 Calculate the relative change in the mass of the sample due to its drying t. R and in the process of testing t. W and sample density r. U. By formulas:

t. R \u003d. (M. 1 ¾ M. 2 ) / M. 2 , (2)

t. W. \u003d (M. 2 ¾ M. 3 ) / M. 3 , (3)

Volume of test sample V U. Calculate according to the results of the measurement of its length and width after the end of the test, and thickness - during the test.

8.2 Calculate the difference in temperature facial temperatures D. T U. and middle temperature of the test sample T Mu. By formulas:

D. T U. = T. 1u. ¾ T. 2u. , (5)

T Mu.= (T. 1u. + T. 2U.) / 2 (6)

8.3 When calculating the thermophysical indicators of the sample and the density of the stationary heat flux into the calculated formulas substitute the average-generic values \u200b\u200bof the results of five measurements of the temperature difference sensors and a signal of the heat meter or electrical power, made after establishing a stationary heat flux through the test sample.

8.4 When testing on the device assembled by asymmetric scheme, thermal sample resistance R U. Calculate by formula

(7)

where R K. take equal to 0.005m 2 × K / W, and for thermal insulation materials and products - zero.

8.5 Efficient thermal conductivity l. effu. Calculate by formula

(8)

8.6 Thermal resistance R U. and efficient thermal conductivity l. effu. Sample of bulk material is calculated by formulas:

, (9)

. (10)

8.7 The density of stationary heat flux q U. Through the sample, tested on the device assembled by asymmetric and symmetric schemes, is calculated according to the formulas:

q u \u003d f u e u , (11)

. (12)

8.8 When testing on a device with a hot security zone, in which the density of the heat flux is determined by measuring the electrical power supplied to the heater of the hot plate measuring zone, thermal resistance, efficient thermal conductivity and the density of stationary heat flux through the sample are calculated by formulas:

, (13)

, (14)

When testing bulk materials in formula (13) and (14) instead R K. Substitute value R L ..

8.9 For the result of the test, the average thermometic values \u200b\u200bof thermal resistance and the effective thermal conductivity of all tested samples are taken.

9 Test Protocol

The test report must contain the following information:

Name of material or product;

The designation and name of the regulatory document on which the material or product is made;

Enterprise manufacturer;

Party number;

Manufacturing date;

Total number of test samples;

The type of device on which the test was carried out;

The position of the test samples (horizontal, vertical);

Method of making samples of bulk material with a thermal resistance of the bottom and the lid of the drawer, in which samples were experiencing;

Sizes of each sample;

The thickness of each sample before the start of the test and in the process of testing indicating whether the test was carried out at a fixed pressure on the sample or at a fixed thickness of the sample;

Fixed pressure (if it was fixed);

The average size of inhomogeneous inclusions in samples (if any);

Methods of drying samples;

Relative change in the mass of each sample due to its day;

The humidity of each sample before the start and after the end of the test;

Density of each sample in the process of testing;

Relative change in the mass of each sample that occurred in the process of testing;

The temperature of the hot and cold facial edges of each sample;

The difference in the temperatures of hot and cold facial edges of each sample;

The average temperature of each sample;

Heat flux density through each sample after establishing stationary thermal regime;

Thermal resistance of each sample;

Effective thermal conductivity of the material of each sample;

The average thermometic value of the thermal resistance of all tested samples;

The average temperature of the effective thermal conductivity of all tested samples;

Direction of heat flux;

Date of testing;

Date of the last graduation of the device (if the test is carried out on a used device equipped with a heather);

For standard samples used in the graduation of the device, it must be specified: type, thermal resistance, calibration date, validity period, organization conducted by calibration;

Assessment of the error of measuring thermal resistance or efficient thermal conductivity;

Statement on full compliance or partial discrepancy of the test procedure to the requirements of this Standard. If there were deviations from the requirements of this Standard during the test, they must be specified in the test report.

10 Error definition of effective thermal conductivity

and thermal resistance

The relative error in determining the effective thermal conductivity and thermal resistance for this method does not exceed ± 3% if the test is carried out in full compliance with the requirements of this standard.

Appendix A.

(mandatory)

Requirements for instruments for determining effective thermal conductivity and thermal resistance with stationary thermal mode

BUT.1 instrument schemes

To measure efficient thermal conductivity and thermal resistance with stationary thermal mode, appliances are used:

Assembled by an asymmetric scheme equipped with one heat meter, which is located between the test sample and the cold plate of the device or between the sample and the hot plate of the device (Figure A.1);

The symmetric circuit, equipped with two heat meters, one of which is located between the test sample and the cold plate of the device, and the second - between the sample and the hot plate of the device (Figure A.2);

The device in which the density of the heat flux passing through the test sample is determined by measuring the electrical power supplied to the heater of the zone measurement of the hot plate of the device (the device with a hot security zone) (Figure A.3).

1 - heater; 2 - heat meter; 3 - test sample; 4 - refrigerator

Figure A.1 - Diagram of the device with one heat meter

1 - heater; 2 - heat meters; 3 - refrigerator; 4 - Test sample

Figure A.2. - The diagram of the device with two heat meters

1 - refrigerator; 2 - test samples; 3 - Plates of the heater measuring zone;

4 - Winding of the measuring zone heater; 5 - plates of the heater of the security zone;

6 - Highlighting the Heater of the Security Zone

Figure A. 3 - Device diagram with hot security zone

A.2 Heater and refrigerator

A.2.1 Plates of the heater or refrigerator may have a square shape, the side of which should be at least 250 mm, or a circle, the diameter of which should be at least 250 mm.

A.2.2 Working surfaces of the plates of the heater and the refrigerator must be made of metal. The deviation from the flatness of the working surfaces should be no more than 0.025% of their maximum linear size.

A.2.3 Relative hemispherical emitting ability of the working surfaces of the plates of the heater and the refrigerator coming into contact with the test sample must be more than 0.8 at the temperatures that these surfaces have during the test.

BUT.3 heat meters

A.3.1 The size of the working surfaces of the heatherers should be equal to the size of the working surfaces of the plates of the heater and the refrigerator.

A. 3.2 Relative hemispherical radiative ability of the front face of a heat meter that comes in contact with the test sample must be more than 0.8 at the temperatures that this face has during the test.

A. 3.3 The zone of measuring the heat meter should be located in the central part of its face. Its area should be at least 10% and no more than 40% of the entire facial area.

A.3.4 The diameter of thermal wires used in the manufacture of the thermoelectric battery of the heat meter should be no more than 0.2 mm.

A.4 Temperature sensors

The number of temperature sensors on each work surface of the plates of the heater or refrigerator and the front face of the heat meter coming into contact with the test sample must be equal to the whole part of the number 10 Ö a and be at least two. The diameter of the wires suitable for these sensors should be no more than 0.6 mm.

A.5 Electrical Measuring System

The electrical measuring system should ensure the measurement of the sensor signal of the surface temperature difference with an error of no more than 0.5%, the signal of the heat meter - with an error of no more than 0.6% or electrical power supplied to the heater of the measuring zone of the hot plate of the device - with an error of not more than 0 , 2%.

The total error in measuring the difference in temperature of the surfaces of the plates of the device and the heat meter coming into contact with the front grades of the test sample should not be more than 1%. The total error is the sum of the errors arising from the distortion of the temperature field near the temperature sensors, changes in the characteristics of these sensors under the influence of external conditions and errors made by an electrical measuring system.

A.6 Device for measuring the thickness of the test sample

The device must be equipped with a device that allows you to measure the thickness of the sample in the process of its test by a caliper with an error of no more than 0.5%.

A.7 Framework

The device must be equipped with a frame that allows you to save different orientation in the space of the instrument block containing a test sample.

A.8 Device for fixing the test sample

The device must be equipped with a device that or creates a constant specified pressure on the device placed in the device, or supports a constant clearance value between the operating surfaces of the instrument slabs.

The maximum pressure created by this device on the test sample must be 2.5 kPa, the minimum - 0.5 kPa, the error of the pressure task is not more than 1.5%.

A.9 Device for reducing side heat loss or heat gain of the test sample

Side heat loss or heat gain in the process of testing must be limited by insulation of the side faces of the test sample with a layer of thermal insulation material, the thermal resistance of which is no less thermal sample resistance.

A. 10 Casing of the device

The device must be equipped with a casing, the air temperature in which is maintained equal to the average temperature of the test sample.

Appendix B.

(mandatory)

Graduation of the device equipped with a heather

B.1 General requirements

The graduation of the device equipped with a heather should be carried out by means of three certified standard samples of thermal resistance made in the prescribed manner made of optical quartz glass, organic glass and foam or fiberglass.

The dimensions of standard samples should be equal to the sample size to be tested. In the process of graduate, the temperature of the facial faces of standard samples must be respectively equal to those temperatures that during the test will have facial edges of the test sample.

The entire range of thermal resistance values \u200b\u200bthat can be measured on the instrument should be divided into two subbands:

the lower boundary of the first subband is the minimum thermal resistance value, which can be measured on this device; upper boundary - the value of the thermal resistance of a standard sample made of organic glass and having a thickness equal to the thickness of the sample to be tested;

the lower boundary of the second subband is the upper boundary of the first subadapaz; The upper bound is the maximum thermal resistance value that can be measured on this device.

B.2 Conditioning of the device collected by asymmetric scheme

Prior to the beginning of the graduation, it is necessary to estimate the numerical value of the thermal resistance of the sample to be tested according to known reference data and determine which subadiapan is the value belongs. The hydraulic graduation is carried out only in this subadapazone.

If the thermal resistance of the sample to be tested belongs to the first subband, heat-dryer graduation

conducted using standard samples made of optical quartz and organic glass. If the thermal resistance of the sample refers to the second subband, the calibration is carried out with standard samples made of organic glass and thermal insulation material.

The first standard sample with smaller thermal resistance is placed in the device. R S. 1 , D. T. 1 of its front faces and output signal of the heat meter e. 1 according to the method described in section 7. Then the second standard sample with large thermal resistance is placed in the device. R S. 2 , Measure the difference in temperature D. T. 2 of its facial faces and output signal of the heat meter e. 2 On the same technique. According to the results of these measurements, gradual coefficients are calculated f. 1 I. f. 2 heat meters by formulas:

The value of the caller's calibration coefficient f U, corresponding to the value of the heat flux flowing through the test sample after the establishment of a stationary heat flux is determined by linear interpolation by the formula

. (B.3)

B.z graduation of the device assembled by symmetric scheme

The method of determining the calibration coefficient of each heatherers of the device assembled according to a symmetric diagram is similar to the method of determining the caller coefficient of the heat meter described in B.2.

B.4 frequency of graduation device

The graduation of the device must be carried out within 24 hours preceding the test or follow-up.

If according to the results of the calibrations carried out within 3 months, the change in the calibration coefficient of the heat meter does not exceed ± 1%, this device can be graded once every 15 days. In this case, the test results can be transmitted to the Customer only after carrying out a graduation, followable for the test, and if the value of the calibration coefficient determined by the results of the subsequent graduation differs from the value of the coefficient determined by the results of the previous graduation, not more than ± 1%.

The calibration coefficient used in the calculation of the thermophysical indicators of the test sample is determined as the average-agent value of the two specified values \u200b\u200bof this coefficient.

If the difference between the value of the calibration coefficient exceeds ± 1%, the results of all tests performed in the period of time between these two graduations are considered invalid and the tests must be re-conducted.

Appendix B.

Bibliography

ISO 7345: 1987 thermal insulation. Physical quantities and definitions

ISO 9251: 1987 thermal insulation. Heat transfer modes and material properties

ISO 8301: 1991 thermal insulation. Determination of thermal resistance and related thermophysical indicators with inpatient thermal mode. Heat meter

ISO 8302: 1991 thermal insulation. Determination of thermal resistance and related thermophysical indicators. Device with a hot security zone

Keywords: thermal resistance, efficient thermal conductivity, standard sample

Introduction

1 area of \u200b\u200buse

3 Definitions and Designations

4 General

5 Means of measurement

6 Test preparation

7 Testing

8 Test Results Processing

9 Test Protocol

10 Error of determining effective thermal conductivity and thermal resistance

Appendixa and instrument requirements for determining effective thermal conductivity and thermal resistance in stationary thermal mode

Appendix B Conditioning of the device equipped with a heather

Appendix in Bibliography

purpose of work: Study of the technique of experimental definition of the coefficient

thermal conductivity of solid materials by plate method.

The task:one. Determine the thermal conductivity coefficient of the material under study.

2. Determine the dependence of the coefficient of thermal conductivity on temperature

the material under study.

Basic provisions.

Heat exchange- This is a spontaneous irreversible process of heat transfer in space in the presence of temperature difference. There are three main methods of heat transfer, substantially differing among themselves in their physical nature:

thermal conductivity;

convection;

heat radiation.

In practice, the heat, as a rule, is transferred simultaneously in several ways, but the knowledge of these processes is impossible without studying the elementary heat exchange processes.

Thermal conductivityit is called the process of heat transfer due to the thermal motion of microparticles. In gases and liquids, heat transfer thermal conductivity is carried out by diffusion of atoms and molecules. In solids, the free movement of atoms and molecules throughout the volume of the substance is impossible and reduced only to their oscillatory movement relative to certain equilibrium positions. Therefore, the process of thermal conductivity in solids is due to the increase in the amplitude of these oscillations distributed in the body volume due to the perturbation of the power fields between the oscillating particles. In metals, heat transfer thermal conductivity occurs not only due to oscillations of ions and atoms located in the nodes of the crystal lattice, but also due to the movement of free electrons forming the so-called "electronic gas". Due to the presence of additional thermal energy carriers in metals in the form of free electrons, the thermal conductivity of metals is significantly higher than solid dielectrics.

When studying the thermal conductivity process, the following basic concepts are used:

Quantity of heat (Q.  ) - Thermal energy, passing over the entire process of the surface of an arbitrary area. In the SI system measured in Joules (J).

Thermal stream (thermal power) (Q.) - The amount of heat passing per unit time through the surface of an arbitrary area.

In the system, the heat flux is measured in watts (W).

The density of the heat flux (q.) - The amount of heat passing per unit time through the surface unit.

In the system SI is measured in W / m 2.

Temperature field- The set of temperature values \u200b\u200bat the moment of time in all points of space occupied by the body. If the temperature at all points of the temperature field over time does not change, then this field is called stationaryif changing, then - nonstationary.

Surfaces formed by points having the same temperature are called isothermal.

Temperature gradient (grad.T.) - The vector directed by normal to the isothermal surface towards the increase in temperature and numerically, defined as the limit of the ratio of the temperature change between two isothermal surfaces by the distance between them by normal, when this distance tends to zero. Or in other words, the temperature gradient is derived from temperature in this direction.

The temperature gradient characterizes the rate of temperature in the direction of normal to the isothermal surface.

The thermal conductivity process characterizes the main law of thermal conductivity - fourier law(1822). According to this law, the density of the heat flux transmitted by means of thermal conductivity is directly proportional to the temperature gradient:

where -thermal conductivity of the substance, W / (MGrad).

The sign (-) shows that the heat flux and temperature gradient are opposite to the direction.

Coefficient of thermal conductivityshows which amount of heat is transmitted per unit of time through the unit of the surface at a temperature gradient equal to one.

The thermal conductivity coefficient is an important thermophysical characteristic of the material and its knowledge is necessary when performing thermal calculations associated with the definition of heat losses through the enclosing structures of buildings and structures, walls of machines and devices, the calculation of thermal insulation, as well as when solving a plurality of other engineering problems.

Another important law of thermal conductivity - fourier-Kirchhoffdetermining the nature of temperature changes in space and in time with thermal conductivity. Other his name - differential equation of thermal conductivityBecause it is obtained by methods of mathematical analysis theory based on the Fourier law. For a 3-dimensional nonstationary temperature field, the differential equation of thermal conductivity is as follows:

where
- temperature coefficient characterizing the thermal properties of the material,

, C p, , respectively, the coefficient of thermal conductivity, the isobaric heat and the density of the substance;

- Laplace operator.

For a one-dimensional stationary temperature field (
) Differential thermal conductivity equation acquires a simple form

Integrating equations (1) and (2), it is possible to determine the density of the heat flux through the body and the law of changes in the temperature inside the body with heat transfer heat transfer. To obtain a solution, you must task conditions of unambiguity.

Terms of unambiguous- These are additional private data characterizing the task in question. They include:

Geometric conditions characterizing the shape and size of the body;

Physical conditions characterizing the physical properties of the body;

temporary (initial) conditions characterizing temperature distribution at the initial moment of time;

boundary conditions characterizing the features of heat exchange at the borders of the body. Distinguish the boundary conditions of the 1st, 2nd and 3rd clan.

For border conditions of the 1st genusthe temperature distribution on the body surface is set. In this case, it is necessary to determine the density of the heat flux through the body.

For boundary conditions of the 2nd kindthe density of the heat flux and the temperature of one of the surfaces of the body is given. It is required to determine the temperature of another surface.

Under the boundary conditions of the 3rd kindconditions of heat transfer between the surfaces of the body and media that wash them outside are known. According to this data, the density of the heat flux is determined. This case refers to the joint heat transfer process with thermal conductivity and convection, called heat transfer.

Consider the simplest example for the case of thermal conductivity through a flat wall. Flatthey call the wall, the thickness of which is significantly less than two other sizes - length and widths. In this case, the conditions of unambiguity can be given as follows:

geometric: Known wall thickness. The temperature field is one-dimensional, consequently the temperature varies only in the direction of the axis x and the heat flux is directed by normal to the wall surfaces;

physical: Known wall material and its thermal conductivity coefficient, and for the whole body \u003d const;

temporary: The temperature field in time does not change, i.e. is stationary;

border conditions: 1st genus, wall temperature components of 1 IT 2.

It is required to determine the law of temperature change in the thickness of the wall T \u003d F (x) and the density of the heat flux through the wallq.

To solve the problem, use equations (1) and (3). Taking into account the received boundary conditions (at x \u003d 0t \u003d t 1; at x \u003d t \u003d t 2) after double integration of the equation (3) we obtain the law of changes in the thickness of the wall

The temperature distribution in the flat wall is shown in Fig. 1.

Fig.1. Temperature distribution in a flat wall.

The density of the heat flux is then determined according to the expression

The determination of the thermal conductivity coefficient cannot give the accuracy of the result required for modern engineering practice, so its experimental definition remains the only reliable way.

One of the known experimental methods of determination is flat layer method. According to this method, the coefficient of thermal conductivity of the material of the plane wall can be determined on the basis of equation (5)

;

In this case, the obtained value of the thermal conductivity coefficient refers to the average temperature value T M \u003d 0.5 (T 1 + T 2).

Despite its physical simplicity, the practical implementation of this method has its own difficulties associated with the difficulty of creating a one-dimensional stationary temperature field in the studied samples and taking into account thermal losses.

Description of the laboratory stand.

The determination of the thermal conductivity coefficient is carried out on a laboratory setup based on the method of simulation modeling of real physical processes. The installation consists of a PEVM associated with the operating plot layout, which is displayed on the monitor screen. The working plot was created by analogy with the real and its scheme presented in Fig. 2.

Fig.2. Installation Scheme Installation

The working plot consists of 2 fluoroplastic samples 12, made in the form of disks thick  \u003d 5 mm and diameterd \u003d 140 mm. Samples are placed between the heater 10 height \u003d 12 mm and the diameter of H \u003d 146 mm and the refrigerator 11, cooled with water. Creating a heat flux is carried out by a heating element with an electrical resistanceR \u003d 41 Ohm and a refrigerator 11 with spiral grooves for the directional circulation of cooling water. Thus, the heat flux passing through the fluoroplastic samples studied is carried out through the refrigerator with water. Part of the heat from the heater goes through the end surfaces into the environment, therefore, to reduce these radial losses, the heat-insulating casing 13, made of asbecement ( K \u003d 0.08 W / (MGrad)), is provided. The casing height К \u003d 22 mm is made in the form of a hollow cylinder with an inner diameter of H \u003d 146 mm and an external diameter of K \u003d 190 mm. The temperature is measured by seven chromel-Copel thermocouple (Type of HC) Pos. 1 ... 7, installed at various points of the working area. The temperature sensor switch 15 allows you to successively measure the thermo-emf of all seven temperature sensors. Thermocouple 7 is installed on the outer surface of the heat-insulating casing to determine thermal leaks through it.

The procedure for carrying out work.

3.1. The temperature mode of the installation is selected by setting the temperature of the hot surface of the plates T g ranging from 35 ° C to 120 ° C.

3.2. On the installation remote control, the power supply devices of the indicator devices record the voltage on the electrical heater U, the thermo-emf temperature sensors are turned on the heating toggle switch.

3.3. Smoothly rotating the rug of the rheostat, the desired voltage is installed on the heater. The retake is made in the stepping version, so the voltage changes step by step. The tension of the temperature should be in accordance with each other according to the dependence on Fig.3.

Fig.3. Working zone of heating.

3.4. By a sequential polling of temperature sensors using the switch 15, thermo-emf values \u200b\u200bof seven thermocouples are determined, which, together with the value, are inscribed in the experiment protocol (see Table 1). Registration of readings is made by indicator devices on the control panel, the readings of which are duplicated on the PEVM monitor.

3.5. At the end of the experience, all regulatory installation authorities are transferred to its original position.

3.6. Repeated experiments are carried out (their number must be at least 3) and with other values \u200b\u200bof T r in the manner prescribed by P.P. 3.1 ... 3.5.

Processing measurement results.

4.1. By graduation characteristics of a chromel-copiel thermocouple temperature sensor readings they are transferred to degrees on the Kelvin scale. .

4.2. The average temperatures of the inner hot and outer cold surfaces of the samples are determined.

where the i was the thermocouple number.

4.3. The complete thermal stream created by an electric heater is determined.

, T.

where U is the voltage of the electric current, in;

R \u003d 41 Ohm - the resistance of the electric heater.

4.4. The thermal stream is determined due to heat transfer through the casing

where the K- coefficient characterizing the heat transfer process through the casing.

, W / (m 2 Grad)

where  K \u003d 0.08 W / (MGrad) is the coefficient of thermal conductivity of the material of the casing;

d H \u003d 0.146 m - the outer diameter of the heater;

d k \u003d 0.190 m - the outer diameter of the casing;

h \u003d 0.012 m - heater height;

h K \u003d 0.022 m - the height of the casing.

T T - the temperature of the outer surface of the casing, determined by the 7th thermocouple

4.5. The thermal stream passing through the samples under study is determined by thermal conductivity

, T.

4.6. The thermal conductivity coefficient of the test material is determined.

, W / (MGrad)

where q  is a heat flux passing through the studied sample by means of thermal conductivity, W;

 \u003d 0.005 m - sample thickness;

- surface area of \u200b\u200bone sample, m 2;

d \u003d 0.140 m - sample diameter;

T g, t x - temperature, respectively, hot and cold surfaces of the sample, K.

4.7. The thermal conductivity coefficient depends on the temperature, so the values \u200b\u200bobtained are connected to the average sample temperature.

The results of the processing of experienced data are recorded in Table 1.

Table 1

Results of measurements and processing experienced data

Thermopar testimony, MV / K

E. 1

4.8. Using the grafoanalytic method for processing the results obtained, the dependence of the thermal conductivity coefficient of the studied material of the average temperature of the sample M in the form of

where  0 ib- are determined by graphically based on the analysis of the characterization of the dependence \u003d f (t m).

CONTROL QUESTIONS

What are the main methods of heat transfer?

What is called thermal conductivity?

What are the features of the thermal conductivity mechanism in conductors and solid dielectrics?

What laws describe the heat conduction process?

What is called flat wall?

What are the boundary conditions?

What is the character of temperature change in a flat wall?

What is the physical meaning of the thermal conductivity coefficient?

What is the knowledge of the coefficient of thermal conductivity of various materials and how is its value determined?

What are the methodological features of the flat layer method?

Proceedings in free convection

purpose of work: To study the patterns of convective heat exchange on the example of heat transfer at free convection for cases of transverse and longitudinal flow of a heated surface. Purchase the skills of processing the results of experiments and representing them in a generalized form.

The task:

1. To determine the experimental values \u200b\u200bof heat transfer coefficients from the horizontal cylinder and the vertical cylinder to the medium with free convection.

2. By processing experimental data to obtain the parameters of the criterion equations characterizing the process of free convection relative to the horizontal and vertical surface.

Basic theoretical provisions.

There are three main methods of heat transfer, significantly different from each other in their physical nature:

thermal conductivity;

convection;

heat radiation.

With thermal conductivity, thermal energy carriers are microparticles of the substance - atoms and molecules, with heat radiation - electromagnetic waves.

Convection- This is a method of heat transfer due to moving macroscopic amounts of a substance from one point of space to another.

Thus, convection is possible only in environments with the property of fluidity - gases and liquids. In the theory of heat exchange, they are generally indicated by the term "liquid"without conducting differences if it is not necessary to negotiate, between drip liquids and gases. The process of transferring heat convection, as a rule, is accompanied by thermal conductivity. Such a process is called convective heat exchange.

Convective heat exchange- This is a joint process of heat transfer convection and thermal conductivity.

In engineering practice, it is most often dealing with the process of convective heat exchange between the solid surface (for example, the surface of the furnace wall, heating device, etc.) and a fluid is washing this surface. This process is called heat Press.

Heat Pot.- A special case of convective heat exchange between the surface of the solid (wall) and the flushing fluid.

Distinguish forced and free (natural)convection.

Forced convectionit occurs under the action of pressure forces, which are created forcibly, for example, a pump, a fan, etc.

Free or natural convectionit occurs under the action of mass forces having a different nature: gravitational, centrifugal, electromagnetic, etc.

On Earth, free convection occurs in the conditions of gravity, so it is called thermal gravity convection. The driving force of the process in this case is the lifting force, which occurs in the medium in the presence of inhomogeneity in the distribution of density within the volume under consideration. With heat exchange, such inhomogeneity occurs due to the fact that individual elements of the medium can be at different temperatures. At the same time, more heated, and therefore, less dense elements of the medium under the action of the lifting force will move upwards, carrying with them warmth, and coolest, and therefore, the more dense elements of the medium will flow to the freed place, as shown in Fig. one.

Fig. 1. The nature of the movement of streams in fluid during free convection

If a permanent heat source is located in this place, then when heated, the density of heated elements of the medium will decrease, and they will also begin to pop up. So, while the difference of densities of individual elements of the medium will take place, their cycle will continue, i.e. Free convection will continue. Free convection occurring in large areas of the environment, where nothing prevents the development of convective flows, is called free convection in unlimited space. Free convection in an unlimited space, for example, takes place when heating the premises, heating water in hot water boilers and many other cases. If the development of convective flows prevented the walls of the channels or grounds, which are filled with a fluid, then the process in this case is called free convection in limited space. Such a process takes place, for example, with heat exchange inside the aircrafts between the window frames.

The main law describing the convective heat exchange process - newton Richmana Law. In analytical form for the stationary temperature regime of heat exchange, it has the following form:

where
- Elementary amount of heat, given for an elementary period of time
from the elementary surface area
;

- temperature of the wall;

- fluid temperature;

- The heat transfer coefficient.

The heat transfer coefficientshows how the amount of heat is given per unit of time from the unit of the surface with the temperature difference between the wall and the liquid in one degree. The unit of measurement of the heat transfer coefficient in the system C - W / m 2 ∙ Grad. With the steady steady process, the heat transfer coefficient can be determined from the expression:

, W / m 2 ∙ hail

where - thermal stream, W;

- surface area of \u200b\u200bheat exchange, m 2;

- Temperature pressure between the surface and liquid, hail.

The heat transfer coefficient characterizes the intensity of the heat exchange between the wall and the liquid washing it. In its physical nature, convective heat exchange is a very complex process. The heat transfer coefficient depends on the very large number of different parameters - the physical properties of the fluid, the nature of the flow of fluid, the flow rate of the fluid, size and shape of the channel, as well as many other factors. In this regard, it is impossible to give overall dependence to find the heat transfer coefficient theoretical

The heat transfer coefficient is most accurate and reliably determined by an experimental pathway based on equation (2). However, in engineering practice, when calculating heat exchange processes in various technical devices, as a rule, it is not possible to perform an experimental determination of the value of the heat transfer coefficient in a real field facility due to the complexity and high cost of this experiment. In this case, to solve the task of determining the assistance comes theory of similarity.

The main practical importance of the theory of similarity is that it allows us to summarize the results of a separate experience conducted on the model in laboratory conditions, on the entire class of real processes and objects similar to the process studied on the model. The concept of similarity, well known for geometric shapes, can also be distributed to any physical processes and phenomena.

Class of physical phenomena- This is a combination of phenomena that can be described by one common system of equations and having the same physical nature.

Unit phenomenon- This is part of the class of physical phenomena, distinguished by certain conditions of unambiguity (geometric, physical, initial, boundary).

Similar phenomena- A group of phenomena of one class with the same unambiguing conditions, except for the numerical values \u200b\u200bof the values \u200b\u200bcontained in these conditions.

The theory of similarity is based on the fact that the dimensional physical quantities characterizing the phenomenon can be combined into dimensional complexes, So, so that the number of these complexes will be less than the number of dimensional values. Received dimensionless complexes are called criteria like. The similarity criteria have a certain physical meaning and reflect the effect of not one physical quantity, and all of their combination, which is included in the criterion, which significantly simplifies the analysis of the process being studied. The process itself in this case can be represented as an analytical dependence.
between the criteria of similarity
characterizing its individual sides. Such dependencies are called criteria equations. The criteria of similarity received names on the names of scientists who have made a significant contribution to the development of hydrodynamics and the theory of heat exchange - Nusselt, Prandtle, Graolsgof, Reynolds, Kirpicheva and others.

The theory of similarity is based on the 3th similarity theorems.

1st theorem:

Similar phenomena have the same similarity criteria.

This theorem shows that in experiments, only those physical quantities that are contained in the similarity criteria should be measured.

2nd theorem:

The initial mathematical equations characterizing this physical phenomenon can always be represented as a relationship between the similarity criteria characterizing this phenomenon.

These equations are called criteria. This theorem shows that experiments should be submitted in the form of criteria equations.

3rd theorem.

Those phenomena in which the similarity criteria drawn up from the definition conditions are equal.

This theorem defines the condition necessary to establish a physical similarity. The criteria of similarities compiled from the conditions of unambiguity are called defining. They determine equality of all others or definedthe criteria of similarity, which actually is already the subject of the 1st similarity theorem. Thus, the 3rd similarity theorem develops and deepens the 1st theorem.

When studying convective heat exchange, the following similarity criteria are most often used.

Reynolds criterion (Re.) - characterizes the ratio between the inertia forces and viscous friction forces acting in the liquid. The value of the Reynolds criterion characterizes the flow of fluid flow during forced convection.

where - fluid speed;

- the coefficient of the kinematic viscosity of the fluid;

- determining size.

Grasgood criterion (GR.) - characterizes the ratio between the viscous friction forces and the lifting force acting in the liquid, during free convection. The value of the Grasgood criterion characterizes the flow of fluid flow during free convection.

where - acceleration of gravity;

- determining size;

- Temperature coefficient of volume expansion of the fluid (for gases
where - determining temperature on the Kelvin scale);

- temperature head between the wall and liquid;

- respectively, the temperature of the wall and liquid;

- The coefficient of the kinematic viscosity of the fluid.

Nusselt criterion (Nu.) - characterizes the relationship between the amount of heat transmitted by means of thermal conductivity and the amount of heat transmitted by convection under convective heat exchange between the surface of the solid (wall) and liquid, i.e. With heat transfer.

where - heat transfer coefficient;

- determining size;

- The coefficient of thermal conductivity of the liquid on the edge of the wall and liquid.

Pakele's criterion (PE) - characterizes the relationship between the amount of heat taken (given) by the flow of fluid and the amount of heat transmitted (given) by means of convective heat exchange.

where - fluid flow rate;

- determining size;

- temperature coefficient;

- respectively, the coefficient of thermal conductivity, the isobaric heat, the density of the liquid.

Prandtl criterion (Pr.) - characterizes the physical properties of the liquid.

where - coefficient of kinematic viscosity;

- The coefficient of temperature fluid.

From the considered criteria, similarity shows that the most important parameter characterizing the intensity of the process, namely, the heat transfer rate is in the expression for the criterion of nusselt. This led to the fact that to solve the problems of convective heat transfer engineering methods based on the use of similarity theory, this criterion is the most important of the defined criteria. The value of the heat transfer coefficient in this case is determined according to the following expression

In this regard, the criterion equations are usually written in the form of a solution relative to the criterion of Nusselt and have a type of power function.

where
- the values \u200b\u200bof the criteria of similarities characterizing different sides of the process under consideration;

- Numeric constants defined on the basis of experimental data obtained when studying the class of similar phenomena on models by experimental means.

Depending on the type of convection and the specific conditions of the process, the set of similarity criteria included in the criteria equation, the values \u200b\u200bof the constants and the correction factor may be different.

With the practical application of criteria equations, the question of the right choice of the decisive size and the decisive temperature is important. The determining temperature is necessary to correctly determine the values \u200b\u200bof the physical properties of the liquid used in the calculation of the values \u200b\u200bof the similarity criteria. The choice of determining size depends on the mutual location of the fluid flow and the washed surface, that is, on the nature of its flowing. This should be guided by the existing recommendations for the following characteristic cases.

Forced convection when moving fluid inside a round tube.

- the inner diameter of the pipe.

Forced convection when the fluid moves in the channels of an arbitrary section.

- equivalent diameter,

where - the cross-sectional area of \u200b\u200bthe channel;

- Perimeter of section.

Transverse flow of a round tube with free convection (horizontal pipe (see Fig. 2) with heat gravitational convection)

- Outer diameter of the pipe.

Fig.2. The nature of the flow around the horizontal pipe with thermal gravitational convection

Longitudinal flow around a flat wall (pipe) (see Fig. 3) with thermal gravitational convection.

- Wall height (pipe length).

Fig. 3. The nature of the flow around the vertical wall (pipe) with thermal gravitational convection.

Determining temperature it is necessary for the correct determination of the thermophysical properties of the medium, the values \u200b\u200bof which vary depending on the temperature.

In the heat transfer as a decisive temperature, the arithmetic average of the temperature of the wall and liquid is taken.

In convective heat exchange between the individual elements of the medium within the volume of the volume under consideration, the arithmetic temperature between the temperatures of the medium elements participating in the heat exchange is taken as the determining temperature.

In this paper, the procedure for conducting a laboratory experiment and the method of obtaining criteria equations for 2 characteristic cases of flow around the heated surface (transverse and longitudinal) with free convection of various gases relative to horizontal and vertical cylinders were considered.

EXPERIMENTAL PART.

In the process of their thermal traffic. In liquids and solid telescotters - heat transfer is carried out by direct transmission of thermal motion of molecules and atoms to neighboring particles of the substance. In gaseous bodies, the propagation of heat thermal conductivity is due to the exchange of energy during the collision of molecules having a different heat movement speed. In metals, thermal conductivity is carried out mainly due to the movement of free electrons.

A number of mathematical concepts are included in the main height of thermal conductivity, which is advised to remind and explain.

Temperature field - This is a co-intensity of temperature values \u200b\u200bat all points of the body at the moment time-nor. Mathematically it is described by imide t. = f.(x, Y, Z, τ). Distinguish stationary temperature the field when the temperature in all points of the body does not depend on the time (does not change over time), and nonstationary temperature field. In addition, if the temperature changes only by one or two spatial coordinates, the temperature field is based on one or two-dimensional, respectively.

Isothermal surface - This is a geometric point of points, the temperature in which is the same.

Temperature gradient — grad T.there is a vector directed by Nor-Mali to an isothermal surface and numerically equal to the derivative of the temperature in this direction.

According to the basic law of heat-conductivity - the law Fourier (1822), the density vector of the heat flux transmitted by thermal conductivity is proportional to the temperature gradient:

Q. = - λ grad T., (3)

where λ - coefficient of thermal water-water substance; His unit of measure T./(m · K.).

The minus sign in equation (3) will indicate that vector q. Directed oppositely vector grad T.. Regarding the greatest temperature reduction.

Heat flow ΔQ. through arbitrary-but-oriented elementary light spare df.equal to the scalar product q. On the vector of elementary platform df., and full heat flow Q.through the entire surface F.determined by the integration of this product on the surface F:

COEFFICIENT OF THERMAL CONDUCTIVITY

Coefficient of thermal conductivity λ in law Fourier (3) characterizes the proportion of this substance to carry out heat. The values \u200b\u200bof heat-wire coefficients are given in reference books in the thermal properties of substances. Numerically coefficient of thermal conductivity λ \u003d q /grad. t. equal to the density of heat flux q. under temperature gradient grad T. = 1 K / M.. The highest heat conduction is the light gas - hydrogen. Under room conditions, the thermal conductivity coefficient of hydrogen λ = 0,2 T./(m · K.). In heavier gases, the thermal conductivity is less - the WHO λ = 0,025 T./(m · K.), in UG-Leroda dioxide λ = 0,02 T./(m · K.).

Clean silver and copper are the highest thermal conductivity coefficient: λ = 400 T./(m · K.). For carbon steels λ = 50 T./(m · K.). In liquids, the thermal conductivity coefficient is usually less than 1 T./(m · K.). Water is one of the best liquid heat conductors for her λ = 0,6 T./(m · K.).

The coefficient of thermal conductivity of non-metallic solid materials is usually below 10 T./(m · K.).

Porous materials - cork, various fibrous fillers such as organic wool - have the smallest thermal conductivity coefficients λ <0,25 T./(m · K.), approaching at low damping density to the thermal conductivity coefficient of air flowing.

A significant effect on the coefficient of thermal conductivity can be the temperature, pressure, and the porous materials also humidity. The reference books always provide conditions under which the thermal conductivity coefficient of this substance was determined, and for other conditions it is impossible to use. Ranges of values λ For various materials, shown in Fig. one.

Fig.1. Intervals of the values \u200b\u200bof thermal conductivity coefficients of various substances.

Heat transfer thermal conductivity

Uniform flat wall.

The pro-stealth and very common Duma, solved the theory of heat exchange, is determining the density of the heat-thread transmitted through the flat wall thickness δ , in the inference of which are supported by tempo t W1 and t W2.(Fig.2). Temperature varies only by plate thickness - by one coordinate x.Such a dacha is called one-dimensional, solutions to their most simple, and in this course we will restrict ourselves to considering only one-number tasks.

Considering that for one-number case:

Grad T. = dT / DX, (5)

and using the basic law of thermal conductivity (2), we obtain the differentiation equation of stationary heat-wires for a flat wall:

In stationary conditions, when the energy is not spent on heating, the density of the heat flux q.unchanged in the thickness of the wall. In most practical tasks, it is estimated that the heat-conductivity coefficient is λ It does not depend on the temperature and the same over the entire thickness of the wall. Value λ Find in reference books at temperatures:

middle between the surface temperatures of the wall. (The error of calculations is usually less than the error of the source data and table values, and with the linear dependence of the thermal conductivity coefficient of temperature: λ \u003d a + btaccurate calculation formula for q.it does not differ from the approximate). For λ \u003d const.:

(7)

those. Temperature dependence t.from coordinate h. Linene (Fig. 2).

Fig.2. Stationary distribution of the temperature of a flat wall thickness.

Dividing the variables in equation (7) and injecting t. from t W1 before t W2. and in h. from 0 to δ :

, (8)

we obtain addiction to calculate the density of the heat flux:

, (9)

or thermal stream power (heat flux):

(10)

Consequently, the amount of heat transmitted after 1 m 2. Walls, directly proportional to the thermal conductivity coefficient λ and the difference in the temperature of the outer surfaces of the wall ( t W1 - T W2) and inversely proportional to the wall thickness δ . The total amount of heat through the wall area F. also in proportion to this area.

The resulting simplest formula (10) is very widespread in heat calculations. According to this formula, not only calculate the density of the heat flux through the flat walls, but also make estimates for cases of more complex, adjustably replacing in the calculations of the wall of a complex configuration onto a flat wall. Sometimes on the basis of the assessment, one or another is rejected without long-term time spent on its detailed elaboration.

Body temperature at point h.determined by the formula:

t x \u003d t w1 - (t w1 - t w2) × (x × d)

Attitude λf / Δ. called the heat-woofer, and the inverse Δ / λf. thermal or thermal resistance of the wall and is indicated R λ.. Using the concept of thermal approximation, the formula for calculating the thermal stream can be represented as:

Dependence (11) is similar to the law Omar In the electrical-ke (the power of the electric current is equal to the differentness of the potentials divided by the electric resistance of the conductor, the current flows).

Very often, thermal resistance is called the magnitude Δ / λ, which is equal to the thermal resistance of the flat wall area 1 m 2..

Examples of calculations.

Example 1.. Determine the heat flux through the concrete wall of the building 200 mM.Height H. = 2,5 m. and 2 long m.If the temperatures on its surfaces: t C1. \u003d 20 0 s, t C2. \u003d - 10 0 C, and heat-conducting coefficient λ =1 T./(m · K.):

= 750 T..

Example 2.. Determine the coefficient of thermal conductivity of the material of the wall thick 50 mM., if the density of the heat flux through it q. = 100 T./m 2., and temperature difference on surfaces {!LANG-10a53fd472da7245a2fdeccce48d4fd2!}{!LANG-9cc67daa7ff5b94e0548544d23b08a72!}

T./(m · K.).

{!LANG-de82191546e64e222aaa0cab948f7b54!}.

{!LANG-26fea2b32c46c5b97e2660de025a4e83!} {!LANG-fe13119fb084fe8bbf5fe3ab7cc89b3b!}{!LANG-da63fa64cd8900d2194b3e3398fa3715!}

{!LANG-4b5bd8e2f2ac561b65108c6ba4079881!}

{!LANG-b474dc3afda2ab86effefb707849ab6d!}

(12)

{!LANG-271dc387133cec5fc949ac561f12561e!} t W1{!LANG-8eefcd256d51352e9a043cb316173645!} {!LANG-03d35be2df984143ab28d258bc686984!}:

, (13)

where {!LANG-372e25f23b5a8ae33c7ba203412ace30!}{!LANG-16a4ec5d6cb9087ad45bd744dfd1427c!}

{!LANG-3289732defa9be3252af96d36f805045!}

. (14)

{!LANG-1d26ecdb360db3113fd01580f2722130!}

{!LANG-1ec56ad01ce6dce4259e8d06011ce101!} {!LANG-46e29e7e0aa8e6033dbbc285732f3cab!}{!LANG-4e47b8589dc7cf42814cd82b0d141f39!} {!LANG-46e29e7e0aa8e6033dbbc285732f3cab!}{!LANG-fe0849e3bccbe2b4c0d42534b27b6ecb!} Q.{!LANG-7b1f81cfbafb692871a47790a2e2e3fe!}

{!LANG-441399c303842d9df49e12fd08c6e7a3!} {!LANG-73951227866a84ed83267e5d7639c602!}){!LANG-baee7a91bdadc37f4fc42a8e77c2e2d8!} = - {!LANG-544edffecbe2e0a8ba0085e7f4e0402e!}{!LANG-739755b8b0e133e1b10f9994e93749ca!}

{!LANG-4ad9ca3381354e905e6d5a97db45c745!}

{!LANG-d2652dca5ed93cf2e3c4ed158753a1ae!}

{!LANG-6878de25640c19f39907c55793409f07!}

{!LANG-647308d71ee10d2c130d36f5da1f47ac!}

{!LANG-591d3e682474dd01779b29355fd61100!}{!LANG-f905460d3e7352f7bfc23ea3a412657c!}

{!LANG-9f1acc8726fb266f207a023e70bfafa9!} {!LANG-01fbdc44ef819db6273bc30965a23814!}{!LANG-77b5c6cdefb9337057e81a693a3f43c2!} {!LANG-7dc8acd332b2ed80c46334ffbc06a5bb!}{!LANG-e9e0887ab6c2874be873fe1d48cb513e!} mM.{!LANG-73d452817753800cc435f61195e5953b!} mM..

{!LANG-0fd59dd52dd43c20d7997c73a6456008!}

{!LANG-a97b37b5988113c657e61cf61ea5d33a!}{!LANG-23d6c44b93d852b79edcd1619948db21!}

{!LANG-4b29dd8045756ead8ff2b3cba9a57ddd!}

{!LANG-ad4f426437559d5edfaf92ece7514e1b!}

{!LANG-1833faf6f0f85b52de688595a2bd9cde!}

{!LANG-be8bd0138ee710859f93833ce4436ca6!}

{!LANG-43630723e09f46a3b885dfe109b2df77!}

{!LANG-7a0e7c65eb36dbc7f31461858a771bf2!}

{!LANG-59d1fe75b2c9ce9d1128e01be16fb5d1!}

{!LANG-2bf40b5ffce954f0007f0e55b09f7662!}.

{!LANG-b4e3428ac3033513e1bad4bd7c09350d!}

{!LANG-998611924c890ce0eae0caf36736ddc5!} l.{!LANG-be7f48c1374d7cd9dd8d0a51aafe57de!}

{!LANG-499bf36b884b1ad6b437848191453ba7!}

. (15)

{!LANG-2f0769b4b2810100c155e553bfd2c84a!} λ {!LANG-e3a325ae6bd454b95d155bb08cd5e19b!} l.{!LANG-257aa6b664267ee22d4e9cac21b1e6c7!} t W1 - T W2{!LANG-302f844a0302771a2a2690d2477349c8!} {!LANG-11f367cca671a8444ee81da6110ab94c!}{!LANG-cdd80e6fe018be54dc9e781b712a8a2e!} {!LANG-6e7f53d973518db8071e4af332663253!}.

{!LANG-a8979c4b03e026aa7b1beaa57cad2da7!}

For λ {!LANG-6d4ae57a8c7d6bd020f8eebe9275677b!} r.{!LANG-08ce00291b4804a4798a937b87295f44!}

{!LANG-b02b2c3b13eb9430c2ad95845098c405!}{!LANG-0f9408b39a7ae833b7b9bd3aff03c654!} mM.{!LANG-9fae8f02a9d9f375d7c923ec48fc5a41!} mM.{!LANG-81ed3f16ec51579ec3dc37538ab8383e!} {!LANG-49d19f5c4c365d4f27acd86bcc810571!} . = 0,5 T./(m · K.); {!LANG-0f0d07afc7201f882e4cb15966f6a262!} . = 0,05 T./(m · K.).