Determination of the thermal conductivity of a liquid by the hot wire method. Some methods for determining thermal conductivity. Equipment and materials

Thermal conductivity is the most important thermophysical characteristic of materials. It must be taken into account when designing heating devices, choosing the thickness of protective coatings, taking into account heat losses. If there is no corresponding reference book at hand or available, and the composition of the material is not known exactly, its thermal conductivity must be calculated or measured experimentally.

Components of thermal conductivity of materials

Thermal conductivity characterizes the process of heat transfer in a homogeneous body with certain dimensions. Therefore, the initial parameters for the measurement are:

Area in the direction perpendicular to the direction of the heat flow.
The time during which the transfer of heat energy occurs.
Temperature difference between separate, most distant from each other parts of a part or test sample.
Heat source power.

To maintain the maximum accuracy of the results, it is required to create stationary (established in time) heat transfer conditions. In this case, the time factor can be neglected.

Thermal conductivity can be determined in two ways - absolute and relative.

Absolute method for assessing thermal conductivity

In this case, the direct value of the heat flux is determined, which is directed to the sample under study. Most often, the sample is taken as a rod or plate, although in some cases (for example, when determining the thermal conductivity of coaxially placed elements) it may look like a hollow cylinder. The disadvantage of lamellar samples is the need for strict plane-parallelism of opposite surfaces.

Therefore, for metals characterized by high thermal conductivity, a sample in the form of a rod is more often accepted.

The essence of the measurements is as follows. On opposite surfaces, constant temperatures are maintained arising from a heat source that is located strictly perpendicular to one of the sample surfaces.

In this case, the sought parameter of thermal conductivity λ will be
λ = (Q * d) / F (T2-T1), W / m ∙ K, where:
Q is the power of the heat flow;
d is the thickness of the sample;
F is the sample area affected by the heat flux;
Т1 and Т2 are temperatures on the sample surfaces.

Since the power of the heat flux for electric heaters can be expressed through their power UI, and temperature sensors connected to the sample can be used to measure the temperature, it will not be difficult to calculate the thermal conductivity λ.

In order to eliminate unproductive heat loss and improve the accuracy of the method, the sample and heater assembly should be placed in an effective heat-insulating volume, for example, in a Dewar vessel.

Relative method for determining thermal conductivity

The factor of heat flux power can be excluded from consideration if one of the methods of comparative assessment is used. For this purpose, a reference sample is placed between the rod, the thermal conductivity of which is required to be determined, and the heat source, the thermal conductivity of the material of which λ 3 is known. To eliminate measurement errors, the samples are tightly pressed against each other. The opposite end of the sample to be measured is immersed in a cooling bath, after which two thermocouples are connected to both rods.

Thermal conductivity is calculated from the expression
λ = λ 3 (d (T1 3 -T2 3) / d 3 (T1-T2)), where:
d is the distance between thermocouples in the test sample;
d 3 is the distance between thermocouples in the reference sample;
T1 3 and T2 3 - readings of thermocouples installed in the reference sample;
T1 and T2 - readings of thermocouples installed in the test sample.

Thermal conductivity can also be determined from the known electrical conductivity γ of the sample material. For this, a wire conductor is taken as a test sample, at the ends of which a constant temperature is maintained in any way. A constant electric current with a force I is passed through the conductor, and the terminal contact should be close to ideal.

Upon reaching a stationary thermal state, the temperature maximum T max will be located in the middle of the sample, with the minimum values of T1 and T2 at its ends. By measuring the potential difference U between the extreme points of the sample, the value of thermal conductivity can be established from the dependence

The accuracy of assessing thermal conductivity increases with an increase in the length of the test sample, as well as with an increase in the current that is passed through it.

Relative methods for measuring thermal conductivity are more accurate than absolute ones, and are more convenient in practical application, however, they require a significant investment of time to perform measurements. This is due to the duration of the establishment of a stationary thermal state in the sample, the thermal conductivity of which is determined.

Many methods have been used to measure thermal conductivity in the past. At present, some of them are outdated, but their theory is still of interest, since they are based on solutions of the heat conduction equations for simple systems, which are often encountered in practice.

First of all, it should be noted that the thermal properties of any material are manifested in various combinations; however, if considered as characteristics of the material, then they can be determined from various experiments. Let us list the main thermal characteristics of bodies and the experiments from which they are determined: a) the coefficient of thermal conductivity measured in a stationary mode of the experiment; b) heat capacity per unit volume, which is measured by calorimetric methods; c) the value measured in a periodic stationary mode of experiments; d) thermal diffusivity x, measured in a non-stationary mode of experiments. In fact, most experiments carried out in a non-stationary mode, in principle, admit both the definition and the definition

We will briefly describe the most common methods here and indicate the sections in which they are discussed. In essence, these methods are divided into those in which measurements are carried out in a stationary mode (stationary mode methods), with periodic heating and in a non-stationary mode (non-stationary mode methods); further they are divided into methods used in the study of bad conductors and in the study of metals.

1. Methods of a stationary regime; bad guides. In this method, the conditions of the main experiment described in § 1 of this chapter should be exactly fulfilled, and the material under study should have the shape of a plate. In other versions of the method, it is possible to investigate a material in the form of a hollow cylinder (see § 2, Chapter VII) or a hollow sphere (see § 2, Chapter IX). Sometimes the material under study, through which heat passes, has the shape of a thick rod, but in this case the theory turns out to be more complicated (see §§ 1, 2 of Chapter VI and § 3 of Chapter VIII).

2. Thermal methods of stationary regime; metals. In this case, a rod-shaped metal sample is usually used, the ends of which are maintained at different temperatures. A semi-bounded rod is considered in § 3 of Ch. IV, and a rod of finite length - in § 5 of Ch. IV.

3. Stationary electrical methods, metals. In this case, a metal sample in the form of a wire is heated by passing an electric current through it, and its ends are maintained at specified temperatures (see § 11 Chapter IV and Example IX § 3 Chapter VIII). You can also use the case of a radial heat flux in a wire heated by an electric current (see example V, § 2, Chapter VII).

4. Methods of stationary regime of moving fluids. In this case, the temperature of the liquid moving between two reservoirs is measured, in which different temperatures are maintained (see § 9, Ch. IV).

5. Methods of intermittent heating. In these cases, the conditions at the ends of the rod or plate change with a period after reaching a steady state, the temperatures are measured at certain points of the sample. The case of a semi-bounded rod is considered in § 4 of Ch. IV, and a rod of finite length - in § 8 of the same chapter. A similar method is used to determine the thermal diffusivity of soil during temperature fluctuations caused by solar heating (see, § 12, Chapter II).

Recently, these methods have begun to play an important role in measurements of low temperatures; they also have the advantage that in the theory of relatively complex systems one can use the methods developed for the study of electric waveguides (see § 6, Ch. II).

6. Methods of non-stationary regime. In the past, transient-mode methods have been used somewhat less than steady-state methods. Their disadvantage lies in the difficulty of establishing how the actual boundary conditions in the experiment agree with the conditions postulated by the theory. It is very difficult to take into account such a discrepancy (for example, when it comes to contact resistance at the boundary), and this is more important for the indicated methods than for the methods of the stationary regime (see § 10, Chapter II). At the same time, the methods of the unsteady mode themselves have certain advantages. Thus, some of these methods are suitable for very fast measurements and for taking into account small temperature changes; in addition, a number of methods can be used "on site" without sample delivery to the laboratory, which is highly desirable, especially when examining materials such as soil and rocks. Most older methods use only the last portion of the temperature versus time graph; in this case, the solution of the corresponding equation is expressed by one exponential term. In Section 7, Ch. IV, § 5 Ch. VI, § 5 chap. VIII and § 5 chap. Section IX considers the case of cooling a body of simple geometrical shape with linear heat transfer from its surface. In Section 14, Ch. IV, the case of unsteady temperature in a wire heated by electric current is considered. In some cases, the entire temperature curve at a point is used (see § 10 Ch. II and § 3 Ch. III).

GOST 7076-99

UDC 691: 536.2.08: 006.354 Group Ж19

INTERSTATE STANDARD

CONSTRUCTION MATERIALS AND PRODUCTS

Method for determining thermal conductivity and thermal resistance

at stationary thermal conditions

BUILDING MATERIALS AND PRODUCTS

Method of determination of steady-state thermal

conductivity and thermal resistance

Date of introduction 2000-04-01

Foreword

1 DEVELOPED by the Research Institute of Building Physics (NIISF) of the Russian Federation

INTRODUCED by Gosstroy of Russia

2 ADOPTED by the Interstate Scientific and Technical Commission for Standardization, Technical Regulation and Certification in Construction (ISTC) on May 20, 1999

State name	The name of the state body construction management
Republic of Armenia	Ministry of Urban Development of the Republic of Armenia
The Republic of Kazakhstan	Committee for Construction of the Ministry of Energy, Industry and Trade of the Republic of Kazakhstan
Republic of Kyrgyzstan	State Inspection for Architecture and Construction under the Government of the Kyrgyz Republic
The Republic of Moldova	Ministry of Territorial Development, Construction and Public Utilities of the Republic of Moldova
Russian Federation	Gosstroy of Russia
The Republic of Tajikistan	Committee for 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 REPLACE GOST 7076-87

4 PUT INTO EFFECT from April 1, 2000 as a state standard of the Russian Federation by the decree of the Gosstroy of Russia dated December 24, 1999 No. 89

Introduction

This International Standard is harmonized with ISO 7345: 1987 and ISO 9251: 1987 in terms of terminology and complies with the main provisions of ISO 8301: 1991, ISO 8302: 1991, which establish methods for determining thermal resistance and effective thermal conductivity using a device equipped with a heat meter and a device with a hot security zone.

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

1 area of use

This standard applies to building materials and products, as well as 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.

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

GOST 166-89 Calipers. Technical conditions

GOST 427-75 Measuring metal rulers. Technical conditions

GOST 24104-88 Laboratory balance for general purpose and exemplary. General specifications

3 Definitions and symbols

3.1 For the purposes of this standard, the following terms are used with appropriate definitions.

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

Heat flux density- heat flux passing through a unit of area.

Stationary thermal conditions- a mode in which all the considered thermophysical parameters do not change with time.

Thermal resistance of the sample- the ratio of the temperature difference between the front faces of the sample to the heat flux density under the conditions of a stationary thermal regime.

Average sample temperature- the arithmetic mean of the temperatures measured on the front faces of the sample.

Effective thermal conductivityl eff material(corresponds to the term "coefficient of thermal conductivity", adopted in the current standards for building heat engineering) - the ratio of the thickness of the test sample of material dTo its thermal resistance R.

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

Table 1

Designation	The quantity	unit of measurement
l eff	Effective thermal conductivity	W / (m × K)
	Thermal resistance	m 2 × K / W
	Sample thickness before testing
	Thermal resistance of standard samples	m 2 × K / W
D T 1, D T 2	The temperature difference between the front faces of standard samples
e 1, e 2	Output signals of the heat meter of the device when it is calibrated using standard samples
f 1, f 2	Calibration coefficients of the heat meter of the device when it is calibrated using standard samples	W / (mV × m 2)
	Sample thickness during testing
	Thermal resistance of the test piece	m 2 × K / W
	Relative change in sample weight after drying
	The relative change in the mass of the sample during the test
	Sample weight when received from the manufacturer
	Sample weight after drying
	Sample weight after testing
D T u	Temperature difference between the faces of the test specimen
	Average temperature of the test piece
	Temperature of the hot face of the test specimen
	Cold face temperature of the test specimen
	The value of the calibrating coefficient of the heat meter of the device, corresponding to the value of the heat flux flowing through the test specimen after the establishment of a stationary thermal regime (with an asymmetric test scheme)	W / (mV × m 2)
	The output signal of the heat meter of the device after the establishment of a stationary heat flux through the test sample (with an asymmetric test scheme)
	Thermal resistance between the face of the sample and the working surface of the device plate
l effu	Effective thermal conductivity of the test piece material	W / (m × K)
	Thermal resistance of sheet material from which the bottom and cover of the box for a sample of bulk material are made	m 2 × K / W
f ¢ u , f² u	The values of the calibration coefficient of the first and second heat meters of the device corresponding to the value of the heat flux flowing through the test specimen after the establishment of a stationary thermal regime (with a symmetric test scheme)	W / (mV × m 2)
e ¢ u , e² u	The output signal of the first and second heat meters after the establishment of a stationary heat flux through the test sample (with a symmetrical test scheme)
	Density of the stationary heat flux passing through the test specimen
	Measurement area
	Electric power supplied to the heater of the measuring zone of the hot plate of the device

4 General

4.1 The essence of the method consists in creating a stationary heat flux passing through a flat sample of a certain thickness and directed perpendicular to the front (largest) faces of the sample, measuring the density of this heat flux, the temperature of the opposite front faces and the thickness of the sample.

4.2 The number of samples required to determine the effective thermal conductivity or thermal resistance and the sampling procedure should be specified in the standard for the specific material or product. If the standard for a specific material or product does not specify the number of samples to be tested, the effective thermal conductivity or thermal resistance is determined on five samples.

4.3 The temperature and relative humidity of the air in the room in which the tests are carried out should be (295 ± 5) K and (50 ± 10)%, respectively.

5 Measuring instruments

To carry out the test, apply:

a device for measuring effective thermal conductivity and thermal resistance, certified in accordance with the established procedure and meeting the requirements given in Appendix A;

a device for determining the density of fibrous materials in accordance with GOST 17177;

a device for determining the thickness of flat fibrous products in accordance with GOST 17177;

drying electrical cabinet, the upper heating limit of which is not less than 383 K, the limit of the permissible error of setting and automatic temperature control is 5 K;

vernier caliper in accordance with GOST 166:

For measuring external and internal dimensions with a measurement range of 0-125 mm, a vernier reading value - 0.05 mm, a margin of error - 0.05 mm;

For measuring external dimensions with a measurement range of 0-500 mm, a vernier counting value - 0.1 mm, a permissible error limit of -0.1 mm;

metal measuring ruler in accordance with GOST 427 with an upper measurement limit of 1000 mm, a limit of permissible deviation from the nominal values of the length of the scale and the distance between any stroke and the beginning or end of the scale - 0.2 mm;

laboratory scales for general use in accordance with GOST 24104:

With the maximum weighing limit of 5 kg, the division value is 100 mg, the standard deviation of the readings of the scales is no more than 50.0 mg, the error from the unequal balance of the rocker arm is no more than 250.0 mg, the limit of permissible error is 375 mg;

With the maximum weighing limit of 20 kg, the division value is 500 mg, the standard deviation of the balance readings is no more than 150.0 mg, the error from the unequal balance of the rocker arm is no more than 750.0 mg, the limit of permissible error is 1500 mg.

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

6 Test preparation

6.1 A sample is made in the form of a rectangular parallelepiped, the largest (front) faces of which have the shape of a square with a side equal to the side of the working surfaces of the device plates. If the working surfaces of the plates of the device have the shape of a circle, then the largest edges of the sample should also have the shape of a circle, the diameter of which is equal to the diameter of the working surfaces of the plates of the device (Appendix A, clause A. 2.1).

6.2 The thickness of the test piece shall be at least five times less than the face edge length or diameter.

6.3 The edges of the sample in contact with the working surfaces of the plates of the device should be flat and parallel. The deviation of the front faces of the rigid specimen from parallelism should not be more than 0.5 mm.

Rigid specimens with thickness differences and deviations from flatness are ground.

6.4 The thickness of the parallelepiped specimen is measured with a caliper with an error of not more than 0.1 mm in four corners at a distance of (50.0 ± 5.0) mm from the corner apex and in the middle of each side.

The thickness of the disk specimen is measured with a caliper with an error of not more than 0.1 mm along the generatrices located in four mutually perpendicular planes passing through the vertical axis.

The arithmetic mean of the results of all measurements is taken as the thickness of the sample.

6.5 The length and width of the specimen in the plan are measured with a ruler with an error of not more than 0.5 mm.

6.6 The correctness of the geometric shape and dimensions of a sample of heat-insulating material is determined in accordance with GOST 17177.

6.7 The average size of inclusions (filler granules, large pores, etc.) that differ in their thermophysical characteristics from the main sample should not exceed 0.1 of the sample thickness.

It is allowed to test a sample with heterogeneous inclusions, the average size of which exceeds 0.1 of its thickness. The test report shall state the average size of the inclusions.

6.8 Determine the mass of the sample M 1 when received from the manufacturer.

6.9 The sample is dried to constant weight at the temperature specified in the normative document for the material or product. A sample is considered dried to constant weight if its weight loss after another drying for 0.5 h does not exceed 0.1%. At the end of drying, determine the mass of the sample M 2 and its density r u, after which the sample is immediately placed either in a device for determining its thermal resistance, or in a sealed vessel.

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

6.10 A sample of the dried bulk material should be placed in a box, the bottom and lid of which are made of thin sheet material. The length and width of the box should be equal to the corresponding dimensions of the working surfaces of the plates of the device, the depth - to the thickness of the test specimen. The thickness of a sample of bulk material should be at least 10 times the average size of the granules, grains and flakes that make up this material.

The relative hemispherical emissivity of the bottom and lid surfaces of the box shall 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 lid of the box are made must be known.

6.11 A sample of bulk material is divided into four equal parts, which are poured into the box one by one, compacting each part so that it occupies the corresponding part of the inner volume of the box. The box is closed with a lid. The lid is attached to the side walls of the box.

6.12 Weigh the box containing the bulk sample. The density of the bulk material sample is calculated from the determined value of the mass of the box with the sample and the predetermined values of the internal volume and the mass of the empty box.

6.13 The error in determining the mass and size of samples shall not exceed 0.5%.

7 Testing

7.1 The tests shall be carried out on a pre-calibrated instrument. The order and frequency of calibration are given in Appendix B.

7.2 Place the sample to be tested in the instrument. Sample location - horizontal or vertical. When the sample is placed horizontally, the direction of the heat flow is from top to bottom.

During the test, the temperature difference between the face faces of the sample D T u should be 10-30 K. The average temperature of the sample during testing should be indicated in the normative document for a specific type of material or product.

7.3 Set the preset values of the temperatures of the working surfaces of the plates of the device and sequentially every 300 s carry out measurements:

heat meter signals e u and temperature sensors of the faces of the sample, if the heat flux density through the test sample is measured using a heat meter;

the power supplied to the heater of the measurement zone of the hot plate of the device and signals from the temperature sensors of the front faces of the sample, if the density of the heat flux through the test sample is determined by measuring the electric power supplied to the heater of the measurement zone of the hot plate of the device.

7.4 The heat flux through the test sample is considered steady (stationary) if the values of the thermal resistance of the sample, calculated from the results of five successive measurements of the signals from the temperature sensors and the heat flux density, differ from each other by less than 1%, while these values do not increase and do not decrease monotonously.

7.5 After reaching a stationary thermal regime, measure the thickness of the sample placed in the device. d u with a caliper with an error of no more than 0.5%.

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

8 Expression of test results

8.1 Calculate the relative weight change of the sample due to drying T r and during testing T w and sample density r u by the formulas:

Tr =(M 1 ¾ M 2 ) / M 2 , (2)

Tw= (M 2 ¾ M 3 ) / M 3 , (3)

Test sample volume V u calculated from the results of measuring its length and width after the end of the test, and thickness - during the test.

8.2 Calculate the temperature difference between the faces D T u and the average temperature of the test piece T mu by the formulas:

D T u = T 1u ¾ T 2u , (5)

T mu= (T 1u + T 2u.) / 2 (6)

8.3 When calculating the thermophysical parameters of the sample and the density of the stationary heat flux, the arithmetic mean values of the results of five measurements of the signals of the temperature difference sensors and the signal of the heat meter or electric power, performed after the establishment of a stationary heat flux through the test sample, are substituted into the calculation formulas.

8.4 When tested on an asymmetric instrument, the thermal resistance of the sample R u calculated by the formula

(7)

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

8.5 Effective thermal conductivity of the sample material l effu calculated by the formula

(8)

8.6 Thermal resistance R u and effective thermal conductivity l effu a sample of bulk material is calculated by the formulas:

, (9)

. (10)

8.7 Density of stationary heat flux q u through a sample tested on a device assembled according to asymmetric and symmetric schemes, calculate, respectively, by the formulas:

q u = f u e u , (11)

. (12)

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

, (13)

, (14)

When testing bulk materials in formulas (13) and (14) instead of R k substitute the value R L ..

8.9 The arithmetic mean values of thermal resistance and effective thermal conductivity of all tested samples are taken as the test result.

9 Test report

The test report shall include the following information:

The name of the material or product;

Designation and name of the regulatory document according to which the material or product was manufactured;

Manufacturing company;

Batch number;

Manufacturing date;

The total number of samples tested;

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

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

The method of making samples of bulk material with an indication of the thermal resistance of the bottom and lid of the box in which the samples were tested;

Dimensions of each sample;

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

Fixed pressure (if it was fixed);

Average size of heterogeneous inclusions in samples (if any);

Sample drying technique;

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

The moisture content of each sample before and after the test;

The density of each sample during testing;

The relative change in the mass of each sample that occurred during the test;

The temperature of the hot and cold faces of each sample;

The temperature difference between the hot and cold faces of each sample;

Average temperature of each sample;

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

Thermal resistance of each sample;

Effective thermal conductivity of the material of each sample;

The arithmetic mean of the thermal resistance of all tested samples;

The arithmetic mean of the effective thermal conductivity of all tested samples;

Heat flow direction;

Date of testing;

Date of the last calibration of the device (if the test was carried out on a device equipped with a heat meter);

For standard samples used in the calibration of the device, the following should be indicated: type, thermal resistance, date of verification, validity period of verification, organization that performed verification;

Evaluation of the error in measuring thermal resistance or effective thermal conductivity;

A statement of full compliance or partial non-compliance of a test procedure with the requirements of this standard. If, during the test, deviations from the requirements of this standard were admitted, then they should be indicated in the test report.

10 Error in determining the effective thermal conductivity

and thermal resistance

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

APPENDIX A

(required)

Requirements for devices for determining the effective thermal conductivity and thermal resistance in a stationary thermal regime

A.1 Instrument diagrams

To measure the effective thermal conductivity and thermal resistance in a stationary thermal regime, the following devices are used:

Assembled in 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);

Assembled according to a symmetrical scheme, 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);

A 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 hot plate measurement zone of the device (device with a hot security zone) (Figure A.3).

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

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

1 - heater; 2 - heat meters; 3 - fridge; 4 - test sample

Figure A.2 - Diagram of a device with two heat meters

1 - fridge; 2 - test samples; 3 - heater plates for the measurement zone;

4 - measurement zone heater winding; 5 - heater plates of the security zone;

6 - guard zone heater winding

Figure A. 3 - Diagram of a device with a hot security zone

A.2 Heater and cooler

A.2.1 The plates of the heater or refrigerator may be in the form of a square, the side of which must be at least 250 mm, or a circle, the diameter of which must be at least 250 mm.

А.2.2 The working surfaces of the heater and refrigerator plates should 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 The relative hemispherical emissivity of the working surfaces of the heater and refrigerator plates in contact with the test specimen should be more than 0.8 at the temperatures that these surfaces have during the test.

A.3 Heat meter

А.3.1 The dimensions of the working surfaces of the heat meter should be equal to the dimensions of the working surfaces of the heater and refrigerator plates.

A. 3.2 The relative hemispherical emissivity of the front face of the heat meter in contact with the test specimen shall be more than 0.8 at the temperatures that this face has during the test.

A. 3.3 The measuring area of the heat meter should be located in the central part of its front edge. Its area should be at least 10% and not more than 40% of the entire front face area.

А.3.4 The diameter of the thermocouple 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 working surface of the heater or refrigerator plates and the front face of the heat meter in contact with the test sample should be equal to an integer part of 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 must ensure the measurement of the signal from the sensors of the difference in surface temperatures 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 the electric power supplied to the heater of the measurement zone of the hot plate of the device - with an error of no more than 0 , 2%.

The total error in measuring the temperature difference between the surfaces of the plates of the device and the heat meter in contact with the front faces of the test specimen should not be more than 1%. The total error is the sum of 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 the error introduced by the electrical measuring system.

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

The device should be equipped with a device that allows you to measure the thickness of the sample during its testing with a caliper with an error of no more than 0.5%.

A.7 Instrument frame

The device should be equipped with a frame that allows maintaining different orientations in space of the device block containing the test sample.

A.8 Device for fixing the test specimen

The device must be equipped with a device that either creates a constant specified pressure on the test specimen placed in the device, or maintains a constant gap between the working surfaces of the plates of the device.

The maximum pressure created by this device on the test specimen should be 2.5 kPa, the minimum - 0.5 kPa, the error in setting the pressure - no more than 1.5%.

A.9 Device for reducing lateral heat loss or heat gain of the test specimen

Lateral heat loss or heat gain during the test should be limited by insulating the side faces of the test specimen with a layer of heat-insulating material, the thermal resistance of which is not less than the thermal resistance of the specimen.

A. 10 Instrument cover

The instrument shall be equipped with an enclosure, the air temperature in which is maintained equal to the average temperature of the test specimen.

APPENDIX B

(required)

Calibration of a device equipped with a heat meter

B.1 General requirements

The calibration of the device equipped with a heat meter should be carried out using three standard samples of thermal resistance, certified in accordance with the established procedure, made of optical quartz glass, organic glass and foam or fiberglass, respectively.

The dimensions of the reference materials must be equal to the dimensions of the sample to be tested. In the process of calibrating the device, the temperature of the front faces of the standard samples should be correspondingly equal to those temperatures that during the test will have the front faces of the test sample.

The entire range of thermal resistance values that can be measured on the device should be divided into two sub-ranges:

the lower limit of the first sub-range is the minimum value of thermal resistance that can be measured on this device; the upper limit is 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 limit of the second sub-band is the upper limit of the first sub-band; the upper limit is the maximum value of thermal resistance that can be measured on this device.

B.2 Calibration of the device assembled according to the asymmetric scheme

Before starting the calibration, the numerical value of the thermal resistance of the sample to be tested should be estimated using known reference data and to determine which sub-range this value belongs to. Calibration of the heat meter is carried out only in this sub-range.

If the thermal resistance of the specimen to be tested is in the first sub-range, the calibration of the heat meter

carried out using standard samples made of optical quartz and organic glass. If the thermal resistance of the sample belongs to the second sub-range, the calibration is carried out using standard samples made of organic glass and heat-insulating material.

Place the first standard sample with lower thermal resistance into the instrument. R S 1 , D T 1 of its faces and a heat meter output e 1 according to the procedure described in section 7. Then a second standard sample with high thermal resistance is placed in the instrument. R S 2 , measure the temperature difference D T 2 of its faces and a heat meter output e 2 using the same technique. Based on the results of these measurements, the calibration coefficients are calculated f 1 and f 2 heat meters according to the formulas:

Calibration coefficient value of the heat meter f u, corresponding to the value of the heat flux flowing through the test specimen after the establishment of a stationary heat flux, is determined by linear interpolation using the formula

... (B.3)

B.3 Calibration of the device assembled according to the symmetrical scheme

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

B.4 Frequency of instrument calibration

The device must be calibrated within 24 hours preceding the test or following the test.

If, according to the results of calibrations carried out within 3 months, the change in the calibrating coefficient of the heat meter does not exceed ± 1%, this device can be calibrated once every 15 days. In this case, the test results can be transferred to the customer only after the calibration that follows the test, and if the value of the calibration coefficient determined from the results of the subsequent calibration differs from the value of the coefficient determined from the results of the previous calibration by no more than ± 1%.

The calibration coefficient used in calculating the thermophysical parameters of the test sample is determined as the arithmetic mean of the two indicated values of this coefficient.

If the difference in the value of the calibration factor exceeds ± 1%, the results of all tests carried out in the time interval between these two graduations are considered invalid and the tests must be repeated.

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 parameters in a stationary thermal regime. A device equipped with a heat meter

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

Key words: thermal resistance, effective thermal conductivity, standard sample

Introduction

1 area of use

3 Definitions and symbols

4 General

5 Measuring instruments

6 Test preparation

7 Testing

8 Expression of test results

9 Test report

10 Error in determining effective thermal conductivity and thermal resistance

Appendix A Requirements for devices for determining the effective thermal conductivity and thermal resistance in a stationary thermal regime

Appendix B Calibration of a device equipped with a heat meter

Appendix B Bibliography

Until now, a unified classification has not been developed, which is associated with the variety of existing methods. The well-known experimental methods for measuring the thermal conductivity of materials are divided into two large groups: stationary and non-stationary. In the first case, the quality of the calculation formula uses particular solutions of the heat conduction equation

under the condition, in the second - under the condition where T is the temperature; f - time; - coefficient of thermal diffusivity; l - coefficient of thermal conductivity; C is the specific heat capacity; g is the density of the material; - the Laplace operator written in the corresponding coordinate system; - specific power of the volumetric heat source.

The first group of methods is based on the use of a stationary thermal regime; the second is a non-stationary thermal regime. Stationary methods for determining the thermal conductivity coefficient by the nature of the measurements are direct (i.e., the thermal conductivity coefficient is directly determined) and are divided into absolute and relative. In absolute methods, the parameters measured in the experiment make it possible to obtain the required value of the thermal conductivity coefficient using the calculation formula. In relative methods, the parameters measured in the experiment allow using the calculation formula to obtain the desired value of the thermal conductivity coefficient. In relative methods, the measured parameters are not enough to calculate the absolute value. Two cases are possible here. The first is to observe the change in the coefficient of thermal conductivity in relation to the initial one, taken as a unit. The second case is the use of a reference material with known thermal properties. In this case, the thermal conductivity coefficient of the standard is used in the calculation formula. Relative methods have some advantage over absolute methods because they are simpler. Further division of stationary methods can be carried out according to the nature of heating (external, volumetric, and combined) and according to the form of isotherms of the temperature field in the samples (flat, cylindrical, spherical). The subgroup of methods with external heating includes all methods that use external (electrical, volumetric, etc.) heaters and heating the sample surfaces by thermal radiation or electron bombardment. The subgroup of methods with volumetric heating unites all methods that use heating by a current passed through the sample, heating the sample under study from neutron or r-radiation, or by microwave currents. The subgroup of methods with combined heating can include methods that simultaneously use external and volumetric heating of samples, or intermediate heating (for example, by high-frequency currents).

In all three subgroups of stationary methods, the temperature field

may be different.

Plane isotherms are formed when the heat flux is directed along the symmetry axis of the sample. Methods using flat isotherms in the literature are called methods with axial or longitudinal heat flux, and the experimental setups themselves are called flat devices.

Cylindrical isotherms correspond to the propagation of the heat flux in the direction of the radius of the cylindrical sample. In the case when the heat flux is directed along the radius of a spherical sample, spherical isotherms appear. Methods using such isotherms are called spherical, and devices are called spherical.

The ability of materials and substances to conduct heat is called thermal conductivity (X,) and is expressed by the amount of heat passing through a wall with an area of 1 m2, 1 m thick in 1 hour with a temperature difference on opposite wall surfaces of 1 degree. The unit for measuring thermal conductivity is W / (m-K) or W / (m- ° C).

The thermal conductivity of materials is determined

Where Q- the amount of heat (energy), W; F- cross-sectional area of the material (sample) perpendicular to the direction of the heat flow, m2; At is the temperature difference on opposite surfaces of the sample, K or ° C; b - sample thickness, m.

Thermal conductivity is one of the main indicators of the properties of thermal insulation materials. This indicator depends on a number of factors: the total porosity of the material, the size and shape of the pores, the type of solid phase, the type of gas filling the pores, temperature, etc.

The dependence of thermal conductivity on these factors is expressed in the most universal form by the Leeb equation:

_______ Ђs ______ - і

Where Kr is the thermal conductivity of the material; Xs is the thermal conductivity of the solid phase of the material; Rs- the number of pores in the section perpendicular to the heat flow; Pi- the number of pores in the section parallel to the heat flow; b - radial constant; є - emissivity; v is a geometric factor affecting. radiation inside the pores; Tt- average absolute temperature; d is the average pore diameter.

Knowledge of the thermal conductivity of a particular heat-insulating material allows you to correctly assess its heat-insulating qualities and calculate the thickness of a heat-insulating structure made of this material for given conditions.

Currently, there are a number of methods for determining the thermal conductivity of materials based on the measurement of stationary and non-stationary heat fluxes.

The first group of methods allows measurements in a wide temperature range (from 20 to 700 ° C) and obtain more accurate results. The disadvantage of methods for measuring the stationary heat flux is the long duration of the experiment, measured in hours.

The second group of methods allows you to conduct an experiment v within a few minutes (up to 1 h), but it is suitable for determining the thermal conductivity of materials only at relatively low temperatures.

Measurement of the thermal conductivity of building materials by this method is carried out using the device shown in Fig. 22. At the same time, with the help of low-inertia heat meter is produced measurement of the steady-state heat flux passing through the tested material sample.

The device consists of a flat electric heater 7 and a low-inertia heat meter 9, installed at a distance of 2 mm from the surface of the refrigerator 10, through which water continuously flows at a constant temperature. Thermocouples are embedded on the surfaces of the heater and heat meter 1,2,4 and 5. The device is housed in a metal casing. 6, filled with thermal insulation material. Sample snug fit 8 to the heat meter and heater is provided with a clamping device 3. Heater, heat meter and the refrigerator is in the form of a disc with a diameter of 250 mm.

The heat flux from the heater through the sample and the low-inertia heat meter is transferred to the refrigerator. The magnitude of the heat flux passing through the central part of the sample is measured by a heat meter, which is a thermopile on a paranite disk, or heat - a measure with a reproducing element, in which a flat electric heater is mounted.

The device can measure thermal conductivity at a temperature on the hot surface of the sample from 25 to 700 ° C.

The set of the device includes: a RO-1 thermostat, a KP-59 potentiometer, a RNO-250-2 laboratory autotransformer, an MGP thermocouple switch, a TC-16 thermostat, a technical alternating current ammeter up to 5 A and a thermos.

Samples of material to be tested shall be circular in plan with a diameter of 250 mm. The thickness of the samples should be no more than 50 and no less than 10 mm. The thickness of the samples is measured with an accuracy of 0.1 mm and is determined as the arithmetic mean of the results of four measurements. The surfaces of the samples should be flat and parallel.

When testing fibrous, free-flowing, soft and semi-rigid heat-insulating materials, the selected samples are placed in holders with a diameter of 250 mm and a height of 30-40 mm, made of asbestos cardboard with a thickness of 3-4 mm.

The density of the sample taken under specific load should be uniform throughout the volume and correspond to the average density of the material under test.

Before testing, the samples must be dried to constant weight at a temperature of 105-110 ° C.

The sample prepared for testing is placed on a heat meter and pressed with a heater. Then the thermostat of the device heater is set to the specified temperature and the heater is switched on to the network. After establishing a stationary mode, in which the heat meter readings will be constant for 30 minutes, the thermocouple readings are noted on the potentiometer scale.

When using a low-inertia heat meter with a reproducing element, the readings of the heat meter are converted to a zero-galvanometer and the current through the rheostat is switched on, and the milliammeter for compensation, while achieving the position of the arrow of the zero-galvanometer at 0, after which the readings on the scale of the device in mA are recorded.

When measuring the amount of heat with a low-inertia heat meter with a reproducing element, the thermal conductivity of the material is calculated according to the formula

Where b is the thickness of the sample, m; T - temperature of the hot surface of the sample, ° С; - temperature of the cold surface of the sample, ° С; Q- the amount of heat passing through the sample in the direction perpendicular to its surface, W / m2.

Where R is the constant resistance of the heat meter heater, Ohm; / - current strength, A; F- area of the heat meter, m2.

When measuring the amount of heat (Q) with a graduated low-inertia heat meter, the calculation is made according to the formula Q= AE(W / m2), where E- electromotive force (EMF), mV; A - constant of the device indicated in the calibration certificate for the heat meter.

The temperature of the surfaces of the sample is measured with an accuracy of 0.1 C (under the condition of a steady state). The heat flux is calculated with an accuracy of 1 W / m2, and the thermal conductivity - up to 0.001 W / (m- ° C).

When working on this device, it is necessary to periodically check it by testing standard samples, which are provided by research institutes of metrology and laboratories of the Committee of Standards, Measures and Measuring Instruments under the Council of Ministers of the USSR.

After conducting the experiment and obtaining the data, a material test certificate is drawn up, which should contain the following data: the name and address of the laboratory that conducted the tests; date of the test; name and characteristics of the material; average density of the material in a dry state; the average temperature of the sample during the test; thermal conductivity of the material at this temperature.

The two-plate method allows obtaining more reliable results than those discussed above, since two twin samples are tested at once and, in addition, thermal flow through samples, has two directions: through one sample it goes from bottom to top, and through the other - from top to bottom. This circumstance greatly contributes to the averaging of test results and brings the experimental conditions closer to the real conditions of material service.

A schematic diagram of a two-plate device for determining the thermal conductivity of materials by the stationary mode method is shown in Fig. 23.

The device consists of a central heater 1, a security heater 2, cooling discs 6, which one

Newly pressed material samples 4 to heaters, insulating backing 3, thermocouple 5 and casing 7.

The set of the device includes the following regulating and measuring equipment. Voltage stabilizer (CH), autotransformers (T), wattmeter (W), Ammeters (A), security heater temperature controller (P), thermocouple switch (I), galvanometer or potentiometer for temperature measurement (G) And a jar of ice (C).

To ensure the same boundary conditions at the perimeter of the tested samples, the shape of the heater was adopted as a disk. For the convenience of calculation, the diameter of the main (working) heater is taken equal to 112.5 mm, which corresponds to an area of 0.01 m2.

Testing the material for thermal conductivity is performed as follows.

From the material selected for testing, two twin samples are made in the form of disks with a diameter equal to the diameter of the guard ring (250 mm). The thickness of the samples should be the same and range from 10 to 50 mm. Sample surfaces should be flat and parallel, free from scratches or dents.

Testing of fibrous and bulk materials is carried out in special casings made of asbestos cardboard.

Before testing, the samples are dried to constant weight and their thickness is measured with an accuracy of 0.1 mm.

The samples are placed on both sides of the electric heater and pressed against it with cooling discs. Then the voltage regulator (latr) is set to the position at which the preset temperature of the electric heater is provided. Turn on the circulation of water in the cooling discs and after reaching a steady state, observed by the galvanometer, measure the temperature at the hot and cold surfaces of the samples, for which they use the appropriate thermocouples and a galvanometer or potentiometer. Electricity consumption is measured at the same time. After that, the electric heater is turned off, and after 2-3 hours, the water supply to the cooling discs is stopped.

Thermal conductivity of the material, W / (m- ° C),

Where W- power consumption, W; b - sample thickness, m; F- the area of one surface of the electric heater, m2;. t is the temperature at the hot surface of the sample, ° С; І2- temperature at the cold surface of the sample, ° С.

The final results for determining the thermal conductivity are referred to the average temperature of the samples.
where t - temperature at the hot surface of the sample (average of two samples), ° С; t 2 - temperature at the cold surface of the samples (average of two samples), ° С.

Pipe method. To determine the thermal conductivity of heat-insulating products with a curved surface (shells, cylinders, segments), an installation is used, the schematic diagram of which is shown in

Rice. 24. This installation is a steel pipe with a diameter of 100-150 mm and a length of at least 2.5 m. Inside the pipe, a heating element is mounted on a refractory material, which is divided into three independent sections along the length of the pipe: the central (working) one, occupying approximately] / from the length of the pipe, and lateral, serving to eliminate heat leakage through the ends of the device (pipe).

The pipe is installed on hangers or on stands at a distance of 1.5-2 m from the floor, walls and ceiling of the room.

The temperature of the pipe and the surface of the material to be tested is measured with thermocouples. During the test, it is necessary to regulate the power of electricity consumed by the security sections in order to exclude a temperature difference between the working and security sections.
mi. The tests are carried out under steady-state thermal conditions, in which the temperature on the surfaces of the pipe and the insulating material is constant for 30 minutes.

The power consumption of the working heater can be measured both with a wattmeter and separately with a voltmeter and ammeter.

Thermal conductivity of the material, W / (m ■ ° С),

X -_____ D

Where D - outer diameter of the tested product, m; d - Internal diameter of the tested material, m; - temperature on the pipe surface, ° С; t 2 - temperature on the outer surface of the tested product, ° С; I is the length of the working section of the heater, m.

In addition to thermal conductivity, this device can measure the amount of heat flux in a heat-insulating structure made of one or another heat-insulating material. Heat flux (W / m2)

Determination of thermal conductivity based on non-stationary heat flow methods (dynamic measurement methods). Methods based on measurement of non-stationary heat fluxes (methods of dynamic measurements), have recently been increasingly used to determine thermophysical quantities. The advantage of these methods is not only the comparative speed of the experiments, but and a greater amount of information obtained in one experience. Here, one more parameter is added to the other parameters of the controlled process - time. Due to this, only dynamic methods make it possible to obtain, based on the results of one experiment, the thermophysical characteristics of materials, such as thermal conductivity, heat capacity, thermal diffusivity, cooling (heating) rate.

Currently, there are a large number of methods and devices for measuring dynamic temperatures and heat fluxes. However, they all require know
Specific conditions and the introduction of corrections to the results obtained, since the processes of measuring thermal quantities differ from the measurement of quantities of a different nature (mechanical, optical, electrical, acoustic, etc.) in their significant inertia.

Therefore, the methods based on the measurement of stationary heat fluxes differ from the considered methods by a much greater identity between the measurement results and the true values of the measured thermal quantities.

Improvement of dynamic measurement methods goes in three directions. First, this is the development of methods for analyzing errors and introducing corrections to the measurement results. Second, the development of automatic correcting devices to compensate for dynamic errors.

Let us consider the two most widespread methods in the USSR, based on the measurement of an unsteady heat flux.

1. Method of regular thermal regime with bicalorimeter. When applying this method, various types of bicalorimeter design can be used. consider one of them - a small-sized flat bicalory - meter of the type MPB-64-1 (Fig. 25), which is designed
for determining the thermal conductivity of semi-rigid, fibrous and bulk thermal insulation materials at room temperature.

The MPB-64-1 device is a cylindrical split shell (body) with an inner diameter of 105 mm, v the center of which is a built-in core with a built-in v it with a heater and a battery of differential thermocouples. The device is made of D16T duralumin.

The thermopile of differential thermocouples of the bicalo-rimeter is equipped with copper-copel thermocouples, the diameter of the electrodes of which is 0.2 mm. The ends of the turns of thermopiles are brought out to the brass petals of a glass cloth impregnated with BF-2 glue, and then through the wires to the plug. Heating element made of Nichrome wire with a diameter of 0.1 mm, sewn onto a round plate impregnated with BF-2 glue glass fabrics. The ends of the heating element wire, as well as the ends of the thermopile wire, are brought out to the brass petals of the ring and then, through the plug, to the power source. The heating element can be powered by 127 VAC.

The device is hermetically sealed thanks to a vacuum rubber seal inserted between the body and covers, as well as a gland packing (red lead) between the handle, boss and body.

Thermocouples, heater and their leads must be well insulated from the case.

The dimensions of the test pieces should not exceed in diameter 104 mm and 16 mm in thickness. On the device, two twin samples are tested simultaneously.

The operation of the device is based on the following principle.

Cooling process of a solid heated to a temperature T° and placed in an environment with a temperature ©<Ґ при весьма большой теплопередаче (а) от телаTo The environment («-> - 00) and at a constant temperature of this environment (0 = const), is divided into three stages.

1. Temperature distribution v the body is at first random, i.e., there is a disordered thermal regime.

2.In the course of time, cooling becomes ordered, i.e., a regular regime sets in, at which
The temperature change at each point of the body obeys an exponential law:

Q - AUe .- "1

Where © - elevated temperature at any point in the body; U - some function of the coordinates of the point; e-base of natural logarithms; t is the time from the beginning of body cooling; t is the cooling rate; A is the constant of the device, depending on the initial conditions.

3. After a regular regime, cooling is characterized by the onset of thermal equilibrium of the body with the environment.

Cooling rate m after differentiating the expression

By T in coordinates InV-T expressed as follows:

Where A and V - device constants; WITH is the total heat capacity of the material under test, equal to the product of the specific heat of the material by its mass, J / (kg- ° C); t is the cooling rate, 1 / h.

The test is carried out as follows. After placing the samples in the instrument, the instrument covers are pressed tightly against the body using a knurled nut. The device is lowered into a thermostat with a stirrer, for example, a TC-16 thermostat filled with water at room temperature, then a thermopile of differential thermocouples is connected to the galvanometer. The device is kept in a thermostat until the temperatures of the outer and inner surfaces of the samples of the test material equalize, which is recorded by the indication of the galvanometer. After that, the core heater is turned on. The core is heated to a temperature 30-40 ° higher than the temperature of the water in the thermostat, and then the heater is turned off. When the pointer of the galvanometer returns to the scale, a record is made of the decaying readings of the galvanometer. A total of 8-10 points are recorded.

In the 1n0-t coordinate system, a graph is built, which should have the form of a straight line intersecting at some points the abscissa and ordinate axes. Then the tangent of the slope of the obtained straight line is calculated, which expresses the value of the rate of cooling of the material:

__ In 6t - In O2 __ 6 02

ТІЬ- - j

T2 - Tj 12 - "El

Where Bi and 02 are the corresponding ordinates for the time Ti and T2.

The experiment is repeated again and the cooling rate is determined once more. If the discrepancy in the values of the cooling rate calculated in the first and second experiments is less than 5%, then they are limited to these two experiments. The average value of the rate of cooling is determined from the results of two experiments and the value of the thermal conductivity of the material is calculated, W / (m * ° C)

X = (A + YCuP) / u.

Example. The test material is a mineral wool mat on a phenolic binder with an average dry density of 80 kg / m3.

1. We calculate the value of the sample of the material placed in the device,

Where Pp is a sample of material placed in one cylindrical container of the device, kg; Vn - the volume of one cylindrical container of the device, equal to 140 cm3; pcr is the average density of the material, g / cm3.

2. We define work BCYP , where V - device constant equal to 0.324; C is the specific heat capacity of the material, equal to 0.8237 kJ / (kg-K). Then WSSD = =0,324 0,8237 0,0224 = 0,00598.

3. Results observations of by cooling the samples in the device in time we enter in the table. 2.

The discrepancies in the values of the cooling rate m and m2 are less than 5%, therefore, repeated experiments can be omitted.

4. Calculate the average cooling rate

T = (2.41 + 2.104) / 2 = 2.072.

Knowing all the necessary values, we calculate the thermal conductivity

(0.0169 + 0.00598) 2.072 = 0.047 W / (m-K)

Or W / (m- ° C).

In this case, the average temperature of the samples was 303 K or 30 ° C. In the formula 0.0169 -L (instrument constant).

2. Probe method. There are several varieties of the probe method for determining the heat pipe
thermal insulation materials differing from each other by the devices used and the principles of heating the probe. Let's consider one of these methods - the method of a cylindrical probe without an electric heater.

This method is as follows. A metal rod with a diameter of 5-6 mm (Fig. 26) and a length of about 100 mm is introduced into the thickness of the hot heat-insulating material and with the help of a rod mounted inside

Thermocouples determine the temperature. The temperature is determined in two steps: at the beginning of the experiment (at the time of the heating of the probe) and at the end, when the equilibrium state occurs and the temperature rise of the probe stops. The time between these two counts is measured using a stopwatch. h Thermal conductivity of the material, W /(m ° C), , R2CV

Where R- rod radius, m; WITH- specific heat capacity of the material from which the rod is made, kJ / (kgX HK); V is the volume of the rod, m3; t - time interval between temperature readings, h; tx and U - temperature values at the moment of the first and second readings, K or ° C.

This method is very simple and allows you to quickly determine the thermal conductivity of a material both in laboratory and in production conditions. However, it is suitable only for a rough estimate of this indicator.