Flame movement through the gas mixture called flame spread. Depending on the speed of flame propagation, combustion can be deflagration with a speed of several m / s, explosive - a speed of the order of tens and hundreds of m / s, and detonation - thousands of m / s.
For deflagration or normal combustion propagation heat transfer from layer to layer is characteristic, and the flame arising in the mixture heated and diluted with active radicals and reaction products moves in the direction of the initial combustible mixture. This is due to the fact that the flame, as it were, becomes a source that emits a continuous flow of heat and chemically active particles. As a result, the flame front moves towards the combustible mixture.
Deflagration combustion subdivided into laminar and turbulent.
Laminar combustion is characterized by a normal flame propagation rate.
The normal speed of flame propagation, according to GOST 12.1.044 SSBT, is called flame front speed relatively unburned gas, in a direction perpendicular to its surface.
The value of the normal speed of flame propagation, being one of the indicators of fire and explosiveness of substances, characterizes the danger of industries associated with the use of liquids and gases, it is used in calculating the rate of increase in the explosive pressure of gas, vapor-air mixtures, critical (extinguishing) diameter and in the development of measures ensuring fire and explosion safety of technological processes in accordance with the requirements of GOST 12.1.004 and GOST 12.1.010 SSBT.
The normal speed of flame propagation - the physicochemical constant of the mixture - depends on the composition of the mixture, pressure and temperature and is determined by the rate of chemical reaction and molecular thermal conductivity.
Temperature relatively weakly increases the normal speed of flame propagation, inert impurities reduce it, and an increase in pressure leads either to an increase or to a decrease in the speed.
In laminar gas flow the velocities of gases are low, and the combustible mixture is formed as a result of molecular diffusion. The burning rate in this case depends on the rate of formation of the combustible mixture. Turbulent flame is formed with an increase in the speed of flame propagation, when the laminarity of its movement is violated. In a turbulent flame, the swirling of gas jets improves the mixing of the reacting gases, since the surface through which molecular diffusion occurs increases.
As a result of the interaction of a combustible substance with an oxidizer, combustion products are formed, the composition of which depends on the initial compounds and the conditions of the combustion reaction.
With the complete combustion of organic compounds, CO 2, SO 2, H 2 O, N 2 are formed, and with the combustion of inorganic compounds, oxides are formed. Depending on the melting point, the reaction products can either be in the form of a melt (Al 2 O 3, TiO 2), or rise into the air in the form of smoke (P 2 O 5, Na 2 O, MgO). The molten solids create the luminosity of the flame. In the combustion of hydrocarbons, the strong luminosity of the flame is provided by the glow of particles of technical carbon, which is formed in large quantities. A decrease in the content of carbon black as a result of its oxidation reduces the luminosity of the flame, and a decrease in temperature makes it difficult to oxidize carbon black and leads to the formation of soot in the flame.
In order to interrupt the combustion reaction, it is necessary to violate the conditions for its occurrence and maintenance. Usually, for extinguishing, a violation of two basic conditions of a steady state is used - a decrease in temperature and a mode of gas movement.
Lowering the temperature can be achieved by introducing substances that absorb a lot of heat as a result of evaporation and dissociation (for example, water, powders).
Gas movement mode can be changed by reducing and eliminating oxygen supply.
Explosion, according to GOST 12.1.010 " Explosion proof", - rapid transformation of matter (explosive combustion), accompanied by the release of energy and the formation of compressed gases capable of performing work.
An explosion, as a rule, leads to an intense increase in pressure. A shock wave is generated and propagated in the environment.
Shock wave has a destructive capacity if the excess pressure in it is higher than 15 kPa. It spreads in the gas ahead of the flame front at a sonic speed of 330 m / s. In an explosion, the initial energy is converted into the energy of heated compressed gases, which is converted into the energy of motion, compression and heating of the medium. Various types of initial energy of the explosion are possible - electrical, thermal, elastic compression energy, atomic, chemical.
The main parameters characterizing the explosion hazard in accordance with GOST 12.1.010 are pressure at the shock front, maximum explosion pressure, average and maximum rate of pressure rise during an explosion, crushing or high-explosive properties of an explosive environment.
The general effect of the explosion manifests itself in the destruction of equipment or premises caused by a shock wave, as well as in the release of harmful substances (explosion products or contained in equipment).
Maximum explosion pressure(P max) - the highest pressure arising from a deflagration explosion of a gas, steam or dust-air mixture in a closed vessel at an initial pressure of the mixture of 101.3 kPa.
Burst pressure rise rate(dР / dt) is the time derivative of the explosion pressure in the ascending section of the dependence of the explosion pressure of a gas, steam, dust-air mixture in a closed vessel on time. At the same time, a distinction is made between the maximum and average rates of pressure rise during an explosion. When establishing the maximum speed, the pressure increment is used on the straight-line section of the explosion pressure versus time, and when determining the average speed, the section between the maximum explosion pressure and the initial pressure in the vessel before the explosion.
Both of these characteristics are important factors for ensuring explosion protection. They are used when establishing the category of premises and buildings for explosion and fire hazard, when calculating safety devices, when developing measures for fire and explosion safety of technological processes.
Detonation is the process of chemical transformation of the oxidizer - reducing agent system, which is a combination of a shock wave propagating at a constant speed and exceeding the speed of sound, and following the front of the zone of chemical transformations of the initial substances. Chemical energy released in the detonation wave feeds the shock wave, preventing it from damping. The detonation wave velocity is a characteristic of each specific system.

The theory states that the explosion of a gas or vapor-air mixture is not an instantaneous phenomenon. When the ignition source is introduced into the combustible mixture, the oxidation reaction of the fuel with the oxidizer begins in the zone of action of the ignition source. The rate of the oxidation reaction in some elementary volume of this zone reaches a maximum - combustion occurs. Combustion at the boundary of the elementary volume with the medium is called the flame front. The front of the flame looks like a sphere. Flame front thickness calculated by Ya.B. Zeldovich , equal to 1-100 microns. Although the thickness of the combustion zone is small, it is still sufficient for the combustion reaction to proceed. The temperature of the flame front due to the heat of the combustion reaction is 1000-3000 ° C and depends on the composition of the combustible mixture.

When the flame front moves, the temperature of the unburned part of the combustible mixture increases, since the pressure of the mixture rises. Near the flame front, the temperature of the mixture also increases, which is due to the
heat transfer by thermal conductivity, diffusion of heated molecules and radiation. On the outer surface of the flame front, this temperature is equal to the autoignition temperature of the combustible mixture.

After the ignition of the combustible mixture, the spherical shape of the flame is very quickly distorted and more and more stretched towards the not yet ignited mixture. The stretching of the flame front and the rapid increase in its surface are accompanied by an increase in the speed of the central part of the flame. This acceleration lasts until the flame touches the walls of the pipes or, in any case, comes close to the wall of the pipe. At this moment, the size of the flame decreases sharply, and only a small part of the flame remains, covering the entire section of the pipe. Pulling out the front of the flame
and its intense acceleration immediately after ignition by a spark, when the flame has not yet reached the pipe walls, is caused by an increase in the volume of combustion products. Thus, in the initial stage of the process of flame front formation, regardless of the degree of combustibility of the gas mixture, acceleration and subsequent deceleration of the flame occurs, and this deceleration will be the greater, the greater the flame speed.

The development of the subsequent stages of combustion is influenced by the length of the pipe. The elongation of the pipe leads to the appearance of vibrations and the formation of a cellular structure of the flame, shock and detonation waves.

The width of the heating zone (in cm) can be determined from the dependence

1 = a / v

where a- coefficient of thermal diffusivity; v- the speed of propagation of the flame.

Linear travel speed v(in m / s) can be determined by the formula

V = V t /

where V t- mass burning rate, g / (cm 3); - the density of the initial combustible mixture, kg / m 3.

The linear speed of movement of the flame front is not constant, it changes depending on the compositions. Mixtures and impurities of inert (non-combustible) gases, mixture temperature, pipe diameter, etc. The maximum flame propagation speed is observed not at a stoichiometric concentration of the mixture, but in a mixture with an excess of fuel. When inert gases are introduced into the combustible mixture, the flame propagation speed decreases. This is explained by a decrease in the combustion temperature of the mixture, since part of the heat is spent on heating the inert impurities not participating in the reaction.

With an increase in the diameter of the pipes, the flame propagation speed increases unevenly. With an increase in the diameter of the pipes to 0.1-0.15 m, the speed grows rather quickly. The temperature increases until the diameter reaches a certain limiting diameter,
above which the increase in speed does not occur. With a decrease in the pipe diameter, the flame propagation speed decreases, and at a certain small diameter, the flame does not propagate in the pipe. This phenomenon can be explained by an increase in heat losses through the walls.
pipes.

Therefore, in order to stop the propagation of the flame in the combustible mixture, it is necessary in one way or another to lower the temperature of the mixture by cooling the vessel (in our example, a pipe) from the outside or diluting the mixture with a cold inert gas.

The normal speed of flame propagation is relatively low (no more than tens of meters per second), but under some conditions, the flame in pipes propagates at a tremendous speed (from 2 to 5 km / s), exceeding the speed of sound in a given environment. This phenomenon was named detonation... The distinctive features of detonation are as follows:

1) constant burning rate regardless of the pipe diameter;

2) high flame pressure caused by a detonation wave, which can exceed 50 MPa, depending on the chemical nature of the combustible mixture and the initial pressure; moreover, due to the high burning rate, the developed pressure does not depend on the shape, capacity and tightness of the vessel (or pipe).

As the flame accelerates, the shock wave amplitude also increases, the compression temperature reaches the autoignition temperature of the mixture.

The increase in the total amount of gas combusting per unit time is explained by the fact that in a jet with a variable cross-sectional velocity, the flame front bends, as a result of which its surface increases and the amount of combustible substance increases proportionally.

When gas mixtures are burning in a closed volume, the combustion products do not perform work; the explosion energy is spent only on heating the explosion products. In this case, the total energy is determined as the sum of the internal energy of the explosive mixture Q vn.en.cm. and the heat of combustion of a given substance ΔQ g. The value of Q int.en.cm. is equal to the sum of the products of the heat capacities of the components of the explosive mixture at constant volume by the initial temperature
mixture temperature

Q int.cm. = C 1 T + C 2 T + ... + C p T

where С 1, С 2, С п are the specific heat capacities of the components that make up
explosive mixture, kJ / (kg K); T - initial temperature of the mixture, K.

The explosion temperature of gas mixtures at constant volume is calculated by the same method as the temperature of combustion of the mixture at constant pressure.

The explosion pressure is found from the explosion temperature. The pressure during the explosion of a gas-air mixture in a closed volume depends on the explosion temperature and the ratio of the number of molecules of combustion products to the number of molecules in the explosive mixture. In the explosion of gas-air mixtures, the pressure usually does not exceed 1.0 MPa, if the initial pressure of the mixture was normal. When replacing the air in the explosive mixture with oxygen, the explosion pressure increases sharply, since the combustion temperature increases.

Explosion pressure of stoichiometric mixtures of methane, ethylene, acetone and
methyl ether with oxygen is 1.5 - 1.9 MPa, and their stoichiometric mixtures with air is 1.0 MPa.

The maximum explosion pressure is used in calculating the explosion resistance of equipment, as well as in calculating safety valves, explosion membranes and enclosures of explosion-proof electrical equipment. Burst pressure R vzr (in MPa) gas-air mixtures are calculated by the formula

R adult =

where p 0- initial pressure of the explosive mixture, MPa; T 0 and T adult- the initial temperature of the explosive mixture and the temperature of the explosion, K;

The number of molecules of combustion gases after the explosion;
- the number of molecules of gases in the mixture before the explosion.

The study of combustion processes of combustible mixtures by Russian and foreign scientists made it possible to theoretically substantiate many phenomena accompanying the combustion process, including the speed of flame propagation. The study of the speed of flame propagation in gas mixtures makes it possible to determine the safe speed of gas-air flows in ventilation, recuperation, aspiration pipelines and in pipelines of other installations, through which gas and dust-air mixtures are transported.

In 1889, the Russian scientist V.A. Michelson considered two limiting cases of flame propagation during normal or slow combustion and during detonation.

The theory of normal flame propagation and detonation was further developed in the works of N.N. Semenova, K.I. Shchelkina, D.A. Frank-Kamenetsky, L.N. Khitrina, A.S. Sokolik, V.I. Skobelkin and other scientists, as well as foreign scientists B. Lewis, G. Elbe and others. As a result, a theory of ignition of explosive mixtures was created. However, attempts to interpret the phenomena of flame propagation as diffusion of active centers or to explain the limits of flame propagation by the conditions of chain breakage are not convincing enough.

In 1942, the Soviet scientist Ya.B. Zeldovich formulated the provisions of the theory of combustion and detonation of gases. Combustion theory provides an answer to the main questions: will a mixture of a given composition be combustible, what will be the burning rate of an explosive mixture, what features and forms of flame should be expected. The theory states that the explosion of a gas or vapor-air mixture is not an instantaneous phenomenon. When the ignition source is introduced into the combustible mixture, the oxidation reaction of the fuel with the oxidizer begins in the zone of action of the ignition source. The rate of the oxidation reaction in some elementary volume of this zone reaches a maximum - combustion occurs. Combustion at the border of an elementary volume with a medium is called a flame front. The front of the flame looks like a sphere. Flame front thickness, calculated by Ya.B. Zeldovich, is equal to 1 - 100 microns. Although the thickness of the combustion zone is small, it is still sufficient for the combustion reaction to proceed. The temperature of the flame front due to the heat of the combustion reaction is 1000 - 3000 0 С and depends on the composition of the combustible mixture. The temperature of the mixture also increases near the flame front, which is caused by heat transfer by thermal conductivity, diffusion of heated molecules, and radiation. On the outer surface of the flame front, this temperature is equal to the autoignition temperature of the combustible mixture. The change in the temperature of the mixture along the pipe axis at the moments of time is graphically shown in Fig. 4.1. Gas layer QC 1, in which the temperature of the mixture rises, is the flame front. As the temperature rises, the flame front expands (up to QC 2) towards the end walls of the pipe A and M, displacing at some speed the unburned mixture towards the wall M, and the burnt gas towards the wall A... After the ignition of the combustible mixture, the spherical shape of the flame is very quickly distorted and more and more stretched towards the not yet ignited mixture. The stretching of the flame front and the rapid increase in its surface is accompanied by an increase in the speed of movement

the central part of the flame. This acceleration lasts until the flame touches the walls of the pipes or, in any case, comes close to the wall of the pipe. At this moment, the size of the flame decreases sharply, and only a small part of the flame remains, covering the entire section of the pipe. The stretching of the flame front and its intense acceleration immediately after ignition by a spark, when the flame has not yet reached the pipe walls, is caused by an increase in the volume of combustion products. Thus, in the initial stage of the process of flame front formation, regardless of the degree of combustibility of the gas mixture, acceleration and subsequent deceleration of the flame occurs, and this deceleration will be the greater, the greater the flame speed.

Rice. 4.1. Temperature change in front of and behind the flame front: 1 - zone

combustion products; 2 - flame front; 3 - self-ignition zone;

4 - preheating zone; 5 - initial mixture

The development of the subsequent stages of combustion is influenced by the length of the pipe. The elongation of the pipe leads to the appearance of vibrations and the formation of a cellular structure of the flame, shock and detonation waves.

Consider the width of the heating zone ahead of the flame front. In this zone, no chemical reaction takes place and no heat is generated. Heating zone width l(in cm) can be determined from the dependence:

where a–The coefficient of thermal diffusivity; v- the speed of propagation of the flame.

For a methane-air mixture, the width of the heating zone is 0.0006 m, for a hydrogen-air mixture it is much smaller (3 microns). Subsequent combustion occurs in a mixture, the state of which has already changed as a result of thermal conductivity and diffusion of components from adjacent layers. Mixing the reaction products does not have any specific catalytic effect on the speed of flame movement.

Let us now consider the speed of movement of the flame front through the gas mixture. Linear travel speed v(in m / s) can be determined by the formula

where is the mass combustion rate, g / (cm × m 2), p is the density of the initial combustible mixture, kg / m 3.

The linear velocity of the flame front is not constant, it changes depending on the composition of the mixture and the admixture of inert (incombustible) gases, the temperature of the mixture, the diameter of the pipes, etc. The maximum speed of flame propagation is observed not at the stoichiometric concentration of the mixture, but in the mixture with an excess of fuel. When inert gases are introduced into the combustible mixture, the flame propagation speed decreases. This is explained by a decrease in the combustion temperature of the mixture, since part of the heat is spent on heating the inert impurities not participating in the reaction. The speed of flame propagation is influenced by the heat capacity of the inert gas. The higher the heat capacity of the inert gas, the more it lowers the combustion temperature and the more it reduces the flame propagation speed. Thus, in a mixture of methane with air diluted with carbon dioxide, the flame propagation rate is approximately three times lower than in a mixture diluted with argon.

When the mixture is preheated, the flame propagation speed increases. It was found that the flame propagation speed is proportional to the square of the initial temperature of the mixture.

With an increase in the diameter of the pipes, the flame propagation speed increases unevenly.

With an increase in the diameter of the pipes to 0.10 - 0.15 m, the speed grows rather quickly; with a further increase in the diameter of the pipes, it continues to increase, but to a lesser extent. An increase in temperature occurs until the diameter reaches a certain limiting diameter, above which an increase in speed does not occur. With a decrease in the pipe diameter, the flame propagation speed decreases, and at a certain small diameter, the flame does not propagate in the pipe. This phenomenon can be explained by an increase in heat losses through the pipe walls.

Therefore, in order to stop the propagation of the flame in the combustible mixture, it is necessary in one way or another to lower the temperature of the mixture by cooling the vessel (in our example, a pipe) from the outside or diluting the mixture with a cold inert gas.

The normal speed of flame propagation is relatively low (no more than tens of meters per second), but under some conditions, the flame in pipes propagates at a tremendous speed (from 2 to 5 km / s), exceeding the speed of sound in a given environment. This phenomenon was called detonation. The distinctive features of detonation are as follows:

1) constant burning rate regardless of the pipe diameter;

2) high flame pressure caused by a detonation wave, which can exceed 50 MPa, depending on the chemical nature of the combustible mixture and the initial pressure; moreover, due to the high burning rate, the developed pressure does not depend on the shape, capacity and tightness of the vessel (or pipe).

Let us consider the transition from rapid combustion to detonation in a long tube of constant cross-section when the mixture is ignited at the closed end. Under the pressure of the flame front, compression waves - shock waves - appear in the combustible mixture. In the shock wave, the gas temperature rises up to values ​​at which the mixture self-ignites far ahead of the flame front. This combustion mode is called detonation. As the flame front moves, the movement of the layers adjacent to the wall is slowed down and, accordingly, the movement of the mixture in the center of the tube is accelerated; distribution of speed

the cross-sectional growth becomes uneven. There are jets of gas mixtures, the speed of which is less than the average speed of the gas mixture during normal combustion, and jets moving faster. Under these conditions, the speed of the flame relative to the mixture increases, the amount of gas burning per unit time increases, and the movement of the flame front is determined by the maximum speed of the gas jet.

As the flame accelerates, the shock wave amplitude also increases, the compression temperature reaches the autoignition temperature of the mixture.

The increase in the total amount of combustion gas per unit time is explained by the fact that in a jet with a variable cross-sectional velocity, the flame front bends; as a result of this, its surface increases and the amount of combustible substance increases proportionally.

One of the ways to reduce the combustion rate of combustible mixtures is the action of inert gases on the flame, but due to their low efficiency, chemical combustion inhibition is currently used by adding halogenated hydrocarbons to the mixture.

Combustible gas mixtures have two theoretical combustion temperatures - at constant volume and at constant pressure, and the first is always higher than the second.

The method for calculating the calorimetric combustion temperature at constant pressure is considered in Section 1. Let us consider the method for calculating the theoretical combustion temperature of gas mixtures at a constant volume, which corresponds to an explosion in a closed vessel. The calculation of the theoretical combustion temperature at a constant volume is based on the same conditions that are indicated in Sec. 1.7.

When gas mixtures are burning in a closed volume, the combustion products do not perform work; the explosion energy is spent only on heating the explosion products. In this case, the total energy is determined as the sum of the internal energy of the explosive mixture Q int.en.cm and the heat of combustion of the given substance. The value of Q int.en.cm is equal to the sum of the products of the heat capacities of the components of the explosive mixture at constant volume by the initial temperature of the mixture

Q int.en.cm = s 1 T + s 2 T + ... + s n T,

where s 1, s 2, s n are the specific heat capacities of the components that make up the explosive mixture, kJ / (kg × K); T is the initial temperature of the mixture, K.

The value of Q int.en.cm can be found in the reference tables. The explosion temperature of gas mixtures at constant volume is calculated by the same method as the temperature of combustion of the mixture at constant pressure.

The explosion pressure is found from the explosion temperature. The pressure during the explosion of a gas-air mixture in a closed volume depends on the explosion temperature and the ratio of the number of molecules of combustion products to the number of molecules in the explosive mixture. In an explosion of a gas-air mixture, the pressure usually does not exceed 1.0 MPa, if the initial pressure of the mixture was normal. When replacing the air in the explosive mixture with oxygen, the explosion pressure increases sharply, since the combustion temperature increases.

In the explosion of even a stoichiometric gas-air mixture, a significant amount of heat is spent on heating the nitrogen in the mixture; therefore, the explosion temperature of such mixtures is much lower than the explosion temperature of mixtures with oxygen. Thus, the explosion pressure of a stoichiometric mixture of methane, ethylene, acetone, and methyl ether

ra with oxygen is 1.5 - 1.9 MPa, and their stoichiometric mixtures with air is 1.0 MPa.

The maximum explosion pressure is used in calculating the explosion resistance of equipment, as well as in calculating safety valves, explosion membranes and enclosures of explosion-proof electrical equipment.

The explosion pressure P explosion (in MPa) of gas-air mixtures is calculated by the formula

,

where Р 0 - the initial pressure of the explosive mixture, MPa; T 0 and T blast - the initial temperature of the explosive mixture and the temperature of the explosion, K; - number of molecules of gases of combustion products after explosion; - the number of molecules of gases in the mixture before the explosion.

Example 4.1 ... Calculate the pressure during the explosion of a mixture of ethyl alcohol vapor and air.

.

P 0 = 0.1 MPa; T adult = 2933 K; T 0 = 273 + 27 = 300 K; = 2 + 3 + 11.28 = 16.28 mol; = 1 + 3 + 11.28 = 15.28 mol.

1 The method consists in determining the upper limits for the maximum and average rate of pressure rise of the explosion of gas and vapor-air mixtures in a spherical reaction vessel of constant volume.

The upper limit for the maximum rate of pressure rise in kPa s -1 is calculated by the formula

where p i- initial pressure, kPa;

S and. i- normal speed of flame propagation at initial pressure and temperature, m · s -1;

a-radius of a spherical reaction vessel, m;

Dimensionless maximum explosion pressure;

R  -maximum absolute explosion pressure, kPa;

 and- the adiabatic index for the test mixture;

-thermokinetic exponent as a function of the normal speed of flame propagation on pressure and temperature. If the value  unknown, it is taken equal to 0.4.

The upper limit for the average rate of pressure rise in kPa s -1 is calculated by the formula

, (98)

where is a function of parameters  e ,  and ,  , the values ​​of which are found using the nomograms shown in Fig. 26 and 27.

The values  e and  and are found by thermodynamic calculation or, in case of impossibility of calculation, are taken equal to 9.0 and 1.4, respectively.

The relative root-mean-square error of calculation by formulas (97) and (98) does not exceed 20%.

2. The maximum rate of increase in the pressure of the explosion of gas and vapor-air mixtures for substances consisting of atoms C, H, O, N, S, F, Cl is calculated by the formula

, (99)

where V- the volume of the reaction vessel, m 3.

The relative root-mean-square error of calculation by formula (99) does not exceed 30%.

The method of experimental determination of the conditions of thermal spontaneous combustion of solids and materials

1. Apparatus.

The equipment for determining the conditions of thermal spontaneous combustion includes the following elements.

1.1. A thermostat with a working chamber capacity of at least 40 dm 3 with a thermostat that allows maintaining a constant temperature from 60 to 250 ° C with an error of no more than 3 ° C.

1.2. Baskets made of corrosion-resistant metal, cubic or cylindrical, 35, 50, 70, 100, 140 and 200 mm high (10 pieces of each size) with lids. The diameter of the cylindrical basket should be equal to its height. The wall thickness of the basket is (1.0 ± 0.1) mm.

1.3. Thermoelectric converters (at least 3) with a maximum working junction diameter of no more than 0.8 mm.

2. Preparation for the test.

2.1. A calibration test is carried out to determine the correction ( t T) to indications of thermoelectric converters 2 and 3 ... For this, a basket with a non-flammable substance (for example, calcined sand) is placed in a thermostat heated to a predetermined temperature. Install thermoelectric converters (Fig. 2) so that the working junction of one thermoelectric converter is in contact with the sample and is located in its center, the second is in contact with the outer side of the basket, and the third is at a distance of (30 ± 1) mm from the wall of the basket. Working junctions of all three thermoelectric converters must be located at the same horizontal level, corresponding to the center line of the thermostat.

1 , 2 , 3 - working junctions of thermoelectric converters.

The basket with a non-combustible substance is kept in a thermostat until a steady state is established, at which the readings of all thermoelectric

transducers remain unchanged for 10 minutes or fluctuate with a constant amplitude around average temperatures t 1 , t 2 , t 3 ... Amendment  t T is calculated by the formula

, (100)

2.2. Samples for testing should characterize the average properties of the test substance (material). When testing sheet material, it is collected in a stack corresponding to the inner dimensions of the basket. In samples of monolithic materials, a hole with a diameter of (7.0 ± 0.5) mm is pre-drilled to the center for a thermoelectric converter.