RU2322662C2 - Thermal diffusivity measurement method and device (variants) - Google Patents

Abstract

FIELD: construction.

SUBSTANCE: method involves creating one-dimensional heat flow; measuring heat-carrier temperature-time dependence at heat-exchanger inlet and measuring time-dependent temperature between heat-exchanger and heat insulation.

EFFECT: increased measurement accuracy and reliability.

6 cl, 5 dwg

Description

The invention relates to construction equipment and can be mainly used to measure the thermophysical characteristics of various building structures, for example walls, ceilings, floors, bulkheads, ceiling, etc.

A known method of drilling wells and a device for its implementation [1], allowing to obtain samples of materials from various depths. By measuring the parameters of these samples, one can obtain information on the physical and chemical properties and configuration of the deep layers. A disadvantage of the known method and device for its implementation is that they do not provide non-destructive testing of the studied object.

Numerous variants of ultrasonic flaw detection methods and devices that implement them, for example, are known [2, 3], which make it possible to determine the presence of inhomogeneities in various structures and the configuration of these inhomogeneities, however, devices of this kind do not allow measuring the thermophysical characteristics of the materials under study, in particular thermal resistance.

Numerous options are known for measuring the thermophysical characteristics of various electronic devices, for example, the method described in [4] for determining the thermal resistance of the junction-case semiconductor diodes. A disadvantage of the known technical solution lies in a narrow scope: it can be used to measure the thermophysical characteristics of only radio-electronic devices, and only one of their class is semiconductor diodes.

Numerous variants of devices are known for measuring the thermophysical characteristics of various electronic devices, for example, a device for measuring the thermal resistance of transistors [5]. A disadvantage of the known technical solution lies in a narrow scope: it can be used to measure the thermal resistance of only electronic devices, and only one of their class - transistors.

A device for determining the characteristics of materials described in [6] is known, comprising a source of pulsed heating, a thermocouple, and an electronic processing unit. A thermocouple is located on the surface of the test sample. The output of the thermocouple is connected to the input of the electronic processing unit. The main disadvantage of the known device is that when using pulsed heating requires complex processing of the measurement results, which requires sophisticated equipment. This leads to a significant increase in the cost of measurements. In addition, the great complexity of processing the measurement results leads to a decrease in their accuracy and reliability.

Closest to the technical nature of the claimed method is a method of non-contact non-destructive testing of the thermophysical properties of materials [7]. The known method consists in measuring the temperature at predetermined points on the surface of the sample and the ambient temperature with two thermal detectors, using the results obtained, determining the correction factor, then acting on the surface of the sample with a fixed point source of heat. At a given point in time, the excess temperatures of the heated surface at predetermined points are measured with two thermal detectors. Continue heating and measure the point in time when the temperature farther from the heating spot of the thermal detector increases by a predetermined value. The measured values determine the coefficients of thermal diffusivity and thermal conductivity.

The closest in technical essence to the claimed device is a thermal probe for non-destructive testing of the thermophysical properties of materials [8], containing a linear heater and two thermocouples located symmetrically relative to the linear heater on both sides of it. The main disadvantage of the known device is that the linear heater generates a heat flux diverging in a plane perpendicular to the axis of the linear heater.

Known technical solutions (method and device) have low consumer properties due to the narrow scope, low accuracy and low reliability of the measurements. The presence of these disadvantages is due to the following factors. Known technical solutions use a point or linear source of thermal energy, so it can be used only when measuring the thermophysical characteristics of homogeneous objects. If the object under study has various inhomogeneities (for example, a reinforced concrete wall), then the measurement results will differ when a point or linear source of thermal energy acts on various points on the surface of the object under study. Low accuracy and reliability of the known technical solutions due to the high complexity of the model that describes their work. So, in the known method, it is necessary to conduct preliminary measurements to determine the correction factor. The work of the known method and device are described by the heat balance equation, in which it is necessary to take into account the loss of thermal power to the environment due to convective and radiant heat transfer. In addition, in the heat balance equation of the known method, it is necessary to take into account the loss of thermal power due to the partial absorption of laser radiation by the environment and the partial reflection of laser radiation by the surface of the object under study. These components are taken into account approximately by calculation. When processing the measurement results in a known manner, an error function is used, calculated by expanding it into a Taylor series. The description of the known method indicates that the calculation of this function in an analytical form is very difficult. It is also indicated there that only for materials with a thermal diffusivity coefficient a≥10 ^-7 m ² / s it is possible to limit oneself to the first member of the Taylor series, and only then can the working formula be used to process the measurement results. All these factors together lead to low consumer properties of the known method and device due to the narrow scope and low accuracy and reliability.

The objective of the invention is to increase consumer properties by expanding the scope and increasing accuracy and reliability.

The solution of the problem in accordance with claim 1 is ensured by the fact that in the known method, which consists in heating the surface of the test object, measuring the dependence of time on temperature by a first temperature meter, measuring the dependence of time on temperature by a second temperature meter, the following improvements are made: the section is heated the surface of the object under study through the use of a coolant entering the heat exchanger, the external surface of which, except for the surface the tee adjacent to the surface area of the object under study is provided with thermal insulation, in this case, the time dependence of the temperature of the coolant at the inlet of the heat exchanger is measured by the first temperature meter, the time dependence of temperature is measured by the second temperature meter located between the heat exchanger and thermal insulation, and the thermal diffusivity is determined - a from the ratio:

a = 4Fo * τ {V ₀ (T ₀ -T _w ) / [2 (T _w -T _n ) Fo * + V _w τ]} ² ,

where Fo * is the characteristic value of the criterion Fourier number, τ is the current time, V ₀ = CM / (cγF), C is the specific heat of the coolant, M is the mass flow of the coolant, s is the specific heat of the material of the test object, γ is the density of the material of the test object , F is the contact area of the heat exchanger and the test object, T ₀ is the temperature measured by the first temperature meter, T _w is the temperature measured by the second temperature meter, T _n is the ambient temperature, V _w is the rate of change of temperature measured by the second meter temperature controller.

In the particular case, in accordance with paragraph 2 of the claims, the time dependence of the temperature of the coolant at the outlet of the heat exchanger is measured by a third temperature meter and conditions are provided for the readings of the second and third temperature meters to be equal.

The solution of the problem in accordance with paragraph 3 of the claims is ensured by the fact that in the known method, which consists in heating the surface of the test object, measuring the dependence of time on temperature by a first temperature meter, measuring the dependence of time on temperature by a second temperature meter, the following improvements are made: the section is heated the surface of the object under study through the use of a coolant entering the heat exchanger, the external surface of which, except for the surface the tee adjacent to the surface area of the object under study is provided with thermal insulation; in this case, the time dependence of the temperature of the coolant at the inlet of the heat exchanger is measured by a first temperature meter, the time dependence of temperature is measured by a second temperature meter located between the heat exchanger and thermal insulation, and the thermal diffusivity a is determined from ratios:

a = Na _e

N = {c _oe (T ₀ -T _w ) [2 (T _wе -T _н ) Fo * + V _wе τ]} ² / {c _o (T ₀ -T _wе ) [2 (T _wе -T _н ) Fo * + V _w τ]} ² ,

where a _e is the coefficient of thermal diffusivity of the reference object with _oe is the volumetric heat capacity of the material of the reference object, T _ve is the temperature measured by the second temperature meter in the study of the reference object, V _we is the rate of change of temperature measured by the second temperature meter in the study of the reference object, c ₀ - volumetric heat capacity of the material of the studied object, Fo * is the characteristic value of the criterion Fourier number, τ is the current time, T ₀ is the temperature measured by the first temperature meter, T _w is the temperature measured by the second temperature meter, T _n - ambient temperature.

In the particular case, in accordance with paragraph 4 of the claims, the time dependence of the temperature of the coolant at the outlet of the heat exchanger is measured by a third temperature meter and conditions are provided for the readings of the second and third temperature meters to be equal.

The solution of the problem in accordance with paragraph 5 of the claims is ensured by the fact that the following improvements are made to the known device comprising a heater, a first temperature meter, a second temperature meter: it further comprises an inlet pipe, a connecting pipe, a heat exchanger, an output pipe, wherein the output of the inlet pipe is connected to the input of the heater, configured to heat the coolant, and the output of the heater is connected to the input of the connecting pipe an ode whose outlet is connected to the inlet of the heat exchanger, the outlet of the heat exchanger is connected to the inlet of the outlet pipe, the outer surface of the heat exchanger, except for the surface adjacent to the surface area of the object under study, is provided with thermal insulation, the first temperature meter is placed in the connecting pipe, and the second temperature meter is placed between the heat exchanger and thermal insulation.

In the particular case in accordance with paragraph 6 of the claims, the device further comprises a third temperature meter located in the outlet pipe.

We show that the task of the invention is really solved in the claimed technical solution.

The primary thermal conductivity determined by the data of the measured parameters is usually thermal conductivity, and the thermal conductivity is calculated by the known thermal diffusivity coefficient using the relation [7]:

where λ is the coefficient of thermal conductivity of the material of the studied object, [W / (m · K)]; a is the coefficient of thermal diffusivity of the material of the investigated object [m ² / s]; c ₀ is the volumetric heat capacity of the material of the test object [J / (m ³ · K)], s is the specific heat of the material of the test object [J / (kg · K)]; γ is the density of the material of the investigated object [kg / m ³ ].

It should be noted that abroad the thermal diffusivity is accepted as a reference value, and in Russia the thermal conductivity standard is traditionally used.

Note that in some cases, the final value to be determined is the specific thermal resistance, determined by the formula:

where r is the specific thermal resistance [m ² K / W], the reciprocal of the heat transfer coefficient α _s [W / m ² K]; L is the thickness of the heated layer of the investigated object [m].

In practice, the value of the specific thermal resistance g should be determined experimentally, in particular, by experimentally determined thermal diffusivity or thermal conductivity.

The physical model of the studied object can be represented as a uniform unlimited plate with a thickness L. At the initial moment of time, one of the surfaces of the plate is brought into contact with a heat source - a heat exchanger having the same temperature as the studied plate.

We introduce the following assumptions and restrictions.

1. The length of the source of thermal power along the surface of the object under study is large enough so that it can be assumed that the heat flux through the wall is one-dimensional and uniform in cross section.

2. The thermophysical parameters of the material of the studied object are the same in all directions and are constant in time.

3. The heat exchanger is thermally insulated from the environment, except for part of its surface in contact with the surface of the object under study; Thus, the external medium for the heat exchanger is the studied object in contact with it, deep into which (in the transverse direction) the heat flux released in the heat exchanger due to the transfer of thermal energy from the heated heat carrier extends.

4. The heat exchanger is isothermal, as a result of which it is possible to limit oneself to an analysis of its volumetric average temperature.

The volumetric average temperature of the flow heat exchanger within the framework of the accepted assumptions will be described by an equation of the form:

where C _w is the total heat capacity of the heat exchanger [J / K]; T _w - heat exchanger temperature [K]; T _c - the surface temperature of the investigated object adjacent to the working surface of the heat exchanger [K]; T _f - coolant temperature [K]; σ is the thermal conductivity between the surfaces of the heat exchanger and the test object [W / K]; α _to - contact heat transfer coefficient at the interface between the surfaces of the heat exchanger and the test object [W / m ² K]; F is the contact area of the heat exchanger and the test object [m ² ]; α is the convective heat transfer coefficient in the heat exchanger [W / m ² K]; S is the internal surface area of the heat exchanger [m ² ], τ is the current time.

In the future, the average heat transfer coefficient α is considered over the heat exchanger.

In real cases, the thermal inertia of the coolant can be neglected, then the temperature distribution in the coolant along the z axis, which coincides with the average direction of motion of the coolant in the heat exchanger, will be described by the equation:

where C is the specific heat of the coolant [J / (kg · K)]; M is its mass flow rate [kg / s]; l is its length [m]; T ₀ - temperature of the hot coolant at the inlet.

From (4), considering the heat exchanger isothermal, and the distribution of α along the z axis constant, we can obtain the distribution of the temperature of the coolant along the z axis:

- средняя по оси z температура теплоносителя.Where

- the average coolant temperature along the z axis.

From (3), taking into account (5), we obtain:

Considering the parameters included in (6) and determining the heat regime of the heat exchanger, it is necessary to note the difficulties of a priori estimation of φ, an important characteristic that determines the cooling efficiency. Estimation of φ requires the calculation of the convective heat transfer coefficient according to the criterion relations.

The dimensionless parameter φ is a very general characteristic that is present in the mathematical description of the volumetric average temperatures of heat exchangers. For φ> 3 ... 4, the task of predicting the thermal regime is extremely simplified, since in this case E≈1 and in addition to a significant simplification of the calculated ratios, there is no need to calculate the convective heat transfer coefficient.

It is convenient to write down the basic formula describing the heating or cooling process after transforming relation (6) under condition E = 1 in the form:

Formula (7) includes the heating rate m ₀ , which is the sum of the own heating rate m _n and an additional component that increases with increasing mass flow rate of the coolant.

Note that when the condition φ> 3 ... 4 is satisfied, it follows from (5) that the temperature of the coolant at the outlet of the heat exchanger T _in is equal to the temperature of the heat exchanger T _w , that is, the condition:

The heat output given by the heat exchanger under the condition E = 1 can be determined from the relation:

Substituting in (9) the expression for T _w from (7), we obtain:

From (10) it follows that at the initial moment of time and at the end of the process of reaching the stationary thermal regime, the value of the heat output is determined by the relation:

As analysis shows, for the effective operation of the measurement system in question, it is necessary to ensure that the following conditions are met:

where from

However, in the course of measurements, taking into account assumption 3, the studied object is the heat-receiving medium for the heat exchanger, due to the final (and usually low) thermal conductivity of its material, the surface of the studied object, which is in contact with the working surface of the heat exchanger, is heated especially quickly.

Thus, the value of T _c in the relations (7), (10) - (12) is nothing but the temperature of the heated surface of the investigated object.

Due to the increase in T _c in time, the heat output given by the heat exchanger decreases during the measurement process from the initial value

to the end

It follows that relations (7) and (10) must be supplemented by the dependence T _c (τ), for which it is necessary to determine the non-stationary temperature field in the wall, described by the following boundary-value problem:

The following notation is introduced in the boundary-value problem: x — coordinate along the axis orthogonal to the contact plane of the heat exchanger and the object under study [m]; q is the specific heat flux [W / m ² ]; P is the total heat flux given off by the heat exchanger and determined by relation (10).

Boundary condition (17c) was introduced for the reason that the measurement process takes much less time than is required for heating the entire wall thickness.

We transform (10), taking into account the notation σ _Σ in (12), to the following form:

notice, that

where σ _m is the newly introduced designation of the parameter having the dimension of thermal conductivity.

Substituting (19) in (18), we obtain:

After substituting (20) into (17b), we can obtain the boundary condition in the following form:

We obtain an estimate of the unsteady surface temperature of the object under study by integrating both sides of equation (17a) over x in the range from 0 to L:

Substituting (22) taking into account (21) into (17a), we obtain:

To simplify equation (23), it is convenient to pass from temperatures to overheating ϑ = Т ₀ -Т _с0 :

under the initial condition, determined taking into account (17 g):

We multiply all terms of equation (24) by L and move on to dimensionless parameters

where Fo is the dimensionless criterion Fourier number.

The solution of equation (26) has the form:

As a result of integration in (27), taking into account the initial condition (25) after the transformations, we obtain the final solution of equation (26) in the form:

From (28) it is possible to determine the unsteady temperature of the heated surface of the investigated object:

Substituting (29) into (7), taking into account the identity T _c ≡T _co , we obtain the expression for the temperature of the heat exchanger T _w , which, as follows from (5), for φ> 4 coincides with the temperature of the heat carrier at the outlet of the heat exchanger:

From (10), taking into account (29), it is also possible to determine the heat flux given by the heat exchanger:

The initial stage of heating, by definition, will be limited to such a length of time from the start of the heating process, within which the following inequalities are satisfied:

For very small values of the argument, after expanding the exponential in a series, we can restrict ourselves to a linear approximation, i.e.

therefore, the functions f ₁ and f ₂ with a high degree of accuracy can be represented in the form:

where f _in is the approximate value of the function f _i at the initial time interval (i = 1, 2); C _c is the total heat capacity of the heated wall layer within the area F of contact with the heat exchanger [J / K].

When conditions (34) are satisfied, expressions (29) and (30) can be reduced to the form:

From (35) it is possible to determine the rate of growth of the surface temperature of the investigated object V _c :

The relations obtained for the temperatures T _u and T _w, and a surface temperature rise rate test object suitable for determining the thermophysical properties (thermal conductivity, thermal diffusivity) of the material of the test object, as considered model of the physical process relates only to the surface, leaving no consideration of process temperature field propagation deep into the investigated object. Therefore, it is necessary to obtain a solution for an unsteady temperature field in the material layer of the object under study adjacent to the surface.

We introduce one more assumption (in addition to the four previously formulated).

Assumption 5:

which allows us to move on to considering the average rate of temperature increase in the heated layer.

We substitute V _c from (38) into the left-hand side of equation (17a), assuming, taking into account (37), that V _c = const. As a result, we obtain a new equation:

The solution of equation (39) has the form:

where C ₁ and C ₂ are the integration constants.

From the boundary condition (17b) we obtain:

We find the integration constant C ₂ from the additional boundary condition:

Given the boundary conditions (41) and (42), solution (40) will take the form:

From (43) it is easy to determine the surface temperature T _w = T _a (x = 0); taking into account the obvious relation λ = acγ, we obtain:

From (44) it is easy to determine the value of the coefficient of thermal diffusivity:

The obtained relation (45) includes, as a parameter, the thickness of the heated layer L, the value of which should be recognized as undefined, since over time this thickness continuously increases.

With good thermal contact between the surfaces of the heat exchanger and the test object, characterized by the values of the criterion Bi> 50, a constant characteristic value of the criterion Fourier number Fo * is always maintained, which characterizes the relationship between the depth of heating and the time from the beginning of the supply of thermal energy [7]. As a result, the thickness of the heated layer L is related to the current time τ by the following relation [7]:

Substituting (46) into (45), after the transformations, we can obtain a new equation:

From equation (47) it is possible to determine the coefficient of thermal diffusivity:

In equation (48) are difficult to define in experimental quantities of the test object surface temperature T and temperature change _from the surface speed V _c. This is due to difficulties in installing a temperature meter between the surfaces of the heat exchanger and the test object in such a way that it only records the temperature of the test object, excluding the influence of the heat exchanger on the temperature meter readings.

The task is greatly simplified if we take into account the results of numerical mathematical modeling of the heating process under consideration, according to which the temperature difference between the surfaces of the heat exchanger and the test object during the whole process is small compared with the temperature levels. This allows us to make the following assumption:

Assumption 6:

which in principle makes it possible to confine oneself to measuring only the temperature of the heat exchanger. The rate of temperature increase is determined from the ratio:

or

за все время от начала прогрева, а в формуле (51) - мгновенное V_wм, при этом ΔT_w - приращение температуры теплообменника в течение выбранного промежутка времени Δτ; индекс «н» соответствует начальному значению; Т_н - начальная температура всей измерительной схемы и окружающей среды в самом распространенном случае, когда все начальные температуры совпадают.In the formula (50) appears the average value of speed

for all the time from the start of heating, and in formula (51) - instantaneous V _wm , while ΔT _w is the increment of the heat exchanger temperature over a selected period of time Δτ; index “n” corresponds to the initial value; T _n - the initial temperature of the entire measuring circuit and the environment in the most common case, when all the initial temperatures coincide.

We present the formula for determining the heat flux density taking into account (9) and (17b) in the form:

We also introduce the notation

where V ₀ - has a dimension of velocity [m / s].

Taking into account (49), (50), (52) and (53), formula (48) can be represented as:

In order to verify the operability of the formula for determining the thermal diffusivity (54), mathematical modeling was carried out, during which the unsteady temperature field in the studied object in contact with the heat exchanger, whose unsteady temperature was calculated by the formula (10), was calculated by the finite-difference method.

The calculation was carried out in a nonlinear setting using iterations.

Numerical studies were carried out with the following parameter values:

As a result of numerical studies, it was determined, in particular, that Fo * = 0.0665 at ΔТ = 0.5 K. Taking into account the parameters adopted in (55), the value V ₀ = 8.2 · 10 ^-6 m / s.

Substituting in (54) all the indicated parameter values, we obtain a working calculation formula:

Formula (56) includes the instantaneous value V _w = V _wm .

The research results are presented in table 1.

Table 1. The results of numerical calculations of T _w from current time τ, instantaneous growth rates of the heat exchanger temperature V _wm and values of the thermal diffusivity a, calculated by the formula (56). τ min 3 5 8 10 13 twenty thirty 40 fifty I τ, s 180 300 480 600 780 1200 1800 2400 3000 T _wm , ° С 27.5 33 38.7 41.3 44 48.3 52 54.5 56.4 Δτ, s 180 120 180 120 180 420 600 600 600 II ΔТ, ° С 7.5 5.5 5.7 2.6 2.7 4.3 3,7 2,5 1.9

V _w

0.0417 0,0458 0,0317 0,0217 0.015 0.01 0.0062 0.004 0.0032 III a, 10 ^-7

1.23 0.5 0.47 0.64 0.82 0.87 1,0 1.39 1.43

The data in table 1 are divided into three groups (I, II, III). The first group consists of data from primary studies (calculation, experiment). The second group includes values determined on the basis of primary data and serving as initial data for determining the instantaneous speed; wherein Δτ _n = τ _n -τ _n-1 ; ΔТ _n = Т _n -Т _n-1 , where n is the number of this column of table 1. The third row of group II contains data on the calculated instantaneous speed. In the third group, the values of the final determined physical quantity (a) are given.

The instantaneous velocity values presented in Table 1 (for simplicity, in the table and in formulas (54) and (56), the index “m” at V _{w is} omitted) were determined by formula (51).

From table 1 we can draw the following conclusions.

1. The rate of increase in temperature of the heat exchanger (and, accordingly, the heated wall surface) decreases with time - from the beginning of the process to the fiftieth minute - by an order of magnitude.

2. The greatest difference between the thermal diffusivity coefficient determined by the obtained analytical formula (54), and in this particular case, by the formula (56), from the initial value a = 1.28 · 10 ^-7 m ² / s (55), is observed in the time interval from the fifth to the twentieth minute. It is in this time period, as shown by the results of numerical calculations, that the maximum difference between the temperatures of the heat exchanger and the adjacent surface of the object under study takes place, and assumption 6, described by relations (49), is least rigorous.

3. In general, the proposed methodology for determining the thermal diffusivity, based on the use of formula (54), allows one to obtain sufficiently accurate values of the sought-for value, even though the instantaneous velocities were determined very approximately. From this we can conclude that the developed physical model is adequate to the physical process.

The accuracy of determining the coefficient of thermal diffusivity can be improved by introducing the necessary corrections for the nonlinearity of the thermal process based on the use of the results of numerical analysis, and also using the derivatives of the temperature with respect to time at each given moment to determine instantaneous velocities.

The most effective method of error compensation is the application of the method of comparison with a reference sample. This method is based on a preliminary measurement of the value of a sample from a material with well-known thermophysical properties.

In the latter case, the processing of measurement results is carried out according to the formula:

In (57), the “e” index refers to a reference sample.

When using relation (57), the error in determining the coefficient of thermal diffusivity should be significantly reduced due to the mutual compensation of systematic errors for the reference and studied samples of materials.

The value of the coefficient of thermal conductivity λ is determined by the measured value of the coefficient of thermal diffusivity a using relation (1). This is possible because the parameters c and γ of most materials are well known, in contrast to λ and a.

Thus, the claimed device provides a quick, simple and reliable determination of the coefficient of thermal diffusivity.

Let us analyze the basic laws of thermal processes in the framework of the considered physical model.

First of all, we note that thanks to the introduced assumption 5, expressed by relation (38), it is possible to arrive at the coordinate dependence (43), which contains the heating rate V _c . The use of the instantaneous velocity V _cm in the calculations eliminates the need to take into account the significant nonlinearity of the heating process, and the introduction of assumption 6 allows us to significantly simplify the method of measuring and processing experimental data.

The nonlinearity of the process is manifested in a continuous decrease in the heating rate, as can be seen from the data in table 1. This nonlinearity can be analyzed using relations (37) and (46).

We write relation (37) in the form:

where can we determine the thickness of the heated layer in the approximation of infinitely large thermal conductivity:

Substituting in (59) all the parameter values from (55), we obtain, for example, for τ = 3000 s: V _w = 0.0032 K / s (Table 1), whence L = 0.15 m = 15 cm.

On the other hand, the same calculation by formula (46) leads for L = 0.058 m ≈ 6 cm for τ = 3000 s, which is 2.5 times less than the value determined by formula (59). Such a result is understandable, given the low thermal conductivity and thermal diffusivity of the wall material in question.

The following conclusion can be drawn from the analysis: for the studied object 30 cm thick, the proposed measurement method provides the result at the stage when the studied object warms up only 1/5 of the thickness.

We complete the description of the essence of the claimed technical solution by analyzing the results of numerical studies.

Figure 1 presents the calculated dependences of temperature differences between the heat exchanger and the test object on time from the start of the heating process with the values of the thermal conductivity of the test object in W / (m · K): 0.05 (line L ₁ ); 0.1 (line L ₂ ); 0.3 (line L ₃ ); 0.5 (line L ₄ ); 1 (line L ₅ ); 2 (line L ₆ ); 3 (line L ₇ ). All the lines shown correspond to the same values of thermal diffusivity multiplied by 10 ⁶ . The maximum values of the temperature difference are realized just on that time interval within which the errors in calculating the coefficient of thermal diffusivity were maximum (table 1).

Figure 2 presents the initial sections of the dependences of the temperature of the heat exchanger on the heating time for the values of the coefficient of thermal conductivity of the studied object in W / (m · K): 0.05 (line L ₈ ); 0.1 (line L ₉ ); 0.3 (line L ₁₀ ); 0.5 (line L ₁₁ ); 1 (line L ₁₂ ); 2 (line L ₁₃ ); 3 (line L ₁₄ ). As can be seen from figure 2, at the initial time interval, these dependencies are nonlinear, due to the presence of the second degree of the parameter τ, as can be seen from formula (36). From figure 2 it follows that the dynamics of heating can determine the thermophysical properties of the material of the wall.

Figure 3 shows the inverse dependences - a (T _w ) for different time instants from the beginning of receipt of the coolant in the heat exchanger, in minutes: 10 (line L ₁₅ ); 20 (line L ₁₆ ); 30 (line L ₁₇ ); 40 (line L ₁₈ ); 50 (line L ₁₉ ); 60 (line L ₂₀ ), and Fig. 4 shows similar dependences of a on the heat dissipation power in the heat exchanger at different times from the start of the flow of heat carrier into the heat exchanger in minutes: 10 (line L ₂₁ ); 20 (line L ₂₂ ); 30 (line L ₂₃ ); 40 (line L ₂₄ ); 50 (line L ₂₅ ); 60 (line L ₂₆ ). The data of these figures can serve as a guide for the experimental determination of the coefficient of thermal diffusivity of a material.

Thus, the claimed technical solution can be implemented both using the obtained analytical formulas, and based on the results of numerical calculations.

The claimed method and device in comparison with the prototype method and the prototype device have higher consumer properties by expanding the scope and increasing the accuracy and reliability of measurements. In contrast to the prototype method and prototype device, the described technical solution uses a thermal energy source extended in two mutually perpendicular directions, therefore, measurements give effective (average) values of thermophysical characteristics, and in most cases, in practice, not point, but effective values of thermal characteristics, for example, when studying the walls of buildings. The claimed method and device provides the formation of a one-dimensional heat flux (the heat flux propagates linearly from the inner surface of the test object to the outer surface of the test object), which is described by a much simpler mathematical model than in the prototype. Thus, compared with the known technical solutions, the claimed method and device have a wider scope. In the described method and device uses a simpler and more reliable model. In the known method, it is necessary to carry out preliminary measurements to determine the correction factor, and in the described technical solutions this is not necessary. In the heat balance equation describing the operation of the claimed method and device, it is not necessary to take into account the loss of thermal energy into the environment due to convective and radiant heat transfer due to the fact that an extended source of thermal energy equipped with thermal insulation is used. In the heat balance equation of the described method there are no terms necessary for the heat balance equation of the prototype method that describe the partial absorption of thermal power by the environment during the transfer of thermal energy from the energy source to the surface of the object under study and with partial reflection from the surface of the object under study. In the model describing the operation of the claimed method, there is no need to expand any functions in a Taylor series. Thus, the described technical solutions have higher accuracy and reliability. The expansion of the scope and increase the accuracy and reliability of measurements leads to an increase in consumer properties of the claimed technical solutions in comparison with prototypes.

The invention is illustrated by a description of an embodiment of the claimed device and drawings, in which:

- figure 1-4 are graphs explaining the invention;

- figure 5 shows a diagram of a device corresponding to paragraph 5 of the claims.

A device for measuring the thermal diffusivity contains (Fig. 5) a heater 1, a first temperature meter 2, a second temperature meter 3, an inlet pipe 4, a connecting pipe 5, a heat exchanger 6, an output pipe 7, while the output of the input pipe 4 is connected to the input of the heater 1 made with the possibility of heating the coolant, the output of the heater 1 is connected to the input of the connecting pipe 5, the output of which is connected to the input of the heat exchanger 6, the output of the heat exchanger 6 is connected to the input of the output pipeline 7, the outer surface of the heat exchanger 6, in addition to the surface adjacent to the surface area of the test object 8, is provided with thermal insulation 9, the first temperature meter 2 is placed in the connecting pipe 5, and the second temperature meter 3 is placed between the heat exchanger 6 and thermal insulation 9. The third meter temperature 10 is placed in the outlet pipe 7. In figure 5, arrows 11 and 12 show the direction of movement of the coolant.

A device for measuring thermal diffusivity works as follows. The heat carrier under pressure passes through the inlet pipe 4 to the heater 1, where it is heated, and then through the connecting pipe 5 it enters the heat exchanger 6. A part of its heat energy is transferred to the test object 8. From the heat exchanger 6, the heat carrier enters the outlet pipe 7. The first temperature meter 2 measures the time dependence of the temperature of the coolant at the inlet of the heat exchanger 6. The second temperature meter 3 measures the time dependence of the temperature between the heat exchanger 6 and those heat insulation 9. The thermal diffusivity is determined from relation (54).

If the data on the study of the reference object is known, that is, an object whose parameters are known with sufficient accuracy, then the thermal diffusivity of the studied object can be determined from relation (57).

The calculated formulas are substituted with the measured values of T _w , V _w , T _wе , V _wе , measured by a second 3 or third 10 temperature meter. Measurements are reliable only if the readings of the second 3 and third 10 temperature meters are equal. If during the measurement process this condition is not satisfied from the first measurement, it is necessary to achieve this equality by adjusting the mass flow rate of the coolant.

INFORMATION SOURCES

1. Sukhov R.I., Lebedkin Yu.M., Kuznetsov V.G. and others. A method of drilling wells and a device for its implementation. RF patent for invention No. 2237148, prior. 1999.10.06, publ. 2001.07.20, IPC7 Е21В 6/02, Е21В 7/00, Е21В 10/36.

2. Pilin B.P., Markov A.A., Molotkov S.L. The method of ultrasonic inspection and a device that implements it. RF patent for the invention No. 2131123, prior. 1996.01.12, publ. 1999.05.27, IPC6 G01N 29/04.

3. Bobrov V.T., Tarabrin V.F., Ordynets S.A., Kuleshov R.V. Ultrasonic flaw detector "Swallow". RF patent for invention No. 2231783, prior. 2001.08.09., Publ. 2003.07.10, IPC7 G01N 29/04.

4. Sergeev V.A. A method for determining the thermal resistance of the transition-case of semiconductor devices. RF patent for invention No. 2178893, prior. 2001.03.13, publ. 2002.01.27, IPC7 G01R 31/26.

5. Sergeev V.A. Device for measuring the thermal resistance of transistors. Application for a patent of the Russian Federation for invention No.2000127414 / 09, prior. 2000.10.31, publ. 2002.10.10, IPC7 G01R 31/26.

6. Medvedev VV, Troitsky O.YU. Device for determining the characteristics of materials. RF patent for invention No. 2212653, prior. 2002.05.28, publ. 2003.09.20, IPC7 G01N 25/18.

7. Chernyshev V.P., Sysoev E.V., Pavlov R.V. Method of non-destructive non-destructive testing of thermophysical properties of materials. RF patent for invention No. 2251098, prior. 2003.11.17, publ. 2005.04.27, MPK7 G01N 25/18.

8. Chudinov Yu.V., Ishuk I.N., Fesenko A.I. Thermal probe for non-destructive testing of thermophysical properties of materials. Application for a patent of the Russian Federation for invention No. 2003129494, prior. 10/02/2003, publ. 03/27/2005, IPC7 С01N 25/18.

9. Lykov A.V. Theory of thermal conductivity. Graduate School. - M .: 1967. - 599 p.

Claims (6)

1. The method of measuring the coefficient of thermal diffusivity, which consists in heating the surface of the investigated object, measuring the dependence of the time on the temperature by the first temperature meter, measuring the dependence of the time on the temperature by the second temperature meter, characterized in that they heat the surface area of the test object by using a coolant entering the heat exchanger the surface of which, in addition to the surface adjacent to the surface area of the investigated object, is equipped with lovoy insulation, the measured time dependence of the coolant temperature at the inlet of the heat exchanger a first temperature measurement, the temperature dependency is measured from the time the second temperature measuring device, disposed between the heat exchanger and thermal insulation, and the thermal diffusivity a is determined from the relation a = 4Fo * τ {V ₀ (T ₀ -T _w ) / [2 (T _w -T _n ) Fo * + V _w τ]} ² , where Fo * is the characteristic value of the criterion Fourier number, τ is the current time, V ₀ = CM / (cγF), C is the specific heat of the coolant, M is the mass flow of the coolant, s is the specific heat of the material of the test object, γ is the density of the material of the test object , F is the contact area of the heat exchanger and the test object, T ₀ is the temperature measured by the first temperature meter, T _w is the temperature measured by the second temperature meter, T _n is the ambient temperature, V _w is the rate of change of temperature measured by the second meter temperature controller. 2. The method of measuring the thermal diffusivity coefficient according to claim 1, characterized in that the time dependence of the temperature of the coolant at the outlet of the heat exchanger is measured by a third temperature meter and the conditions for the equality of the readings of the second and third temperature meters are provided. 3. The method of measuring the coefficient of thermal diffusivity, which consists in heating the surface of the test object, measuring the dependence of the time on the temperature by the first temperature meter, measuring the dependence of the time on the temperature by the second temperature meter, characterized in that the surface area of the test object is heated by using a coolant entering the heat exchanger the surface of which, in addition to the surface adjacent to the surface area of the investigated object, is equipped with lovoy insulation, the measured time dependence of the coolant temperature at the inlet of the heat exchanger a first temperature gauge measures the temperature dependence on time second temperature measuring device, disposed between the heat exchanger and the thermal insulation, the thermal diffusivity a is determined from the relation a = Na _e N = {c _oe (T ₀ -T _w ) [2 (T _wе -T _н ) Fo * + V _wе τ]} ² / {c _o (T ₀ -T _wе ) [2 (T _wе -T _н ) Fo * + V _w τ]} ² , where a _e is the coefficient of thermal diffusivity of the reference object, O _oe is the volumetric heat capacity of the material of the reference object, T _ve is the temperature measured by the second temperature meter in the study of the reference object, V _we is the rate of change of temperature measured by the second temperature meter in the study of the reference object, c _о is the volumetric heat capacity of the material of the studied object, Fo * is the characteristic value of the criterion Fourier number, τ is the current time, T ₀ is the temperature measured by the first temperature meter, T _w - t the temperature measured by the second temperature meter, T _n - ambient temperature. 4. The method of measuring the coefficient of thermal diffusivity according to claim 3, characterized in that the time dependence of the temperature of the coolant at the outlet of the heat exchanger is measured by a third temperature meter and conditions are provided for the readings of the second and third temperature meters to be equal. 5. A device for measuring the coefficient of thermal diffusivity, comprising a heater, a first temperature meter, a second temperature meter, characterized in that it further comprises an inlet pipe, a connecting pipe, a heat exchanger, an output pipe, wherein the output of the input pipe is connected to an input of a heater configured to heating medium, and the output of the heater is connected to the input of the connecting pipe, the output of which is connected to the input of the heat exchanger, the output of the heat ennika output connected to an input pipe, the outer surface of the heat exchanger, except the surface adjacent to the surface of the object portion is provided with thermal insulation, a first temperature gauge placed in the connecting pipe, and a second temperature gauge is arranged between the heat exchanger and thermal insulation. 6. A device for measuring the thermal diffusivity according to claim 5, characterized in that it further comprises a third temperature meter located in the outlet pipe.

Images