RU60730U1 - DEVICE FOR MEASURING TEMPERATURE CONDUCTIVITY COEFFICIENT - Google Patents

Abstract

The utility model relates to construction equipment and can be mainly used to measure the thermal diffusivity of various building structures, for example, walls, ceilings, floors, bulkheads, ceilings, etc. The objective of the utility model is to increase consumer properties by expanding the scope and increasing the accuracy and reliability of measurements . The essence of the utility model is the formation of a one-dimensional heat flow, measuring the time dependence of the temperature of the coolant at the inlet of the heat exchanger, and measuring the time dependence of the temperature between the heat exchanger and thermal insulation.

Description

The utility model 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 device for drilling wells [1], which allows 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 device is that it does not provide non-destructive testing of the studied object.

Numerous versions of devices for ultrasonic flaw detection are known, for example, [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 thermal characteristics of the materials under study, in particular, thermal diffusivity.

Numerous variants of devices for measuring the thermal resistance of various electronic devices are known, for example, a device for measuring the thermal resistance of transistors described in [4]. A disadvantage of the known technical solution lies in a narrow scope: it can only be used to measure the thermal resistance of transistors.

A device for determining the characteristics of materials described in [5] is known, comprising a source of pulse heating, a thermocouple, and an electronic processing unit. Thermocouple located on the surface

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.

The closest in technical essence to the claimed device for measuring thermophysical characteristics is a thermal probe for non-destructive testing of thermophysical properties of materials [6], containing a linear heater and two thermocouples located symmetrically relative to the linear heater on both sides of it.

The known technical solution has 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. In the known technical solution, a linear source of thermal energy is used, therefore, 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 linear source of thermal energy acts on different parts of the surface of the object under study. The low accuracy and reliability of the known technical solutions are also due to the high complexity of the model that describes their work. The operation of the known device is 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. These losses are taken into account approximately by calculation. The linear heater generates a heat flux diverging in a plane perpendicular to the axis of the linear heater. This mode of heat distribution is characterized by a complex mathematical model,

for practical application, a number of simplifications have to be introduced into the mathematical model, which are not always implemented in practice. All these factors together lead to low consumer properties of the known device due to the narrow scope and low accuracy and reliability.

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

The solution to the problem in accordance with paragraph 1 of the utility model is ensured by the fact that the following improvements are made to the known device comprising a heater, thermal insulation, a first temperature meter, a second temperature meter: it further comprises an inlet pipe, a connecting pipe, a heat exchanger, an output pipe , the output of the inlet pipe is connected to the input of the heater, the output of the heater is connected to the input of the connecting pipe, the output of the connecting pipe is connected it is connected with 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 is provided with thermal insulation in addition to the outer surface of the heat exchanger adjacent to the surface of the test object, the first temperature meter is placed in the connecting pipe, the second temperature meter is located between the heat exchanger and thermal insulation, and the thermal diffusivity and determined by the ratios:

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

or

a = Na _e

N = {s _oe (T ₀ -T _w ) [2 (T _we- T _n ) Fo * + V _wе τ]} ² / {s _o (T _o -T _we ) [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

mass flow rate of the coolant, s is the specific heat of the material of the studied object, γ is the density of the material of the studied object, F is the contact area of the heat exchanger and the studied 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 temperature meter, and _e is the thermal diffusivity of the reference object c _oe is volumetric the heat capacity of the material of the reference object, T _ve - temperature measured by the second temperature meter in the study of the reference object, V _we - the rate of change of temperature measured by the second temperature meter in the study the reference object, С ₀ - volumetric heat capacity of the material of the studied object.

In the particular case, in accordance with claim 2 of the utility model, the device for measuring the thermal diffusivity coefficient further comprises a third temperature meter located in the outlet pipe.

Such a construction of a device for measuring thermal diffusivity allows to increase consumer properties by expanding the scope and increasing the accuracy and reliability of measurements due to the formation of a one-dimensional heat flow and measuring the time difference of the temperature difference of the coolant at the inlet and outlet of the heat exchanger. It will be shown below that the temperature measured by the second temperature meter when it is placed between the heat exchanger and the thermal insulation is equal to the temperature measured by the third temperature meter when it is placed in the outlet pipe.

Simultaneous measurement of temperature on the surface of the heat exchanger (between the heat exchanger and thermal insulation) and at the outlet of the heat carrier flow from the heat exchanger - in the outlet pipe allows to increase the accuracy and reliability of measurements, since this allows you to verify that the conditions for ensuring full heat transfer are met

(full recovery). If the temperature of the coolant at the outlet would be higher than the temperature of the heat exchanger, then this would mean that the contact area of the coolant with the heat-absorbing surface of the heat exchanger is not large enough. In this case, as will be shown below, the algorithm for processing the measurement results is complicated, since the heat transfer coefficient must be taken into account in the calculated ratios., And when the fact that the coolant temperature at the outlet and the heat exchanger temperature is equal, the heat transfer coefficient drops out of the calculated ratios, which greatly simplifies the processing algorithm measurement results. In the experiment, it is possible to achieve in the experiment the equality of these temperatures by adjusting the mass flow rate of the coolant. To ensure this possibility, it is necessary to use a second temperature meter and a third temperature meter.

We show that the task of the utility model is really solved in the claimed device.

The primary thermal conductivity determined from the measured data is usually thermal conductivity, and the thermal conductivity is calculated from the known thermal diffusivity using the relation [7]

where λ is the coefficient of thermal conductivity of the material of the studied object, [W / mK]; a is the coefficient of thermal diffusivity of the material of the investigated object [m ² / s]; с ₀ - volumetric heat capacity of the material of the studied object [J / m ³ K], s - specific heat capacity of the material of the studied 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 specific thermal resistance r should be determined experimentally, in particular, by experimentally determined thermal diffusivity or thermal conductivity.

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

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, into the depths of which (in the transverse direction) the heat flux released in the heat exchanger due to the transfer of thermal energy from the heated coolant is distributed.

4. The heat exchanger is isothermal, as a result of which it is possible to limit oneself to 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 _with - 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]; 1 - its length [m]; T _about - the temperature of the hot coolant at the inlet.

From (4), assuming that the heat exchanger is isothermal, and the distribution of α along the z axis is constant, we can obtain the distribution of the temperature of the coolant along the z axis.

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 heat carrier 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 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 condition

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 is the coordinate

along the axis orthogonal to the plane of contact of the heat exchanger and the test object [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 it takes to warm 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) into (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) with (21) in (17a) into account, 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 (17g):

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 heat exchanger temperature T _w , which, as follows from (5), for φ> 4 coincides with the temperature of the heat carrier at the heat exchanger outlet:

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 as

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].

Under conditions (34), expressions (29) and (30) can be reduced to

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 the propagation of the temperature field 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) takes 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 indefinite, 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 surface temperature value T _from the object under study and the rate of change of the surface temperature of 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, in accordance

with which the temperature difference between the surfaces of the heat exchanger and the test object during the entire 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

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. - Results of numerical calculations of T _w from current time τ, instantaneous growth rates of heat exchanger temperature V _wm and values of thermal diffusivity a calculated according to formula (56). I τ min 3 5 8 10 13 twenty thirty 40 fifty 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 II Δτ, s 180 120 180 120 180 420 600 600 600 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 but,
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 ; ΔT _n = T _n -T _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. On the whole, the proposed methodology for determining the thermal diffusivity, based on the use of formula (54), allows one to obtain fairly 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 results of numerical analysis, as well as using the derivatives of the temperature with time at each given moment to determine the 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 measurement and processing of 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 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 / mK: 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 investigated object in W / mK: 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 this Fig., At the initial time interval these dependencies are nonlinear, which is 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 from these FIGs. 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 essence of the utility model is illustrated by a description of an embodiment of the claimed device and drawings, in which:

- figures 1-4 are graphs explaining the essence of the utility model;

- figure 5 shows a diagram of a variant of the structural implementation of the claimed device.

A device for measuring the coefficient of thermal diffusivity, contains (Fig. 5) a heater 1, thermal insulation 2, a first temperature meter 3, a second temperature meter 4. It further comprises an inlet pipe 5, a connecting pipe 6, a heat exchanger 7, an output pipe 8, a third temperature meter 9. The output of the input pipe 5 is connected to the input of the heater 1, the output of the heater 1 is connected to the input of the connecting pipe 6, the output of the connecting pipe 6 is connected to the input of the heat exchanger 7, the output of the heat exchanger 7 is connected to the inlet of the outlet pipe 8, the outer surface of the heat exchanger 7 is provided with thermal insulation 2 in addition to the outer surface of the heat exchanger 7 adjacent to the surface of the test object 10, the first temperature meter 3 is placed in the connecting pipe 6, the second temperature meter 4 is placed between the heat exchanger 7 and thermal insulation 2. The third temperature meter 9 is located in the outlet pipe 8. 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 5 to the heater 1, where it is heated, and then through the connecting pipe 6 it enters the heat exchanger 7. A part of its heat energy is transferred to the test object 10. From the heat exchanger 7, the heat carrier enters the outlet pipe 8. The first temperature meter 3 measures the time dependence of the temperature of the coolant at the inlet of the heat exchanger 7. The second temperature meter 4 measures the time dependence of the temperature between the heat exchanger 7 and t 2. Thermal insulation pilaf is determined from the 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 4 temperature meter. Measurements are reliable only if the readings of the second 3 and third 4 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

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Claims (2)

1. A device for measuring the coefficient of thermal diffusivity, comprising a heater, thermal insulation, 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, the output of the input pipe is connected to the input of the heater, the output of the heater connected to the inlet of the connecting pipe, the output of the connecting pipe is connected to the input of the heat exchanger, the output of the heat exchanger is connected to the input ohm output conduit, the outer surface of the heat exchanger is provided with thermal insulation in addition to the adjacent surface of the object outer surface of the heat exchanger, the first temperature measuring instrument disposed in the connecting pipe, a second temperature gauge is arranged between the heat exchanger and the thermal insulation, the thermal diffusivity a is determined by the relationships: a = 4Fo * τ {V ₀ (T ₀ -T _w ) / [2 (T _w -T _n ) Fo * + V _w τ]} ² or 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 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 studied object, y is the density of the material of the studied object , F - the contact area of the heat exchanger and the test object, Vo - dimension rate of temperature change, T ₀ - temperature measured by the first temperature measurement, T _w is the temperature measured by the second temperature measurement, T _n - ambient temperature, V _w - rates temperature change, measured by a second temperature measuring device, and _e - thermal diffusivity reference object, c is the volume specific heat _oo reference object material _we T - temperature measured by the second temperature measuring device in the study of the reference object, V _we - speed of temperature change measured by the second temperature measuring device in the study of the reference object, c ₀ is the volumetric heat capacity of the material of the studied object. 2. The device for measuring the thermal diffusivity according to claim 1, characterized in that it further comprises a third temperature meter located in the outlet pipe.