RU54193U1 - DEVICE FOR MEASURING THERMOPHYSICAL CHARACTERISTICS (OPTIONS) - Google Patents

Abstract

The utility model relates to construction equipment and can be mainly used to measure the thermal resistance of various building structures, for example, walls, ceilings, floors, various bulkheads, partitions, ceilings, etc. The objective of the utility model is to increase consumer properties by increasing accuracy and reliability. The solution to this problem is provided by measuring time from the moment the heating of the inner surface of the test object begins to the moment the temperature rises at a given point located on the outer surface or side surface of the test object.

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 the measurement of the thermophysical characteristics of the materials under study, in particular, thermal resistance.

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 only.

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

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. In the prototype, the linear heat source and the points at which the temperature is measured are on the same surface of the object under study, therefore, the known technical solution is not applicable for measuring the thermophysical characteristics of multilayer objects. If the studied object has different thermophysical characteristics in the direction perpendicular to its surface and in the direction parallel to its surface, then the known technical solution can measure the thermophysical characteristics only in the direction parallel to the surface of the studied object and

perpendicular to the heating line. Thus, the known technical solution is not applicable for measuring the thermophysical characteristics of anisotropic objects. Low accuracy and reliability of the known technical solutions 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 of the problem in accordance with claim 1 of the utility model is ensured by the following improvements made to the known device comprising a heater, a heat-sensitive element, and thermal insulation: it additionally contains a second heat-sensitive element, the heater contains a heat exchanger equipped with an inlet pipe for the coolant inlet to the heat exchanger, an outlet pipe for the heat carrier to exit the heat exchanger, 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 inner surface of the test object, a contact temperature meter, used between the inner surface of the test object and the outer surface of the heat exchanger, is used as a heat-sensitive element

the surface of the heat exchanger, a thermal imager was used as the second heat-sensitive element.

In the particular case, in accordance with paragraph 2 of the utility model formula, the optical axis of the thermal imager is directed to the outer surface of the object under study.

In the particular case, in accordance with paragraph 3 of the utility model formula, the optical axis of the thermal imager is directed to the side surface of the object under study.

The solution of the problem in accordance with paragraph 4 of the utility model is ensured by the fact that the following improvements are made to the known device containing a heater, a heat-sensitive element, thermal insulation: it additionally contains a second heat-sensitive element, the heater contains a heat exchanger equipped with an inlet pipe for the coolant inlet to the heat exchanger, an outlet pipe for the heat carrier to exit the heat exchanger, the outer surface of the heat exchanger is provided with thermal insulation In addition to the external surface of the heat exchanger adjacent to the inner surface of the test object, a contact temperature meter is used as a heat-sensitive element, located between the inner surface of the test object and the external surface of the heat exchanger, and a second contact temperature meter is used as a second heat-sensitive element.

In the particular case, in accordance with paragraph 5 of the utility model formula, a second contact temperature meter is located on the outer surface of the object under study.

In the particular case, in accordance with paragraph 6 of the utility model formula, a second contact temperature meter is located on the side surface of the object under study.

The utility model formulas described in paragraph 1 and paragraph 4 of the constructive embodiment of a device for measuring thermophysical characteristics

ensure 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).

The utility model formulas described in clauses 2 and 5 of the embodiment of the device for measuring thermophysical characteristics provide a fixation of the time at which the temperature rises at a given point located on the outer surface of the object under study.

The utility model formulas described in Section 3 and Section 6 of the constructive embodiment of a device for measuring thermophysical characteristics provide a fixation of the time at which a temperature rise begins at a given point located on the side surface of the object under study.

When measuring the thermophysical characteristics of the studied object, the primary thermal conductivity determined by the measured parameters is usually the thermal diffusivity, and the thermal conductivity is calculated by the known thermal diffusivity using the relation [7]

where λ is the thermal conductivity of the material of the measured layer [W / mK]; a - thermal diffusivity of the material of the investigated layer [m ² / s]; with ₀ - volumetric heat capacity [J / m ³ 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 the final value to be determined in building envelopes 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 _c [W / m ² K]; 1 - thickness of the investigated object [m].

For a multilayer object under investigation, the specific thermal resistance is determined from the relation

where r _i - thermal resistance of the i-th layer of the investigated object, and r _k - contact thermal resistance between adjacent layers; n is the number of layers.

It is the presence of the second term that mainly prevents the calculation of thermal resistance from the known values of the thermal conductivity of each layer using the obvious equality

where l _i is the thickness of the i-th layer [m]; λ is the thermal conductivity of the material of the i-th layer [W / mK].

Since the determination of contact thermal resistances between the layers is a serious problem, the use of relation (4) for estimating specific thermal resistances is unacceptable. In practice, the value of the effective specific thermal resistance r should be determined experimentally, in particular, from the experimentally determined effective thermal diffusivity.

It would seem that for a single-layer object under study it is not difficult to determine the specific thermal resistance calculated using formula (2). However, the purpose of experimental methods for measuring r is precisely to investigate the actual thermal resistance, which depends on the observance of manufacturing technology, not

to mention the presence of reinforcement, inclusions, the possible porosity of the material and other factors.

Given the foregoing, it becomes urgent to develop experimental methods for measuring specific thermal resistances that would combine the maximum speed of measurements with high accuracy and complete reliability.

We formulate the problem of developing a device operating in an unsteady mode and free from the disadvantages of pulsed methods. The task includes the development of a physical and mathematical model and the design of the device, providing information content, simplicity and accuracy of determining the effective thermal diffusivity of the studied object.

The idea is to fix the moment of completion of the first stage of the thermal regime of the object under study, when, as a result of the supply of heat power to the internal surface at a given point on the external or side surface, the first signs of overheating appear, that is, the temperature begins to differ from the initial one. Determination of the criterion for the end of the first stage, that is, an estimate of the amount of overheating sufficient for registration and subsequent processing, will be carried out a posteriori.

Consideration of the case of a single-layer homogeneous object under study of constant thickness.

The purpose of the research is to solve the following problems:

1) Development of a research method and an algorithm for processing measurement results;

2) Determination of the time required for measurements, depending on the thermophysical properties of the material of the studied object;

3) Development of the constructive implementation of the device and the definition of requirements for its elements and the device as a whole.

The physical model of the object under consideration - a single-layer object under investigation - can be represented as a uniform unlimited plate with a width L. At the initial moment of time, the inner surface of the plate is brought into contact with a heat source with a temperature t _cl .

We introduce the following restrictions:

1. The contact of the heat source with the heated surface area of the investigated object is carried out so quickly that this contact process can be considered instantaneous.

2. The temperature of the heat source from the beginning to the end of the process remains unchanged.

3. The thermal power of the source is constant throughout the process.

4. The length of the source of thermal power along the surface of the investigated object is much greater than the thickness of the studied object.

These limitations are best satisfied by a compact flow-through heat exchanger, whose thermal inertia is much less than the thermal inertia of the object under study. We also introduce the following assumptions:

1. The heat flux through the object under study is one-dimensional and uniform in cross section.

2. The thermophysical parameters of the material of the studied object are constant.

3. The heat transfer coefficient from the source to the adjacent surface of the investigated object remains unchanged at any time.

4. Until the completion of the first stage of the unsteady thermal regime, the external (opposite to the one in contact with the source)

heat) the surface of the investigated object can be considered insulated.

Taking into account the accepted assumptions and limitations, the physical model of the object turns out to be similar to that adopted in the book [7] for a plate heated or cooled on both surfaces by media with the same temperature and heat transfer coefficients.

The mathematical model of the described object within the framework of the described physical model can be represented by a boundary-value problem, including the heat equation

with boundary conditions

and initial condition

where t is the temperature of the plate; θ is the time [s].

We introduce dimensionless variables

where Fo is the criterial Fourier number; y is the dimensionless coordinate of the plate; Bi - Biot criterion; θ is the dimensionless temperature of the plate.

We transform (5) - (8) to the form

We solve the problem by the method of separation of variables. We represent temperature as the product of two functions, the first of which depends only on the variable y, and the second only on the variable Fo.

After substituting (14) into (10) - (13), the problem splits into two.

one)

where k is the parameter we introduced.

2)

The solution of equation (15) will be equal to

We substitute (19) into (16) and (17), then

kA = 0 => A = 0, Y (y) = Bcoskу at y = 0,

-kBsink = -BiBcosk for y = 1.

From here we get

This transcendental equation has innumerable roots and can be solved graphically or numerically.

Taking into account the solution of equation (18), we write the general solution as the sum of all particular solutions

The constants C _n can be determined from the initial condition (13) in the form [7]

Where

Then, taking into account (22) - (24), we rewrite (21)

Expression (25) with allowance for (9) allows us to calculate the temperature distribution in the plate at any moment of time for the given thermophysical properties of the plate and the medium.

The process of heating a flat layer is depicted in figure 1, where the abscissa represents the distance from the inner surface of the test object to a certain point inside the test object, and the temperature at this point along the ordinate. In Fig. 1, each moment in time t _i corresponds to its own coordinate X _i , more than which the material layer remains isothermal and preserves the initial temperature, while the temperature field is already higher than the initial one within the coordinate coordinate 0≤X <X _i . Within this layer 0≤X <X _{i the} temperature decreases from the value of T _i to T ₀ . Figure 1 shows five lines characterizing the temperature distribution in a layer of thickness L for time instants t ₁ , t ₂ , t ₃ , t ₄ and t ₅ . The values of the heating depth corresponding to these time instants are, respectively, X ₁ , X ₂ , X ₃ , X _4, and L. Since L corresponds to

the total thickness of the investigated layer, then the time t ₅ corresponds to the duration of complete heating of the investigated layer.

Thus, based on the considered model, it is possible to calculate the warm-up time with any given accuracy. We assume that the heating time has come if the temperature on the insulated surface is t = t ₀ + Δt ₃ , where 0.01 ° С≤Δt ₃ ≤2 ° С. This will correspond to the value of the Fourier number Fo ₁ . The choice of the range is preliminary and corresponds to an a priori idea of the sensitivity of the applied temperature measurement technique.

By measuring the duration of the initial stage of heating τ ₁ experimentally, one can determine the thermal diffusivity by the formula following from the first expression in (9):

To solve the problems formulated earlier, it is necessary to carry out computational studies of characteristic dependencies.

The most important parameters that determine the course of the process are the quality of contact of the heated portion of the test object with the heat power source and the temperature of the heat power source, characterized by Bi ₁ criterion.

Figure 2 shows the dependence of the criterion Fourier number Fo ₁ on criterion Bi ₁ , which characterizes the heat transfer rate of the working surface of the heat source with the inner surface of the object under study (this surface is accepted by the surface U ₁ , and the outer surface of the object under study will be denoted by the number U ₂ ). The value of Fo _{1 was} determined from the dependence of the surface overheating Us on the Fourier number, and then the inverse dependence was constructed. In this case, the value of surface overheating U ₂ relative to the initial value of 0.1 K was selected as a criterion for completing the first heating stage

were carried out at three operating temperatures t _{cl of the} heat source: 40 ° C (line L ₁ ), 60 ° C (line L ₂ ), 80 ° C (line L ₃ ). As can be seen from figure 2, in a wide range of changes in the value of the Bio criterion - from 0.01 to 1000, the value of Fo ₁ changes only an order of magnitude. Although this change is very small in comparison with the variation of Bi ₁ values, this circumstance nevertheless creates a practical uncertainty in estimating the duration of the process.

Figure 3 presents fragments of the same dependence in a slightly more limited range, namely, when Bi ₁ > 10. As can be seen from this FIG, in the range of variation of the Biot criterion from 10 to 100, the value of Fo ₁ changes at any temperature of the heat source by no more than 10%, and when Bi ₁ > 100, the dependences become saturated at constant levels. From the data of figure 3 it should be concluded about the need to ensure good thermal contact. In particular, in the case under consideration, in order to fulfill the condition Bi ₁ > 100, it is sufficient to ensure that the contact heat transfer coefficient is not less than 50 W / m ² K. Such contact heat transfer intensity is easily achieved in practice when using a heat-conducting paste of the type at the contact point of the surface of the heat exchanger with the test object sealant.

In Fig. 4, the dependences of the value of Fo ₁ , which characterizes the duration of the first stage of heating a flat object under investigation up to heating its opposite thermally insulated surface by 0.1 K, are plotted against the operating temperature of the heat source with a value of criterion Bi ₁ equal to 10 (line L ₄ ) , 100 (line L ₅ ) and 1000 (line L ₆ ). From the data of Fig. 4 it can be seen that at a temperature of the heater above 60 ° C, sufficient stability of the considered dependence is ensured with slight fluctuations (within 1 ... 2 ° C) of the temperature of the heat source.

Based on the data of FIG. 3 and FIG. 4, it can be concluded that the practical implementation of a thermal power source with a desired temperature level and thermal contact quality is practical.

The dependences considered above were obtained for one particular case of the criterion for completing the first stage of heating. For practice, it is important to evaluate the dependence of the value of Fo ₁ on the value of overheating of the opposite surface of the studied object selected as a criterion for completing the first stage. Such dependences are presented in FIGS. 5 and 6. FIG. 5 shows the dependence of the criterion number Fo ₁ , selected as a criterion for the duration of the first stage of heating a flat test object, on overheating of the external surface of the test object at criterion Bi ₁ = 10 and at source temperature equal to: 40 ° C (line L ₇ ), 60 ° C (line L ₈ ) and 80 ° C (line L ₉ ). Figure 6 shows the dependence of the criterion number Fo ₁ on overheating of the external thermally insulated surface of the flat object under investigation for large values of the Biot criterion (Bi ₁ > 100) and at a temperature of the heat source equal to: 40 ° C (line L ₁₀ ), 60 ° C (line L ₁₁ ) and 80 ° C (line L ₁₂ ). The data of these graphs show that for the criterion Bi ₁ > 100, the value of Fo ₁ does not depend on the further growth of the Biot criterion, as the calculations showed, even if more significant overheating of the opposite surface of the test object is chosen as the criterion for completing the first stage of heating.

From Fig.6 it can be seen that at Δt ₂ = 1K and at a heater temperature of 40 ° C Fo ₁ = 0.11, which exactly corresponds to the data of [8], which presents the results of the development and research of an approximate analytical method for calculating unsteady heat regime based on the ideas of two stages of heating. However, this coincidence is particular, and although with large Biot values, there is no further change in the value of Fo ₁ (as shown in [8]), however

there is a dependence on the temperature of the source and, as expected, on the value of Δt ₂ . From this we can conclude that in the experiment it is useful to record not only the fact of overheating of the opposite surface of the studied object, but also to take the time dependence of this overheating value with the subsequent construction of the inverse relationship. Such dependences, obtained on the basis of a calculation from the dependences of Fig. 6, are shown in Fig. 7, which shows the dependence of the duration of the first stage of heating τ ₁ in hours on overheating of the external thermally insulated surface of the flat object under investigation at Bi ₁ > 100 and at the temperature of the heat source equal to: 40 ° C (line L ₁₃ ), 60 ° C (line L ₁₄ ) and 80 ° C (line L ₁₅ ).

As can be seen from Fig.7, with overheating Δt ₂ = 0.1 ... 0.4K, the measurement time at 40 ° C <t _cl <80 ° C will be from 9 to 15 hours. Moreover, the accuracy of determining the thermal diffusivity of the material of the studied object can be significantly increased by removing the dependence of τ ₁ (Δt ₂ ). From Fig.7 it follows that in the range of changes in overheating from 0.1 to 1K at a temperature of the heat source equal to 80 ° C, such measurements will take about 14 hours.

The algorithm for determining thermal diffusivity is as follows: the experimental dependence of the form shown in Fig. 7 is removed. Then a theoretical relationship is used, similar to that shown in Fig.6. Then, using the formula (26), the thermal diffusivity values for different time instants are calculated. In this case, either the constancy of thermal diffusivity is established, or its average value is taken.

The conducted studies relate to the model of the studied object with a thermally insulated external surface. However, as the overheating of this surface increases, the intensity of natural radiant-convective heat transfer increases. In order to take this factor into account,

calculation of the dependence of the criterion Bi ₂ on the outer surface of the test object from its overheating, the results of which are presented in Fig. 8.

From the data of this FIG. it follows that even with small overheating of the external surface of the object under study, neglecting heat transfer, strictly speaking, is undesirable, although permissible. Therefore, in some cases it is advisable to solve the problem in a more general and rigorous setting. The evaluation results show that within the limits of overheating Δt ₂ = 1 ... 2K, the result can be refined to 5%.

Let us determine the gain in time spent on measuring the thermal resistance of the object under study in the described thermal regime compared with the stationary method.

The time spent on the preparation and conduct of measurements is an important economic indicator of the measurement method used. If more than two or three working shifts are spent from the start of turning on the heat source to the readings of the sensors by recording devices (more than 16 ... 24 hours), then for each subsequent shift the measurement complexity increases, the reliability of the equipment decreases, extraneous factors that interfere with reliability can interfere measurements.

To estimate the gain in time spent on measuring the thermal resistance of the proposed device in comparison with a device operating in a stationary mode, we carry out the following computational comparisons:

1) we compare the duration of the first stage of heating, characterized by the corresponding values of Fo ₁ and τ ₁ with the time of establishment of the stationary thermal regime in the above model of the studied object, characterized by its value of τ ₂ , and in dimensionless form - by the value of Fo ₂ ;

2) compare the values of Fo ₁ and τ ₁ with Fo ₃ and τ ₃ - the latter will characterize the duration of the transition (non-stationary) mode until a specified temperature t _{2 is} established on the external surface of the object under study.

Consider each of the options.

In the first case, the duration of the first stage of heating is compared with the time τ _{2 of} the temperature field equalization in the studied object; leveling criterion is the achievement of the temperature level on the heat-insulated surface of the studied object of such a value that with a given degree of accuracy is close to t _cl .

Such a value must inevitably be established, since the considered surface of the studied object is thermally insulated. A further increase in temperature is impossible, since a source of thermal power with a constant temperature is selected, and the process ends with the establishment of a uniform temperature t _cl over the entire thickness of the test object, after which the process of heat supply to the test object stops.

The calculation was carried out when choosing two options for establishing a stationary thermal regime.

Criterion 1. It is believed that the stationary thermal regime has occurred if the temperature of the thermally insulated surface of the test object t ₂ differs from the stationary value t _cl by a given value Δt ₃ = t _cl -t ₂ . We take Δt ₃ = 1K.

Criterion 2. Using the theory of regular thermal regime [9], we take into account that from a certain moment in time, the change in the temperature field in time is described by one exponent. This follows from the relation . Then, in solution (25), the factor can be taken out of the sum sign . In the case of a regular thermal regime, the natural criterion for establishing a stationary thermal regime is

we can assume that the condition , where Fo ₂ is the value of the Fourier number that corresponds to the indicated inequality.

When using the first criterion, it is necessary to carry out numerical calculations, and the second criterion, although somewhat less defined, allows quick estimates.

Table 1 shows the results of calculations by both temperature criteria, which, with an error of ~ 1K, will be considered stationary. The calculation results of Fo ₂ and τ _{2 are} presented in tables 2 and 3.

Table 1
The temperature of the thermally insulated surface of the investigated object by the time the stationary thermal regime sets in according to two criteria Source temperature, t ° C 40 60 80 t ₂ ° C by criterion 1 39 59 79 by criterion 2 39.5 59.1 78.6 table 2
The values of the criterial numbers Fo ₂ characterizing the total duration of the transitional thermal regime of the investigated object depending on the temperature of the source for different values of the criterion Bi ₁ Source temperature, t ° С 40 60 80 By criterion 1 1,58 1.92 2.18 Bi ₁ = 10 By criterion 2 1.96 By criterion 1 1.34 1,62 1.79 Fo ₂ Bi ₁ = 100 By criterion 2 1.65 By criterion 1 1.31 1,59 1.76 Bi ₁ = 1000 by criterion 2 1,62

Table 3
The time to establish a stationary thermal regime in a flat object under investigation with a thermally insulated external surface Source temperature, t ° C 40 60 80 by criterion 1 12.9 15.6 17,2 Bi ₁ = 10 by criterion 2 15.9 by criterion 1 10.9 13,2 14.6 τ ₂ day Bi ₁ = 100 by criterion 2 13.5 by criterion 1 10.7 13.0 14.3 Bi ₁ = 1000 by criterion 2 13,2

From the tables it can be seen that in the best way the results of calculations of Fo ₂ and τ ₂ according to two criteria coincide at t _cl = 60 ° C, when the values of t ₂ differ by only 0.1 ° C. At t _cl = 40 ° C, the value of t ₂ according to the second criterion is 0.5 ° C higher, and the duration of the process is 2.5 ... 3 days higher. At t _cl = 80 ° C, the opposite trend is observed, although the difference in τ ₂ in the opposite direction is only 1.1 ... 1.2 days.

Compare the data of table 3 and Fig.7. Consider the situation when Bi ₁ > 100 and t _cl = 60 ° C. From table 3 it is seen that the stationary thermal regime with an accuracy of 1 ° C is set for 13 ... 13.5 days. At the same time, the initial stage of heating with the criterion Δt ₂ = 1K is completed 16 hours after the start of the process. Thus, the total duration of the process is ~ 20 times the duration of the initial stage.

We perform the same calculations using the analytical formula derived in [10] based on the general method presented in [8]:

where μ is an eigenvalue, for Bi ₁ > 100 the equality μ = 2.5 holds; Fo _* = 0.11; t ₃ - a given temperature level on a thermally insulated surface. The value of Fo _* has the same physical meaning as Fo ₁ .

Setting Δt ₃ = 1K, instead of (26), we can write

We take the value t ₀ = 20 ° C, and for t _{cl the} values from tables 1 ... 3. The calculation results showed that the values of Fo ₂ calculated by the formula (27) differ from those shown in table 2 by 0.4% at t _cl = 40 ° C; by 0.5% at t _cl = 60 ° С and by 0.6% at t _cl = 80 ° С.

Thus, it can be concluded with absolute certainty that the duration of the initial heating stage for the considered thermal and mathematical models of the studied object is only ~ 5% of the total time until the establishment of the stationary thermal regime.

We proceed to compare the values of Fo ₁ and τ ₁ with the values of Fo ₃ and τ ₃ characterizing the duration of the process of establishing a given temperature t ₂ on the external surface of the object under study.

In this case, the measurement rates are practically compared by two methods. The first relates to non-stationary methods and is described above. The second method refers to the stationary one and uses the passage of heat flux through the object under study from the heater (heat exchanger with a coolant duct) to a thermostat (the same heat exchanger, but receiving heat flux).

The calculations were carried out for temperatures t _cl = 60 ° С and t ₂ = 40 ° С and at Bi ₁ = 1000.

The calculation according to the formula (25) showed that the temperature t ₂ = 40.0012 ° C is reached at Fo ₃ = 0.3795, which corresponds to τ ₃ = 3.088 days. The calculation according to the formula (26) gave the result: Fo ₃ = 0.387 or τ ₃ = 3.15 days.

At the same time, it can be seen from Fig. 7 that at Bi ₁ > 100 and at t _cl = 60 ° C, 16 hours are required to register overheating Δt ₂ = 1 K, and in the case of Δt ₂ = 0.3 K, no more than 12 hours or 0.5 days, which is 6 times less than the time spent on measuring thermal resistance in a stationary thermal mode.

The claimed device in comparison with the prototype has higher consumer properties by expanding the scope and increasing the accuracy and reliability of measurements. In contrast to the prototype, 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 are needed, for example, in the study of the walls of buildings. Due to the fact that in the claimed technical solution, the inner surface is heated, and the temperature is measured on the outer or side surface, the described device is applicable for the study of multilayer objects. Therefore, the claimed device can be used to measure the thermophysical characteristics of anisotropic objects. The claimed device provides for 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 solution, the claimed device have a wider scope. In the described device uses a simpler and more reliable model. In the heat balance equation describing the operation of the claimed 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. 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 solution compared to the prototype.

The essence of the utility model is illustrated by a description of specific embodiments of the claimed device and drawings, in which:

- figure 1-8 shows graphs explaining the essence of the utility model;

- figure 9 shows a diagram of a device corresponding to claim 1 and claim 2 of the formula of the utility model;

- figure 10 shows a view in axonometric projection of the device corresponding to claim 1 and claim 2 of the formula of the utility model;

- figure 11 shows a diagram of the device corresponding to claim 1 and claim 3 of the formula of the utility model;

- Fig.12 is a view in axonometric projection of the device corresponding to claim 1 and claim 3 of the formula of the utility model.

In accordance with claim 1 of the utility model formula, a device for measuring thermal characteristics includes (FIG. 9 and FIG. 10) a heater, a heat-sensitive element, thermal insulation 1, a second heat-sensitive element, and the heater comprises a heat exchanger 2 provided with an inlet pipe 3 for entering the coolant into the heat exchanger 2, the outlet pipe 4 for the exit of the coolant from the heat exchanger 2, the outer surface of the heat exchanger 2 is provided with thermal insulation 1 except adjacent to the inner surface 5 of the investigated object 6 of the outer surface of the heat exchanger 2, a contact temperature meter 7 is used as the heat-sensitive element, located between the inner surface of the test object 6 and the outer surface of the heat exchanger 2, the thermal imager 8 is used as the second heat-sensitive element. Figures 9-12 also have the following notation: 9 - the optical axis of the thermal imager, 10 - the outer surface of the investigated object 6, 11 - the lateral surface of the studied object 6, arrows 12 and 13 show the direction of movement of the heat carrier la.

In the particular case (claim 2 of the utility model formula), the optical axis 9 of the thermal imager 8 is directed to the outer surface 10 of the investigated object 6.

In the particular case (claim 3 of the utility model formula), the optical axis 9 of the thermal imager 8 is directed to the side surface 11 of the investigated object 6.

The variants of the device for measuring thermophysical characteristics described in claims 1-3 of the formula of the utility model work as follows. The heated heat carrier passes through the inlet pipe 3 to the heat exchanger 2, transfers heat energy to the test object 6 through a portion of the inner surface 5 of the test object 6, adjacent to the outer surface of the heat exchanger 2. The contact temperature meter 7 records the moment the temperature rises in the contact area of the outer surface of the heat exchanger 2 s the inner surface 5 of the test object 6. The thermal imager 8 captures the moment the temperature rises at a given point. In accordance with Clause 2 of the utility model formula, the specified point is selected on the outer surface 10 of the test object 6. In accordance with Clause 3 of the utility model formula, the set point is selected on the lateral surface 11 of the test object 6. The thermophysical characteristics of the test object are calculated according to the method described above. .

Corresponding to clauses 4, 5 and 6, embodiments of a device for measuring thermal characteristics have a design and operating principle similar to those described above, the difference is that instead of a thermal imager 8, a second contact temperature meter is used.

In accordance with paragraph 5 of the utility model formula, a second contact temperature meter is located on the outer surface 10 of the test object 6.

In accordance with claim 6 of the utility model formula, a second contact temperature meter is located on the side surface 11 of the test object 6.

INFORMATION SOURCES

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Claims (6)

1. A device for measuring thermophysical characteristics, comprising a heater, a thermosensitive element, thermal insulation, characterized in that it further comprises a second thermosensitive element, the heater comprises a heat exchanger equipped with an inlet pipe for entering the heat carrier into the heat exchanger, an outlet pipe for exiting the heat carrier from the heat exchanger, external the surface of the heat exchanger is provided with thermal insulation in addition to the external surface adjacent to the inner surface of the studied object the surface of the heat exchanger, a contact temperature meter used between the inner surface of the test object and the outer surface of the heat exchanger is used as a heat-sensitive element, and a thermal imager is used as the second heat-sensitive element. 2. A device for measuring thermophysical characteristics according to claim 1, characterized in that the optical axis of the thermal imager is directed to the outer surface of the object under study. 3. A device for measuring thermal characteristics according to claim 1, characterized in that the optical axis of the thermal imager is directed to the side surface of the object under study. 4. A device for measuring thermophysical characteristics, comprising a heater, a thermosensitive element, and thermal insulation, characterized in that it further comprises a second thermosensitive element, the heater comprises a heat exchanger equipped with an inlet pipe for entering the heat carrier into the heat exchanger, an outlet pipe for exiting the heat carrier from the heat exchanger, external the surface of the heat exchanger is provided with thermal insulation in addition to the external surface adjacent to the inner surface of the studied object surface of the heat exchanger, a contact temperature meter used between the inner surface of the test object and the outer surface of the heat exchanger is used as a heat-sensitive element, and a second contact temperature meter is used as a second heat-sensitive element. 5. A device for measuring thermophysical characteristics according to claim 4, characterized in that the second contact temperature meter is located on the outer surface of the test object. 6. A device for measuring thermophysical characteristics according to claim 4, characterized in that the second contact temperature meter is located on the side surface of the test object.