RU2245538C1 - Nondestructive control method for thermal and physical properties of building materials for multilayer building structure erection - Google Patents

Abstract

FIELD: building, particularly for investigating or analyzing materials.

SUBSTANCE: method involves performing adiabatic thermal action on surface of outer structure layer with the use of disc heater arranged in plane of test probe surrounded by protective heat-insulation ring; recording time dependence of investigated material surface temperature; arranging heat flow sensor on contact surface of the second probe instead of disc heater; installing two linear heaters at a distance from disc heater of the first probe and two linear heaters at a distance from heat flow sensor of the second probe; arranging thermoelectric batteries at fixed distance from linear heaters along line parallel to line of heaters location; applying single heat impulse from linear heat sources to outer structure layers to determine heat and physical properties thereof; determining time of temperature field relaxation in controlled points; performing action of heat pulses in both probes from linear heat sources; changing heat pulse frequency up to obtain temperature in points spaced the same distances from linear heaters equal to two pre-determined values along with determining frequencies of heat pulses for the first and the second outer layers correspondingly; determining heat and physical properties of outer structure layers with the use of above information and obtained mathematical relations; performing heat action on inner structure layer with the use of disc heater of the first probe to define heat and physical properties of inner layer; recording heat flux value by sensor arranged on contact surface of the second probe; measuring temperature in points located correspondingly under disc heater and on contact surface of heat flux sensor with the use of pre-measured temperatures in above points, pre-measured value of heat flux passing through structure layers and previously obtained values of heat and physical properties of outer structure layers; determining heat and physical properties of inner structure layer on the base of mathematical relations describing temperature drop in each of three layers.

EFFECT: increased accuracy of heat and physical properties determination in multi-layer articles.

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Description

The invention relates to building heat engineering, in particular to measurements of thermophysical properties (TFS) of multilayer walling (external ceilings, partitions, coatings, floors, etc.).

A known method of non-destructive testing of TPS materials (see AS USSR N 1390555, class 4 G 01 N 25/18, 1988), which consists in the thermal effect on the surface of the body, which is semi-infinite in the heat ratio, from a point heat source, measuring the time to reach maximum excess temperature at a given point on the surface of the body, measuring the power of the heat source, while providing a constant power of the heat source until the maximum excess temperature is reached at a given point on the surface, then the power of the heat source The lasers are inversely proportional to the square root of time and the maximum excess temperature is measured at the point of application of the heat source, and the required TPS are determined by the appropriate formulas taking into account the measured parameters.

The disadvantages of this method are, firstly, that a point heat source is used as a heater, while a heat source with a large active (heat transfer) is needed to determine the TPS of building materials (concrete, brick, heaters such as foam, etc.) surface, since the required heating time of such materials subject to the conditions of their thermal destruction is very large - more than an hour, and secondly, the scope of this method is limited to single-layer structures.

There is also a method for determining the TPS of building materials of structures (see AS USSR No. 1122956, class G 04 N 25/18, 1984), according to which the surfaces of the reference body and the structure under study are brought into contact, a heat pulse is applied, and a temperature change is recorded in the plane of their contact, the coefficients of thermal activity are calculated, and then the required TFS are calculated, and the temperature changes are recorded in two different time intervals.

The disadvantages of this method are the limited scope of its application by single-layer structures and, in addition, the complexity of the algorithm for calculating the required TFS, since the critical time is calculated first, thermal activity coefficients are calculated using cumbersome formulas, and only then TFS is calculated from the data obtained.

The prototype adopted a method for determining the thermophysical characteristics of multilayer building structures (see patent RU No. 2147070 C1, class G 01 N 25/18), which consists in adiabatic action on the surface of each outer layer with a corresponding disk heater located in the cavity of the probe bordered by a heat-insulating ring , and recording the dependence of the surface temperature of the investigated material on time. To determine the thermal diffusivity coefficients of the outer layers of the structure, the time dependence of temperature is recorded at four surface points: under both heaters and at two surface points located under the corresponding guard rings and spaced from the heater edge at distances equal to the corresponding thicknesses of the outer layers of the structure. To determine the thermophysical characteristics of the inner layers of the structure, one of the heaters is turned off and the dependence of the surface temperature on time is recorded at two of the indicated points.

The prototype disadvantages are a large methodological error in determining the required TFS, due to the inadequacy of the mathematical model used to describe the temperature field by the thickness of the product to the physics of real thermal processes, as well as the complexity and cumbersome calculations in determining controlled TFS, which significantly complicates the implementation of the prototype method.

The technical task of the invention is to increase the accuracy of determining the desired thermophysical properties of multilayer products.

The stated technical problem is achieved by the fact that in the method of non-destructive testing of the thermophysical properties of building materials of multilayer structures, consisting in adiabatic thermal action on the surface of the outer layer of the structure with a disk heater located in the plane of the measuring probe bordered by a protective heat-insulating ring, and recording the dependence of the surface temperature of the material on time, on the contact surface of the second probe instead of a disk heater placed the heat flux sensor is installed and two linear heaters are additionally installed at a predetermined distance from the disk heater of the first probe and the heat flux sensor of the second probe, and thermopiles placed on a line parallel to the line of heaters are placed at a fixed distance from the linear heaters, in order to determine the thermophysical properties the outer layers of the structure effect one thermal pulse from linear heat sources, determine the relaxation time of the temperature field in rooted points, then in both probes they act as thermal pulses from linear heat sources, change the frequency of thermal pulses until the temperature value at points located at specified distances from linear heaters becomes equal to two predetermined values, and the frequencies are determined thermal pulses, respectively, for the first and second outer layer and, using this information, using the obtained mathematical dependencies determine the thermophysical properties of the outer structural layers, in order to determine the thermophysical properties of the inner layer, the heat is applied by the disk heater of the first probe, the heat flux is recorded using a sensor located on the contact plane of the second probe, and the temperature is measured at points located respectively under the disk heater and on the contact surface of the thermal sensor flow, using the measured temperature values at the indicated points and the measured value of the heat flux penetrating the layers studied of the proposed structure, as well as previously obtained values of the thermophysical properties of the outer layers, using mathematical dependencies describing the temperature difference in each of the three layers, determine the desired thermophysical properties of the inner layer of the studied structure.

The essence of the proposed method is as follows.

One probe is installed on each of the outer surfaces of a heat-insulated multilayer multilayer structure (see Fig. 1), in the contact plane of the first of which there is a DN heater and two linear heaters LN1 and LN2, fixed at a small predetermined distance from the disk heater, as well as a thermocouple Тп ₁ , placed in the center of the contact plane of the disk heater, and two thermopiles Тб ₁ and Тб ₂ , located along the line at a given distance x _1, respectively, from linear sources ЛН1 and ЛН2 (cm Fig. 1). In the contact plane of the second thermal probe, the heat flux sensor Tq and two linear heaters LN3 and LN4 are located at a given distance from it. In addition, at a given distance x ₁ from the line of action of these heaters, thermopiles Tb ₃ and Tb ₄ are placed, respectively, and a second thermocouple Tp ₂ is mounted in the center of the circle of the heat flux sensor.

Heaters, thermocouples and thermopiles of both the first and second probes are closed on the side external from the contact plane by heat-insulating material such as ripor or asbestos, causing the directional movement of heat fluxes to the outer surface of the structure and preventing heat transfer in other directions, thereby ensuring the implementation of the adiabatic heating mode.

The probes are pressed against the outer surface of the test product by a certain force, reported by a load or spring (not shown in the drawing). In Fig.1, 1-4 are indicated - serial numbers of the surfaces of the layers, R1-R3 - thicknesses of the layers. The distance x ₁ from linear heaters to thermal batteries is taken 3-5 times less, respectively, of the thicknesses of layers R1 and R3, because in this case, the results of measuring temperature fields from the action of linear heat sources will practically not be affected by the influence of the inner (second) layer of the structure, i.e. the first and third layers can be considered with great reliability semi-infinite bodies relative to thermal processes caused by the action of linear heat sources.

где k - коэффициент, задаваемый в диапазоне от 2 до 5, τ _рел - интервал времени от момента нанесения теплового импульса до момента, когда избыточная температура в точке контроля станет равной порогу чувствительности ε контрольно-измерительной аппаратуры (см. фиг.2,а). Осуществляют тепловое воздействие от линейного источника тепла, увеличивая частоту тепловых импульсов в соответствии с зависимостьюThe determination of the TFS of the outer layers of structures is carried out in accordance with the measurement algorithm, the essence of which is as follows. At the beginning of the thermal effects is performed by a single pulse of a predetermined thermal capacity of q _n, which is not more than 10-15% of the power _term Q at which the actions of the heat source side temperature reaches the thermal degradation temperature of the material, and determining the relaxation time τ _rel (see. figure 2, a) the temperature field at a point on the surface of the investigated body, located at a given distance x ₁ from the line of action of the heat source. Then determine the minimum repetition rate of thermal pulses in accordance with the dependence

where k is a coefficient specified in the range from 2 to 5, τ _rel is the time interval from the moment of applying the heat pulse to the moment when the excess temperature at the control point becomes equal to the sensitivity threshold ε of the control and measuring equipment (see figure 2, a) . Carry out thermal action from a linear heat source, increasing the frequency of thermal pulses in accordance with the dependence

где К₁-К₄ коэффициенты пропорциональности, значения которых определяются экспериментально на эталонных изделиях или задаются соответственно в диапазонах К₁=1-10; К₂=1-100; К₃=1-50, К₄=0,1-1, τ _min – минимальный интервал времени определения разности Δ T_i (задается от 1 до 3 с).where Δ T (τ) = T _{ass 1} -T (τ) is the difference between the previously set value T _{ass 1} and the current value of the controlled temperature; Δ T _i = T _{ass 1} -T (τ _i ) is the difference between the set and the current temperature at time points (see figure 2, b), determined by the ratio

where K ₁ -K ₄ proportionality coefficients, the values of which are determined experimentally on standard products or are set respectively in the ranges K ₁ = 1-10; K ₂ = 1-100; K ₃ = 1-50, K ₄ = 0.1-1, τ _min is the minimum time interval for determining the difference Δ T _i (set from 1 to 3 s).

The increase in the repetition rate of thermal pulses in accordance with dependence (1) is carried out until the steady-state quasistationary temperature value at the control point reaches in advance the set value T _set1 , i.e. Δ T _i = T _set1 -T (τ _i ) = 0 (cm Fig. 2, b). The steady-state temperature value at the control point is reached when the next heat pulse from a series of pulses supplied by a linear source changes the temperature at this point by an amount lower than the sensitivity threshold ε of the instrumentation (ε ≤ 0.01 ° C). The frequency of thermal pulses F _{x1 is} determined, after which, in accordance with dependence (1), an increase in the frequency of thermal pulses is started until the value of the excess controlled temperature at the same point x ₁ reaches the second predetermined value T _ass2 (see Fig. 2, c). In this case, the frequency of thermal pulses F _x2 is determined, and the desired thermophysical properties are determined by the dependences obtained on the basis of the following considerations.

The process of heat propagation in a thermally insulated from an outer surface of a semi-infinite medium in thermal relation to the body when the linear heat source _and q action described by the solution of the problem of heat conduction, which has the form [see., E.g., AV Luikov Theory of thermal conductivity. - M .: Higher. school, 1967. - 599 p.]:

where x is the distance from the linear heat source to the control point, m; τ is the time, s; τ _i - the moment of application of the i-th thermal pulse to the surface of the body; λ is the thermal conductivity of the product, W / mK; α is the coefficient of thermal diffusivity, m ² / s.

When applying one heat pulse, the temperature change at the control point is determined by the ratio

Using relation (3), for a given value of ε - the sensitivity of the measuring equipment - from solving the equation

the time interval of relaxation of the temperature field τ _{rel is determined} at the point x ₁ from the action of a thermal pulse with a power of q _and .

The obtained interval τ _rel completely determines the number of pulses affecting the steady-state temperature at the control point at the time of measurement of τ, i.e. if τ _i - the pulse supply time does not belong to the interval [τ -τ _imp. , τ], then it does not affect the temperature at the control point. The number of pulses supplied in the interval τ _rel with frequency F is determined by the relation

where E (y) is the function of the integer part of the number y.

The _steady-state temperature as a result of a series of pulses at the control point x ₁ based on (2), for two setpoints T _set1 and T _set2 will be determined by the relations

where Δ τ _i = 1 / Fx _i is the distance between the leading edges of thermal pulses.

а так как значение x₁ мало (0,005-0,008 м), ограничимся в разложении двумя слагаемымиTo solve system (5) - (6) with respect to α and λ, we use the series expansion

and since the value of x _{1 is} small (0.005-0.008 m), we restrict ourselves in the expansion to two terms

Dividing (7) by (8), we obtain the expression for thermal conductivity

To determine the coefficient of thermal conductivity, the found value of coefficient a is substituted into (5) and the relation

Included in relations (7) and (8) and, respectively, in the final formulas for determining the thermal and thermal diffusivity of the materials under study (9) and (10), the number of thermal pulses n ₁ and n ₂ participating in the formation of the temperature field at the control point x ₁ for two steady states of the thermal system T (x, τ) = T _ass1 and T (x, τ) = T _ass2 , determined in accordance with expression (4) by the formula n _i = E (τ _rel · Fx _i ), where i = 1, 2.

Thus, using relations (9) and (10) and having information about the frequency of thermal pulses F _x1 and F _x2 for the first outer layer and about the frequencies F _x3 and F _x4 for the second outer layer, it is easy to determine the TPS of both outer layers.

To determine the TPS of the materials of the inner layer of the structure, a DN heater is turned on and a specific heat flow through the circle is supplied to the surface of the structure until a heat flow appears on the opposite surface of the structure. In this case, the heat flux q _{x is} measured, as well as the temperature in planes 1 and 4 using thermocouples Tп1 and Тп2.

The temperature difference on the first layer of the structure is determined by the following relation [see, for example, G. Dulnev Heat and mass transfer in electronic equipment. M: Higher. school, 1984. 247 p.]:

Hence the temperature in plane 2 is determined from the relation

By analogy with expression (11), the temperature in plane 3 is determined from the relation

those.

Using expressions (12) and (13), the temperature difference on the inner layer of the structure is determined by the expression

From expression (14), the desired coefficient of thermal conductivity of the inner layer of the structure is determined by the relation

To determine the thermal diffusivity coefficient of the inner layer of the structure, we use an analytical solution [see, for example, Kozlov V.P. Two-dimensional axisymmetric non-stationary problems of heat conduction./ Ed. A.G. Shashkova. Mn .: Nauka i tekhnika, 1986. 392 pp.], Describing the temperature distribution over the thickness R _{2 of the} material layer and over time τ when using the half-space model and having the form

Having information about λ and q _and using well-known detailed tables to determine the function of the multiple probability integral ierfc z, it is easy to determine the desired thermal diffusivity α ₂ from the expression (16) _.

Thus, having information about the power and frequency of thermal pulses of a linear heat source and measuring the temperature at given points on the surface of the investigated product, we determine the TFS of the outer layers of the structure using relations (9) and (10), and measuring the heat flux on the side of the product opposite to the disk heater and temperatures on both external sides of the structure under the action of a disk heater, by the relations (15) and (16) we determine the TFS of the inner layer of the structure.

To test the operability of the proposed method of non-destructive testing of TFS, experiments were conducted on a three-layer product, the outer layers of which are made of polymethylmethacrylate 20 mm thick and the inner one is made of ripor 30 mm thick.

The experimental data for the outer layers of the structure are shown in table No. 1, and for the inner layer in table No. 2.

Table number 1 q _and n ₁ n ₂ T _ass1 T _ass2 F _x1 F _x2 λ _1.3 α _1,3 δ λ,% δ α,% The outer layer No. 1 0.5 864 3132 thirty fifty 11,4 41.5 0.186 1.05-10 ^-7 4.6 7 The outer layer No. 2 0.5 860 3126 thirty fifty 10,2 42.3 0.187 1,07-10 ^-7 4.1 5.3

Table number 2 T ₁ T ₂ T ₃ T ₄ q _and λ ₂ α ₂ δ λ ₂ ,% δ a ₂ ,% The inner layer 64 60.7 25.1 21.8 32 0,026 4.86-10 ^-7 3,7 5.65

Experimental verification showed the correctness of the main theoretical conclusions underlying the proposed method of non-destructive testing of TPS multilayer structures without violating their integrity.

The main disadvantage of the prototype method is the inadequacy of the mathematical model used to describe the temperature field over the thickness of the product z to the physics of real thermal processes, because the prototype uses a one-dimensional solution for a semi-limited body under the action of a constant heat flux over the entire surface and having the form [for example, Kozlov V.P. Two-dimensional axisymmetric non-stationary problems of heat conduction./ Ed. A.G. Shashkova. - Mn .: Science and Technology. 1986. 392 p .; see formula 5-83, p.226]:

whereas when heat is supplied to the surface of the product through a circle of radius r ₀ , which takes place in the prototype method, the solution describing the temperature distribution over the thickness z in time τ has the following form [Kozlov V.P. Two-dimensional axisymmetric non-stationary problems of heat conduction./ Ed. A.G. Shashkova. - Mn .: Science and Technology. 1986. 392 p .; see formula 5-81, p.226]:

Hence the inadequacy of the mathematical description of thermal processes in the test product (the difference between expressions (1) and (2)) gives rise to the methodological error of the prototype method, the value of which, as shown by calculations and experiments on materials with known TPS, is 40-50% with an experiment duration of about 15 minutes (table 3).

Initial data: r ₀ = 0.02 m; λ = 0.197 W / m · K; α = 4.54 · 10 ^-7 m ² / s; z = 0.04 m; q ₀ = 1.5 kBt / m ² .

Table 3 Time τ, s 150 200 250 300 350 400 450 500 Temperature T according to (17) (prototype), ° С 0.077 0.377 1.116 2.568 5.102 9.191 15.421 24.482 Temperature Т according to (18), ° С 0.064 0.285 0.796 1.77 3.452 6.164 10.304 16.333 Rel burr.% twenty 32 40 45 47 49 49 49 Continuation of Table 3 Time τ, s 550 600 650 700 750 Temperature T according to (17) (prototype), ° С 37.168 54.365 77.046 106.27 143.169 Temperature Т according to (18), ° С 24.773 36.2 51.239 70.559 94.87 Rel burr.% fifty fifty fifty fifty fifty

A significant disadvantage of the prototype method is the equalization of the temperature at the boundary of the first and second layers of the product to the temperature on the surface of the product at a distance from the edge of the disk heater equal to the thickness of the first layer of the product (see expressions (4) of the prototype: T _b2 ≈ T _b1 ). These product points are not in the same conditions with respect to the disk heater: the heat flux from the heater to the first point passes through a heat-limited solid homogeneous body - the first layer of the product, and the heat flux to the second point goes along the boundary of this layer and the protective heat insulator. The temperature field at the first point is described by expression (18), and at the second point it is described by formula (5-64) in the book of V.P. Kozlov Two-dimensional axisymmetric non-stationary problems of heat conduction./ Ed. A.G. Shashkova. - Mn .: Science and Technology. 1986.- 392 p.

The calculation for products made of materials with known and stable TPS (plexiglass, ripore) showed that the methodological error due to the incorrect T _b2 ≈ T _b1 is at least 25-30%, and the larger the TFS of the material under study (first layer) differ from the TFS values of the protective heat insulator, the greater the temperature difference at the indicated control points, i.e. more methodological error in determining the required TFS.

In the claimed technical solution, when determining the TFS of the first and third outer layers of the product, the well-known correct solution is used to describe the temperature field under the action of a linear pulsed heat source (2) [see Lykov A.V. Theory of thermal conductivity. - M .: Higher. shk., 1967], and to determine the TFS of the inner layer of the product, expression (16), also known in the theory of electrical thermal analogy, is used [see Dulnev G.I. Heat and mass transfer in electronic equipment. - M .: Higher. school., 1984]. Therefore, in the developed technical solution, the methodological error from the inadequacy of the description by mathematical relationships of physical processes in the test product is minimized, which ultimately allows to significantly increase the accuracy of the measurement of the desired TPS in multilayer products without violating the integrity and operational characteristics. In addition, the advantage of the claimed technical solution in comparison with the prototype is that simple mathematical expressions are used to determine the TFS of all layers, which greatly simplifies the implementation and increases the metrological level of the developed method, whereas in the prototype method the TFS of all layers is determined by processing a heating thermogram using piecewise linear approximation, complex cumbersome calculations, determination of the desired TPS layers through thermal activity, etc., which naturally reduces It provides accuracy in determining the desired properties and creates additional costs when introducing this method into the practice of thermophysical measurements.

A significant advantage of the claimed technical solution compared to the prototype is the receipt of measurement information in a number- and frequency-pulse form, which, firstly, increases the noise immunity when implementing the developed measurement method, and secondly, significantly reduces the random component of the total measurement error, which, as a result, increases the accuracy and reliability of the desired TFS.

The above results of numerical and physical experiments showed the efficiency of the proposed method and its significant advantages compared with the known technical solutions, which allows us to conclude that the developed method is promising and effective in determining the heat-shielding properties of multilayer building structures of buildings and structures, as well as in other sectors of the national households.

Claims (1)

Method of non-destructive testing of the thermophysical properties of building materials of multilayer structures, consisting in adiabatic thermal action on the surface of the outer layer of the structure with a disk heater located in the plane of the measuring probe bordered by a protective heat-insulating ring, and recording the time dependence of the surface temperature of the material under study, characterized in that on the contact the surface of the second probe instead of a disk heater is placed heat flow sensor and at this distance from the disk heater of the first probe and the heat flux sensor of the second probe, two additional linear heaters are additionally installed, and thermopiles located on a line parallel to the line of heaters are placed at a fixed distance from the linear heaters, and in order to determine the thermophysical properties of the outer layers of the structure, one thermal pulse from linear heat sources, determine the relaxation time of the temperature field at controlled points, then at both Ndah carry out the action of thermal pulses from linear heat sources, change the frequency of thermal pulses until the temperature at points located at predetermined distances from the linear heaters becomes equal to a predetermined two values, determine the frequency of thermal pulses, respectively, for the first and the second outer layer and, using this information, using the obtained mathematical dependencies determine the thermophysical properties of the outer layers of the structure, to determine the thermophysical properties of the inner layer carry out the heat exposure by the disk heater of the first probe, register the heat flux using a sensor located on the contact plane of the second probe, and also measure the temperature at points located respectively under the disk heater and on the contact surface of the heat flux sensor using the measured values temperatures at the indicated points and the measured value of the heat flux penetrating the layers of the investigated structure, as well as earlier The obtained values of the thermophysical properties of the outer layers, using mathematical dependencies describing the temperature difference in each of the three layers, determine the desired thermophysical properties of the inner layer of the studied structure.

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