RU2140070C1 - Process determining thermal and physical characteristics of construction materials in multilayer structures without breakdown of their integrity - Google Patents

Abstract

FIELD: construction heat engineering. SUBSTANCE: process consists in adiabatic action on surface of each external layer with proper disk heater located in space of probe hemmed with heat insulation ring and in recording of dependence of temperature of surface of examined material on time. Dependence of temperature on time to determine coefficient of thermal diffusivity of external layers of structure is recorded in four points on surface: under both heaters and in two points of surface located under corresponding guarding rings and away from edges of heaters by distances equal to corresponding thickness of external layers of structure. For determination of thermal and physical characteristics of internal layers of structure one of heaters is disconnected and dependence of surface temperature on time is recorded in two of mentioned points. EFFECT: increased speed of determination of thermal and physical characteristics of construction materials. 1 dwg

Description

 The invention relates to construction heat engineering, in particular to measurements of thermophysical characteristics (TFH) of multilayer walling (external ceilings, partitions, coatings, floors, etc.).

A known method of non-destructive testing of the thermal conductivity characteristics of materials, consisting in the thermal effect on the surface of a thermally semi-infinite investigated body from a point heat source, measuring the time to reach the maximum excess temperature at a given point on the body surface τ _max , measuring the power of the heat source, while providing a constant power to the heat source W _o until the maximum temperature in excess given point of the surface, then the heat source strength change inversely propor- ionalno square root of the time and measure the value of maximum temperature in excess application point heat source T _{o max,} and the required TPC determined by the respective formulas given measured W _o, τ _max and T _{o max} (cm. AS USSR N 1390555, C. 4 G 01 N 25/18, 1988).

The disadvantages of this method are:
1) A point heat source is used as a heater, while for determining the thermal conductivity characteristics of building materials (concrete, brick, heat-insulating materials such as polystyrene, etc.) a heat source with a large active (heat-releasing) surface is required, since the required heating time for such materials is compliance with the conditions for the exclusion of their thermal destruction is very large - more than an hour.

 2) The scope is limited to single-layer structures.

 3) The complexity of the hardware design is required to ensure a given law of regulating the power of the heat source, starting from the moment of reaching the maximum excess temperature.

 There is also a method for determining the thermal characteristics of building materials of constructions, according to which the surfaces of the reference body and the structure under study are brought into contact, a heat pulse is applied and the temperature change is recorded in the plane of their contact, thermal activity coefficients are calculated, and then the sought thermal characteristics are calculated, and the temperature changes are calculated at two different time intervals (see AS of the USSR N 1122956 class G 04 N 25/18, 1984).

The disadvantages of this method are:
1) The scope is limited to single-layer structures.

 2) The complexity of the calculation algorithm: first, the critical time is calculated, the thermal activity coefficients are cumbersome formulas, and only then the TFC is calculated from the obtained data.

The prototype adopted a method for determining the thermophysical materials of structures by introducing the surfaces of the reference body and the structure under study, creating a heat pulse in the plane of contact, recording the temperature in this plane over time and simultaneously measuring the dielectric constant of the material of the structure, characterized in that in order to expand the scope on structures having a finishing (facing) layer, the temperature is recorded in the plane of contact at three instants of time τ ₁ , τ ₂ , τ ₃ , and the time instants are selected based on the condition τ ₁ = yτ ₂ = (2y-1) τ ₃ , where 0.5 <y <1.0, and the dielectric constant is measured twice, moreover, one measurement is carried out at a depth of the control zone commensurate with the thickness of the finishing layer, and the other at a greater depth, the volume moisture of the materials of the finishing layer and the body of the structure is determined from a predetermined dependence and the thermophysical characteristics of the materials of construction are found from the calculated data (see A.S. USSR N 922606, G 01 N 25/18, 1982).

The disadvantages of the prototype are:
1. The scope is limited to two-layer structures, since the used design relations, being non-linear, are fundamentally not applicable for three-layer structures.

 2. The large required time of experiments and the complexity of the hardware design due to the need, in addition to measuring temperature, to twice measure the dielectric constant of the layer materials (at two different depths of the control zone).

 In addition, the large required time is due to the use of pulsed heat: building materials - concrete, brick, foam-type insulation, etc. - have low thermal conductivity; therefore, it is advisable to use a continuously operating heater (as in the proposed application).

 3. The increased error due to the need to recalculate the dielectric constant in the volumetric humidity of both layer materials.

 The objective of the invention is to expand the scope (use of the method for the study of a three-layer structure) and increase performance.

 The presence of a set of essential features: conducting research on a three-layer structure, using a heater in a continuous heating mode, eliminating the need to measure the dielectric constant of layer materials will provide an extension of the field of application and increase speed.

Compared with the prototype (see AS USSR N 922606 class. G 01 N 25/18, 1982), the proposed method has:
1) a wider scope: the object of study is a three-layer design instead of a two-layer;
2) increased performance, due to:
a) the use of a heater in continuous heating instead of pulsed,
b) the exclusion of time spent on measuring the dielectric constant of the materials of the layers.

 The technical and economic efficiency of the method follows from the expansion of the scope and increase speed.

 The specified technical result is achieved by the fact that they carry out an active thermal effect on the surface of each outer layer adiabatically from disk heaters located in the cavity of the probes bordered by guard (heat-insulating) rings, and register the dependence of the surface temperature of the material on time, in order to determine the thermal diffusivity of the outer the layers of the structure record the dependence of temperature on time at four surface points: under both fir trees and at two surface points located under the corresponding guard rings at predetermined distances from the heaters, and to determine the thermophysical characteristics of the inner layers of the structure, one of the heaters is turned off and the surface temperature versus time is recorded at two of these points, one of which is located under the guard ring from the side of the switched on heater, and the other under the switched off heater, in addition, the determination of the thermal conductivity of the outer and inner layers Design is carried out using a special approximation of the obtained temperature dependency of the time and the desired thermal characteristics determined by appropriate formulas.

 The presence of a set of essential features: conducting a study of a multilayer structure, using a disk heater, implementing a continuous heating mode, eliminating the need to achieve a steady-state temperature at predetermined control points, will expand the scope and increase speed.

The essence of the proposed method is as follows. One probe is installed on each of the outer surfaces of a thermally semi-infinite multilayer structure, in the cavity of which a disk heater and a thermocouple are located (see drawing). 3ond is pressed to the outer surface by a certain force imparted by a load or spring (not shown in the drawing). In FIG. marked: 1 - 4 - serial numbers of the surfaces of the layers, h ₁ - h ₃ - thickness of the layers.

In the case of different thicknesses h ₁ and h ₃ and the values of thermophysical characteristics of the materials of the layers, the required heat flux densities q ₁ and q _{4 of the} heaters are also different. The drawing shows the case of h ₁ > h ₃ and q ₁ > q ₄ .

Temperature measurements carried out by means of thermocouples depending on time t are carried out at points O ₁ , O ₄ located under the corresponding heaters, and at points b ₁ and b ₄ located under the guard (heat-insulating) rings. Points b ₁ and b ₄ are separated from the cutoff of the corresponding heater at distances h ₁ and h ₃ .

 Security rings made of a foam-type insulating material surround a heat source, providing directional movement of the heat flux to the outer surface of the structure and preventing heat transfer in other directions, thereby allowing the adiabatic heating mode to be realized.

 Electric heaters are turned on and thermocouples measure the temperature at the indicated points on the external surfaces at predetermined intervals of time, followed by registration with a secondary device. Based on the obtained dependences of temperature on time, the thermophysical characteristics of the outer layers of the structure are found.

To determine the thermophysical characteristics of the materials of the inner layers of the structure, one of the heaters (usually having less power) is turned off and the temperature is measured only at points b ₁ and O ₄ . By mathematical processing of the obtained experimental temperature dependences of time, the thermophysical characteristics of the materials of the inner layers are found.

где

erfu - функция ошибок Гаусса;

ε - тепловая активность материала тела;
T_o - начальная температура тела.The analytical solution describing the temperature distribution over the body thickness z and in time t when using the half-space model has the following form [A. Lykov Theory of thermal conductivity. - M .: Higher. school 1967.]:

Where

erfu - Gaussian error function;

ε is the thermal activity of the body material;
T _o - initial body temperature.

так как при z = 0, u = 0,

Разделив соотношение (1) на (2), получим для 1-го слоя (z = h₁):

где

Благодаря теплоизоляционному (охранному) кольцу можно записать:

(4)
Из равенств (3) - (4), используя экспериментальные термограммы

и табулированные значения функции ierfc u₁, определяем коэффициент температуропроводности a₁ первого слоя.For the surface of the body z = 0 according to (1) we get:

since for z = 0, u = 0,

Dividing relation (1) by (2), we obtain for the 1st layer (z = h ₁ ):

Where

Thanks to the heat-insulating (security) ring, you can write:

(4)
From equalities (3) - (4), using experimental thermograms

and tabulated values of the function ierfc u ₁ , we determine the coefficient of thermal diffusivity a _{1 of the} first layer.

Moreover, to increase the accuracy, it is advisable to first calculate several values of (a ₁ ) _i , using the points of thermograms corresponding to different points in time t _i .

The desired value a ₁ is then determined by averaging the obtained values (a ₁ ) _i .

, достаточной для надежного ее измерения (3 - 4^oC).Our experiments show that the accuracy of determination of a ₁ is in the range of 2–3% when using 5–10 points of thermograms. In order to shorten the procedure, it is advisable to take the first point at the minimum possible time, namely, the time to reach the excess temperature

sufficient for reliable measurement (3 - 4 ^o C).

где

;

Для определения коэффициента температуропроводности а₂ внутреннего слоя следует отключить нагреватель меньшей мощности. В нашем случае, как отмечалось, таким является нагреватель с плотностью теплового потока q₄. Температурная волна будет распространяться от более нагретой поверхности контакта 2 к менее нагретой 3. Тогда по аналогии с (3) получим следующие два соотношения:

где

Из этих выражений имеем:

Искомый коэффициент температуропроводности а₂ находим из (8). Таким образом, величина а₂ определяется посредством обработки двух термограмм:

Для определения коэффициентов тепловой активности ε₁ и ε₃ наружных слоев используем решение системы дифференциальных уравнений теплопроводности для двух полуограниченных тел, приведенных в контакт, т.е. подчиняющихся граничному условию 4-го рода (ГУ-4). При этом начальные температуры тел отличаются друг от друга. Теплопередача осуществляется через поверхность контакта от тела, имеющего более высокую температуру.Similarly, we find the coefficient of thermal diffusivity a _{3 of the} third layer:

Where

;

To determine the coefficient of thermal diffusivity a _{2 of the} inner layer, a heater of lower power should be turned off. In our case, as noted, this is a heater with a heat flux density q ₄ . The temperature wave will propagate from the warmer contact surface 2 to the less heated 3. Then, by analogy with (3), we obtain the following two relations:

Where

From these expressions we have:

The required thermal diffusivity coefficient a _{2 is} found from (8). Thus, the value of a ₂ is determined by processing two thermograms:

To determine the thermal activity coefficients ε ₁ and ε _{3 of the} outer layers, we use the solution of the system of differential heat equations for two semi-bounded bodies brought into contact, i.e. obeying the boundary condition of the 4th kind (GU-4). In this case, the initial temperatures of the bodies differ from each other. Heat transfer is carried out through the contact surface from the body having a higher temperature.

 With GU-4, the solution has the simplest form when the condition on which the temperature is constant over the entire heat transfer process is imposed on a hotter body [A. Lykov Theory of thermal conductivity. - M .: Higher. school 1967. - S. 865].

 In our case, the bodies corresponding to GU-4 are, first of all, the heater and the layer with which it is in contact, and then paired combinations of subsequent layers.

где

Для соблюдения ГУ-1 по отношению к нагревателю следует экспериментальную термограмму

нагревателя аппроксимировать отрезками кусочно-ступенчатой функции с шагом по времени Δt. Тогда (9) будет иметь вид:

где i - номер шага по времени, i = 1 - n; целесообразно шаг по времени Δt выбрать равномерным;
t_i= t_i-1+Δt;

Учитывая, что коэффициент температуропроводности a₁ определен ранее (см. соотношение (3)), находим из (10) относительную тепловую активность в дискретных точках по времени t_i:

где обозначено:

Тогда дискретна тепловая активность для первого слоя

Искомую тепловую активность для первого слоя в целом находим усреднением:

Коэффициент теплопроводности

Аналогично для материала третьего слоя

Для определения тепловой активности ε₂ внутреннего слоя следует использовать указанный ранее подход при нахождении температуропроводности a₂ этого слоя - отключить нагреватель меньшей мощности, т.е. с плотностью теплового потока q₄.For the first layer, provided that the temperature of the heater is constant (that is, imposing a boundary condition of the first kind on the heater), the solution has the form [A. Lykov Theory of thermal conductivity. - M .: Higher. school 1967. - S. 366]:

Where

To comply with GU-1 with respect to the heater, an experimental thermogram follows

approximate the heater by segments of a piecewise-step function with a time step Δt. Then (9) will have the form:

where i is the number of time steps, i = 1 - n; it is advisable to choose a time step Δt uniformly;
t _i = t _i-1 + Δt;

Given that the thermal diffusivity a _{1 was} previously determined (see relation (3)), we find from (10) the relative thermal activity at discrete points in time t _i :

where indicated:

Then discrete thermal activity for the first layer

The desired thermal activity for the first layer as a whole is found by averaging:

Coefficient of thermal conductivity

Similarly for the material of the third layer

To determine the thermal activity ε _{2 of the} inner layer, one should use the previously mentioned approach when finding the thermal diffusivity a _{2 of} this layer - turn off the heater of lower power, i.e. with a heat flux density q ₄ .

откуда с учетом (7) имеем:

ьThen, by analogy with (10), we obtain:

whence, taking into account (7), we have:

b

Claims (1)

 A method for determining the thermophysical characteristics of building materials of multilayer structures without violating their integrity, consisting in the thermal effect on the surface of the outer layer by a disk heater and recording the dependence of the surface temperature of the test material on time, characterized in that for the implementation of the adiabatic heating mode, the disk heater is placed in the cavity of the probe bordered guard ring, additionally installed on the opposite surface of another outer layer d a skovy heater, structurally similar to the first one, in order 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 respective guard rings and at a distance equal to the corresponding the thickness of the outer layers of the structure, and to determine the thermophysical characteristics of the inner layers of the structure, one of the the oil is turned off and the time dependence of the surface temperature is recorded at two of the indicated points, one of which is located under the guard ring on the side of the switched on heater and the other under the switched off heater, in addition, to determine the thermal conductivity of the outer and inner layers of the structure, the experimental temperature dependences on time for surface points having a higher temperature is approximated by piecewise-step functions with a given time step, and the desired thermophysical Kie characteristics determined by appropriate formulas.