RU2698947C1 - Method for nondestructive inspection of thermophysical characteristics of construction materials and articles - Google Patents

Abstract

FIELD: measurement technology.

SUBSTANCE: invention can be used in practice of thermophysical measurements, in construction heat engineering and various industries. Essence of the invention consists in heating the analyzed object by the action of a pulse of microwave radiation focused in a line of specified dimensions with a lens from radiotransparent dielectric material, measurement of excess temperature on the surface of the analyzed object, which is insulated from the environment, at two points located at given distances x₁ and x₂ from the electromagnetic action line and determining the desired thermophysical characteristics from the obtained mathematical relationships. Surface of analyzed object is exposed to electromagnetic field of microwave range with frequency of not less than 20 GHz, at that, at the beginning, single thermal pulse is applied to power, which is set by time of supply of electromagnetic field of microwave radiation of specified frequency. Method then includes determining a relaxation time interval from the beginning of the thermal action to a point in time when the temperature at the control point x₁ becomes equal to initial temperature. Then, the minimum frequency of supply of UHF radiation pulses to the analyzed object is determined. Further, the analyzed object is exposed to pulses of specified power, increasing the frequency of their supply until the steady-state value of the controlled temperature at the point x₁ becomes equal to a predetermined value. Pulse repetition rate is determined, wherein the measured excess temperature at the control point x₂ is also measured on the surface of the analyzed object, and the desired thermophysical characteristics are determined from the obtained mathematical relationships.

EFFECT: high accuracy of determining unknown thermophysical characteristics of construction materials and articles.

1 cl, 3 dwg, 3 tbl

Description

The alleged invention relates to thermophysical measurements and can be used to determine such thermophysical characteristics (TF) of building materials and products, such as coefficients of thermal and thermal diffusivity.

A known method of non-destructive testing of the complex of thermal characteristics of solid building materials [see, for example, RF patent No. 2263901, class. G01N 25/18, 2004], consisting in heating the test sample in the form of a rectangular prism by applying heat to its surface, measuring the temperature and density of the heat flux on the same surface, determining the desired thermophysical characteristics from the corresponding dependencies.

The disadvantages of this method are the limited functionality due to the need to manufacture building materials from samples and in the form of a square prism (parallelepiped), this is possible only if the integrity and operational characteristics of the products under investigation are violated, the accuracy of thermal conductivity measurement is low due to the influence of the heater’s own heat capacity and unaccounted for heat loss from the surface of the investigated product into the environment.

A known method for determining the TFC of building materials [see, for example, RF patent No. 2399911 G01N 25/18, 2010], which consists in heating the test sample in the form of a square prism (parallelepiped) through an uninsulated end face by exposure to a high-frequency electromagnetic field (microwave radiation) from an alternating-phase multi-slit antenna, while one of the end and all side faces of the studied sample is thermally insulated from the environment, gradually increase the power of electromagnetic microwave radiation and control the change temperatures on the face free from thermal insulation and the opposite end face of the test sample, determine the microwave radiation power at which the temperature changes at the controlled points, the steady-state temperatures at the controlled points of the sample are measured, as well as the ambient temperature and the power reflected from the surface face of microwave radiation and the measured data on the basis of the obtained mathematical relationships determine the desired thermophysical characteristics. In the second variant of this method, the sample is symmetrically heated in the form of a prism through opposite end faces with insulated lateral sides of the prism by the action of microwave radiation from two alternating-phase multi-slot antennas, gradually increase the power of electromagnetic microwave radiation, control the temperature change on faces free from thermal insulation and in the middle section of the test sample in the form of a prism and determine the value of the power of microwave radiation, at which the temperature changes in controlled points, measure the steady-state temperature values at the controlled points of the sample, as well as the ambient temperature and the power of the microwave radiation reflected from the surfaces of the faces, and determine the sought-for characteristic characteristics based on the obtained mathematical relationships.

The disadvantages of this method are the need to destroy the studied products in order to produce samples in the form of a square prism (parallelepiped) to control the desired thermophysical characteristics and low accuracy of the TFC measurement due to the influence on the results of unaccounted heat losses from the surface of the studied samples to the environment, the value which is proportional to the time of the experiment.

The prototype is a method of non-destructive testing of the performance characteristics of building materials and products [US Pat. 2399911 RF, IPC G01N 25/18], which consists in exposing the test object to a pulse of a high-frequency electromagnetic field (microwave radiation) along a line, heating the body under investigation in a thermal sense, which is half-thermally limited along a plane perpendicular to the external surface of the body and going inside this body. To organize this effect, the electromagnetic radiation of the horn antenna of the microwave generator is focused by a lens from a dielectric material into a line of predetermined parameters. The magnitude of the microwave line length is set an order of magnitude greater than the distance from this line to the temperature control points, so that the end effects due to the limited length of the thermal line do not affect the controlled temperature field, and the line width is determined by the minimum possible microwave focusing resolution radiation in a line, which depends on the radiation wavelength, the distance from the lens to the surface of the investigated product and a number of other parameters.

The disadvantage of the prototype method is the low accuracy of determination of the desired TFH due to the need to measure low level excess temperatures in a thermophysical experiment due to a single-pulse thermal effect on the objects under study, whose power is limited from above by the temperature of thermal decomposition of the materials under study. In addition, when a single heat pulse of limited power is applied, a small volume of the studied building material (surface layer) is warmed up, which also reduces the accuracy and reliability of the results, since due to heterogeneity, dispersion, anisotropy, etc. building materials in order to obtain reliable results, it is necessary to warm up a large volume of the investigated materials and use the measurement information (temperature) averaged over this volume to determine the desired TFH.

The technical task of the proposed invention is to increase the accuracy of determination of the required TFH of building materials and products.

, где k - коэффициент, задаваемый в диапазоне от 2 до 5, далее воздействуют на исследуемый объект импульсами заданной мощности q_и, увеличивая частоту их подачи по закону

где ΔT(τ)=Т_зад-Т(τ) - разность между наперед заданным значением Т_зад и текущим значением контролируемой температуры; ΔT_i=Т_зад-T(τ_i) - разность между заданной и текущей температурой в моменты времени, определяемые соотношением

, где K₁÷K₄ - коэффициенты пропорциональности, значения которых определяются экспериментально на эталонных изделиях, либо задаваемые соответственно в диапазоне K₁=1÷10; K₂=1÷100; K₃=1÷50; K₄=0,1÷1, увеличение частоты подачи тепловых импульсов осуществляют до тех пор, пока установившееся значение контролируемой температуры в точке x₁ станет равным наперед заданному значению Т_зад, определяют при этом частоту следования тепловых импульсов F_x, при этом измеряют также установившуюся избыточную температуру Т_изм в точке контроля х₂ на поверхности исследуемого объекта, а искомые теплофизические характеристики определяют по полученным математическим зависимостям.The stated technical problem is achieved by the fact that in the method of non-destructive testing of the thermal conductivity characteristics of building materials and products, which consists in heating the objects under study by the influence of a microwave pulse, focused in a line of the given dimensions by a lens made of a radio-transparent dielectric material, measuring excess temperature on the surface of the object under study that is insulated from the environment at two points located at given distances x ₁ and x ₂ from the line of electromagnetic exposure and the determination of the desired heat Of the official characteristics according to the obtained mathematical relationships, the surface of the test object is affected by an electromagnetic field of the microwave range with a frequency of at least 20 GHz, and at the beginning it is exposed to a single heat pulse with a power of q _and , which is set by the time of applying the electromagnetic field of microwave radiation of a specified frequency, then the time interval is determined _rel τ from the beginning of exposure to heat up to the time when the temperature at the control point _x1 becomes equal to the initial temperature T ₀ = ε, where ε - Sensitivity lnost test equipment then determines the minimum frequency supplying microwave pulses to the examined object from the relationship

, where k is a coefficient specified in the range from 2 to 5, then they act on the object under study by pulses of a given power q _and , increasing the frequency of their supply according to the law

where ΔT (τ) = T _ass -T (τ) is the difference between the previously set value of T _ass and the current value of the controlled temperature; ΔT _i = T _ass -T (τ _i ) is the difference between the set and the current temperature at time instants determined by the relation

, where K ₁ ÷ K ₄ are the proportionality coefficients, the values of which are determined experimentally on the reference products, or set accordingly in the range K ₁ = 1 ÷ 10; K ₂ = 1 ÷ 100; K ₃ = 1 ÷ 50; K ₄ = 0.1 ÷ 1, an increase in the frequency of the supply of thermal pulses is carried out until the steady-state value of the controlled temperature at point x ₁ becomes equal to the predetermined value T _ass , the frequency of thermal pulses F _{x is} determined, and the steady-state excess temperature T _ISM at the control point x ₂ on the surface of the investigated object, and the desired thermophysical characteristics are determined by the obtained mathematical dependencies.

The essence of the proposed method is as follows.

Since traditional building materials (brick, concrete, foam concrete, expanded clay concrete, etc.) are dielectrics, then under the influence of high-frequency electromagnetic radiation of the microwave range they heat up and the specific power dissipation in the studied object in accordance with the work [Pyushner, G. Heating with energy superhigh frequencies / G. Puschner. - M .: Energy, 1968. - 312 p.] Is determined by the expression:

where E is the intensity of the alternating electric field, ƒ is the frequency of microwave radiation, ε _m is the dielectric constant of the investigated material.

From the theory of propagation of electromagnetic waves in the microwave range, it is known that an electromagnetic wave in a dielectric is attenuated in the direction of propagation in accordance with the dependence:

where α is the attenuation coefficient, determined by the formula:

и

- действительная и мнимая составляющие диэлектрической проницаемости смеси (вода + исследуемый материал).where γ is the wavelength

and

- the real and imaginary components of the dielectric constant of the mixture (water + test material).

An analysis of relations (1) and (2) showed that the depth of penetration of the electromagnetic field of the microwave range, and, consequently, the rate of scattering (loss) along the depth of the dielectric, is most dependent on the frequency of microwave radiation. In FIG. Figure 1 shows how the depth of penetration of electromagnetic waves depends on the frequency of microwave radiation, and, consequently, the depth of the heat-generating region on the frequency of radiation of a microwave generator when exposed to traditional building materials, for example, foam concrete, of known humidity. Based on the calculations and the results (graphs) obtained, it can be concluded that when exposed to the studied building materials with electromagnetic radiation of the microwave range in the form of a circle with a frequency of at least 10 GHz, almost all thermal power is released in the surface layer with a depth of 1-2 mm.

In the beginning, they act on the surface of the object under study with a pulse lasting 2-3 seconds of a high-frequency electromagnetic field (microwave radiation) with a frequency of at least 20 GHz along a line whose length is set to at least 8-10 cm and a width of about 0.2 cm. The length of the line microwave exposure is set an order of magnitude greater than the distance from this line to the temperature control points, so that the end effects due to the limited length of the heat exposure line do not affect the controlled temperature field (Fig. 2).

In order to obtain the most narrow radiation pattern and improve the characteristics of the antenna (for phase equalization in the mouth of the horn), a lens made of radio-transparent dielectric material is built into it. This technical solution allows to obtain an antenna with a given radiation pattern.

Heating of the test object 1 is carried out by a pulsed action of a high-frequency electromagnetic field along line 2 for 2-3 seconds from a radiating antenna 3 with a lens 4 mounted in it and connected to a microwave generator 5 (see Fig. 2).

After applying a pulse of a given power, determine the time interval τ _rel from the beginning of exposure to a microwave pulse to the time when the temperature at the control point at a given distance x ₁ from the line of action of the microwave pulse (see Fig. 2) becomes equal to the initial temperature T ₀ ± ε, where ε is the sensitivity of the measuring equipment, i.e. determine the relaxation time of the temperature field at the point x ₁ (see Fig. 3A).

, 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 контрольно-измерительной аппаратуры. Осуществляют тепловое воздействие от линейного источника тепла, увеличивая частоту тепловых импульсов в соответствии с законом

Then determine the minimum frequency of the supply of pulses of microwave radiation to the studied object from the ratio

, where k is a coefficient specified in the range from 2 to 5, and _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 of the control and measuring equipment. Carry out a thermal effect from a linear heat source, increasing the frequency of thermal pulses in accordance with the law

, где K₁÷K₄ - коэффициенты пропорциональности, значения которых определяются экспериментально на эталонных изделиях, либо задаваемые соответственно в диапазоне K₁=1÷10; K₂=1÷10; K₃=1÷50; K₄=0,1÷1. Значение коэффициента K₄ определяет частоту вычисления разности ΔT_i между наперед заданной температурой Т_зад и текущей избыточной температурой в точке контроля, K₄ - коэффициент, значение которого задают от 0,1 до 5, причем для материалов с большой теплопроводностью значение K₄ целесообразно брать <1, а для теплоизоляторов - >1, т.к. в первом случае теплограмма нагрева изменяется динамичнее и для определения равенства установившейся температуры заданному значению необходимо чаще определять ΔT_i, а при исследовании же теплоизоляционных материалов температурно-временные изменения в исследуемом теле происходят менее динамично, вследствие чего определять разность ΔT_i можно через большие интервалы времени. Как показали эксперименты, коэффициенты K₂ и K₃ целесообразно задавать соответственно в диапазоне от 0,2 до 5 и от 10 до 50, причем для материалов с большой теплопроводностью следует брать нижние пределы указанных диапазонов, а для теплоизоляционных материалов - верхние значения этих диапазонов.where ΔТ (τ) = T _ass -T (τ) is the difference between the previously set value of T _ass and the current value of the controlled temperature; ΔT _i = T _ass -T (τ _i ) is the difference between the set and the current temperature at time instants determined by the relation

, where K ₁ ÷ K ₄ are the proportionality coefficients, the values of which are determined experimentally on the reference products, or set accordingly in the range K ₁ = 1 ÷ 10; K ₂ = 1 ÷ 10; K ₃ = 1 ÷ 50; K ₄ = 0.1 ÷ 1. The value of the coefficient K ₄ determines the frequency of calculating the difference ΔT _i between the predetermined temperature T _ass and the current excess temperature at the control point, K ₄ is a coefficient whose value is set from 0.1 to 5, and for materials with high thermal conductivity it is advisable to take the value of K ₄ <1, and for heat insulators -> 1, because in the first case, the heating thermogram changes more dynamically and to determine the equality of the steady-state temperature to a given value, it is necessary to determine ΔT _i more often, and when studying heat-insulating materials, the temperature-time changes in the body under study occur less dynamically, as a result of which the difference ΔT _i can be determined at large intervals. As experiments showed, it is advisable to set the coefficients K ₂ and K ₃ in the range from 0.2 to 5 and from 10 to 50, respectively, and for materials with high thermal conductivity, the lower limits of these ranges should be taken, and for thermal insulation materials, the upper values of these ranges.

The increase in the thermal pulse repetition rate in accordance with the law (4) is carried out until the steady-state quasistationary temperature value at the control point reaches the predetermined value T _{ass in} advance, i.e. ΔT _i = T _ass -T (τ _i ) = 0 (see Fig. 3b). 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 control and measuring equipment (ε≤0.01 ° C). The frequency of thermal pulses F _{x is} determined (see Fig. 3b).

Further, as the established measured temperature T _edited at the point X ₂ on the surface of the test object (see. FIG. 1) and the required TPC determined by the formulas 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 under the action of a linear heat source _and q is described a solution heat conduction problem, which has the form [Luikov AV Theory of thermal conductivity. - M .: Higher. school, 1967. - 599 s]:

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 / (m⋅K); a - thermal diffusivity, m ² / s.

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

Using relation (5), for a given value of ε - the sensitivity of the measuring equipment - from solving the equation

the relaxation time interval of the temperature field τ _{rel is determined} at a point at a distance 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 ratio:

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 , x ₂ based on (7) for two given values of T _ass and T _ISM will be determined by the relations:

- интервал времени между передними фронтами тепловых импульсов.Where

- the time interval between the leading edges of the thermal pulses.

, а так как значение x₁ и х₂ малы (не более 0,05…0,1 м), ограничимся в разложении двумя слагаемыми:To solve system (8) - (9) with respect to a and λ, we use the series expansion

, and since the values of x ₁ and x ₂ are small (not more than 0.05 ... 0.1 m), we restrict ourselves in the expansion to two terms:

Dividing (10) by (11), we obtain the expression for thermal conductivity in the form:

To determine the thermal conductivity coefficient, the found values of the thermal diffusivity coefficient а are substituted in (8) and we obtain the relation:

To test the efficiency of the proposed method, experiments were carried out on building materials from silicate and red bricks, expanded clay concrete. The samples were heated at ambient temperature of 20 ± 2 ° C. The pulse repetition rate is 0.025 Hz, the relaxation time of the heating temperature for silicate brick is 440 s, for red brick - 480 s, for expanded clay concrete - 560 s.

Tables 1-3 show the experimental data.

Experimental verification showed the correctness of the main theoretical conclusions underlying the proposed method of non-destructive testing of the thermal characteristics of building materials and products.

The advantage of the claimed method in comparison with the prototype method is the adaptive conclusion in the process of the experiment of a thermal system to a given thermal regime, i.e. when the controlled excess temperature at the point x ₁ will be equal to a predetermined value of T _ass , the value of which is taken so that, firstly, the error of its measurement was minimal when using available measuring means (equipment), and secondly, the value of this temperature is taken much lower than the temperature of thermal destruction of the studied materials, which allows you to control their thermal characteristics without violating the integrity (melting, burning, deformation, etc.) and operational characteristics.

In addition, 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, easily allows you to convert the measurement information into digital form, and secondly, increases the noise immunity when implementing the developed method measurements, thirdly, significantly reduces the random component of the total measurement error, which, in the end, increases the accuracy and reliability of the desired TFH.

The above results of numerical and physical experiments showed the operability of the proposed method and its significant advantages compared to the known technical solutions, which allows us to conclude that the developed method is promising and effective in determining the heat-shielding properties of building structures of buildings and structures, as well as in building heat engineering, heat power engineering etc.

Claims (3)

A method of non-destructive testing of the thermophysical characteristics of building materials and products, which consists in heating the objects under study by the influence of a microwave pulse focused in a line of given dimensions by a lens made of a radiolucent dielectric material, measuring the excess temperature on the surface of the object under investigation that is thermally insulated from the environment at two points located at specified distances x ₁ and x ₂ from the line determining electromagnetic effects and the desired thermal characteristics f derived mathematical relationships, characterized in that on the surface of the object affect the electromagnetic field of the microwave range with a frequency of at least 20 GHz, and at the beginning of impact single thermal pulse power q _and that given time feeding the electromagnetic field of the microwave radiation of said frequencies is then determined interval time τ _rel from the onset of heat exposure to the point in time when the temperature at the control point x ₁ becomes equal to the initial temperature T ₀ = ε, where ε is the sensitivity l instrumentation, then determine the minimum frequency of the supply of pulses of microwave radiation to the studied object from the ratio

, where k is a coefficient specified in the range from 2 to 5, then they act on the object under study by pulses of a given power q _and , increasing the frequency of their supply according to the law

, where ΔT (τ) = T _ass -T (τ) is the difference between the previously set value of T _ass and the current value of the controlled temperature; ΔT _i = T _ass -T (τ _i ) is the difference between the set and the current temperature at time instants determined by the ratio

, where K ₁ ÷ K ₄ are the proportionality coefficients, the values of which are determined experimentally on the reference products, or set accordingly in the range K ₁ = 1 ÷ 10; K ₂ = 1 ÷ 100; K ₃ = 1 ÷ 50; K ₄ = 0.1 ÷ 1, an increase in the frequency of the supply of thermal pulses is carried out until the steady-state value of the controlled temperature at point x ₁ becomes equal to the predetermined value T _ass , the frequency of thermal pulses F _{x is} determined, and the steady-state excess temperature T _ISM at the control point x ₂ on the surface of the investigated object, and the desired thermophysical characteristics are determined by the obtained mathematical dependencies.

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