CN111307858A - Unsteady multi-unit thermal conductivity tester and testing method for constant-power planar heat source - Google Patents
Unsteady multi-unit thermal conductivity tester and testing method for constant-power planar heat source Download PDFInfo
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- CN111307858A CN111307858A CN201911235255.5A CN201911235255A CN111307858A CN 111307858 A CN111307858 A CN 111307858A CN 201911235255 A CN201911235255 A CN 201911235255A CN 111307858 A CN111307858 A CN 111307858A
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Abstract
The invention discloses an unsteady multi-unit thermal conductivity tester for a constant-power plane heat source, which comprises a G-shaped clamp frame; the G-shaped clamp frame is placed on the working table surface, a lower insulation board and a side insulation board are arranged on the bottom plate surface of the G-shaped clamp frame, and a clamp bolt is vertically connected to the upper plate surface of the G-shaped clamp frame. The invention has the beneficial effects that: can realize the quick test of several minutes and contain wet material thermal conductivity, avoided the several hours long-term heating in the steady state method to cause wet material internal moisture distribution to destroy, and have the heated board overspeed device tensioner of G type screw caliper and activity hinge, quick replacement to the sample that awaits measuring has been realized effectively, and according to different material that awaits measuring the tensile force between the instant upper/lower heated board of adjusting, supplementary sample and the sample that awaits measuring, can only test a sample at every turn usually with conventional transient test method and compare, this test box has realized the high efficiency test of four samples of at every turn test.
Description
Technical Field
The invention relates to a tester, in particular to a tester for testing unsteady multi-unit thermal conductivity of a normal-power plane heat source, belonging to the technical field of thermal conductivity testing of moisture-containing porous materials.
Background
Thermal conductivity, also known as thermal conductivity, is a physical quantity that measures how easily a material conducts heat. The thermophysical properties of the material are abbreviated as thermophysical properties. Generally refers to the fundamental parameters describing the thermophysical properties of the material: such as thermal conductivity, thermal diffusivity, specific heat capacity, thermal storage coefficient, etc. The measurement of the thermophysical properties is generally carried out by a steady-state method at present. I.e. a sufficiently long time (in hours) has elapsed, the temperature of the object does not change with time in the state of thermal equilibrium. At this time, the temperature difference and the spacing distance between two points of the uniform material are measured, the heat flow intensity of a parallel plane passing through the two points and being perpendicular to the heat flow direction is measured, and the thermal conductivity of the material is calculated by using the Fourier law. The thermal conductivity of the material is measured by a hot plate method, namely, the instrument and the method are representative of the steady-state measurement method.
The steady-state measurement method of the thermophysical property of the material is mature at present and is widely applied. However, this measurement method also has some inherent disadvantages: firstly, the method is easily influenced by environmental conditions, a real constant temperature environment is not easily realized, and the measurement accuracy is not high and is generally about 3%; secondly, the measurement takes a long time, the consumption is high, and the popularization and the application are difficult. In most of today's cases, measurements can only be made in a few large laboratories. For example, the Chinese test institute, the Henan institute for building design, etc. Therefore, the existing situation is that in the field of industrial and agricultural production, data are rarely obtained by adopting a measurement mode actually, but the situation is met by searching data, and the general requirements of engineering technology are met by adopting outdated data; third, the thermal diffusivity cannot be measured, only the thermal conductivity. The method is not suitable for the requirements of the vigorous development of the current scientific technology on the thermophysical property measurement.
Transient measurement of material thermophysical properties is relative to steady state measurements. Transient measurement refers to measuring the thermophysical properties of a material in the process of heating or cooling the material. It is well known that the temperature of an object varies with time at various points in space during heat transfer. Therefore, measuring the thermophysical properties of a material in this process means measuring the temperature field in the material as a function of time. This is somewhat difficult with respect to the steady state measurement method. This is one of the reasons why transient measurement methods, which are not fully developed in the late stage, are also used. For practical purposes, there must be heat conduction as long as there is a temperature difference. The temperature of each point in space changes with time. This type of heat transfer problem does not have many heat transfer equations that can be solved accurately due to the diversity of boundary conditions and initial conditions. Many require numerical computation to solve for the approximation. This difficulty is significant. Generally, the method is not suitable for measuring the thermophysical properties of materials. This difficulty in mathematics is also a further reason why transient measurement methods have not been fully developed in time.
At present, transient methods are used for measuring the thermophysical properties of materials, and a simple one-dimensional heat transfer model is generally selected. This simple model allows to find its exact solution. There are two classes of such simple models: firstly, a plane constant-current heat source is used for heating in a large uniform medium to form a one-dimensional semi-infinite heat transfer model vertical to the plane heat source. The model is precisely solvable. Therefore, two measuring methods of measuring the thermophysical property of the material by a pulse method and measuring the thermophysical property of the material by a constant current method are developed accurately. Secondly, a long, straight and thin heat ray source is placed in a uniform large medium, and a one-dimensional cylindrical surface heat transfer model which takes the heat ray as an axis and is perpendicular to the radial direction of the axis can be constructed under the condition that the heating of the heat ray does not change along with time. The model can also find its exact solution. Thus, a measuring instrument and a measuring method for measuring the thermal conductivity of a material by a hot wire method have also been developed. In addition, the accurate solution deformation is rewritten into a linear equation of temperature change and logarithm of measurement time, and a least square method is used for linear fitting to calculate the slope and intercept of a fitting straight line, so that the thermophysical property of the material can be obtained.
However, none of the above methods is suitable for measuring thermal conductivity of moisture-containing porous materials. From a microscopic perspective, gases, liquids, conductive solids, and non-conductive solids differ in their heat transfer mechanisms. The gas heat conduction mechanism is irregular thermal motion of gas molecules, the conductive solid is free electron motion in crystal lattices, and the liquid and the non-conductive solid are elastic wave vibration. The heat conduction mechanism of the moisture-containing porous medium is more complex, and the influences of solid heat conduction of the skeleton matrix, gas heat conduction of vapor and air in pores, liquid heat conduction of liquid water in the pores, latent heat of phase change generated by evaporation and condensation of water in the pores and the like exist. The defects of measuring the thermal conductivity of the moisture-containing porous material by using a steady state method comprise: the damage to the constant temperature environment, the measurement time, the high cost, the large and difficult movement of the equipment, the temperature rise and the damage to the wet distribution, the error increase, etc. The defects of measuring the thermal conductivity of the moisture-containing porous material by adopting a pulse method and a hot wire method comprise: the price of the test equipment is high, the sample to be tested is not easy to process, the calibration of the instrument is complicated, and the like.
Disclosure of Invention
One of the objectives of the present invention is to provide a non-steady-state multi-cell thermal conductivity tester for a constant power planar heat source to solve the above problems.
The invention realizes the purpose through the following technical scheme: a constant-power plane heat source unsteady state multi-unit thermal conductivity tester comprises a G-shaped clamp frame; the G-shaped clamp frame is placed on the working table surface, a lower insulation board and a side insulation board are arranged on the bottom plate surface of the G-shaped clamp frame, and a clamp bolt is vertically connected with the upper plate surface of the G-shaped clamp frame;
the side heat-insulation plates are vertically arranged on four sides of the lower heat-insulation plate respectively, the side heat-insulation plates are connected with the lower heat-insulation plate through movable hinges, two layers of polyimide heating films are arranged in parallel in a box body formed by the side heat-insulation plates and the lower heat-insulation plate, a thermocouple I and a thermocouple II are arranged on the two layers of polyimide heating films respectively, the thermocouple I and the thermocouple II are electrically connected with an external adjustable voltage-stabilized power supply, and the polyimide heating films are electrically connected with the data processing terminal;
the bottom end of the caliper bolt is fixedly provided with an upper insulation board, the upper insulation board is located right above the lower insulation board, and the top end of the caliper bolt is fixedly connected with a caliper handle.
Based on the above tester, another object of the present invention is to provide a testing method, comprising:
semi-infinite large objectThe one-dimensional unsteady heat conduction problem has a definite solution under the second class of boundary conditions, and a half infinite object is assumed to have uniform initial temperature T0From the time t-0, a constant heat flux q is applied at x-00The planar heat source of (2) heats the object surface, the differential equation and the solution conditions for the thermal conductivity problem:
the temperature rise within the object away from the surface x is then:
order to
Here, ierfc (ξ) is the first integral of the Gaussian error complement function of variable ξ, i.e.
Where t is 0, x is 0,
from the equations and equations, one can derive
If t is measured separately1Time x equals 0 and t2The temperature rise at time x δ is then known from the equation and the equation
If order
Here, ω can be determined experimentally. Then
The equation can be transformed into
For this function, the variable ξ can be determined by solving a mathematical function table or MATLAB1The value of (c). Thereby obtaining thermal diffusivity
The thermal conductivity can be obtained by substituting the equation into the equation:
as a further scheme of the invention: upper strata the top of polyimide heating film is equipped with auxiliary sample a, the upper strata the below of polyimide heating film is equipped with auxiliary sample c, and is equipped with the sample b that awaits measuring between the two-layer polyimide heating film, and the material and the length of side of auxiliary sample a, sample b and auxiliary sample c all are the same, and the surface is ground flat and stacks in proper order to make their thickness satisfy ha+hb＝hcAnd the side length is ten times of the thickness.
As a further scheme of the invention: the first thermocouple and the second thermocouple form a thermocouple pair and are arranged at the centers of the upper surface and the lower surface of the sample b to be measured.
The invention has the beneficial effects that: this unsteady many units of normal power plane heat source thermal conductivity tester reasonable in design can realize that several minutes tests moisture containing material thermal conductivity fast, it causes moisture containing material internal moisture distribution to destroy to have avoided the long-term heating of several hours in the steady state method, and the heated board overspeed device tensioner who has G type screw caliper and activity hinge, compare with traditional A type spring clip (invariable tensile force), quick replacement sample that awaits measuring has effectively been realized, and the heated board about adjusting immediately according to different materials that await measuring, the tensile force between auxiliary sample and the sample that awaits measuring, can only test a sample at every turn usually with conventional transient test method and compare, this test box has realized the high efficiency test of four samples of at every turn.
Drawings
FIG. 1 is a schematic front view of the present invention;
FIG. 2 is a schematic top view of the sample box of the present invention.
In the figure: 1. g type calliper frame, 2, lower heated board, 3, side heated board, 4, activity hinge, 5, polyimide heating film, 6, thermocouple I, 7, thermocouple II, 8, data processing terminal, 9, adjustable constant voltage power supply, 10, last heated board, 11, calliper bolt and 12, calliper handle.
Detailed Description
The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the drawings in the embodiments of the present invention, and it is obvious that the described embodiments are only a part of the embodiments of the present invention, and not all of the embodiments. All other embodiments, which can be derived by a person skilled in the art from the embodiments given herein without making any creative effort, shall fall within the protection scope of the present invention.
Referring to fig. 1-2, a constant power plane heat source unsteady state multi-unit thermal conductivity tester includes a G-type caliper frame 1; the G-shaped clamp frame 1 is placed on a working table surface, a lower insulation board 2 and a side insulation board 3 are arranged on the bottom plate surface of the G-shaped clamp frame 1, and a clamp bolt 11 is vertically connected to the upper plate surface of the G-shaped clamp frame 1;
the side heat-insulation plates 3 are vertically arranged on four sides of the lower heat-insulation plate 2 respectively, the side heat-insulation plates 3 are connected with the lower heat-insulation plate 2 through movable hinges 4, two layers of polyimide heating films 5 are arranged in parallel in a box body formed by the side heat-insulation plates 3 and the lower heat-insulation plate 2, a thermocouple I6 and a thermocouple II 7 are arranged on the two layers of polyimide heating films 5 respectively, the thermocouple I6 and the thermocouple II 7 are electrically connected with an external adjustable voltage-stabilized power supply 9, and the polyimide heating films 5 are electrically connected with a data processing terminal 8;
the bottom end of the caliper bolt 11 is fixedly provided with an upper insulation board 10, the upper insulation board 10 is located right above the lower insulation board 2, and the top end of the caliper bolt 11 is fixedly connected with a caliper handle 12.
The test method comprises the following steps:
the problem of one-dimensional unsteady heat conduction of a semi-infinite object has a definite solution under a second class of boundary conditions, and a uniform initial temperature T is obtained by assuming the semi-infinite object0From the time t-0, a constant heat flux q is applied at x-00The planar heat source of (2) heats the object surface, the differential equation and the solution conditions for the thermal conductivity problem:
the temperature rise within the object away from the surface x is then:
order to
Here, ierfc (ξ) is the first integral of the Gaussian error complement function of variable ξ, i.e.
Where t is 0, x is 0,
from the equations and equations, one can derive
If t is measured separately1Time x equals 0 and t2The temperature rise at time x δ is then equal toEquation and equation can be known
If order
Here, ω can be determined experimentally. Then
The equation can be transformed into
For this function, the variable ξ can be determined by solving a mathematical function table or MATLAB1The value of (c). Thereby obtaining thermal diffusivity
The thermal conductivity can be obtained by substituting the equation into the equation:
further, in the embodiment of the present invention, an auxiliary sample a is disposed above the upper polyimide heating film 5, an auxiliary sample c is disposed below the upper polyimide heating film 5, a sample b to be measured is disposed between the two polyimide heating films 5, the auxiliary sample a, the sample b to be measured, and the auxiliary sample c are made of the same material and have the same side length, and the surfaces of the auxiliary sample a, the sample b to be measured, and the auxiliary sample c are ground flat and stacked in sequence, and the thicknesses of the auxiliary sample a, the sample b to be measured, and the auxiliary sample c satisfy ha+hb＝hcAnd the side length of the sample is ten times of the thickness of the sample, so that the sample can be regarded as a semi-infinite large flat plate.
Further, in the embodiment of the present invention, the first thermocouple No. 6 and the second thermocouple No. 7 constitute a thermocouple pair, and are disposed at the centers of the upper and lower surfaces of the sample b to be measured, so as to measure the temperature at which x is 0 and x is δ.
The working principle is as follows: when the constant-power plane heat source unsteady multi-unit thermal conductivity tester is used, the samples a and b can be regarded as a whole because the samples a and b are flat and same-material samples clamped and stacked together. When the heating film is heated by electricity, the heating film uniformly transmits heat flow to both sides, and the portion of the sample adjacent to the polyimide heating film 5 first starts to increase in temperature and then gradually extends to both sides away from the polyimide heating film 5. If at the experimental measurement time t1At the moment, the temperature influence depth reaches the upper surface of the sample b to be measured, at this moment, the whole formed by the samples a and b and the auxiliary sample c can be regarded as a semi-infinite object, and the surface of the sample to be measured adjacent to the polyimide heating film 5 is the surface of the semi-infinite object where x is 0.
The polyimide heating film 5 is powered by a direct current stabilized power supply with adjustable voltage U and current I, so that the heat flux density is constant
Here, a is the heating film surface area.
Firstly, a sample is placed in a drying box for drying, the thermal conductivity of the sample in a dry state is tested, and then the sample is sequentially increased by a first-level moisture content for testing. In the actual test, considering the influence of factors such as heat capacity of the heating plate, heat preservation of the test device, sample treatment and the like, the test sets a correction coefficient C' based on the thermal conductivity of the dry material. Namely, it is
λn＝C′·λn\*MERGEFORMAT(0.11)
Here, λnFor the uncorrected test thermal conductivity, lambda, corresponding to the nth set of moisture content in the experimentnFor the corresponding correction of the thermal conductivity test, C' can be given by calibrating the thermal conductivity of the dry sample measured in a differential scanning calorimeter.
It will be evident to those skilled in the art that the invention is not limited to the details of the foregoing illustrative embodiments, and that the present invention may be embodied in other specific forms without departing from the spirit or essential attributes thereof. The present embodiments are therefore to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims rather than by the foregoing description, and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein. Any reference sign in a claim should not be construed as limiting the claim concerned.
Furthermore, it should be understood that although the present description refers to embodiments, not every embodiment may contain only a single embodiment, and such description is for clarity only, and those skilled in the art should integrate the description, and the embodiments may be combined as appropriate to form other embodiments understood by those skilled in the art.
Claims (4)
1. A constant-power plane heat source unsteady state multi-unit thermal conductivity tester comprises a G-shaped clamp frame (1); the method is characterized in that: the G-shaped clamp frame (1) is placed on a working table surface, a lower heat insulation plate (2) and a side heat insulation plate (3) are arranged on the bottom plate surface of the G-shaped clamp frame (1), and a clamp bolt (11) is vertically connected to the upper plate surface of the G-shaped clamp frame (1);
the side heat insulation plates (3) are vertically arranged on four sides of the lower heat insulation plate (2) respectively, the side heat insulation plates (3) are connected with the lower heat insulation plate (2) through movable hinges (4), two layers of polyimide heating films (5) are arranged in parallel in a box body formed by the side heat insulation plates (3) and the lower heat insulation plate (2), a thermocouple I (6) and a thermocouple II (7) are arranged on the two layers of polyimide heating films (5) respectively, the thermocouple I (6) and the thermocouple II (7) are electrically connected with an external adjustable voltage-stabilized power supply (9), and the polyimide heating films (5) are electrically connected with a data processing terminal (8);
calliper bolt (11) bottom mounting is provided with heated board (10), just go up heated board (10) and be located heated board (2) under directly over, the top fixedly connected with calliper handle (12) of calliper bolt (11).
2. The unsteady-state multi-cell thermal conductivity tester of the constant-power planar heat source according to claim 1, characterized in that: upper strata the top of polyimide heating film (5) is equipped with auxiliary sample a, the upper strata the below of polyimide heating film (5) is equipped with auxiliary sample c, and is equipped with the sample b that awaits measuring between two-layer polyimide heating film (5), and auxiliary sample a, the material and the length of a side of sample b and auxiliary sample c all the same, and the surface grinds flat and stacks in proper order to make their thickness satisfy ha+hb＝hcAnd the side length is ten times of the thickness.
3. The unsteady-state multi-cell thermal conductivity tester of the constant-power planar heat source according to claim 2, characterized in that: the first thermocouple (6) and the second thermocouple (7) form a thermocouple pair and are arranged at the centers of the upper surface and the lower surface of the sample b to be measured.
4. A testing method using the unsteady-state multi-unit thermal conductivity tester for the constant-power planar heat source according to any one of claims 1 to 3, wherein the testing method comprises the following steps:
the problem of one-dimensional unsteady heat conduction of a semi-infinite object has a definite solution under a second class of boundary conditions, and a uniform initial temperature T is obtained by assuming the semi-infinite object0From the time t-0, a constant heat flux q is applied at x-00The planar heat source of (2) heats the object surface, the differential equation and the solution conditions for the thermal conductivity problem:
the temperature rise within the object away from the surface x is then:
order to
Here, ierfc (ξ) is the first integral of the Gaussian error complement function of variable ξ, i.e.
Where t is 0, x is 0,
from the equations and equations, one can derive
If t is measured separately1Time x equals 0 and t2The temperature rise at time x δ is then known from the equation and the equation
If order
Here, ω can be determined experimentally, then
The equation can be transformed into
For this function, the variable ξ can be determined by solving a mathematical function table or MATLAB1So that the thermal diffusivity can be obtained
The thermal conductivity can be obtained by substituting the equation into the equation:
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| CN112710695A (en) * | 2020-12-21 | 2021-04-27 | 西安交通大学 | Method for obtaining heat conductivity coefficient of buffer layer for high-voltage XLPE cable under different compression rates |
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Application publication date: 20200619 |