Device for measuring thermophysical parameters of metal in open environment

Abstract

The invention belongs to the technical field of physical parameter measuring devices, and relates to a metal thermophysical parameter measuring method and device. In a traditional experimental device, four thermophysical parameters including the heat conductivity coefficient, the specific heat capacity, the convective heat transfer coefficient and the emissivity are achieved through different measuring devices, measurement of the heat conductivity coefficient is generally achieved through a one-dimensional heat transfer model, and the device is complex in structure. The device comprises a heating system and a temperature measuring system, the bottom end of a to-be-measured sample is arranged on a heat insulation plate, the periphery and the top end of the to-be-measured sample are exposed in an open space, and a heat transfer model is expanded from one dimension to three dimensions. Heating is stopped after the sample is continuously heated, the sample is subjected to three temperature change processes of temperature rise, steady state and temperature reduction, and the temperature measurement system measures the temperatures of different positions of the sample. By analyzing the influence of conduction, convection and radiation in different temperature stages on heat transfer, the four thermophysical parameters are measured in the same experimental device. The device is simple in structure, rich in experiment content, advanced in experiment method and suitable for carrying out teaching experiments in laboratories.

Description

Technical Field

[0001] This invention belongs to the technical field of physical parameter measurement devices, and particularly relates to methods and devices for measuring the thermal properties of metals. Background Technology

[0002] The thermal properties of materials refer to the properties related to heat among the various properties of an object. Common thermal property parameters include: thermal conductivity, which characterizes thermal conductivity; specific heat capacity, which characterizes heat storage capacity; and convective heat transfer coefficient and emissivity, which characterize heat dissipation capacity. The determination of thermal property parameters is one of the important thermal experiments in university physics experimental courses.

[0003] Currently, in university physics experimental teaching at major universities [1，2] Various thermophysical parameters are generally measured using different devices. For example, measuring the thermal conductivity of a good conductor involves both a steady-state method based on Fourier's law of thermal conductivity and a non-steady-state method measuring the internal temperature of the material over time. Both methods require a cooling device at one end of the sample to maintain a constant temperature difference between the two ends, and both study heat conduction in one dimension. Generally, insulation material is added outside the device to minimize heat convection and radiation. These devices are complex in structure and have limited measurement functions. Specific heat capacity, depending on the measurement principle, can be measured using cooling methods or mixed calorimetry. However, convective heat transfer coefficients and emissivity are significantly affected by experimental conditions and environmental factors, and such experiments are rarely conducted in teaching.

[0004] In recent years, my country's science and technology have developed rapidly, and temperature measurement technology and data processing methods have continued to improve, providing feasibility for us to achieve accurate temperature measurement and process complex data. However, in the experimental teaching of major universities, the measurement methods of thermophysical parameters still use traditional measuring devices, which cannot meet the needs of cultivating high-quality talents who can adapt to the development of the times.

[0005] This invention eliminates the insulating material and cooling device at one end of the sample in traditional experimental setups, simplifying the experimental apparatus. The sample is exposed to the natural environment, enabling the measurement of four thermophysical parameters: thermal conductivity, emissivity, convective heat transfer coefficient, and specific heat capacity. The experimental content is richer, allowing students to better understand heat transfer and temperature changes within substances. Through experiments, students deepen their understanding and mastery of the three modes of heat transfer. Summary of the Invention

[0006] The technical problem solved by this invention is achieved through the following technical solution:

[0007] An open-environment metal thermophysical property measurement device includes a heating system and a temperature measurement system. The heating system includes a voltage regulator, a heating rod, an insulation plate, and a control circuit module. The heating rod is placed in a pre-drilled hole at the bottom of the sample to be tested, with the bottom of the sample placed on the insulation plate, and the surrounding area and top exposed in the open space. The voltage regulator, heating rod, and control circuit module are electrically connected. The control circuit module controls the on / off state of the voltage regulator. The voltage regulator and heating rod are electrically connected for steady-state heating of the sample to be tested. The voltage regulator and heating rod are periodically electrically connected for dynamic heating of the sample to be tested in a periodic mode. The sample is continuously heated until it reaches a steady state, at which point heating stops, and the sample cools naturally to room temperature. The sample undergoes three temperature change processes: heating, steady state, and cooling. The temperature measurement system measures the temperature at different locations on the sample. By analyzing the effects of conduction, convection, and radiation on heat transfer at different temperature stages, the thermal conductivity, specific heat capacity, convective heat transfer coefficient, and emissivity are measured.

[0008] Furthermore, the sample to be tested is a slender cylindrical shape.

[0009] Furthermore, the size of the pre-drilled hole at the bottom of the sample to be tested is the same as the size of the heating rod.

[0010] Furthermore, the sample to be tested is a material with a high thermal conductivity, such as a metal.

[0011] Furthermore, holes are drilled at different axial positions on the sample to be tested, with the hole depth reaching the sample's axial center. A K-type thermocouple is used to measure the temperature at the sample's axial center.

[0012] Furthermore, the insulation board has openings, and the power cord of the heating rod passes through the insulation board and is electrically connected to the voltage regulator and control circuit.

[0013] Furthermore, the insulation board uses aluminum silicate ceramic fiber board with a high insulation coefficient.

[0014] Furthermore, the voltage regulator has a voltage regulation range of 0V-220V, providing different heating powers for the heating rod.

[0015] Furthermore, the control circuit module includes a microcontroller and power relays.

[0016] Furthermore, the temperature measurement system includes a microcontroller, a K-type thermocouple, a temperature conversion chip, and a display screen.

[0017] The advantages and positive effects of this invention are:

[0018] 1. This invention removes the insulating material and cooling device from the sample in traditional experimental setups, expanding the heat transfer model from one-dimensional to three-dimensional. It is based on the natural heat dissipation process under natural conditions, which helps students better understand the physical process of heat transfer.

[0019] 2. It enables the simultaneous measurement of four thermophysical parameters: thermal conductivity, emissivity, convective heat transfer coefficient, and specific heat capacity, overcoming the problem of traditional experimental teaching instruments measuring only one physical parameter.

[0020] 3. It can measure the thermophysical properties of metals using both steady-state and periodic heating dynamic methods, offering a wider range of experimental options.

[0021] 4. By changing the heating power, the thermophysical properties of metals at different temperatures can be investigated.

[0022] 5. Through repeated experiments, the measurement errors of specific heat capacity and thermal conductivity are both around 5%, and the convective heat transfer coefficient and emissivity are within the reference range, demonstrating high accuracy and stability.

[0023] 6. It has a simple structure, low cost, low requirements for experimental environment conditions, and simple operation. The experimental principle helps students understand mathematical methods in physics and is suitable for carrying out teaching experiments in the laboratory. Attached Figure Description

[0024] The technical solution of the present invention will be further described in detail below with reference to the accompanying drawings and embodiments. However, it should be understood that these drawings are designed for illustrative purposes only and are not intended to limit the scope of the present invention. Furthermore, unless specifically indicated, these drawings are intended only to conceptually illustrate the structural construction described herein and are not necessarily drawn to scale.

[0025] Figure 1 This is a schematic diagram of the structure of the open environment thermal property parameter measuring device provided in Embodiment 1 of the present invention;

[0026] Figure 2 This is a schematic diagram showing the change of the average temperature of the sample under steady-state heating over time, as provided in Embodiment 2 of the present invention.

[0027] Figure 3 This is a schematic diagram of the sample heat flow provided in Embodiment 2 of the present invention;

[0028] Figure 4 This is a schematic diagram showing the change of the average temperature of the sample under dynamic heating over time, as provided in Embodiment 3 of the present invention. Detailed Implementation

[0029] First, it should be noted that the specific structure, features, and advantages of the present invention will be described in detail below by way of examples. However, all descriptions are for illustrative purposes only and should not be construed as limiting the present invention in any way. Furthermore, any single technical feature described or implied in the embodiments mentioned herein, or any single technical feature shown or implied in the accompanying drawings, can still be arbitrarily combined or deleted among these technical features (or their equivalents) to obtain more other embodiments of the present invention that may not be directly mentioned herein. Additionally, for the sake of simplifying the drawings, the same or similar technical features may be indicated only in one place in the same drawing.

[0030] In the description of this invention, it should be noted that the terms "center," "upper," "lower," "left," "right," "vertical," "horizontal," "inner," and "outer," etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings, or the orientation or positional relationship in which the product of this invention is usually placed during use. They are only for the convenience of describing this invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limitations on this invention.

[0031] It should be noted that, unless otherwise specified, the embodiments and features described in this application can be combined with each other.

[0032] Example 1

[0033] like Figure 1 The open-environment metal thermophysical parameter measuring device provided in this embodiment includes a heating system and a temperature measuring system. The heating system includes a voltage regulator T, a heating rod R, an insulation plate J, and a control circuit module M. The heating rod R is placed in a pre-drilled hole at the bottom of the sample Y to be tested, with the bottom surface of the heating rod R flush with the bottom surface of the sample Y. It is placed on the insulation plate J, and the power cable of the heating rod passes through the pre-drilled hole in the insulation plate J, electrically connecting to the voltage regulator T and the power relay Z. The control circuit module M controls the on and off times of the power relay Z to achieve steady-state heating or dynamic heating. The temperature measuring system measures the temperature at different locations on the sample to measure the thermal conductivity, specific heat capacity, convective heat transfer coefficient, and emissivity.

[0034] The control circuit module M includes a microcontroller, a level isolation converter circuit, and a power relay Z. The microcontroller outputs a level signal, which, after level conversion and isolation, controls the power relay Z. Pins 2 and 3 of the power relay Z are connected, electrically connecting the voltage regulator T to the heating rod R, thus performing steady-state heating of the sample. Alternatively, the microcontroller outputs a periodic level signal, which, after level conversion and isolation, controls the power relay Z. Pins 2 and 3 of the power relay Z are periodically connected, periodically electrically connecting the voltage regulator T to the heating rod R, thus performing dynamic heating of the sample. Changing the period of the microcontroller's output level allows for dynamic heating of the sample under different periodic modes. The microcontroller can be an STM32 series chip, the level converter can be an S8050 transistor, the level isolation can be a TLP-521 chip, and the power relay can be a 9431C12DS chip.

[0035] The temperature measurement system C includes a microcontroller, eight K-type thermocouples, a temperature conversion module, and a display screen. Seven K-type thermocouples are placed in the temperature sensing holes of the sample under test. The K-type thermocouples are electrically connected to the temperature conversion module, the temperature conversion chip is electrically connected to the microcontroller's I / O port, and the microcontroller is electrically connected to the display screen. The K-type thermocouples collect the temperature at different locations on the sample under test, which is then processed by the microcontroller through the temperature conversion module and displayed on the LCD screen. One K-type thermocouple measures the ambient temperature. The microcontroller can be an STM32 series chip, and the core component of the temperature conversion module is the MAX6675.

[0036] The voltage regulator T has a voltage regulation range of 0V-220V, providing different heating powers for the heating rod R. For experimental safety reasons, the heating power shall not exceed 50W. The insulation board J is made of aluminum silicate ceramic fiber board with a high thermal insulation coefficient. The sample Y to be tested is made of metal or other materials with high thermal conductivity.

[0037] In this invention, the sample shape is a slender cylinder. The radial temperature field of the sample is negligible, and the sample diameter should be as small as possible. The sample length determines the heating time; if the length is too long, not only will the heating time increase, but the temperature change will also become gradual at the end of the sample, and the temperature gradient will be insignificant. The sample is measured at equal intervals along its axial direction. The spacing of the temperature measuring holes reflects the axial temperature gradient of the sample. The accuracy of K-type thermocouples is ±0.8℃. Overly dense temperature measuring holes may not clearly show the temperature gradient due to measurement errors, increasing interference in data processing. Furthermore, dense temperature measuring holes require more K-type thermocouples, increasing instrument cost. Considering all these factors, the sample length is 245cm, the diameter is 2cm, the axial temperature measuring interval is 3.5cm, and the temperature measuring holes extend to the axial centerline of the sample, measuring the temperature along the central axis of the sample.

[0038] When one end of the sample is heated, the differential equation for the heat conduction of the sample can be derived based on the law of conservation of energy and Fourier's law of thermal conductivity:

[0039]

[0040] The sample under test will continue to heat up until it reaches a steady state, at which point heating will stop and it will naturally cool to room temperature. The sample will undergo three temperature change processes: heating up, steady state, and cooling down. The three terms on the right side of equation (1) represent the three destinations of the heat source. From left to right, the first term represents the storage of heat in the sample, the second term represents the heat being conducted to the air through thermal convection between the metal surface and the surrounding air, and the third term represents the heat being radiated to the surrounding environment through the surface.

[0041] The proportions of these three factors vary at different temperature stages: In the initial stage of heating, when the temperature is low, the latter two factors account for a smaller proportion, and heat conduction and storage within the sample dominate. In the middle and later stages of heating, as the temperature gradually increases, the proportions of the three factors gradually become more even. During steady-state heating, heat conduction within the sample reaches a steady state, and heat flows out mainly through air convection, conduction, and radiation, with the latter two factors dominating. In the initial stage of cooling, when the temperature is high, the heat outflow is similar to that in steady state; however, in the middle and later stages of cooling, when the temperature is low, heat flows out mainly through air convection, with the second factor dominating.

[0042] Should we consider the dynamic and steady-state methods in thermal conductivity measurement, corresponding to the temperature change term with time in equation (1)?

[0043] Example 2

[0044] The open-environment metal thermophysical property parameter measuring device provided in this embodiment is a further improvement and supplement to Embodiment 1:

[0045] Steady-state method for determining the thermal conductivity of metals: The sample to be tested is continuously heated, and the curve of the average temperature of the sample changing over time is shown in the figure. Figure 2 As shown. Considering the heating and temperature rise stage of the sample, Figure 2 Central Region 1:

[0046] Q=CmΔT (2)

[0047] Where m is the mass of the sample; ΔT is the change in the average temperature of the sample. Differentiating both sides of equation (2) with respect to time:

[0048]

[0049] In equation (3), That is, the output power P0 of the heat source; yes Figure 2 The slope k1 of the temperature-time variation curve in part 1 of the central region:

[0050] P0=Cmk1 (4)

[0051] Equation (4) does not consider the heat loss from the sample surface. Figure 2 Temperature changes in three parts of the sample in the central region:

[0052] During the cooling process, the heat retained in the sample is mainly dissipated in two forms: air convection and metal radiation. At this time, the following exists:

[0053]

[0054] Where P α P is the amount of heat dissipated by thermal convection per unit time. β This refers to the amount of heat dissipated through thermal radiation per unit time.

[0055] P α =Sα(TT) o (6)

[0056] S represents the area of ​​the sample in contact with air.

[0057]

[0058] For the sample under test, the heat dissipation efficiency P loss for:

[0059]

[0060] When the temperature is close to room temperature, i.e., T≈T0, equation (7) is expanded using Taylor:

[0061]

[0062] When the proportion of first-order terms is more than 90% of the total:

[0063]

[0064] Higher-order terms can be ignored; only first-order terms are retained.

[0065]

[0066] P loss ′ is the heat dissipation power that ignores the second-order infinitesimal amount of metal thermal radiation. Equation (5) is transformed into:

[0067]

[0068] The solution to this equation is:

[0069] T-T0=Ce -Bt (13)

[0070] in,

[0071]

[0072] Therefore, the slope of ln(T-T0) with respect to time t is equal to -B:

[0073]

[0074] According to equation (10), the temperature region that satisfies the condition of neglecting higher-order terms can be calculated using the theoretical values ​​of α and β, i.e. Figure 2 In region 3, the slope is obtained by fitting a straight line:

[0075]

[0076] From equation (15), k2·Cm(T-T0) is the sum of the first-order terms of the convective and radiative heat dissipation of the sample under test, which is approximately the total heat dissipation power of the sample.

[0077] Specific heat capacity calculation considerations Figure 2 In region 1, the heating process is linear, and the heat dissipation power also changes approximately linearly with temperature. Higher-order terms of heat dissipation can be ignored. The average value of the temperature region is selected. This means that the average heat dissipation power during this process is... The thermodynamic equation for the heating process of a metal can be written as:

[0078]

[0079] The formula for calculating specific heat capacity is obtained as follows:

[0080]

[0081] The air convective heat transfer coefficient α and the metallic emissivity β vary with temperature; therefore, only the magnitudes of these two parameters at the steady-state temperature can be calculated when the sample reaches this temperature. At steady state, all heat input from the heating rod is lost as heat dissipation from the sample, i.e., P0 = P loss ,but:

[0082]

[0083] T max Given the average temperature of the sample at steady state, β can be calculated using equation (19):

[0084]

[0085] Substituting β into equation (15), we can calculate α:

[0086]

[0087] consider Figure 2 In the steady-state process of region 2, the temperature field of the sample no longer changes with time. According to Fourier's law, the heat flux density through a certain cross-section of the sample is:

[0088]

[0089] The heat flow through a certain cross-section of the sample is equal to the heating power minus the heat flow lost radially from the first half of the sample at that cross-section. For example... Figure 3 As shown, P0 represents the heat flow input to the sample. The heat flow moves along the sample towards the lower temperature range, and simultaneously, the heat flow is lost radially through air convection and thermal radiation. The power lost is represented by P1. Finally, the magnitude of the heat flow reaching and passing through the cross-section is represented by P2. According to the law of conservation of energy, the heat flow through the cross-section is:

[0090] P2 = P0 - P1 (23)

[0091] Figure 3 In the above, if the cross section Q between temperature measurement points T2 and T3 is selected as the reference cross section, then the heat flow lost before the heat flow enters the reference cross section is:

[0092]

[0093] Let the temperatures at measurement points T1 and T2 represent the average temperatures of the sample's contact areas with air, S1 and S2, respectively. The temperature gradient across the cross-section is calculated using the temperatures at both ends of the cross-section. From equation (22), the thermal conductivity can be obtained as:

[0094]

[0095] Where δL is the length interval between two adjacent temperature measurement points, and A is the cross-sectional area of ​​the sample.

[0096] Example 3

[0097] The open-environment metal thermophysical parameter measuring device provided in this embodiment is a further improvement and supplement to Embodiment 1:

[0098] Taking heat dissipation into account, a periodic heat source is applied to the sample:

[0099] T(t) = T s +T n sin(ωt) (26)

[0100] That is, the heat source temperature is at T s Nearby, ±T n It varies periodically with frequency ω within the range.

[0101] Without considering the radial temperature distribution of the sample and heat dissipation from the end face, equation (1) can be expressed as:

[0102]

[0103] This can be further simplified to:

[0104]

[0105] After heating for a period of time, the temperature distribution in the sample is obtained by equation (28):

[0106]

[0107] M is a term related to thermal conductivity and is considered a constant. As can be seen from equation (29), the periodic heat source causes the temperature field in the sample to have a distribution of thermal waves with exponentially decaying amplitude along the axial direction, based on the axial linear gradient distribution.

[0108] Referring to wave theory, the propagation time Δt of the heat wave between two points with a known distance δL can be obtained from the phase difference of the heat wave between the two points, and thus the propagation speed of the heat wave can be obtained:

[0109]

[0110] Knowing the wave speed

[0111]

[0112] According to equations (30) and (31), the thermal conductivity can be obtained as follows:

[0113]

[0114] When the sample temperature is close to the surrounding gas temperature T a That is, T≈T a Then, equation (28) can be approximated as:

[0115]

[0116] The solution to equation (33) is as follows:

[0117]

[0118] Equation (34) is in the form of a Fourier series, where the existence of n reflects the temperature gradient in the rod. When n = 0, the gradient disappears and the temperature in the rod tends to be uniform. The solution to equation (34) is:

[0119]

[0120] The overall temperature of the sample decreases exponentially over time, further yielding the following:

[0121]

[0122] ln(TT a The temperature of the sample is linearly related to t. The temperature change of the sample during the entire process from the start of periodic heating to the cessation of heating is as follows: Figure 4 As shown. Considering T in equation (33) a Choose the temperature of the gas surrounding the sample. Figure 4 The data in the middle region II were fitted, and at near room temperature, T ~ T0:

[0123]

[0124] During the periodic cooling phase of the stable phase, select Figure 4 The data in region I were fitted to approximate the lowest value T of the periodic cooling, T ~ T1:

[0125]

[0126] α / C and β / C can be obtained from equations (37) and (38).

[0127] Returning to the heat conduction equation (28) for the sample, integrating both sides with respect to space, we obtain:

[0128]

[0129] Substitute the boundary conditions:

[0130]

[0131] get:

[0132]

[0133] In the formula and The three items can be obtained from experimental data. The specific heat capacity C can be calculated by combining equations (38) and (39). Then, by substituting the specific heat capacity value back into equations (32) and (27), α, β and κ can be calculated.

[0134] When measuring thermophysical parameters using Example 2 or 3, the experimental content and operating steps are as follows:

[0135] Experiment 1: Measurement of thermophysical parameters under the same heating power using the steady-state method. Experimental steps:

[0136] 1. Calculate the output voltage of the voltage regulator based on the internal resistance of the heating rod and adjust the voltage regulator accordingly.

[0137] 2. The microcontroller starts the steady-state heating program and outputs a high level.

[0138] 3. Set the data acquisition interval of the thermometer to 1 second, start the measurement, observe the temperature reading of the thermometer, and when the readings of each temperature probe no longer change, wait for about 5 minutes to stop heating. Stop data acquisition when the sample temperature drops to about room temperature.

[0139] 4. Using data processing software such as MATLAB, Origin, and Python, write corresponding data processing programs based on the experimental principles of Example 2, and import the experimental data into the programs to obtain the experimental results.

[0140] Experiment 2: Measurement of thermophysical parameters of copper (aluminum) rods under different heating powers using the steady-state method. Experimental steps:

[0141] Following the operating steps in Experiment 1, adjust the voltage regulator according to the required heating voltage at different power levels (10W, 20W, 30W, 40W, 50W), and measure the change in sample temperature over time at different heating powers. Import the experimental data into the program to obtain the experimental results.

[0142] Experiment 3: Measurement of thermophysical parameters under the same heating power using the dynamic method. Experimental steps:

[0143] 1. Calculate the output voltage of the voltage regulator based on the internal resistance of the heating rod and adjust the voltage regulator accordingly.

[0144] 2. The microcontroller starts the dynamic heating program and outputs periodic high and low levels.

[0145] 3. Set the data acquisition interval of the thermometer to 1 second, start the measurement, and observe the temperature reading of the thermometer. At this time, the temperature of the thermometer should change periodically. Record the peak temperature. After the peak temperature is relatively stable, record for 6-8 cycles and then stop heating. Wait for the sample to cool down. Data acquisition can be stopped when the temperature drops to around room temperature.

[0146] 4. Using data processing software such as MATLAB, Origin, and Python, write corresponding data processing programs based on the experimental principles of Example 3, and import the experimental data into the programs to obtain the experimental results.

[0147] Experiment 4: Measurement of thermophysical parameters of copper (aluminum) rods under different heating powers. Experimental steps:

[0148] Following the operating steps in Experiment 3, adjust the voltage regulator according to the required heating voltage at different power levels (10W, 20W, 30W, 40W, 50W), and measure the change in sample temperature over time at different heating powers. Import the experimental data into the program to obtain the experimental results.

[0149] The above embodiments have provided a detailed description of the present invention, but the content described is only a preferred embodiment of the present invention and should not be considered as limiting the scope of the present invention. All equivalent variations and improvements made within the scope of the present invention should still fall within the patent coverage of the present invention.

Claims

1. A device for measuring the thermal properties of metals in an open environment, characterized in that: It includes a heating system and a temperature measuring system.

2. The heating system according to claim 1, characterized in that: The heating system includes a voltage regulator, a sample to be tested, an insulation board, and a control circuit module. A heating rod is placed in a pre-drilled hole at the bottom of the sample to be tested, with the bottom of the sample resting on the insulation board and its surroundings and top exposed in an open space. The voltage regulator, heating rod, and control circuit module are electrically connected. The control circuit module controls the on / off state of the voltage regulator. When the voltage regulator and heating rod are electrically connected, the sample is heated using a steady-state heating method. When the voltage regulator and heating rod are periodically electrically connected, the sample is heated using a dynamic heating method under different periodic modes.

3. The temperature measurement system according to claim 1, characterized in that: The temperature measurement system includes a microcontroller, a K-type thermocouple, a temperature conversion chip, and a display screen. By measuring the temperature changes at different locations on the sample, it can measure the thermal conductivity, specific heat capacity, convective heat transfer coefficient, and emissivity.

4. The sample to be tested according to claim 2, characterized in that: The sample to be tested is a slender cylindrical shape.

5. The sample to be tested according to claim 2, characterized in that: The size of the pre-drilled hole at the bottom of the sample to be tested is the same as the size of the heating rod.

6. The sample to be tested according to claim 2, characterized in that: The sample to be tested is a material with a high thermal conductivity, such as a metal.

7. The sample to be tested according to claim 4, characterized in that: Holes are drilled at different axial positions on the sample to be tested, with the hole depth reaching the central axis of the sample, and the temperature of the central axis of the sample is measured.

8. The insulation board according to claim 2, characterized in that: An opening is made in the insulation board, and the power cord of the heating rod passes through the insulation board and is electrically connected to the voltage regulator and control circuit module.

9. The insulation board according to claim 2, characterized in that: The insulation board is made of aluminum silicate ceramic fiber board with a high insulation coefficient.

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