A testing device and method for testing the convective heat transfer performance of a structural surface under low pressure

Abstract

The application belongs to the technical field of convective heat transfer, and particularly relates to a testing device and a testing method for convective heat transfer performance of a structure surface under rarefied gas pressure. The testing device comprises an environment cabin and a control system for adjusting environment parameter in the environment cabin, and the control system comprises a to-be-tested structure heating assembly, a pressure control system, a temperature control system and a wind speed control system. The testing method obtains total heat transfer power of the to-be-tested structure surface by controlling gas temperature, pressure and flow rate in the environment cabin and adjusting heat balance state of the to-be-tested structure surface, obtains convective heat transfer power of the to-be-tested structure surface under corresponding working conditions, and characterizes convective heat transfer characteristics of the structure surface. The application can realize accurate measurement of structure surface heat transfer, structure surface temperature and gas state parameters such as temperature, pressure and flow rate near the structure under rarefied gas pressure, and the obtained experimental data can provide experimental data support for analysis and calculation of convective heat transfer performance of the structure surface under rarefied gas pressure.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of convective heat transfer, and particularly relates to a testing device and a testing method for convective heat transfer performance of a structure surface under rarefied gas pressure. BACKGROUND

[0002] With the continuous development of science and technology, the living and development area of human beings is continuously expanding to the near space, and the aerospace technology is an important direction of future scientific and technological development.

[0003] In the near space area (working condition range: pressure 100 Pa-101325 Pa, environmental temperature 90 DEG C-90 DEG C, wind speed 0-10 m / s), the gas is in a low-pressure rarefied state, and in the heat transfer structure design of the aerospace vehicle and the aerospace environment simulation device, a large number of convective heat transfer problems between the low-pressure rarefied gas and the solid structure surface are involved, and the convective heat transfer coefficient between the rarefied gas and the heat transfer structure surface must be determined. Because the average molecular free path of the low-pressure rarefied gas is large, the convective heat transfer theory based on the continuous medium assumption is no longer applicable under the low-pressure condition, and the characterization of the convective heat transfer coefficient depends on the experimental correlation, and the existing data are mostly limited to specific working conditions, and there is a lack of universal model suitable for the multi-scale region involving the slip flow, the transition flow state and the molecular flow, therefore, a special experimental device is needed to test the convective heat transfer coefficient between the rarefied gas and the heat transfer structure surface under multiple working conditions.

[0004] At present, the existing convective heat transfer testing devices are mostly designed for normal pressure or higher pressure environment, and there is a lack of special equipment for testing the convective heat transfer of rarefied gas under multiple working conditions. It is very necessary to develop a convective heat transfer testing device integrated with multiple working condition coupling environment control. SUMMARY

[0005] In order to solve the above technical problems, one of the purposes of the present application is to provide a testing device for convective heat transfer performance of a structure surface under rarefied gas pressure.

[0006] The application adopts the following technical scheme:

[0007] A testing device for the convective heat transfer performance of a structural surface under rarefied pressure includes an environmental chamber for providing a testing environment for the structure under test and a control system for adjusting the testing environment parameters within the environmental chamber. The control system includes a heating component for the structure under test, a pressure control system, a temperature control system, and a wind speed control system. The heating component for the structure under test is disposed inside the structure under test and isolated from the space inside the environmental chamber. The pressure control system adjusts the vacuum level inside the environmental chamber through an air exchange port disposed on the environmental chamber body. The temperature control system includes a heat sink heat exchanger disposed inside the environmental chamber and a cold and heat source circulation system disposed outside the environmental chamber. The wind speed control system includes a variable frequency motor, fan blades, and a fan shroud. The fan shroud is disposed around the structure under test. The fan blades are connected to the variable frequency motor and extend into the fan shroud, discharging air toward the structure under test.

[0008] Preferably, the wind hood includes a first wind hood and a second wind hood, which are joined together to form a shell with a shape consistent with the internal shape of the environmental chamber. The heat sink heat exchanger is an expansion plate type heat exchange structure, and the heat sink heat exchanger is respectively disposed at both ends of the first wind hood and the second wind hood, forming a closed hollow cavity structure with the first wind hood and the second wind hood.

[0009] Preferably, the first and second wind hoods are provided with air holes, which are respectively arranged at the center of the line connecting the center point of the curved surface and the vertex of the edge of the curved surface, that is, four air holes are provided on each of the first and second wind hoods.

[0010] Preferably, the pressure control system includes a vacuuming branch and a gas replenishment and repressurization branch. The gas replenishment and repressurization branch includes a gas source shut-off valve, a gas replenishment regulating valve, and a gas replenishment cylinder arranged sequentially from the gas exchange port. The vacuuming branch includes a vacuuming regulating valve, a vacuum pump unit, and a check valve arranged sequentially from the gas exchange port.

[0011] Preferably, the environmental chamber is provided with two air exchange ports, which are respectively connected to the vacuum pumping branch and the gas replenishment and repressurization branch.

[0012] Preferably, the surface of the heat sink heat exchanger is also uniformly provided with electric heating strips.

[0013] A second objective of this invention is to provide a method for testing the convective heat transfer performance of a structural surface under rarefied pressure, the method comprising the following steps:

[0014] S1. The rarefaction of the gas is classified using the Knudsen number, and the critical pressure is calculated according to the following formula. :

[0015] ;

[0016] In the formula, is the Boltzmann constant, with value = 1.38 x 10 -23 J K -1 ; is the thermodynamic temperature of the gas in the environmental chamber; is the average collision diameter of the gas molecules in the environmental chamber, obtained by consulting the NIST database, with unit nm; represents the characteristic length, with value 0.001, is the Knudsen number;

[0017] The value of the calculated is the critical pressure , 100 times the value is , 10000 times the value is , then:

[0018] When the gas pressure in the environmental chamber is higher than , it is considered that the gas in the chamber is in a continuous flow state;

[0019] When the gas pressure in the environmental chamber is between and , it is considered that the gas in the chamber is in a slip flow state;

[0020] When the gas pressure in the environmental chamber is between and , it is considered that the gas in the chamber is in a transition flow state;

[0021] When the gas pressure in the environmental chamber is lower than , it is considered that the gas in the chamber is in a free molecular flow state;

[0022] S2. Start the pressure control system to adjust the gas pressure in the environmental chamber to be lower than the critical pressure and maintain, measure and record the heating power of the heating assembly of the structure to be tested , at this time is equivalent to the radiant heat transfer power of the surface of the structure to be tested ;

[0023] S3. Start the cold and heat source circulation system, adjust the average temperature of the gas in the chamber to the preset value by cooling and maintain for at least 5 min, respectively record the average temperature of the heat sink heat exchanger and the surface of the wind shield , the average temperature of the surface of the structure to be tested , the average temperature of the surface near the structure to be tested and the heating power , the surface near the structure to be tested is within a linear distance of 0.01 m from the surface of the structure to be tested;

[0024] The surface radiation heat transfer coefficient is calculated according to the following formula. :

[0025] ;

[0026] S4. Adjust the cold and heat source circulation system to obtain n sets of different average temperatures of the cabin gas environment. Repeat step S3 to obtain n different sets of surface radiation heat transfer coefficients. And obtain the surface radiation heat transfer coefficient. Average temperature of heat sink heat exchanger and fan shroud surface Average surface temperature of the structure under test Functional relationship:

[0027] ;

[0028] S5. Adjust the air pressure inside the environmental chamber to the predetermined test pressure condition using the pressure control system; adjust the average temperature of the gas environment inside the chamber using the method in step S3. The test temperature was set to the predetermined test temperature and maintained for at least 5 minutes. The average surface temperature of the heat sink heat exchanger was recorded at this time. Average surface temperature of the structure under test Average temperature near the surface of the structure under test and heating power , Equivalent to the total heat transfer power on the surface of the structure under test ;

[0029] The convective heat transfer coefficient of the surface of the structure under test is calculated using the following formula. :

[0030] ;

[0031] In the formula, This represents the outer surface area of ​​the part of the structure under test that comes into contact with the gas inside the chamber.

[0032] S6. Repeat step S5 to obtain test data on the convective heat transfer coefficient under different air pressure, temperature, and wind speed conditions; for a given condition, calculate the Nusselt number under the corresponding condition using the following formula. Reynolds number Prandtl numbers And Grashof :

[0033] ;

[0034] ;

[0035] ;

[0036] ;

[0037] wherein, represents the convective heat transfer coefficient, with the unit of W / (m²·K); represents the characteristic length of the flow field, with the value of 0.001 m; represents the thermal conductivity of the gas in the cabin, with the unit of W / (m²·K); represents the gas density in the cabin, with the unit of kg / m 3 ; represents the gas flow rate in the cabin, with the unit of m / s; represents the dynamic viscosity of the gas in the cabin, with the unit of Pa·s; represents the specific heat capacity at constant pressure of the gas in the cabin, with the unit of J / (kg·K); g represents the gravitational acceleration, with the value of 9.8 m / s 2 ; represents the coefficient of volume expansion of the gas in the cabin, with the unit of K -1 ; represents the difference between the temperature at the surface of the structure to be measured and the temperature of the gas at least 0.25 m away from the surface of the structure to be measured; represents the kinematic viscosity of the gas in the cabin, with the unit of m² / s;

[0038] S7. The function relationship described below is obtained by using a neural network fitting:

[0039] Continuous flow state: ;

[0040] Slip flow state: ;

[0041] Transition flow state: ;

[0042] wherein, a represents the rarefaction effect correction coefficient of the slip flow region, which is related to the gas type and the characteristics of the solid surface, and the value is between 0.5 and 1; b represents the viscosity correction coefficient of the slip flow region, which is related to the gas type, and the value is between 0.5 and 1; c represents the test fitting coefficient of the slip flow region, and the value is between 1 and 2; d represents the boundary correction coefficient of the slip flow region, which is related to the gas type, and the value is between 0.2 and 0.5; α represents the rarefaction effect correction coefficient of the transition flow region, which is related to the gas type and the characteristics of the solid surface, and the value is between 1 and 3; β represents the test fitting coefficient of the transition flow region, and the value is between 0.3 and 1.

[0043] Preferably, for the function relationship obtained in step S7, a correction method is further included, in particular,

[0044] For the slip flow regime, the Nusselt number , Reynolds number , Prandtl number and Grashof number are corrected by the following formula:

[0045] ;

[0046] ;

[0047] ;

[0048] ;

[0049] For the transition flow regime, the Nusselt number , Reynolds number , Prandtl number and Grashof number are corrected by the following formula:

[0050] ;

[0051] ;

[0052] Then, after correction:

[0053] Slip flow region: ;

[0054] Transition flow region: .

[0055] Preferably, in step S3, the specific temperature adjustment method is: the cabin gas temperature is coarsely adjusted by adjusting the cooling power, the cabin gas temperature is adjusted to the preset value ± 5 K, and then the heating power of the to-be-tested structure heating assembly and the electric heating belt is adjusted to perform secondary fine adjustment on the cabin gas temperature, until the average temperature of the cabin gas environment reaches the preset value ± 0.5 K, and is maintained for at least 5 min.

[0056] The present application has the following advantages:

[0057] (1) Compared with the existing device, the device provided by the present application can accurately control a wide range of pressure, temperature and wind speed required for testing, and can perform comprehensive data collection of environmental parameters and surface parameters of the to-be-tested structure, and realize accurate measurement and characterization of the convective heat transfer coefficient of the surface of the to-be-tested structure in the continuous flow, slip flow and transition flow regions.

[0058] (2) The testing device provided in this application adopts a modular design, including an environmental control module and a heat transfer measurement module. Each module has a clearly defined function, which facilitates maintenance and upgrades. The device can also be connected to different types of gas sources and the gas supply rate can be controlled by regulating valves to meet different testing requirements.

[0059] (3) Compared with the prior art, the test method provided in this application combines physical field decoupling measurement, multi-parameter coupling and coordinated control, multi-scale flow model correction, wind field optimization design and neural network data processing to achieve high-precision heat transfer characterization of the entire domain from continuous flow to free molecular flow. Combining the theoretical correction model with the experimental system is beneficial to solving the long-standing problem of rarefied gas convection heat transfer prediction in the aerospace field. Attached Figure Description

[0060] Figure 1 This is a schematic diagram of the test device of this application.

[0061] The correspondence between the reference numerals in the attached figures is as follows:

[0062] 10-Structure to be tested, 11-Environmental chamber, 12-Supporting components, 20-Ventilation port, 21-Gas source shut-off valve, 22-Maintenance gas regulating valve, 23-Maintenance gas cylinder, 24-Extraction regulating valve, 25-Vacuum pump unit, 26-Check valve, 30-Heat sink heat exchanger, 31-Electric heating belt, 40-Cold and heat source circulation system, 51-Variable frequency motor, 52-Fan blades, 53-Fan shroud, 53a-First fan shroud, 53b-Second fan shroud, 531-Air vent.

[0063] In the diagram, P represents the pressure measuring point, T represents the temperature measuring point, and F represents the flow measuring point. Detailed Implementation

[0064] The technical solution of the present invention will be described in more detail below with reference to embodiments:

[0065] like Figure 1 As shown, a testing device for the convective heat transfer performance of a structural surface under rarefied pressure includes an environmental chamber 11 that provides a sealed environment for the structure under test 10 and a control system that regulates the environmental parameters within the environmental chamber 11. The control system includes a heating component for the structure under test, a pressure control system, a temperature control system, and a wind speed control system.

[0066] The structure under test 10 is positioned at the center of the environmental chamber via a support assembly 12. The support assembly 12 can be a thin rod or fin-like structure that facilitates gas flow, thereby reducing its impact on the uniformity of airflow within the environmental chamber 11. An insulation structure is also provided between the structure under test 10 and the support assembly 12 to reduce heat conduction between them. A heating assembly for the structure under test is located inside the structure under test 10 and is isolated from the space within the environmental chamber 11.

[0067] The pressure control system adjusts the vacuum degree in the cabin through the air exchange port 20 arranged on the environmental cabin. The pressure control system comprises a vacuum extraction branch and a gas supplementing and pressure restoring branch, wherein the gas supplementing and pressure restoring branch comprises, in sequence from the air exchange port 20, a gas source cut-off valve 21, a gas supplementing valve 22 and a gas supplementing cylinder 23, and the vacuum extraction branch comprises, in sequence from the air exchange port 20, an air extraction valve 24, a vacuum pump unit 25 and a check valve 26. The vacuum extraction branch and the gas supplementing and pressure restoring branch can be connected to the cabin through the same air exchange port 20 or through independent air exchange ports 20. The pressure control system can be connected to different kinds of gas sources according to the test requirements and control the gas supplementing rate through the gas supplementing valve.

[0068] In this embodiment, the pressure control system can at least realize the regulation of the gas components in the environmental cabin 11 and the precise adjustment of the gas pressure in the range of 100 Pa to 101325 Pa. The selection of the specific device is not specially limited in the present application, and any device that can achieve the purpose of the present application can be used, for example, the vacuum pump unit in a certain device adopts the form of a Roots-screw vacuum pump set to realize the regulation of the gas pressure in the environmental cabin in a large range in a short time.

[0069] The temperature control system comprises a heat sink heat exchanger 30 arranged in the environmental cabin and a cold and heat source circulation system 40 arranged outside the environmental cabin 11. The heat sink heat exchanger 30 adopts a liquid-formed bulging plate heat exchange structure, and the surface temperature distribution of the bulging plate heat exchange structure is uniform.

[0070] The heat sink heat exchanger 30 is further provided with an electric heating belt 31, the electric heating belt 31 is uniformly arranged on the outer surface of the heat sink heat exchanger 30, the heating power of the electric heating belt 31 is adjusted through frequency conversion, the surface temperature of the heat sink heat exchanger 30 is secondarily regulated, and thus the accurate regulation of the gas temperature in the environmental cabin 11 is realized.

[0071] The cold and heat source circulation system 40 is a prior art, and the system flow design, component selection and temperature control logic are not described in detail. For example, a brine unit and a cold and heat source circulation pipeline can be used for regulation, at this time, the heat sink heat exchanger 30 is connected to the cold and heat source circulation pipeline 40 through the liquid inlet pipeline and the liquid outlet pipeline and forms a closed loop with the brine unit.

[0072] In the present application, the temperature control system should at least realize the precise control of the gas temperature in the environmental cabin 11 in the range of-90 to 90℃.

[0073] The wind speed control system comprises a variable frequency motor 51, a fan blade 52 and a wind cover 53, the wind cover 53 is arranged at the periphery of the structure to be measured 10, the fan blade 52 is connected with the variable frequency motor 51, and the fan blade 52 extends into the inside of the wind cover 53 and blows towards the structure to be measured 10.

[0074] The fan blade 52 connected with the variable frequency motor 51 is prior art, for example, the variable frequency motor 51 provides power for the rotation of the fan blade 52 in the environmental cabin 11 through a magnetic bearing device, and the control of the gas flow rate in the environmental cabin 11 is realized through the power adjustment of the variable frequency motor 51.

[0075] The wind cover 53 comprises a first wind cover 53a and a second wind cover 53b, the first wind cover 53a and the second wind cover 53b are folded to form a shell with a shape consistent with the shape of the inside of the environmental cabin 11, in the present application, the heat sink heat exchanger 30 is arranged at the two ends of the first wind cover 53a and the second wind cover 53b respectively, and the first wind cover 53a and the second wind cover 53b form a closed hollow cavity structure.

[0076] Air holes 531 are arranged on the first wind cover 53a and the second wind cover 53b, the air holes 531 are arranged at the center positions of the connecting lines of the center points of the curved surfaces of the wind cover 53 and the vertexes of the curved surface edges, that is, four air holes are arranged on the first wind cover 53a and the second wind cover 53b respectively.

[0077] In the present application, based on the test requirements, temperature, pressure, flow rate and other measuring devices are also included, which can be realized by prior art, and the present application does not make special limitations.

[0078] For example, in the present embodiment, the measuring devices comprise: a surface temperature measuring point of the structure to be measured 10, a gas flow temperature measuring point near the surface of the structure to be measured (distance from the surface of the structure to be measured ≤0.01m), a gas flow rate measuring point near the surface of the structure to be measured (distance from the surface of the structure to be measured ≤0.01m), a gas temperature measuring point away from the surface of the structure to be measured (distance from the surface of the structure to be measured ≥0.25m) in the environmental cabin 11, a surface temperature measuring point of the heat sink heat exchanger 30, and a surface temperature measuring point of the wind cover 53. In order to ensure the accuracy of the test, a plurality of the above measuring points are arranged, and the test results are algebraically averaged.

[0079] Embodiment 1

[0080] The present application provides a test method based on the test device for the convective heat transfer performance of the structure surface under the above-mentioned rarefied gas pressure.

[0081] Taking the test working condition of the gas pressure in the environmental cabin 5kPa, the gas temperature-50℃ and the wind speed 2m / s as an example, the steps are as follows:

[0082] S1. The Knudsen number is used to classify the degree of rarefaction of the gas in the environmental chamber, which is divided into continuous flow (Kn≤0.001), slip flow (0.001≤Kn≤0.1), transition flow (0.1≤Kn≤10) and free molecular flow (Kn≥10). The critical pressure of each flow region is calculated using the following formula P cr Calculation:

[0083] ;

[0084] ;

[0085] ;

[0086] In the formula, is the Boltzmann constant, which is =1.38×10 -23 J K -1 ; is the thermodynamic temperature of the gas in the environmental chamber; is the average collision diameter of the gas molecules in the environmental chamber, which is obtained by consulting the NIST database, such as =0.375nm, =0.361nm, =0.37nm; represents the characteristic length of flow, which is taken as 0.001m here.

[0087] It can be considered that when the gas pressure in the environmental chamber is higher than , the gas in the chamber is in a continuous flow state; when the gas pressure in the environmental chamber is between and , the gas in the chamber is in a slip flow state; when the gas pressure in the environmental chamber is between and , the gas in the chamber is in a transition flow state; when the gas pressure in the environmental chamber is lower than , the gas in the chamber is in a free molecular flow state.

[0088] Before the test starts, the structure to be tested 10 is fixed in a predetermined position, and each subsystem is adjusted to a standby state.

[0089] S2. Start the pressure control system, monitor the gas pressure in the environmental chamber, and adjust the gas pressure in the environmental chamber to be lower than the critical pressure and maintain, which can be considered as the gas in the chamber being in a free molecular flow state. When the gas molecules are in a free molecular flow state, the convective heat transfer between the gas and the solid can be ignored, and the thermal load of the structure to be tested is entirely from the radiation heat transfer. The heating power of the heating assembly of the structure to be tested is measured at this time equivalent to the radiation heat transfer power of the surface of the structure to be measured ;

[0090] S3. Start the temperature control system, and coarsely adjust the cabin gas temperature to the vicinity of the preset value -50℃ (ΔT≤5℃) by adjusting the cooling power, and then finely adjust the cabin gas temperature by dynamically adjusting the heating power of the heating assembly and the electric heating belt of the structure to be measured, until the average temperature of the cabin gas environment reaches the preset value -50℃ (the temperature change range is ≤0.5℃, and the duration is ≥5min, which is the criterion for maintaining stable temperature), the surface temperature of the heat sink heat exchanger and the wind shield, the surface temperature of the structure to be measured, the gas temperature near the surface of the structure to be measured, and the inlet and outlet temperatures of the temperature control subsystem are monitored, and the average temperatures of the heat sink heat exchanger and the wind shield , the surface of the structure to be measured , the average temperature near the surface of the structure to be measured , and the heating power are calculated by using the average method.

[0091] According to the Stefan-Boltzmann law, the radiation heat transfer between the surfaces of objects is only related to the surface emissivity, surface area, surface position relationship and surface temperature. For a specific structure to be measured, when the spatial position and surface temperature are determined, the radiation heat transfer power of the surface of the structure to be measured can be characterized by the surface radiation heat transfer coefficient , and the surface radiation heat transfer coefficient under the condition of a specific spatial position and surface temperature is calculated according to the following formula :

[0092] ;

[0093] S4. In the temperature range of the cabin gas, set multiple different temperatures with a temperature interval of 5℃ preset value T 0 , repeat step S3 to obtain n groups of different surface radiation heat transfer coefficients , and obtain the functional relationship between the surface radiation heat transfer coefficient and the average temperature of the heat sink heat exchanger and the wind shield , the average temperature of the surface of the structure to be measured by fitting:

[0094] ;

[0095] S5. Slowly supplement air in the environmental cabin through the pressure control system, monitor the air pressure in the environmental cabin, adjust the air pressure in the environmental cabin to a predetermined value, such as 500 Pa, adjust the air flow speed in the cabin through the wind speed control subsystem until a predetermined value, such as 2 m / s, and adjust the average temperature of the gas environment in the cabin to a predetermined value, such as -50℃ (±0.5℃) and maintain for at least 5 min, and record the average temperature of the heat sink heat exchanger surface, the average temperature of the surface of the structure to be tested, the average temperature near the surface of the structure to be tested, and the heating power at this time

[0096] The convective heat transfer coefficient of the surface of the structure to be tested under the test working condition of the embodiment is calculated according to the following formula

[0097]

[0098] In the formula, A represents the outer surface area of the part of the structure to be tested in contact with the gas in the cabin.

[0099] S6. Different pressure preset values are selected, and the above step S5 is repeated to obtain the convective heat transfer coefficient test data under different air pressure conditions; in order to reduce the experimental cost, the implementation sequence of different working conditions is performed in order from low to high air pressure.

[0100] Different air temperature and wind speed preset values are selected, and the above steps S1-S6 are repeated to obtain the convective heat transfer coefficient test data under different air pressure, air temperature and wind speed conditions.

[0101] The convective heat transfer characterization method based on the above test method and test data is as follows:

[0102] When the air pressure in the environmental cabin is higher than , in the continuous flow state, for a given working condition, the Nusselt number, Reynolds number, Prandtl number and Grashof number under the corresponding working condition are calculated according to the following formula

[0103]

[0104]

[0105] ​​​​​​​​​​​​​​​​​

[0106] ;

[0107] In the formula, This represents the convective heat transfer coefficient, with units of W / (m²·K). This represents the characteristic length, which is 0.001m in this case. This represents the thermal conductivity of the gas inside the cabin, expressed in W / (m²·K). This indicates the density of the gas inside the cabin, in kg / m³. 3 ; This indicates the gas velocity inside the chamber, in m / s. This indicates the dynamic viscosity of the gas inside the flow chamber, in Pa·s. The constant-pressure specific heat capacity of the gas inside the chamber is expressed in J / (kg·K); g represents the acceleration due to gravity, taken as 9.8 m / s². 2 ; The coefficient of volumetric expansion of the gas inside the cabin, expressed in K. -1 ; This represents the temperature difference between the gas at the surface of the structure under test and at a distance of at least 0.25 m from the surface of the structure under test; The kinematic viscosity of the gas inside the chamber is expressed in m² / s.

[0108] In the slip flow and transition flow regions, the rarefied gas effect has a significant impact on heat transfer and flow. Kn As the dimensionless numbers increase (enhancing the rarefied gas effect), heat transfer efficiency decreases, fluid viscosity decreases, and the continuous medium assumption may even partially fail. Therefore, it is necessary to correct the dimensionless numbers under the continuous medium assumption. The correction methods for each dimensionless number will be introduced below.

[0109] When the air pressure is between and When the flow is in a slip flow state, velocity slip and temperature jumps will occur at the gas-solid interface, requiring modification of the boundary conditions. The following formula is used to adjust the Nusselt number. Reynolds number Prandtl numbers And Grashof Make corrections:

[0110] ;

[0111] ;

[0112] ;

[0113] ;

[0114] In the formula, Knudsen number, taking value between 0.001 and 0.1; a Thin effect correction coefficient of slip flow region, related to gas type and solid surface characteristics, taking value between 0.5 and 1; b Viscosity correction coefficient of slip flow region, related to gas type, taking value between 0.5 and 1; c Test fitting coefficient of slip flow region, taking value between 1 and 2; d Boundary correction coefficient of slip flow region, related to gas type, taking value between 0.2 and 0.5.

[0115] When the ambient pressure is between and , in the transition flow state, the continuum hypothesis is invalid, the Reynolds number and the Grashof number are invalid, the Nusselt number and the Prandtl number are corrected by the following formula:

[0116] ;

[0117] ;

[0118] In the formula, Knudsen number, taking value between 0.1 and 10; α Thin effect correction coefficient of transition flow region, related to gas type and solid surface characteristics, taking value between 1 and 3; β Test fitting coefficient of transition flow region, taking value between 0.3 and 1.

[0119] S7. Obtain a plurality of sets of test data under different pressure, temperature and flow rate conditions according to steps S2-S6, and use neural network fitting to obtain the following function relationship to characterize the convective heat transfer characteristics in the continuous flow state and the slip flow state region, and use neural network fitting to obtain the following function relationship:

[0120] Continuous flow state: ;

[0121] Slip flow region: ;

[0122] Transition flow region: ;

[0123] In the continuous flow state to the transition flow state region, based on the above function relationship, for a specific surface structure, as long as the corresponding temperature, pressure and flow rate condition is given, the corresponding convective heat transfer characteristic coefficient can be obtained.

[0124] In summary, the test device can realize the gas-solid convective heat transfer test under the working condition of pressure 100Pa-101325Pa, gas environment temperature-90℃-90℃, and wind speed 0-10m / s, which covers the working condition range of near space environment and can meet the gas-solid convective heat transfer experimental test requirement in the technical field of near space. Therefore, the application can well meet the heat flow performance test in the near space environment.

[0125] The above is only a preferred embodiment of the present application, and is not used to limit the present application; although the present application is described in detail with reference to the foregoing embodiments, those skilled in the art should understand that any modification, equivalent replacement and improvement within the spirit and principle of the present application should be included in the protection scope of the present application.

Claims

1. A method for testing the convective heat transfer performance of a structured surface under rarefied gas pressure, characterized in that, The test is completed by using a test device, which comprises an environment cabin (11) for providing a test environment for the structure (10) to be tested and a control system for adjusting the test environment parameters in the environment cabin; the control system comprises a structure heating assembly to be tested, a pressure control system, a temperature control system and a wind speed control system, the structure heating assembly to be tested is arranged inside the structure to be tested and is isolated from the space in the environment cabin, the pressure control system adjusts the vacuum degree in the cabin through the air exchange port (20) arranged on the environment cabin body, the temperature control system comprises a heat sink heat exchanger (30) arranged in the environment cabin and a cold and heat source circulation system (40) arranged outside the environment cabin, the wind speed control system comprises a variable frequency motor (51), a fan blade (52) and a fan cover (53), the fan cover (53) is arranged on the periphery of the structure to be tested, the fan blade (52) is connected with the variable frequency motor (51), and the fan blade (52) extends into the inside of the fan cover (53) and blows air towards the structure to be tested (10); The test method comprises the following steps: S1. The Knudsen number is used to classify the rarefaction of the gas, and the critical pressure of the free molecular flow region is calculated according to the following formula : ； wherein represents the Boltzmann constant, having a value of 1.38 x 10 -23 J K -1 ; represents the thermodynamic temperature of the gas inside the environmental chamber; represents the average collision diameter of the gas molecules inside the environmental chamber, obtained by consulting the NIST database, in nm; represents the characteristic length, having a value of 0.001 m, represents the Knudsen number; The value of the calculated critical pressure , 100 times the value of , 10,000 times the value of is: When the air pressure in the environmental cabin is higher than the air in the cabin is considered to be in a continuous flow state; When the pressure in the environment chamber is between and , the gas in the chamber is considered to be in a slip flow regime; When the pressure in the environment cabin is between and , the gas in the cabin is considered to be in a transitional flow state; When the air pressure in the environmental cabin is lower than the gas in the cabin is considered to be in a free molecular flow state; S2. Start the pressure control system to adjust the air pressure in the environmental chamber to below the critical pressure and maintain, measure and record the heating power of the heating assembly of the structure under test At this time equivalent to the radiant heat transfer power of the surface of the structure under test (10) ; S3. Start the cold and heat source circulation system (40), and adjust the average temperature of the cabin gas environment by cooling supply to the preset value and maintain for at least 5 min, and record the average surface temperature of the heat sink heat exchanger (30) and the fan cover (53) at this time , the average surface temperature of the structure to be tested (10) , the average temperature near the surface of the structure to be tested (10) , and the heating power , the surface near the structure to be tested (10) is within 0.01 m from the surface of the structure to be tested (10) The surface radiative heat transfer coefficient is calculated according to the following formula : ； S4. Adjusting the cold and heat source circulating system (40) to obtain n groups of different average temperatures of the cabin gas environment , repeating step S3 to obtain the corresponding n groups of different surface radiation heat transfer coefficients , and obtaining the functional relationship between the surface radiation heat transfer coefficient and the average temperature of the surface of the heat sink heat exchanger (30) and the fan cover (53) , the average temperature of the surface of the structure to be tested (10) ​ ； S5. Adjust the air pressure in the environmental chamber to the predetermined test pressure condition by the pressure control system; adjust the average temperature of the gas environment in the chamber to the predetermined test temperature condition by the method of step S3 and maintain for at least 5 min, and record the average temperature of the surface of the heat sink heat exchanger (30), the average temperature of the surface of the structure to be tested (10), the average temperature near the surface of the structure to be tested (10), and the heating power at this time, respectively ;​​​​​​ The convective heat transfer coefficient of the surface of the structure (10) under test is calculated according to the following formula : ； wherein S represents the outer surface area of the part of the structure (10) in contact with the gas in the cabin. S6. Repeat step S5, and the convective heat transfer coefficient test data under different air pressure, temperature, and wind speed conditions can be obtained; for a given condition, the Nusselt number under the corresponding condition is calculated using the following formula , Reynolds number , Prandtl number , and Grashof number : ； ； ； ； wherein, represents the convective heat transfer coefficient, with the unit of W / (m²·K); represents the characteristic length of the flow field, with the value of 0.001 m; represents the thermal conductivity of the gas in the cabin, with the unit of W / (m²·K); represents the gas density in the cabin, with the unit of kg / m 3 ; represents the gas flow rate in the cabin, with the unit of m / s; represents the dynamic viscosity of the gas in the cabin, with the unit of Pa·s; represents the specific heat capacity at constant pressure of the gas in the cabin, with the unit of J / (kg·K); g represents the gravitational acceleration, with the value of 9.8 m / s 2 ; represents the coefficient of volume expansion of the gas in the cabin, with the unit of K -1 ; represents the difference between the gas temperature at the surface of the structure to be measured and the gas temperature at least 0.25 m away from the surface of the structure to be measured; represents the kinematic viscosity of the gas in the cabin, with the unit of m² / s; S7. The function relationship obtained by using the neural network fitting is as follows: Continuous flow regime: ; Slip flow regime: ; Transition flow regime: ; wherein, a represents the rarefaction effect correction coefficient of the slip flow region, which is related to the gas species and the solid surface characteristics, and takes a value between 0.5 and 1; b represents the viscosity correction coefficient of the slip flow region, which is related to the gas species, and takes a value between 0.5 and 1; c represents the experimental fitting coefficient of the slip flow region, and takes a value between 1 and 2; d represents the boundary correction coefficient of the slip flow region, which is related to the gas species, and takes a value between 0.2 and 0.5; α represents the rarefaction effect correction coefficient of the transition flow region, which is related to the gas species and the solid surface characteristics, and takes a value between 1 and 3; β represents the experimental fitting coefficient of the transition flow region, and takes a value between 0.3 and 1.

2. A method of testing the convective heat transfer performance of a structured surface under rarefied gas pressure as claimed in claim 1, wherein, The fan cover (53) comprises a first fan cover (53a) and a second fan cover (53b), the first fan cover (53a) and the second fan cover (53b) are folded to form a shell with a shape consistent with the shape of the inside of the environment cabin (11), the heat sink heat exchanger (30) is a heat exchanger structure with a rising plate, and the heat sink heat exchanger (30) is arranged at two ends of the first fan cover (53a) and the second fan cover (53b) respectively, and forms a closed hollow cavity structure with the first fan cover (53a) and the second fan cover (53b).

3. A method of testing the convective heat transfer performance of a structured surface under rarefied gas pressure as claimed in claim 2, wherein Air holes (531) are arranged on the first fan cover (53a) and the second fan cover (53b), and the air holes (531) are arranged at the center positions of the connecting lines between the center points of the curved surfaces of the fan covers (53) and the vertexes of the curved surface edges.

4. The method of testing the convective heat transfer performance of a structured surface in a dilute gas pressure according to claim 1, wherein, The pressure control system comprises a vacuumizing branch and a gas supplementing and pressure restoring branch, the gas supplementing and pressure restoring branch comprises, in sequence from the air exchange port (20), a gas source shut-off valve (21), a gas supplementing regulating valve (22) and a gas supplementing cylinder (23), and the vacuumizing branch comprises, in sequence from the air exchange port (20), an air exhaust regulating valve (24), a vacuum pump unit (25) and a check valve (26).

5. A method of testing the convective heat transfer performance of a structured surface under rarefied gas pressure as claimed in claim 2, wherein, The surface of the heat sink heat exchanger (30) is also uniformly provided with an electric heating belt (31).

6. A method of testing the convective heat transfer performance of a structured surface under rarefied gas pressure as claimed in claim 1, wherein, For the function relationship obtained in step S7, a correction method is further included, specifically, For the sliding flow regime, the Nusselt number is modified using the following equation , the Reynolds number , the Prandtl number , and the Grashof number : ； ； ； ； For the transition flow regime, the Reynolds number and the Grashof number Failure, the Nusselt number , Prandtl number are corrected using the following formula: ； ； After correction, Slipstream zone: ; Transition flow region: .

7. A method of testing the convective heat transfer performance of a structured surface under rarefied gas pressure as claimed in claim 6, wherein The specific adjustment method of the temperature in the step S3 is: the temperature of the cabin is coarsely adjusted by adjusting the cooling power, the temperature of the cabin is adjusted to the preset value ± 5K, then the temperature of the cabin is secondarily finely adjusted by adjusting the heating power of the structure heating assembly and the electric heating belt, until the average temperature of the cabin gas environment reaches the preset value ± 0.5K and is maintained for at least 5 min. the preset value ± 0.5K and is maintained for at least 5 min.

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