Long-life operation method of heater for low-density wind tunnel

Abstract

The invention discloses a long-life operation method of a heater for a low-density wind tunnel. The method comprises the following steps: constructing an equivalent model of an operation system of the heater for the low-density wind tunnel; on the basis of the equivalent model, a valve multi-step opening control model in the pressure and temperature adjusting starting stage is built; iteratively solving parameters in the valve multi-step opening control model to obtain an optimal valve multi-step opening control model; and on the basis of the optimal valve multi-step opening control model, the pressure and temperature regulation starting stage of the low-density wind tunnel heater operation system is controlled, and long-life operation of the low-density wind tunnel heater is achieved. The method disclosed by the invention is mainly applied to a pressure-regulating and temperature-regulating starting stage, the pressure-regulating and temperature-regulating starting stage of a heater operation system for the low-density wind tunnel is controlled through an optimal valve multi-step opening control model, and long-life operation of the heater for the low-density wind tunnel is realized; the problem that the graphite heating element is easy to damage under cold airflow thermal vibration and differential pressure coupling impact in the use process of the low-density wind tunnel heater is solved.

Description

Technical Field

[0001] The invention belongs to the technical field of wind tunnel testing, and in particular relates to a long-life operation method for a heater used in a low-density wind tunnel. Background Art

[0002] The low-density wind tunnel uses graphite resistance heater technology to heat nitrogen to obtain a high-temperature pure gas test medium, with a total airflow temperature of more than 1000K. Its operation mode is as follows: (1) Before the test, under vacuum, a low-power electric energy is loaded to the heater, and the heating element is preheated to a high temperature through graphite resistance heating. After preheating, the temperature distribution of the heating element shows a trend of "high temperature at the inlet end - high temperature in the middle - high temperature at the outlet end", for example, "900℃ at the inlet end - 1200℃ in the middle - 900℃ at the outlet end". (2) During the pressure and temperature adjustment start-up stage, the pressure regulating valve is opened to adjust the pressure. After reaching a certain pressure, the heater is loaded with high-power electric energy, and the synchronous cold airflow passes through the inside of the high-temperature heating element, and reaches the heater outlet section through forced convection heat exchange to obtain a high-temperature airflow. (3) After the pressure and temperature are stable, the stable test phase begins. The heater maintains high-power electrical energy loading. Due to the introduction of the cooling effect of the cold air flow, the temperature distribution of the heating element during the stable operation of the test shows a trend of "lower temperature at the inlet - higher temperature in the middle - higher temperature at the outlet", for example, "400°C at the inlet - 900°C in the middle - 1000°C at the outlet". (4) At the end of the test, the heater is turned off, the pressure regulating valve is closed, and the airflow inside the heater is discharged downstream from the nozzle.

[0003] During the temperature and pressure adjustment start-up phase of the low-density wind tunnel, the heating element is prone to crack defects due to the thermal shock of the cold air flow, which in turn shortens its service life. Before the test, the temperature distribution of the heating element showed a trend of "higher temperature at the inlet end - high temperature in the middle - higher temperature at the outlet end". During the stable test phase, the temperature distribution of the heating element showed a trend of "lower temperature at the inlet end - higher temperature in the middle - higher temperature at the outlet end". During the temperature and pressure adjustment phase, the inlet end of the heating element gradually changes from "higher temperature" to "lower temperature", and the air flow temperature here is basically room temperature. Here, it gradually transitions from the initial "large air wall temperature difference" to the "lower air wall temperature difference" state, which causes the inlet section of the heating element to initially generate extremely strong thermal stress and gradually decrease, which is easy to cause crack defects inside the heating element, and then greatly affects the service life of the heating element. At the same time, at the beginning of the temperature and pressure adjustment start-up phase, the internal pressure of the heater is low, and there is a problem of starting shock flow. The cold air flow is large and gradually decreases over time, which has a strong thermal vibration effect on the high-temperature heating element, and also leads to greater thermal stress. These two effects induce the inlet section of the heating element to experience a strong cold air flow thermal shock.

[0004] During the temperature and pressure adjustment start-up phase of the low-density wind tunnel, the heating element is prone to crack defects due to the strong pressure difference impact, which in turn shortens its service life. During the temperature and pressure adjustment start-up phase, the cold air flows through the inlet end of the heating element into the internal channel of the heating element and quickly builds up pressure, usually in a few seconds; afterwards, the higher-pressure airflow in the internal channel of the heating element flows through the micro-slit at the inlet end of the heating element into the blind cavity of the heating element, causing the pressure of the blind cavity of the heater to slowly increase and reach a basic balance after tens of seconds. During this process, the outer sleeve of the heating element is subjected to a strong internal pressure difference and gradually decreases over time, causing the outer sleeve of the heating element to produce large mechanical stress; during the test phase, the pressure of the internal channel of the heating element and the blind cavity of the heater is basically balanced.

[0005] Conventional wind tunnels usually use metal heaters to heat the gas, such as metal thermal storage heaters or metal continuous heaters. Because the total operating temperature is relatively low, usually within 800K, a high-temperature and high-pressure stop valve is usually set downstream of the heater to quickly build up pressure in the wind tunnel. The temperature and pressure adjustment startup operation technology is as follows: before starting the pressure adjustment, close the high-temperature and high-pressure stop valve downstream of the heater, and slowly pre-pressurize the heater; after the pressure in the heater cavity is reached, quickly open the high-temperature and high-pressure stop valve, and synchronously close the loop to quickly adjust the intake pressure regulation system (PID pressure regulation method) to enable the wind tunnel to quickly build up the required total pressure. The conventional wind tunnel has a slow pressurization process in the early stage, and the heater operates in a balanced and stable pressure environment. There is no problem of strong pressure difference shock during the temperature and pressure regulation startup stage of the low-density wind tunnel heater; at the same time, the conventional wind tunnel heater adopts a pre-inflation method, and there is no problem of self-inflation during the temperature and pressure regulation startup stage of the low-density wind tunnel heater. The flow rate in the pressure regulation startup stage is closer to that in the stable operation stage, and there is no problem of thermal vibration shock caused by large cold air flow during the temperature and pressure regulation startup stage of the low-density wind tunnel heater; the energy supply body of the conventional wind tunnel heater is a metal structure (tough material), which has stronger mechanical toughness than the graphite (brittle material) non-metallic energy supply body of the low-density wind tunnel, and its own thermal vibration energy resistance is better, but it does not meet the higher operating total temperature requirements of the low-density wind tunnel heater. Based on the analysis of three aspects, due to the lack of high-temperature and high-pressure stop valves with higher operating temperatures (above 1000K) and the low operating temperature of limited metal materials, the temperature and pressure regulation startup operation technology of conventional wind tunnel heaters cannot be used for the temperature and pressure regulation startup operation of low-density wind tunnel heaters, and cannot solve the problem of short service life caused by thermal vibration of cold air flow and pressure difference coupling impact during the temperature and pressure regulation startup stage of low-density wind tunnel heaters.

[0006] During the use of low-density wind tunnel heaters, during the temperature and pressure adjustment startup stage, the high thermal stress induced by the thermal vibration impact of the cold air flow and the high mechanical stress induced by the pressure difference impact of the cold air flow are coupled, which can easily cause crack defects in the high-temperature graphite heating elements. If its operating technology is not improved, its service life will be only several orders of magnitude. Summary of the invention

[0007] The technology of the present invention solves the problem: overcomes the shortcomings of the prior art and provides a method for long-life operation of a low-density wind tunnel heater. The method is mainly used in the pressure and temperature regulation startup stage. The pressure and temperature regulation startup stage of the low-density wind tunnel heater operation system is controlled through an optimal valve multi-step opening control model to achieve long-life operation of the low-density wind tunnel heater, thereby solving the problem that the graphite heating element is easily damaged under the impact of cold air flow thermal vibration and pressure difference coupling during the use of the low-density wind tunnel heater.

[0008] In order to solve the above technical problems, the present invention discloses a method for long-life operation of a heater for a low-density wind tunnel, comprising:

[0009] Construct an equivalent model of the heater operation system for low-density wind tunnels;

[0010] Based on the equivalent model, a multi-step valve opening control model for the pressure and temperature regulation startup phase is constructed;

[0011] Iteratively solve the parameters in the valve multi-step opening control model to obtain the optimal valve multi-step opening control model;

[0012] Based on the optimal valve multi-step opening control model, the pressure and temperature regulation startup phase of the low-density wind tunnel heater operation system is controlled to achieve long-life operation of the low-density wind tunnel heater.

[0013] In the above-mentioned long-life operation method for a low-density wind tunnel heater, the low-density wind tunnel heater operation system includes: a nitrogen gas source system, a pressure regulating valve, a heater housing, a heater insulation layer, a heating element outer sleeve, a heating element inner sleeve, a heater blind cavity pressure sensor, a heater outlet section, a heater outlet pressure sensor, a nozzle, a test section and a vacuum exhaust system;

[0014] The nitrogen gas source system is connected to the inlet of the pressure regulating valve through the connecting pipe a;

[0015] The outlet of the pressure regulating valve is connected to the inlet end of the outer sleeve of the heating element through a connecting pipe b that passes through the heater shell and the heater insulation layer in sequence; an insulating sealing structure is arranged between the connecting pipe b and the heater shell and the heater insulation layer;

[0016] The inner sleeve of the heating element is supported on the inner surface of the outer sleeve of the heating element by the fins outside the inner sleeve of the heating element, forming a gap air flow channel;

[0017] The outer sleeve of the heating element passes through the insulation layer of the heater, and the outlet end is connected to the heater shell;

[0018] The heater insulation layer is arranged outside the outer sleeve of the heating element, and a gap insulation structure is arranged between the heater insulation layer and the outer sleeve of the heating element, and is supported on the heater shell;

[0019] The inlet of the heater outlet section is connected to the heater shell; the heater outlet section, the nozzle, the test section and the vacuum exhaust system are connected in sequence;

[0020] The heater blind cavity pressure sensor passes through the heater shell and the heater insulation layer and is fixed on the heater shell;

[0021] The heater outlet pressure sensor passes through the heater outlet section shell and is fixed on the heater outlet section shell.

[0022] In the above-mentioned long-life operation method for a heater for a low-density wind tunnel, the equivalent model includes:

[0023] Gas storage capacity, used for equivalent nitrogen gas source system volume and connecting pipeline volume upstream of the pressure regulating valve;

[0024] The throttle throat of the pressure regulating valve is used to equal the throttle area of ​​the pressure regulating valve;

[0025] The volume of the internal passage of the heating element and the outlet section of the heater downstream of the pressure regulating valve is used to equal the volume of the connecting pipe between the outlet of the pressure regulating valve and the outer sleeve of the heating element, the volume of the outer sleeve of the heating element minus the solid area of ​​the inner sleeve of the heating element, the volume of the cavity in the outlet section of the heater, and the volume of the cavity in the front section of the throat of the nozzle;

[0026] The equivalent throttling throat of the micro-slit at the inlet end of the heating element is used to be the equivalent throat of the gap of the connecting structure of the pipe b connecting the outlet of the equivalent pressure regulating valve to the outer sleeve of the heating element;

[0027] The blind cavity volume of the heater is the cavity volume of the closed cavity formed by the outer sleeve of the equivalent heating element and the heater shell minus the solid area of ​​the heater insulation layer;

[0028] Nozzle throttle throat, used for the throat of the equivalent nozzle;

[0029] The vacuum exhaust back pressure chamber is used for the rear section of the equivalent nozzle throat, the test section and the vacuum exhaust system.

[0030] In the above-mentioned long-life operation method for the heater for the low-density wind tunnel, the valve multi-step opening control model is expressed as follows:

[0031]

[0032] Among them, A2 represents the area of ​​the throttling throat of the pressure regulating valve during the start-up stage of temperature and pressure regulation, ε1, ε2, ε3, ε4, ε5, ε6 represent the 1-step, 2-step, 3-step, 4-step, 5-step and 6-step relative openings of the throttling throat of the pressure regulating valve, η represents the opening of the throttling throat of the pressure regulating valve, A 2max Indicates the maximum throttling area of ​​the throttling throat of the pressure regulating valve, τ LIndicates the maintenance time corresponding to the multi-step relative opening of the throttle throat of the pressure regulating valve within the range below the maximum comprehensive stress, τ m Indicates the maintenance time corresponding to the relative opening of the throttle throat of the pressure regulating valve at the maximum comprehensive stress, τ U It indicates the maintenance time corresponding to the multi-step relative opening of the throttling throat of the pressure regulating valve in the range above the maximum comprehensive stress, and t indicates time.

[0033] In the above-mentioned method for long-life operation of a low-density wind tunnel heater, ε1 = 25% ε m , ε2=50%ε m , ε3=75%ε m , ε4=ε m , ε5=50%+50%ε m , ε6=100%; where ε m Indicates the relative opening of the throttling throat of the pressure regulating valve at the maximum comprehensive stress.

[0034] In the long-life operation method for the heater used in the low-density wind tunnel, the parameters in the valve multi-step opening control model are iteratively solved to obtain the optimal valve multi-step opening control model, that is, ε m The optimal solution is to m The optimal solution is substituted into the valve multi-step opening control model to obtain the final optimal valve multi-step opening control model.

[0035] In the above-mentioned method for long-life operation of a low-density wind tunnel heater, ε is obtained as follows m The optimal solution is:

[0036] S31, given ε m The initial value of That is, let

[0037] S32, based on formula (1), is calculated as Under the following conditions, the flow rate of the throttling throat of the pressure regulating valve G2, the flow rate of the equivalent throttling throat of the micro-slit at the inlet end of the heating element G4, the flow rate of the throttling throat of the nozzle G6, the outlet pressure of the throttling throat of the pressure regulating valve P2, the pressure of the internal channel of the heating element downstream of the pressure regulating valve and the outlet section volume of the heater P3, the pressure of the blind cavity volume of the heater P5 and the temperature T3 of the internal channel of the heating element downstream of the pressure regulating valve and the outlet section volume of the heater;

[0038] S33, calculate the graphite temperature T at the inlet end of the outer sleeve of the heating element h ;

[0039] S34, based on the calculation results recorded in step S32 and step S33, the comprehensive stress σ at the inlet end of the outer sleeve of the heating element is calculated;

[0040] S35, σ and set stress threshold For comparison, if Then confirm is m The optimal solution of Then update ε m The value of , returns to steps S31 to S34 until Get ε m The optimal solution

[0041] In the above-mentioned long-life operation method for a low-density wind tunnel heater, based on formula (1), it is calculated that Under the following conditions, the flow rate G2 of the throttling throat of the pressure regulating valve, the flow rate G4 of the equivalent throttling throat of the micro-slit at the inlet end of the heating element, the flow rate G6 of the throttling throat of the nozzle, the outlet pressure P2 of the throttling throat of the pressure regulating valve, the pressure P3 of the internal channel of the heating element downstream of the pressure regulating valve and the outlet section volume of the heater, the pressure P5 of the blind cavity volume of the heater and the temperature T3 of the internal channel of the heating element downstream of the pressure regulating valve and the outlet section volume of the heater, include:

[0042] Determine the pressure P1 of the gas storage volume, the temperature T1 of the gas storage volume, the outlet temperature T2 of the throttling throat of the pressure regulating valve, the throttling area A4 of the equivalent throttling throat of the micro-slit at the inlet end of the heating element, the cross-sectional area A6 of the throttling throat of the nozzle, the test air flow temperature T of the internal channel of the heating element downstream of the pressure regulating valve and the volume requirement of the heater outlet section 3max , the air flow temperature T at the initial moment of the temperature and pressure regulation stage of the internal channel of the heating element downstream of the pressure regulating valve and the outlet section of the heater 30 , the linear temperature adjustment time t of the air flow in the internal channel of the heating element downstream of the pressure regulating valve and the outlet volume of the heater 3max , the volume V3 of the internal passage of the heating element downstream of the pressure regulating valve and the volume of the heater outlet section, the volume V5 of the heater blind cavity volume and the airflow temperature T5 of the heater blind cavity volume;

[0043] Determine the calculation formula for G2, G4, G6, P3 and T3:

[0044]

[0045]

[0046]

[0047] P3=P2…(5)

[0048]

[0049] Where R represents the nitrogen gas constant, and γ represents the nitrogen specific heat ratio;

[0050] The mass conservation equations that determine the volume of the internal passage of the heating element and the outlet section of the heater downstream of the pressure regulating valve are:

[0051]

[0052] The mass conservation equation that determines the volume of the heater blind cavity is:

[0053]

[0054] Will Substitute into formula (1), and according to the determined P1, T1, T2, A4, A6, T3max, T 30 , t3max, V3, V5 and T5, and use the discrete solution method to iteratively solve formulas (2) to (8) to obtain The values ​​of G2, G4, G6, P2, P3, P5 and T3 under the same conditions.

[0055] In the above-mentioned long-life operation method for low-density wind tunnel heaters, the graphite temperature T at the inlet end of the outer sleeve of the heating element is calculated. h ,include:

[0056] According to the thickness δ of the cylindrical wall at the inlet end of the outer sleeve of the heating element, the heat exchange area per unit volume n of the heating element is determined:

[0057]

[0058] According to the heat transfer equivalent diameter d of the gap airflow channel formed by the outer sleeve of the heating element and the inner sleeve of the heating element e , determine the convection heat transfer coefficient α of the inner surface of the inlet end of the outer sleeve of the heating element:

[0059]

[0060] Where λ represents the thermal conductivity of nitrogen, N u The Nusselt number of the airflow in the gap airflow channel formed by the outer sleeve of the heating element and the inner sleeve of the heating element;

[0061]

[0062] Wherein, Pr represents the nitrogen Prandtl number, Re represents the airflow Reynolds number, and L represents the length of the inlet section of the gap airflow channel formed by the outer sleeve of the heating element and the inner sleeve of the heating element;

[0063]

[0064] Wherein, G represents the gas flow rate of the gap gas flow channel formed by the outer sleeve of the heating element and the inner sleeve of the heating element, A represents the cross-sectional area of ​​the gap gas flow channel formed by the outer sleeve of the heating element and the inner sleeve of the heating element, and μ represents the viscosity coefficient of nitrogen;

[0065] G=G2-G4···(13)

[0066] Determine the energy conservation equation at the inlet end of the outer sleeve of the heating element:

[0067]

[0068] Among them, ρ h Indicates the density of graphite material at the inlet end of the outer sleeve of the heating element, C ph Indicates the specific heat capacity of the graphite material at the inlet end of the outer sleeve of the heating element, It represents the electric power density loaded by the power supply at the inlet end of the outer sleeve of the heating element, T g Indicates the air flow temperature at the inlet end of the outer sleeve of the heating element;

[0069] Combining formulas (9) to (13), using G2 and G4 calculated in step S32, we can calculate G, Re, and N. u , α, n, discrete formula (14), given the initial value of the graphite temperature at the inlet end of the outer sleeve of the heating element, T is obtained by iteration h .

[0070] In the above-mentioned long-life operation method for a low-density wind tunnel heater, based on the calculation results of step S32 and step S33, the comprehensive stress σ at the inlet end of the outer sleeve of the heating element is calculated, including:

[0071] Based on the T calculated in step S33 h , combined with the following formulas (15) to (20), the thermal stress σ at the inlet end of the outer sleeve of the heating element is calculated A :

[0072]

[0073]

[0074]

[0075]

[0076]

[0077] D out =D in +2δ···(20)

[0078] Wherein, E represents the Young's modulus of the graphite material of the outer sleeve of the heating element, ε represents the thermal expansion coefficient of the graphite material of the outer sleeve of the heating element, β represents the thermal Poisson's ratio of the graphite material of the outer sleeve of the heating element, and D in Indicates the inner diameter of the cylinder at the inlet end of the outer sleeve of the heating element, D out Indicates the outer diameter of the cylinder at the inlet end of the outer sleeve of the heating element, λ h represents the thermal conductivity of the graphite material of the outer sleeve of the heating element, R1 represents the calculation parameter 1 of the thermal stress of the cylindrical wall of the outer sleeve of the heating element, R2 represents the calculation parameter 2 of the thermal stress of the cylindrical wall of the outer sleeve of the heating element, and R a Indicates the ratio of the diameter of the outer sleeve of the heating element to the cylindrical wall;

[0079] Based on P3 and P5 calculated in step S32, the following formulas (21) to (22) are combined to calculate the mechanical stress σ at the inlet end of the outer sleeve of the heating element: n :

[0080]

[0081] P n =P3-P5···(22)

[0082] Among them, P n Indicates the internal pressure difference at the inlet end of the outer sleeve of the heating element;

[0083] Then, the comprehensive stress σ at the inlet end of the outer sleeve of the heating element is:

[0084] σ=σ A +σ n ···(twenty three).

[0085] The present invention has the following advantages:

[0086] The present invention discloses a method for long-life operation of a heater for a low-density wind tunnel. The method comprises the following steps: quickly evaluating the thermal stress, mechanical stress and comprehensive stress of an outer sleeve at an inlet end of a heating element during the temperature and pressure regulation startup process of the low-density wind tunnel, optimizing and determining the relative opening degree of the maximum comprehensive stress of a multi-step valve opening method; and then based on the optimization and determination of the relative opening degree of the maximum comprehensive stress of the multi-step valve opening method, a long-life operation control strategy for a heater using a multi-step pressure regulating valve opening based on a "shorter-longer-moderate" valve position maintenance time is proposed to replace a single-step valve opening strategy, thereby significantly reducing the comprehensive stress of the outer sleeve at the inlet end of the heating element during the temperature and pressure regulation startup process, avoiding rapid fragmentation and damage of the heating element, and extending the service life of the heater. BRIEF DESCRIPTION OF THE DRAWINGS

[0087] Figure 1 is a flow chart of a method for long-life operation of a heater for a low-density wind tunnel according to an embodiment of the present invention;

[0088] Figure 2 Schematic diagram of the composition of a heater operating system for a low-density wind tunnel according to an embodiment of the present invention;

[0089] Figure 3 is a schematic diagram of an equivalent model in an embodiment of the present invention;

[0090] Figure 4 1 is a schematic diagram of a valve position change curve of a single-step opening valve in an embodiment of the present invention;

[0091] Figure 5 Schematic diagram of a pressure change curve of a single-step opening valve in an embodiment of the present invention;

[0092] Figure 6 It is a schematic diagram of a flow rate change curve of a single-step opening valve in an embodiment of the present invention;

[0093] Figure 7 It is a schematic diagram of a temperature variation curve of an outer sleeve at the inlet end of a heating element of a single-step valve opening embodiment of the present invention;

[0094] Figure 8 It is a schematic diagram of a maximum stress variation curve of an outer sleeve at an inlet end of a heating element of a single-step opening valve in an embodiment of the present invention;

[0095] Fig. 9 1 is a schematic diagram of a valve position change curve of a multi-step opening valve in an embodiment of the present invention;

[0096] Fig.10 Schematic diagram of a pressure change curve of a multi-step opening valve in an embodiment of the present invention;

[0097] Fig.11 It is a schematic diagram of a flow rate variation curve of a multi-step opening valve in an embodiment of the present invention;

[0098] Fig.12 It is a schematic diagram of a temperature variation curve of an outer sleeve at the inlet end of a heating element of a multi-step opening valve in an embodiment of the present invention;

[0099] Fig.13 It is a schematic diagram of a maximum stress variation curve of an outer sleeve at the inlet end of a heating element of a multi-step opening valve in an embodiment of the present invention. DETAILED DESCRIPTION

[0100] In order to make the objectives, technical solutions and advantages of the present invention more clear, the embodiments disclosed in the present invention will be further described in detail below with reference to the accompanying drawings.

[0101] One of the core ideas of the present invention is to disclose a long-life operation method for a heater for a low-density wind tunnel, which adopts a multi-step opening pressure regulating valve operation technology with a "shorter-longer-moderate" valve position maintenance time (i.e., a valve multi-step opening control model), effectively reducing the maximum thermal stress induced by the thermal vibration impact of the cold airflow on the outer sleeve of the inlet end of the heating element during the temperature and pressure regulation startup stage, the maximum mechanical stress induced by the internal pressure difference impact, and the maximum combined stress of the coupled impact of the two, solves the problem of easy crack defects in the outer sleeve of the heating element caused by the thermal vibration of the cold airflow and the coupled impact of the pressure difference, and extends the service life of the heating element.

[0102] Reference Figure 1 In this embodiment, the long-life operation method of the low-density wind tunnel heater includes:

[0103] Step 1: Build an equivalent model of the heater operation system for a low-density wind tunnel.

[0104] In this embodiment, if Figure 2 As shown, the heater operating system for a low-density wind tunnel includes: a nitrogen gas source system 1, a pressure regulating valve 2, a heater shell 3, a heater insulation layer 4, a heating element outer sleeve 5, a heating element inner sleeve 6, a heater blind cavity pressure sensor 7, a heater outlet section 8, a heater outlet pressure sensor 9, a nozzle 10, a test section 11 and a vacuum exhaust system 12. Among them, the nitrogen gas source system 1 is connected to the inlet of the pressure regulating valve 2 through the connecting pipe a; the outlet of the pressure regulating valve 2 is connected to the inlet end of the heating element outer sleeve 5 through the connecting pipe b that passes through the heater shell 3 and the heater insulation layer 4 in sequence; an insulating sealing structure is arranged between the connecting pipe b and the heater shell 3 and the heater insulation layer 4; the heating element inner sleeve 6 is supported on the inner surface of the heating element outer sleeve 5 through the external fins of the heating element inner sleeve 6 to form a gap airflow channel; the heating element outer sleeve 5 passes through the heater insulation layer 4, and the outlet end is connected to the heater shell 3; the heater insulation layer 4 is arranged outside the heating element outer sleeve 5, and a gap insulation structure is arranged between the heater insulation layer 4 and the heating element outer sleeve 5, and supported on the heater shell 3; the inlet of the heater outlet section 8 is connected to the heater shell 3; the heater outlet section 8, the nozzle 10, the test section 11 and the vacuum exhaust system 12 are connected in sequence; the heater blind cavity pressure sensor 7 passes through the heater shell 3 and the heater insulation layer 4, and is fixed on the heater shell 3; the heater outlet pressure sensor 9 passes through the shell of the heater outlet section 8 and is fixed on the shell of the heater outlet section 8.

[0105] like Figure 3As shown in the figure, assuming that the flow velocity in the pipeline of the low-density wind tunnel heater operation system is low and ignoring the air flow resistance loss of the connecting pipelines between structures, the low-density wind tunnel heater operation system is simplified as follows: the gas storage volume 13 upstream of the pressure regulating valve, the pressure regulating valve throttling throat 14, the internal channel of the heating element downstream of the pressure regulating valve and the heater outlet section volume 15, the equivalent throttling throat 16 of the micro-slit at the inlet end of the heating element, the heater blind cavity volume 17, the nozzle throttling throat 18 and the vacuum exhaust back pressure cavity 19. Among them, the gas storage volume 13 is used to equal the volume of the nitrogen gas source system 1 and the connecting pipeline volume upstream of the pressure regulating valve 2. The pressure regulating valve throttling throat 14 is used to equal the throttling area of ​​the pressure regulating valve 2. The internal channel of the heating element downstream of the pressure regulating valve and the volume of the heater outlet section 15 are used to equalize the volume of the connecting pipe between the outlet of the pressure regulating valve 2 and the outer sleeve 5 of the heating element, the volume of the outer sleeve 5 of the heating element minus the solid area of ​​the inner sleeve 6 of the heating element, the cavity volume of the heater outlet section 8, and the cavity volume of the front section of the throat of the nozzle 10. The equivalent throttling throat 16 of the micro-slit at the inlet end of the heating element is used to equalize the throat of the connecting structure gap of the pipe b connecting the outlet of the pressure regulating valve 2 to the outer sleeve 5 of the heating element. The blind cavity volume 17 of the heater is used to equalize the cavity volume of the closed cavity formed by the outer sleeve 5 of the heating element and the heater shell 3 minus the solid area of ​​the heater insulation layer 4. The throttling throat 18 of the nozzle is used to equalize the throat of the nozzle 10. The vacuum exhaust back pressure chamber 19 is used to equalize the rear section of the throat of the nozzle 10, the test section 11 and the vacuum exhaust system 12.

[0106] Step 2: Based on the equivalent model, a multi-step valve opening control model for the pressure and temperature regulation startup phase is constructed.

[0107] In this embodiment, the initially constructed valve multi-step opening control model is expressed as follows:

[0108]

[0109] Wherein, A2 represents the area of ​​the throttle throat 14 of the pressure regulating valve during the start-up phase of temperature and pressure regulation; ε1, ε2, ε3, ε4, ε5, ε6 represent the 1-step, 2-step, 3-step, 4-step, 5-step and 6-step relative openings of the throttle throat 14 of the pressure regulating valve, respectively; η represents the opening of the throttle throat 14 of the pressure regulating valve, which is determined by the total pressure required by the wind tunnel test and is usually 10% to 90%; A 2max Represents the maximum throttling area of ​​the throttling throat 14 of the pressure regulating valve, which is determined by the selected structure of the pressure regulating valve 2; τ L It indicates the maintenance time corresponding to the multi-step relative opening of the throttle throat 14 of the pressure regulating valve within the range below the maximum comprehensive stress, which is usually a short time, such as 5s; τ m The relative opening of the throttle throat 14 of the pressure regulating valve at the maximum comprehensive stress corresponds to the maintenance time, which is a relatively long time, for example 20s; τ UIt indicates the maintenance time corresponding to the multi-step relative opening of the throttling throat 14 of the pressure regulating valve in the range above the maximum comprehensive stress, which is a moderate time length, for example 10s; t indicates time.

[0110] Furthermore, ε1 = 25% ε m , ε2=50%ε m , ε3=75%ε m , ε4=ε m , ε5=50%+50%ε m , ε6=100%; where ε m It indicates the relative opening of the throttle throat 14 of the pressure regulating valve when the maximum comprehensive stress occurs.

[0111] Step 3, iteratively solving the parameters in the valve multi-step opening control model to obtain the optimal valve multi-step opening control model.

[0112] In this embodiment, as mentioned above, ε in the above formula (1) is m Unknown, the purpose of this step is to determine ε m The optimal solution is to m The optimal solution of Substituting into the above formula (1), the final optimal valve multi-step opening control model can be obtained:

[0113]

[0114] Preferably, ε can be obtained by the following method: m The optimal solution is:

[0115] S31, given ε m The initial value of That is, let

[0116] S32, based on formula (1), is calculated as Under the following conditions, the flow rate G2 of the throttling throat 14 of the pressure regulating valve, the flow rate G4 of the equivalent throttling throat 16 of the micro-slit at the inlet end of the heating element, the flow rate G6 of the throttling throat 18 of the nozzle, the outlet pressure P2 of the throttling throat 14 of the pressure regulating valve, the pressure P3 of the internal channel of the heating element downstream of the pressure regulating valve and the volume 15 of the heater outlet section, the pressure P5 of the blind cavity volume 17 of the heater and the temperature T3 of the internal channel of the heating element downstream of the pressure regulating valve and the volume 15 of the heater outlet section.

[0117] First, determine the pressure P1 of the gas storage volume 13 (determined by the test state and the gas source capacity, usually 1.0-1.8 MPa), the temperature T1 of the gas storage volume 13 (approximately 288K), the outlet temperature T2 of the pressure regulating valve throttling throat 14 (approximately 288K), the throttling area A4 of the equivalent throttling throat 16 of the micro-slit at the inlet end of the heating element (determined by the gap characteristics of the connection structure of the pipe b connecting the outlet of the pressure regulating valve to the outer sleeve of the heating element, usually equivalent to the cross-sectional area of ​​2-10 mm micropores), the cross-sectional area A6 of the nozzle throttling throat 18 (determined by the nozzle structure selected by the wind tunnel test state, usually the cross-sectional area of ​​1-50 mm holes), the internal channel of the heating element downstream of the pressure regulating valve and the required test air flow temperature T of the heater outlet section volume 15 3max (usually 900-1400K), the internal channel of the heating element downstream of the pressure regulating valve and the volume of the heater outlet section 15 The air flow temperature T at the initial moment of the temperature and pressure regulation stage 30 (approximately 288K), the linear temperature adjustment time t of the air flow in the inner channel of the heating element downstream of the pressure regulating valve and the outlet section of the heater with a volume of 15 3max (determined by the test state and heater characteristics, usually 20 to 40s), the volume V3 of the internal channel of the heating element downstream of the pressure regulating valve and the volume 15 of the heater outlet section (determined by the heater structure, usually 0.01 to 0.05m 3 ), the volume V5 of the heater blind cavity 17 (determined by the heater structure, usually 0.05 to 0.5 m 3 ) and the air flow temperature T5 (approximately 288K) in the heater blind cavity volume 17.

[0118] Furthermore, the calculation formulas for G2, G4, G6, P3 and T3 are determined as follows:

[0119]

[0120]

[0121]

[0122] P3=P2···(5)

[0123]

[0124] Wherein, R represents the nitrogen gas constant, and γ represents the nitrogen specific heat ratio.

[0125] Furthermore, the mass conservation equation for the internal passage of the heating element downstream of the pressure regulating valve and the volume 15 of the heater outlet section is determined:

[0126]

[0127] Furthermore, the mass conservation equation of the heater blind cavity volume 17 is determined as follows:

[0128]

[0129] Finally, Substitute into formula (1), and according to the determined P1, T1, T2, A4, A6, T 3max 、T 30 ,t 3max , V3, V5 and T5, and iteratively solve formulas (2) to (8) using a discrete solution method to obtain The values ​​of G2, G4, G6, P2, P3, P5 and T3 under the same conditions.

[0130] S33, calculate and obtain the graphite temperature T at the inlet end of the outer sleeve 5 of the heating element h .

[0131] First, according to the thickness of the cylindrical wall δ (usually 0.002-0.015 m) at the inlet end of the outer sleeve 5 of the heating element, the heat exchange area per unit volume n of the heating element is determined:

[0132]

[0133] Further, according to the heat transfer equivalent diameter d of the gap airflow channel formed by the heating element outer sleeve 5 and the heating element inner sleeve 6 e (determined by the heater structure, usually 0.01 to 0.06 m), determine the convection heat transfer coefficient α of the inner surface of the inlet end of the outer sleeve 5 of the heating element:

[0134]

[0135]

[0136]

[0137] G=G2-G4···(13)

[0138] Where λ represents the thermal conductivity of nitrogen, N u represents the Nusselt number of the gap airflow channel formed by the outer sleeve 5 of the heating element and the inner sleeve 6 of the heating element, Pr represents the nitrogen Prandtl number, Re represents the airflow Reynolds number, L represents the aspect ratio of the inlet section of the gap airflow channel formed by the outer sleeve 5 of the heating element and the inner sleeve 6 of the heating element, G represents the gas flow rate of the gap airflow channel formed by the outer sleeve 5 of the heating element and the inner sleeve 6 of the heating element, A represents the cross-sectional area of ​​the gap airflow channel formed by the outer sleeve 5 of the heating element and the inner sleeve 6 of the heating element (determined by the heater structure, 0.0005~0.005m 2 ), μ represents the viscosity coefficient of nitrogen.

[0139] Furthermore, the energy conservation equation at the inlet end of the outer sleeve 5 of the heating element is determined:

[0140]

[0141] Among them, ρ h represents the density of graphite material at the inlet end of the outer sleeve 5 of the heating element, C ph represents the specific heat capacity of the graphite material at the inlet end of the outer sleeve 5 of the heating element, represents the electric power density loaded by the power supply at the inlet end of the outer sleeve 5 of the heating element (determined by the required test state and heater characteristics), T g Indicates the air flow temperature at the inlet end of the outer sleeve 5 of the heating element (288K).

[0142] Finally, by combining formulas (9) to (13), G2 and G4 calculated in step S32 are used to calculate G, Re, and N. u , α, n, discrete formula (14), given the initial value of the graphite temperature at the inlet end of the outer sleeve 5 of the heating element (determined by the test state and the heater structure, usually 900-1800K), T is obtained by iteration h .

[0143] S34, based on the calculation results recorded in step S32 and step S33, the comprehensive stress σ at the inlet end of the outer sleeve 5 of the heating element is calculated.

[0144] First, based on the T calculated in step S33 h , combined with the following formulas (15) to (20), the thermal stress σ at the inlet end of the outer sleeve 5 of the heating element is calculated and determined A :

[0145]

[0146]

[0147]

[0148]

[0149]

[0150] D out =D in +2δ···(20)

[0151] Wherein, E represents the Young's modulus of the graphite material of the outer sleeve 5 of the heating element, ε represents the thermal expansion coefficient of the graphite material of the outer sleeve 5 of the heating element, β represents the thermal Poisson's ratio of the graphite material of the outer sleeve 5 of the heating element, and D inD represents the inner diameter of the cylinder at the inlet end of the outer sleeve 5 of the heating element (determined by the heater structure, usually 0.05 to 0.3 m), out represents the outer diameter of the cylinder at the inlet end of the outer sleeve 5 of the heating element, λ h represents the thermal conductivity of the graphite material of the outer sleeve 5 of the heating element, R1 represents the calculation parameter 1 of the thermal stress of the cylindrical wall of the outer sleeve 5 of the heating element, R2 represents the calculation parameter 2 of the thermal stress of the cylindrical wall of the outer sleeve 5 of the heating element, and R a It represents the ratio of the cylindrical wall diameter of the outer sleeve 5 of the heating element.

[0152] Further, based on P3 and P5 calculated in step S32, the following formulas (21) to (22) are combined to calculate the mechanical stress σ at the inlet end of the outer sleeve 5 of the heating element: n :

[0153]

[0154] P n =P3-P5···(22)

[0155] Among them, P n Represents the internal pressure difference at the inlet end of the outer sleeve 5 of the heating element.

[0156] Then, the comprehensive stress σ at the inlet end of the outer sleeve 5 of the heating element is:

[0157] σ=σ A +σ n ···(twenty three)

[0158] S35, compare σ with the set stress threshold σ. Then confirm is m The optimal solution of Then update ε m The value of , returns to steps S31-34 until Get ε m The optimal solution In the update ε m When taking the value of , +2% each time: ····.

[0159] Step 4: Based on the optimal valve multi-step opening control model, the pressure and temperature adjustment startup phase of the low-density wind tunnel heater operation system is controlled to achieve long-life operation of the low-density wind tunnel heater.

[0160] In this embodiment, the specific application of the method of the present invention is as follows:

[0161] a) Determine the heater preheating temperature and pressure regulating valve opening according to the test requirements;

[0162] b) using the method of the present invention to determine the optimal valve multi-step opening control model, as shown in the above formula (24);

[0163] c) Preheat the heater to the required temperature;

[0164] d) In the range below the maximum comprehensive stress, use multi-step equidistant relative opening The valve position opening mode is different from the conventional one, and each valve position adopts a shorter valve position holding time;

[0165] e) When the maximum comprehensive stress is reached, the relative opening is used. The valve position opening mode adopts a longer valve position holding time;

[0166] f) In the range above the maximum comprehensive stress, use multi-step equidistant relative opening ( , 100%) valve position opening mode, each valve position adopts moderate valve position maintenance time;

[0167] g) After reaching the preset maximum opening required for pressure regulation, the fixed valve position is adopted or switched to the PID feedback pressure regulation control strategy to stabilize the nozzle inlet pressure;

[0168] h) After the nozzle inlet pressure is stable, start the test and measure the model test data within the effective test time;

[0169] i) After the test, close the pressure regulating valve.

[0170] In this embodiment, when the single-step valve opening method is adopted, the valve throttling area change curve is as follows: Figure 4 During the temperature and pressure adjustment startup process, the single-step valve opening method is adopted, and the obtained P3, P5, and P n The pressure change curve is as follows Figure 5 As shown, the flow change curves of G2, G4, G6, G, etc. are as follows Figure 6 As shown, the temperature variation curve of the outer sleeve at the inlet end of the heating element is as follows Figure 7 As shown, the maximum stress variation of the outer sleeve at the inlet end of the heating element is as follows Figure 8 As shown above, when the temperature and pressure regulating startup process adopts the single-step valve opening method, the maximum thermal stress of the outer sleeve at the inlet end of the heating element is about 76MPa, the maximum mechanical stress reaches 14MPa, and the maximum comprehensive stress reaches 79MPa. This stress exceeds the material strength of the outer sleeve of the heating element, which will cause the outer sleeve of the heating element to break and damage.

[0171] In this embodiment, the method of the present invention is used, τ L =5s, τ m =20s, τ U = 10s, after multiple optimizations, ε m The optimal solution The corresponding valve throttling area change curve is as follows: Fig. 9 During the temperature and pressure adjustment startup process, the multi-step valve opening method (i.e., the optimal valve multi-step opening control model) described in the present invention is adopted to obtain P3, P5, P n The pressure change curve is as follows Fig.10 As shown, the flow change curves of G2, G4, G6, G, etc. are as follows Fig.11 As shown, the temperature variation curve of the outer sleeve at the inlet end of the heating element is as follows Fig.12 As shown, the maximum stress variation of the outer sleeve at the inlet end of the heating element is as follows Fig.13 As shown above, when the temperature and pressure adjustment startup process adopts the multi-step valve opening method, the maximum thermal stress of the outer sleeve at the inlet end of the heating element is about 34MPa, the maximum mechanical stress reaches 10MPa, and the maximum comprehensive stress reaches 41MPa. The maximum comprehensive stress of the outer sleeve at the inlet end of the heating element is reduced from 79MPa in a single step to 41MPa in multiple steps, which meets the material use requirements of the heating element, avoids rapid damage to the heating element, and extends the service life of the heater.

[0172] Although the present invention has been disclosed as above in the form of a preferred embodiment, it is not intended to limit the present invention. Any person skilled in the art may make possible changes and modifications to the technical solution of the present invention by using the methods and technical contents disclosed above without departing from the spirit and scope of the present invention. Therefore, any simple modifications, equivalent changes and modifications made to the above embodiments based on the technical essence of the present invention without departing from the content of the technical solution of the present invention shall fall within the protection scope of the technical solution of the present invention.

[0173] The contents not described in detail in the specification of the present invention belong to the common knowledge of the professionals in this field.

Claims

1. A method for long-life operation of a heater for a low-density wind tunnel, characterized in that: include: Construct an equivalent model of the heater operation system for low-density wind tunnels; Based on the equivalent model, a multi-step valve opening control model for the pressure and temperature regulation startup phase is constructed; Iteratively solve the parameters in the valve multi-step opening control model to obtain the optimal valve multi-step opening control model; Based on the optimal valve multi-step opening control model, the pressure and temperature regulation startup phase of the low-density wind tunnel heater operation system is controlled to achieve long-life operation of the low-density wind tunnel heater.

2. The method for long-life operation of a heater for a low-density wind tunnel according to claim 1, characterized in that: A heater operating system for a low-density wind tunnel, comprising: a nitrogen gas source system (1), a pressure regulating valve (2), a heater housing (3), a heater insulation layer (4), a heating element outer sleeve (5), a heating element inner sleeve (6), a heater blind cavity pressure sensor (7), a heater outlet section (8), a heater outlet pressure sensor (9), a nozzle (10), a test section (11) and a vacuum exhaust system (12); The nitrogen gas source system (1) is connected to the inlet of the pressure regulating valve (2) through a connecting pipe a; The outlet of the pressure regulating valve (2) is connected to the inlet end of the outer sleeve (5) of the heating element via a connecting pipe b that passes through the heater shell (3) and the heater insulation layer (4) in sequence; an insulating sealing structure is provided between the connecting pipe b and the heater shell (3) and the heater insulation layer (4); The heating element inner sleeve (6) is supported on the inner surface of the heating element outer sleeve (5) by the external fins of the heating element inner sleeve (6), so as to form a gap air flow channel; The outer sleeve (5) of the heating element passes through the heater insulation layer (4), and the outlet end is connected to the heater shell (3); The heater insulation layer (4) is arranged outside the outer sleeve (5) of the heating element, a gap insulation structure is arranged between the heater insulation layer (4) and the outer sleeve (5) of the heating element, and the gap insulation structure is supported on the heater shell (3); The inlet of the heater outlet section (8) is connected to the heater housing (3); the heater outlet section (8), the nozzle (10), the test section (11) and the vacuum exhaust system (12) are connected in sequence; The heater blind cavity pressure sensor (7) passes through the heater shell (3) and the heater insulation layer (4), and is fixed on the heater shell (3); The heater outlet pressure sensor (9) passes through the shell of the heater outlet section (8) and is fixed on the shell of the heater outlet section (8).

3. The method for long-life operation of a heater for a low-density wind tunnel according to claim 2, characterized in that: Equivalent models include: The gas storage volume (13) is used to equalize the volume of the nitrogen gas source system (1) and the volume of the connecting pipeline upstream of the pressure regulating valve (2); A throttle throat (14) of the pressure regulating valve is used to equal the throttle area of ​​the pressure regulating valve (2); The internal passage of the heating element downstream of the pressure regulating valve and the volume of the heater outlet section (15) are used to equalize the volume of the connecting pipe between the outlet of the pressure regulating valve (2) and the outer sleeve (5) of the heating element, the volume of the outer sleeve (5) of the heating element minus the solid area of ​​the inner sleeve (6) of the heating element, the volume of the inner cavity of the heater outlet section (8), and the volume of the inner cavity of the throat front section of the nozzle (10); The equivalent throttling throat (16) of the micro-slit at the inlet end of the heating element is used as the equivalent throat of the gap of the connecting structure of the connecting pipe b from the outlet of the pressure regulating valve (2) to the outer sleeve (5) of the heating element; The blind cavity volume (17) of the heater is used to deduct the cavity volume of the solid area of ​​the heater insulation layer (4) from the closed cavity formed by the equivalent heating element outer sleeve (5) and the heater shell (3); A nozzle throttle throat (18) is used as a throat of an equivalent nozzle (10); The vacuum exhaust back pressure chamber (19) is used for the throat rear section of the equivalent nozzle (10), the test section (11) and the vacuum exhaust system (12).

4. The method for long-life operation of a heater for a low-density wind tunnel according to claim 3, characterized in that: The valve multi-step opening control model is expressed as follows: Wherein, A2 represents the area of ​​the throttle throat (14) of the pressure regulating valve during the start-up phase of temperature and pressure regulation, ε1, ε2, ε3, ε4, ε5, ε6 represent the 1-step, 2-step, 3-step, 4-step, 5-step and 6-step relative openings of the throttle throat (14) of the pressure regulating valve, η represents the opening of the throttle throat (14) of the pressure regulating valve, A 2max represents the maximum throttling area of ​​the throttling throat (14) of the pressure regulating valve, τ L It represents the maintenance time corresponding to the multi-step relative opening of the throttle throat (14) of the pressure regulating valve below the maximum comprehensive stress, τ m The relative opening of the throttle throat (14) of the pressure regulating valve at the maximum comprehensive stress corresponds to the maintenance time, τ U It represents the maintenance time corresponding to the multi-step relative opening of the throttle throat (14) of the pressure regulating valve in the range above the maximum comprehensive stress, and t represents the time.

5. The method for long-life operation of a heater for a low-density wind tunnel according to claim 4, characterized in that: ε1=25%ε m , ε2=50%ε m , ε3=75%ε m , ε4=ε m , ε5=50%+50%ε m , ε6=100%; where ε m Indicates the relative opening of the throttle throat (14) of the pressure regulating valve when the maximum comprehensive stress occurs.

6. The method for long-life operation of a heater for a low-density wind tunnel according to claim 5, characterized in that: The parameters in the valve multi-step opening control model are iteratively solved to obtain the optimal valve multi-step opening control model, that is, ε m The optimal solution is to m The optimal solution is substituted into the valve multi-step opening control model to obtain the final optimal valve multi-step opening control model.

7. The method for long-life operation of a heater for a low-density wind tunnel according to claim 6, characterized in that: ε is obtained by m The optimal solution is: S31, given ε m The initial value of That is, let S32, based on formula (1), is calculated as Under the following conditions, the flow rate G2 of the throttling throat (14) of the pressure regulating valve, the flow rate G4 of the equivalent throttling throat (16) of the micro-slit at the inlet end of the heating element, the flow rate G6 of the throttling throat (18) of the nozzle, the outlet pressure P2 of the throttling throat (14) of the pressure regulating valve, the pressure P3 of the internal channel of the heating element downstream of the pressure regulating valve and the volume of the heater outlet section (15), the pressure P5 of the blind cavity volume (17) of the heater, and the temperature T3 of the internal channel of the heating element downstream of the pressure regulating valve and the volume of the heater outlet section (15); S33, calculate the graphite temperature T at the inlet end of the outer sleeve (5) of the heating element h ; S34, based on the calculation results recorded in step S32 and step S33, the comprehensive stress σ at the inlet end of the outer sleeve (5) of the heating element is calculated; S35, σ and set stress threshold For comparison, if Then confirm is m The optimal solution of Then update ε m The value of , returns to steps S31 to S34 until Get ε m The optimal solution 8. The method for long-life operation of a heater for a low-density wind tunnel according to claim 7, characterized in that: Based on formula (1), we can calculate Under the following conditions, the flow rate G2 of the throttling throat (14) of the pressure regulating valve, the flow rate G4 of the equivalent throttling throat (16) of the micro-slit at the inlet end of the heating element, the flow rate G6 of the throttling throat (18) of the nozzle, the outlet pressure P2 of the throttling throat (14) of the pressure regulating valve, the pressure P3 of the internal channel of the heating element downstream of the pressure regulating valve and the volume of the heater outlet section (15), the pressure P5 of the blind cavity volume (17) of the heater and the temperature T3 of the internal channel of the heating element downstream of the pressure regulating valve and the volume of the heater outlet section (15), include: Determine the pressure P1 of the gas storage volume (13), the temperature T1 of the gas storage volume (13), the outlet temperature T2 of the throttling throat (14) of the pressure regulating valve, the throttling area A4 of the equivalent throttling throat (16) of the micro-slit at the inlet end of the heating element, the cross-sectional area A6 of the throttling throat (18) of the nozzle, the test air flow temperature T required by the internal channel of the heating element downstream of the pressure regulating valve and the outlet section volume (15) of the heater 3max , the internal channel of the heating element downstream of the pressure regulating valve and the volume of the heater outlet section (15) The air flow temperature T at the initial moment of the temperature and pressure regulating stage 30 , the linear temperature adjustment time t of the air flow in the internal channel of the heating element downstream of the pressure regulating valve and the outlet volume of the heater (15) 3max , the volume V3 of the internal passage of the heating element downstream of the pressure regulating valve and the volume of the heater outlet section (15), the volume V5 of the heater blind cavity volume (17) and the air flow temperature T5 of the heater blind cavity volume (17); Determine the calculation formula for G2, G4, G6, P3 and T3: Where R represents the nitrogen gas constant, and γ represents the nitrogen specific heat ratio; The mass conservation equation for determining the volume of the internal passage of the heating element and the heater outlet section (15) downstream of the pressure regulating valve is: Determine the mass conservation equation for the heater blind cavity volume (17): Will Substitute into formula (1), and according to the determined P1, T1, T2, A4, A6, T 3max 、T 30 ,t 3max , V3, V5 and T5, and iteratively solve formulas (2) to (8) using a discrete solution method to obtain The values ​​of G2, G4, G6, P2, P3, P5 and T3 under the same conditions.

9. The method for long-life operation of a heater for a low-density wind tunnel according to claim 8, characterized in that: The graphite temperature T at the inlet end of the outer sleeve (5) of the heating element is calculated. h ,include: According to the thickness δ of the cylindrical wall at the inlet end of the outer sleeve (5) of the heating element, the heat exchange area per unit volume n of the heating element is determined as follows: According to the heat transfer equivalent diameter d of the gap airflow channel formed by the outer sleeve (5) of the heating element and the inner sleeve (6) of the heating element e , determine the convection heat transfer coefficient α of the inner surface of the inlet end of the outer sleeve (5) of the heating element: Where λ represents the thermal conductivity of nitrogen, N u Indicates the Nusselt number of the air flow in the gap air flow channel formed by the outer sleeve (5) of the heating element and the inner sleeve (6) of the heating element; Wherein, Pr represents the nitrogen Prandtl number, Re represents the airflow Reynolds number, and L represents the length of the inlet section of the gap airflow channel formed by the outer sleeve (5) of the heating element and the inner sleeve (6) of the heating element; Wherein, G represents the gas flow rate of the gap gas flow channel formed by the outer sleeve (5) of the heating element and the inner sleeve (6) of the heating element, A represents the cross-sectional area of ​​the gap gas flow channel formed by the outer sleeve (5) of the heating element and the inner sleeve (6) of the heating element, and μ represents the viscosity coefficient of nitrogen; G=G2-G4···(13) Determine the energy conservation equation at the inlet end of the outer sleeve (5) of the heating element: Among them, ρ h represents the density of graphite material at the inlet end of the outer sleeve (5) of the heating element, C ph represents the specific heat capacity of the graphite material at the inlet end of the outer sleeve (5) of the heating element, represents the electric power density applied by the power supply to the inlet end of the outer sleeve (5) of the heating element, T g Indicates the air flow temperature at the inlet end of the outer sleeve (5) of the heating element; Combining formulas (9) to (13), using G2 and G4 calculated in step S32, we can calculate G, Re, and N. u , α, n, discrete formula (14), given the initial value of the graphite temperature at the inlet end of the outer sleeve (5) of the heating element, T is obtained by iteration h .

10. The method for long-life operation of a heater for a low-density wind tunnel according to claim 8, characterized in that: Based on the calculation results of step S32 and step S33, the comprehensive stress σ at the inlet end of the outer sleeve (5) of the heating element is calculated, including: Based on the T calculated in step S33 h , combined with the following formulas (15) to (20), the thermal stress σ at the inlet end of the outer sleeve (5) of the heating element is calculated A : D out =D in +2δ···(20) Wherein, E represents the Young's modulus of the graphite material of the outer sleeve (5) of the heating element, ε represents the thermal expansion coefficient of the graphite material of the outer sleeve (5) of the heating element, β represents the thermal Poisson's ratio of the graphite material of the outer sleeve (5) of the heating element, and D in Denotes the inner diameter of the cylinder at the inlet end of the outer sleeve (5) of the heating element, D out represents the outer diameter of the cylinder at the inlet end of the outer sleeve (5) of the heating element, λ h represents the thermal conductivity of the graphite material of the outer sleeve (5) of the heating element, R1 represents the calculation parameter 1 of the thermal stress of the cylindrical wall of the outer sleeve (5) of the heating element, R2 represents the calculation parameter 2 of the thermal stress of the cylindrical wall of the outer sleeve (5) of the heating element, and R a Indicates the ratio of the cylindrical wall diameter of the outer sleeve (5) of the heating element; Based on P3 and P5 calculated in step S32, the following formulas (21) to (22) are combined to calculate the mechanical stress σ at the inlet end of the outer sleeve (5) of the heating element: n : P n =P3-P5···(22) Among them, P n represents the internal pressure difference at the inlet end of the outer sleeve (5) of the heating element; Then, the comprehensive stress σ at the inlet end of the outer sleeve (5) of the heating element is: s = s A +s n ...(23).

Images