A multifunctional experimental system for studying supercritical low-temperature nitrogen flow and heat transfer

Abstract

The present invention relates to the field of research on the flow and heat transfer characteristics of supercritical nitrogen in pipelines, and more particularly to a multifunctional experimental system for studying the flow and heat transfer of supercritical low-temperature nitrogen. The outdoor portion of the supercritical low-temperature nitrogen experimental system includes a liquid nitrogen storage tank for storing liquid nitrogen, a cryogenic pump for pressurizing, a high-pressure vaporizer for vaporizing the liquid nitrogen, a liquid nitrogen thermostat for cooling the nitrogen, a buffer tank for stabilizing pressure and flow, and gas-liquid separation, and a cryogenic and high-pressure pressure reducing valve for regulating outlet pressure, all interconnected by pipelines. The system also includes an automatic control box for controlling electrical components. The indoor portion includes a main regulating valve, a flow meter, a third pressure transmitter, and a third temperature transmitter, all interconnected by pipelines. The experimental system itself can directly provide medium- and low-pressure liquid nitrogen as a cold source for cooling the vaporized nitrogen pipeline, providing a large amount of experimental data support for the development of efficient and compact pre-cooling heat exchangers, and is suitable for widespread promotion.

Description

Technical Field

[0001] The present invention relates to the field of research on flow and heat transfer characteristics of supercritical nitrogen in a tube, and in particular to a multifunctional experimental system for studying flow and heat transfer of supercritical low-temperature nitrogen. Background Art

[0002] Hypersonic technology is a strategic development direction for future military and civilian aircraft, including space-to-orbit vehicles, near-space cruise vehicles, and suborbital launch / delivery vehicles. However, due to the extremely high requirements placed on the propulsion system for hypersonic vehicles, such as horizontal takeoff and landing, reusability, compact structure, lightweight, and an extremely wide airspace and speed range, a single engine cannot meet these requirements and achieve stable operation across the entire speed range. This is because hypersonic vehicles experience severe aerodynamic heating effects when flying at high Mach numbers, placing high heat loads on the vehicle's exterior surfaces. Internal systems such as electronic equipment and the combustion chamber also generate significant heat loads. If this excessive heat load cannot be properly cooled, it will significantly reduce the efficiency of equipment such as compressors and pumps, further degrading the vehicle's performance and creating a vicious cycle. Therefore, the development of precooling technology has created opportunities for breakthroughs in hypersonic engine technology. Precooling heat exchangers are one of the core technologies of precooling. Precooling cycle engines utilize supercritical cryogenic fluids to quickly cool incoming air at 1000°C to ambient or even sub-zero temperatures, effectively extending the engine's operating range and further expanding the flight corridor.

[0003] Supercritical nitrogen is a fluid in a special state between gas and liquid. It has the dual properties and advantages of gas and liquid. However, experimental research on the heat transfer characteristics of supercritical nitrogen in microchannels is still insufficient. The experimental data provided in existing literature is limited and poorly systematic. The main reasons may include the low critical point temperature of supercritical nitrogen, approximately 126.2K, which is difficult to cool at such a low temperature. If a traditional refrigerator is used to cool the nitrogen working fluid, the required refrigerator power is large and difficult to obtain while meeting the cooling temperature, and the efficiency is low. Even if a high-power refrigerator is purchased, it is very expensive, making the cost of setting up the experimental system too high and the effect poor.

[0004] After investigation, it is found that there is a lack of experimental systems that can provide dry low-temperature nitrogen close to the critical point and directly participate in the heat exchange cycle. Therefore, it is necessary to construct a supercritical low-temperature nitrogen supply experimental system to further provide a large amount of experimental data support for the development of efficient and compact pre-cooling heat exchangers. Moreover, the experimental system itself can directly provide liquid nitrogen and low-temperature media within the liquid nitrogen temperature range, which can be used as a cold source for other closed cycles. Summary of the Invention

[0005] The present invention proposes a multifunctional experimental system for studying the flow and heat transfer of supercritical low-temperature nitrogen, which provides a large amount of experimental data support for the development of efficient and compact pre-cooling heat exchangers. Moreover, the experimental system itself can directly provide high-pressure liquid nitrogen, which can be used as a cold source for other closed cycles, thus solving the above-mentioned problems.

[0006] In order to achieve the above-mentioned purpose, the present invention adopts the following technical scheme: a multifunctional experimental system for studying the flow and heat transfer of supercritical low-temperature nitrogen, comprising an outdoor supercritical low-temperature nitrogen experimental system and an indoor supercritical low-temperature nitrogen experimental system.

[0007] The outdoor partial supercritical low-temperature nitrogen experimental system includes a liquid nitrogen storage tank for storing liquid nitrogen, a low-temperature pump for pressurizing, a high-pressure vaporizer for vaporizing liquid nitrogen, a liquid nitrogen constant temperature box for cooling nitrogen, a buffer tank for stabilizing pressure, flow and gas-liquid separation, and a low-temperature and high-pressure pressure reducing valve for regulating outlet pressure, which are sequentially connected by pipelines, and also includes an automatic control box for controlling electrical components;

[0008] The indoor partial supercritical low-temperature nitrogen experimental system includes a main regulating valve, a flow meter, a third pressure transmitter and a third temperature transmitter which are sequentially connected through pipelines.

[0009] Preferably, a safety valve is connected to the front end of the main regulating valve, the safety valve is connected to the low-temperature and high-pressure reducing valve, a branch pipeline is connected between the main regulating valve and the flow meter, a branch regulating valve is connected to the branch pipeline, and a stop valve is connected between the flow meter and the third pressure transmitter.

[0010] Preferably, a first temperature transmitter and a first pressure transmitter electrically connected to the automatic control box are connected between the cryogenic pump and the high-pressure vaporizer, and a second temperature transmitter and a second pressure transmitter electrically connected to the automatic control box are also connected to the pipeline connected to the outlet of the cryogenic and high-pressure reducing valve.

[0011] Preferably, the liquid nitrogen constant temperature box includes a box body for containing liquid nitrogen for cooling, and a liquid nitrogen spray system is provided in the box body. The liquid nitrogen spray system is composed of a plurality of nozzles connected in sequence from bottom to top to form a cavity, and the vaporized nitrogen pipeline spirals through the cavity formed by the plurality of nozzles of the liquid nitrogen spray system from bottom to top.

[0012] Preferably, the liquid nitrogen spray system is connected to the liquid nitrogen storage tank through a pipeline, and a pneumatic regulating valve is further connected to the pipeline; the box body is connected to the liquid nitrogen storage tank through a pipeline, and a first solenoid valve is further connected to the pipeline; the bottom of the liquid nitrogen constant temperature box is connected to a manual drain valve through a pipeline; the bottom of the liquid nitrogen constant temperature box is also connected to a residual liquid discharge tank through a pipeline, and a second solenoid valve is further connected to the pipeline, and a tail nitrogen vent is provided on the residual liquid discharge tank; and a liquid level gauge is provided on the top of the liquid nitrogen constant temperature box.

[0013] Preferably, the safety valve is directly connected to the low-temperature and high-pressure reducing valve via a fourth solenoid valve, a pressure relief pipeline is connected between the fourth solenoid valve and the second temperature transmitter, a third solenoid valve is connected to the pressure relief pipeline, and the end of the pressure relief pipeline is connected in sequence to a manually adjustable vent valve and a muffler, and the outlet of the muffler is a debugging vent.

[0014] The beneficial effects of the present invention are:

[0015] The present invention provides a multifunctional experimental system for studying the flow and heat transfer of supercritical low-temperature nitrogen, comprising an outdoor supercritical low-temperature nitrogen experimental system and an indoor supercritical low-temperature nitrogen experimental system. The outdoor supercritical low-temperature nitrogen experimental system comprises a liquid nitrogen storage tank for storing liquid nitrogen, a cryogenic pump for pressurizing, a high-pressure vaporizer for vaporizing liquid nitrogen, a liquid nitrogen constant temperature box for cooling nitrogen, a buffer tank, and a low-temperature and high-pressure reducing valve, all of which are sequentially connected by pipelines. Furthermore, the system also comprises an automatic control box for controlling electrical components. The indoor supercritical low-temperature nitrogen experimental system comprises a main regulating valve, a flow meter, a third pressure transmitter, and a third temperature transmitter, all of which are sequentially connected by pipelines. A safety valve is connected to the front end of the main regulating valve, the safety valve is connected to the low-temperature and high-pressure reducing valve, a branch pipeline is connected between the main regulating valve and the flow meter, the branch pipeline is connected to the branch regulating valve, and a stop valve is connected between the flow meter and the pressure transmitter. The liquid nitrogen thermostat consists of a chamber containing liquid nitrogen for cooling. This chamber houses a liquid nitrogen spray system, which consists of a series of nozzles connected from bottom to top to form a cavity. The vaporized nitrogen pipeline swirls through this cavity. The experimental system itself can directly provide low- and medium-pressure liquid nitrogen, which serves as the cooling source for cooling the vaporized nitrogen pipeline. This system provides extensive experimental data supporting the development of efficient and compact pre-cooling heat exchangers, making it suitable for widespread adoption. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Figure 1 Schematic diagram of the structure of the outdoor supercritical low-temperature nitrogen experimental system.

[0017] Figure 2 Schematic diagram of the structure of the indoor supercritical low-temperature nitrogen experimental system.

[0018] Figure 3It is a schematic structural diagram of the liquid nitrogen constant temperature box of the present invention.

[0019] Figure 4 This is a schematic diagram of the electrical connection between the automatic control box and electrical components of the present invention.

[0020] In the figure: 1-liquid nitrogen storage tank, 2-cryogenic pump, 3-high-pressure vaporizer, 4-liquid nitrogen constant temperature box, 5-buffer tank, 6-cryogenic and high-pressure reducing valve, 7-automatic control box, 8-safety valve, 9-main regulating valve, 10-flow meter, 11-third pressure transmitter, 12-third temperature transmitter, 13-branch regulating valve, 15-stop valve, 16-first temperature transmitter, 17-first pressure transmitter, 18-second temperature transmitter, 19-second pressure transmitter, 20-pneumatic regulating valve, 21-first solenoid valve, 22-manual drain valve, 23-residual liquid discharge box, 24-second solenoid valve, 25-liquid level gauge, 26-fourth solenoid valve, 27-third solenoid valve, 28-manually adjustable vent valve, 29-muffler, 30-first liquid inlet valve, 31-second liquid inlet valve, 32-return air valve, 41-tank, 42-liquid nitrogen spray system. DETAILED DESCRIPTION

[0021] The technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, rather than all the embodiments.

[0022] Reference Figure 1-4 A multifunctional experimental system for studying the flow and heat transfer of supercritical low-temperature nitrogen includes an outdoor supercritical low-temperature nitrogen experimental system and an indoor supercritical low-temperature nitrogen experimental system. The outdoor supercritical low-temperature nitrogen experimental system includes a liquid nitrogen storage tank 1 for storing liquid nitrogen, a low-temperature pump 2 for pressurization, a high-pressure vaporizer 3 for vaporizing liquid nitrogen, a liquid nitrogen constant temperature box 4 for cooling nitrogen, a buffer tank 5 for stabilizing pressure, flow and gas-liquid separation, and a low-temperature and high-pressure reducing valve 6 for regulating outlet pressure, and also includes an automatic control box 7 for controlling electrical components, and the automatic control box 7 is provided with a PLC and a touch control screen; the indoor supercritical low-temperature nitrogen experimental system includes a main regulating valve 9, a flow meter 10, a third pressure transmitter 11 and a third temperature transmitter 12, which are sequentially connected through pipelines. The front end of the main regulating valve 9 is connected to a safety valve 8, which is connected to the low-temperature high-pressure reducing valve 6. A branch line is connected between the main regulating valve 9 and the flow meter 10, and a branch regulating valve 13 is connected to the branch line. A stop valve 15 is connected between the flow meter 10 and the third pressure transmitter 11. The volume of the liquid nitrogen storage tank 1 is 5m 3 , the maximum allowable working pressure is 1.6MPa; the vaporization capacity of high pressure carburetor 3 is 350Nm 3 / h, maximum working pressure 15MPa; cryogenic pump 2 is a variable frequency speed regulating piston pump, 50-150L / min, output control pressure 6MPa; buffer tank 5 has a volume of 1m 3 , the maximum allowable working pressure is 7MPa, the outer shell insulation material is S30408. Liquid nitrogen constant temperature box 4, the temperature setting range is -180 to -140℃, the material is stainless steel inner and outer shell.

[0023] A first temperature transmitter 16 and a first pressure transmitter 17, both electrically connected to the automatic control box 7, are connected between the cryopump 2 and the high-pressure vaporizer 3. A second temperature transmitter 18 and a second pressure transmitter 19, both electrically connected to the automatic control box 7, are also connected to the pipeline connected to the outlet of the cryogenic and high-pressure reducing valve 6. The liquid nitrogen constant temperature tank 4 includes a housing 41 for holding liquid nitrogen for cooling. Liquid nitrogen is contained within housing 41 for cooling the vaporized nitrogen pipeline. A liquid nitrogen spray system 42 is housed within housing 41. This system is composed of a plurality of nozzles connected in sequence from bottom to top, forming a cavity. The vaporized nitrogen pipeline spirals upward through the cavity formed by the nozzles. The nozzles spray liquid nitrogen, cooling the vaporized nitrogen pipeline and, in turn, the nitrogen gas within the vaporized nitrogen pipeline. The liquid nitrogen spray system 42 is connected to the liquid nitrogen storage tank 1 via a pipeline, which also includes a pneumatic control valve 20. The tank body 41 is connected to the liquid nitrogen storage tank 1 via a pipeline, which also includes a first solenoid valve 21. The bottom of the liquid nitrogen constant temperature tank 4 is connected to a manual drain valve 22 via a pipeline. The bottom of the liquid nitrogen constant temperature tank 4 is also connected to a residual liquid discharge tank 23 via a pipeline, which also includes a second solenoid valve 24. The residual liquid discharge tank 23 is equipped with a tail nitrogen vent. A liquid level gauge 25 is installed on the top of the liquid nitrogen constant temperature tank 4. The safety valve 8 is directly connected to the low-temperature and high-pressure pressure reducing valve 6 through a fourth solenoid valve 26. A pressure relief pipeline is connected between the fourth solenoid valve 26 and the second temperature transmitter 18, and the pressure relief pipeline is connected to a third solenoid valve 27. The end of the pressure relief pipeline is connected in sequence to a manually adjustable vent valve 28 and a muffler 29. The outlet of the muffler 29 is a debugging vent.

[0024] Working principle:

[0025] Before the experiment begins, close all valves in the experimental system, open the filling valve and fill about 4.5m of liquid nitrogen into the liquid nitrogen storage tank 1. 3 Liquid nitrogen is added to ensure that the pressure in the tank is constant at 0.4MPa, and the vent valve is opened to test the full amount to prevent danger caused by overfilling; three people are required to carry out the filling process at the same time, one person monitors the operation at one location of the liquid nitrogen storage tank, one person needs to patrol the filling liquid nitrogen pipeline, and one person monitors the operation at the liquid nitrogen filling operation location.

[0026] After the liquid nitrogen is filled, close the filling valve and check whether the stop valve 15 is closed. After confirming that the power supply system is normal, turn on the manual control mode of the automatic control box 7, open the vent valve at the top of the liquid nitrogen constant temperature box 4 and the manual drain valve 22 at the bottom, and manually control the liquid nitrogen constant temperature box 4 to purge. The vent valve and manual drain valve 22 can be closed only after the water vapor is discharged.

[0027] Open the first liquid inlet valve 30 of the cryopump 2 and the vent pre-cooling valve to pre-cool the cryopump 2. After confirming that the pre-cooling temperature is less than -130°C on the touch control screen of the automatic control box 7, open the discharge valve of the cryopump 2. After the drain valve of the buffer tank 5 is opened, purge the pipeline of the buffer tank 5. After confirming that the drain valve pipeline is frosted, close the drain valve of the buffer tank 5.

[0028] Then, the manual control vent valve 28 is opened to purge the system pipeline, and the manual control vent valve 28 is closed after the vent pipeline is frosted.

[0029] Open the second liquid inlet valve 32 to replenish the liquid nitrogen constant temperature box 4 with liquid nitrogen. When the liquid level gauge shows 0.2m, stop replenishing the liquid. If it exceeds 0.2m, it is necessary to vent the liquid nitrogen through the vent valve on the top of the liquid nitrogen constant temperature box 4.

[0030] After confirming again that the pre-cooling temperature of the cryopump 2 is less than -130°C (if the pre-cooling temperature is insufficient, the cryopump 2 cannot operate, and pre-cooling is then performed again through the pre-cooling vent valve of the cryopump 2), open the return air valve 32 of the cryopump 2, and start the cryopump 2 through automatic control. The output pressure control range of the cryopump 2 is 6 to 6.5 MPa.

[0031] After the cryopump 2 is running, open the third solenoid valve 27 on the output pipeline and adjust the output flow rate to 300 Nm3 / h for venting. The pressure and output gas flow of the cryopump 2 must be constant. Observe the output liquid nitrogen on the automatic control box 7. Use the pneumatic regulating valve 20 to control the liquid nitrogen spraying system 42 according to the output gas temperature. The low temperature adjustment range can reach -145°C to -140°C.

[0032] After the indoor partial supercritical low-temperature nitrogen experimental system is prepared, the third solenoid valve 27 is automatically closed, the experimental debugging and nitrogen venting are stopped, and then the fourth solenoid valve 26 is opened to allow the gas to enter the room, and the outdoor operation is completed.

[0033] The indoor partial supercritical low-temperature nitrogen experimental system regulates the flow of nitrogen in the main pipeline through the main regulating valve 9. A branch pipeline is connected between the main regulating valve 9 and the flow meter 10. The branch pipeline is connected to a branch regulating valve 13 for regulating the flow of nitrogen in the main pipeline.

[0034] Open the main regulating valve 9, close the branch regulating valve 13, and keep the stop valve 15 closed. After the nitrogen flows through the flow meter 10, slowly open the branch regulating valve 13 according to the reading of the flow meter 10 to adjust the nitrogen flow rate. After the reading stabilizes, open the stop valve 15 to start the experiment.

[0035] When the experimental section is blocked or the experiment is stopped, close the main regulating valve 9 and slowly open the branch regulating valve 13 to slowly drain the high-pressure nitrogen remaining in the experimental section pipeline through the branch regulating valve 13, and the experiment is stopped;

[0036] Safety valve 8, when the gas in the pipe experiences an abnormal pressure surge, it can be opened in time to provide safety protection;

[0037] The temperature and pressure of each collection point laid in advance on the pipeline are collected and displayed by a visual data acquisition instrument. The parameter status of each point can be monitored in real time, and timely feedback and response can be made to ensure that the nitrogen outlet parameters meet the required indicators of the experiment and remain stable for a period of time before participating in flow and heat transfer research.

[0038] The critical state of supercritical nitrogen is 3.3978 MPa and 126.21 K, so it is necessary to maintain a constant temperature while controlling the pressure. Theoretically, the experimental output temperature can reach a minimum of -145°C, and the output pressure can be adjusted up to 5.5 MPa. The entire experimental system uses a cryopump 2 to first pressurize the liquid nitrogen, then uses the heat absorbed by the liquid nitrogen to form high-pressure nitrogen gas through evaporation. This method achieves the output of the supercritical nitrogen constant state parameters in the high-pressure vaporizer 3.

[0039] Standard liquid nitrogen with a pressure of 0.4 MPa and a temperature of -196°C is preset in the liquid nitrogen storage tank 1. The first liquid inlet valve 30 is opened, and the liquid nitrogen is pressurized to 6 MPa after passing through the cryogenic pump 2. The high-pressure liquid nitrogen is then transported to the high-pressure vaporizer 3. In the high-pressure vaporizer 3, the liquid nitrogen exchanges heat with room temperature air, absorbs heat, and vaporizes to form high-pressure nitrogen (affected by the surrounding environment and the working medium, the temperature of the vaporized nitrogen is generally higher than the operating temperature). Under the regulation of the automatic control box 7 PLC, the generated nitrogen passes through the liquid nitrogen constant temperature box 4, which is equipped with a liquid nitrogen immersion and liquid nitrogen spraying system 42, and the liquid height is displayed by a liquid level indicator. The nitrogen passing through is cooled once to reach the operating temperature, and the operating temperature range can reach -145°C to -140°C. The experimental system's nitrogen temperature regulation method is not limited to using a liquid nitrogen thermostat. A throttling expander can also be used to refrigerate and control the temperature of the nitrogen, greatly improving the flexibility and universality of the experimental system. The nitrogen that has passed through the liquid nitrogen thermostat 4 is then sealed in a buffer tank 5, where the maximum allowable operating pressure of the buffer tank 5 is 7 MPa. This equipment can achieve a gas-liquid separation effect on the two-phase mixture of high-pressure, low-temperature nitrogen and liquid nitrogen flowing into the ground. This is mainly due to the fact that the density of the liquid is greater than that of the gas, so that the liquid nitrogen drips downward and separates from the nitrogen, and the nitrogen without liquid nitrogen flows upward and is discharged, thereby ensuring the purity of the nitrogen output by the system and improving the nitrogen quality. Finally, the high-pressure, low-temperature nitrogen passes through a low-temperature and high-pressure pressure reducing valve to achieve stable output under the working conditions, with a pressure adjustment range of 4.5-5.5 MPa. All state parameters of the working fluid in the entire process can be displayed through a visual data acquisition device, and the parameter status of each point can be observed in real time, which can efficiently and quickly respond to various factors that arise during the experiment. Through the above process, the working condition output of supercritical nitrogen at a certain constant pressure, temperature and flow state can be obtained, which saves energy while ensuring safety and reliability, and also solves the problems of stable adjustment of low-temperature experimental systems and independent collection of experimental data.

[0040] Because supercritical nitrogen is in a low-temperature and high-pressure state, the requirements for the piping, valves, and other equipment required for the experimental system installation are higher than those for normal temperature and pressure experimental systems. In addition, the physical properties of the supercritical fluid itself fluctuate greatly, placing special requirements on the sensitivity and range of the instruments, sensors, and other equipment used in the experimental system data acquisition. Measurement is difficult, and during the experiment, it is necessary to ensure that its temperature, pressure, and flow rate parameters can be accurately controlled. Therefore, while facing huge challenges, the construction of this experimental system can achieve independent and autonomous physical property data collection of supercritical nitrogen under certain state parameters, and it is also conducive to making important contributions to the supplementation and improvement of the supercritical fluid physical property database. Experimental research on precooling heat exchangers using supercritical cryogenic media as the cooling medium is at the forefront of the development of precooling technology internationally. Research using supercritical cryogenic nitrogen experimental systems can help to apply them to scenarios such as aircraft engine precooling systems and new nuclear reactors with limited space requirements and low weight requirements.

[0041] The above description is only a preferred specific embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any technician familiar with the technical field, within the technical scope disclosed by the present invention, who makes equivalent replacements or changes based on the technical solution and inventive concept of the present invention, should be covered by the scope of protection of the present invention.

Claims

1. A multifunctional experimental system for studying supercritical low-temperature nitrogen flow and heat transfer, comprising an outdoor supercritical low-temperature nitrogen experimental system and an indoor supercritical low-temperature nitrogen experimental system, characterized by: The outdoor partial supercritical low-temperature nitrogen experimental system comprises a liquid nitrogen storage tank (1) for storing liquid nitrogen, a low-temperature pump (2) for pressurizing, a high-pressure vaporizer (3) for vaporizing liquid nitrogen, a liquid nitrogen constant temperature box (4) for cooling nitrogen, a buffer tank (5) for stabilizing pressure, flow rate and gas-liquid separation, and a low-temperature high-pressure pressure reducing valve (6) for regulating outlet pressure, which are sequentially connected through pipelines, and also comprises an automatic control box (7) for controlling electrical components; The indoor partial supercritical low-temperature nitrogen experimental system comprises a main regulating valve (9), a flow meter (10), a third pressure transmitter (11) and a third temperature transmitter (12) which are sequentially connected through pipelines; The front end of the main regulating valve (9) is connected to a safety valve (8), the safety valve (8) is connected to the low-temperature high-pressure pressure reducing valve (6), a branch line is connected between the main regulating valve (9) and the flow meter (10), a branch regulating valve (13) is connected to the branch line, and a stop valve (15) is connected between the flow meter (10) and the third pressure transmitter (11); A first temperature transmitter (16) and a first pressure transmitter (17) electrically connected to the automatic control box (7) are connected between the cryogenic pump (2) and the high-pressure vaporizer (3); a second temperature transmitter (18) and a second pressure transmitter (19) electrically connected to the automatic control box (7) are also connected to the pipeline connected to the outlet of the cryogenic and high-pressure reducing valve (6); The liquid nitrogen constant temperature box (4) comprises a box body (41) for containing liquid nitrogen for cooling, wherein a liquid nitrogen spraying system (42) is provided in the box body (41), wherein the liquid nitrogen spraying system is formed by a plurality of nozzles connected in sequence from bottom to top to form a cavity, and a vaporized nitrogen pipeline spirals through the cavity formed by the plurality of nozzles of the liquid nitrogen spraying system from bottom to top; The liquid nitrogen spraying system (42) is connected to the liquid nitrogen storage tank (1) through a pipeline, and a pneumatic regulating valve (20) is also connected to the pipeline; the box (41) is connected to the liquid nitrogen storage tank (1) through a pipeline, and a first solenoid valve (21) is also connected to the pipeline.

2. The multifunctional experimental system for studying supercritical low-temperature nitrogen flow and heat transfer according to claim 1 is characterized by: The bottom of the liquid nitrogen constant temperature box (4) is connected to a manual drain valve (22) through a pipeline; the bottom of the liquid nitrogen constant temperature box (4) is also connected to a residual liquid discharge box (23) through a pipeline, and the pipeline is also connected to a second solenoid valve (24); the residual liquid discharge box (23) is provided with a tail nitrogen vent; and the top of the liquid nitrogen constant temperature box (4) is provided with a liquid level gauge (25).

3. The multifunctional experimental system for studying supercritical low-temperature nitrogen flow and heat transfer according to claim 2 is characterized by: The safety valve (8) and the low-temperature and high-pressure pressure reducing valve (6) are directly connected to a fourth solenoid valve (26); a pressure relief pipeline is connected between the fourth solenoid valve (26) and the second temperature transmitter (18); a third solenoid valve (27) is connected to the pressure relief pipeline; the end of the pressure relief pipeline is connected in sequence to a manually adjustable vent valve (28) and a muffler (29); the outlet of the muffler (29) is a debugging vent.

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