Cryogenic fluid throttling characteristic visualization experimental device

Abstract

A cryogenic fluid throttling characteristic visualization experimental device includes: a vacuum chamber and a connected test chamber, a cryogenic visualization module, a cryogenic fluid filling and discharging module, and a data acquisition system. The vacuum chamber is equipped with a coil heat exchanger and a gas-liquid separator for regulating the subcooling and stability of the fluid before throttling. The cryogenic visualization module is mounted on the test chamber, and the throttling test section to be measured is located within the test chamber. The cryogenic fluid filling and discharging module and the data acquisition system are connected to the vacuum chamber and the test chamber, respectively. This invention enables the visualization study of the throttling cavitation flow characteristics of cryogenic fluids such as liquid hydrogen and liquid nitrogen under different operating conditions.

Description

Technical Field

[0001] This invention relates to a technology in the field of cryogenic fluids, specifically a cryogenic fluid throttling characteristic visualization experimental device. Background Technology

[0002] With the advancement of deep space exploration missions, cryogenic fluids (such as liquid hydrogen and liquid oxygen) are gradually becoming the preferred propellants for new launch vehicles. In propellant delivery, storage, and refueling systems, when fluid flows through components such as valves, orifice plates, and venturi tubes, it is highly susceptible to cavitation phase transitions due to sudden drops in local static pressure, forming complex gas-liquid two-phase flows that significantly affect the fluid's flow rate, pressure, and temperature. Currently, throttling experiments are not applicable to the extreme liquid hydrogen temperature range (-253℃). Furthermore, for fluids like liquid hydrogen with extremely low latent heat of vaporization and high sensitivity to heat, a simple vacuum cannot isolate them from environmental thermal radiation. The fluid is prone to wall boiling before reaching the throttling device, resulting in the fluid entering the orifice not being a pure liquid phase, thus distorting experimental data. Summary of the Invention

[0003] To address the aforementioned shortcomings of existing technologies, this invention proposes a visualization experimental device for cryogenic fluid throttling characteristics, which enables optical observation and parameter measurement of phase changes and flow characteristics of cryogenic media such as liquid hydrogen, liquid nitrogen, and liquid oxygen during throttling.

[0004] This invention is achieved through the following technical solution:

[0005] This invention relates to a cryogenic fluid throttling characteristic visualization experimental device, comprising: a filling vacuum chamber and a test chamber connected thereto, a cryogenic visualization module, a cryogenic fluid filling and discharging module, and a data acquisition system, wherein: the filling vacuum chamber is equipped with a coil heat exchanger and a gas-liquid separator for regulating the subcooling and stability of the fluid before throttling; the cryogenic visualization module is disposed on the test chamber; the throttling test section to be tested is disposed in the test chamber; and the cryogenic fluid filling and discharging module and the data acquisition system are respectively connected to the filling vacuum chamber and the test chamber.

[0006] This invention relates to an experimental method for the above-mentioned apparatus, comprising:

[0007] Step 1: Wrap the piping inside the vacuum filling chamber and test chamber with multiple layers of insulation material (MLI) to further reduce heat leakage under vacuum. Wrap the piping outside the vacuum chamber with rubber-plastic wool, then seal the vacuum chamber and test. Before starting the experiment, turn on the vacuum unit to evacuate the air from the vacuum chamber to achieve a certain vacuum level and ensure proper insulation within the chamber. This includes:

[0008] 1.1 After connecting all the pipes inside the vacuum chamber, wrap them with multiple layers of insulation material. Wrap the pipes outside the vacuum chamber with rubber and plastic cotton and seal all the flanges of the vacuum chamber.

[0009] 1.2 Connect the filling vacuum chamber to a helium mass spectrometer leak detector to test the leak rate inside the vacuum chamber. After passing the test, connect the vacuum pump unit.

[0010] 1.3 First, use the mechanical pump in the vacuum pump unit to perform initial evacuation of the vacuum chamber, reducing the pressure inside the outer vacuum chamber to the level of 1 Pa. Then, activate the molecular pump to perform high-vacuum evacuation of the vacuum chamber. When the vacuum level displayed on the ionization gauge reaches... Then, confirm that the experimental apparatus has a good vacuum seal and complete the vacuuming operation.

[0011] Step 2: Open the valve, introduce cryogenic fluid, purge the air from the pipeline, and pre-cool. After pre-cooling for a period of time, the cryogenic fluid is divided into two streams from the cryogenic source and injected into the coil heat exchanger tank and its internal copper coil channels. The fluid that has undergone heat exchange flows out from the copper coil to the gas-liquid separator. The separated gaseous or liquid fluid enters the throttling test piece in the test chamber, and after throttling, it passes through the retemperature device and flow meter before being discharged. Specifically, this includes:

[0012] 2.1 Drying stage: Introduce dry compressed air to purge the coil heat exchanger and test pipeline, use a hot air gun to heat to accelerate the evaporation of moisture at the bottom, turn on the light source to observe and visualize the test pipeline section, continue purging for 20 minutes until there are no liquid droplets or misty water vapor, and the pipeline drying is completed.

[0013] 2.2 Preliminary Pre-cooling: Keep the coil heat exchanger free of liquid phase fluid. Connect the inlet pipe to the gas phase side of the fluid source and maintain the pressure inside the pipe at approximately 400 kPa. Set the system's cryogenic pressure regulating valve opening to 10% to allow the gas to preferentially cool the upstream piping. Control the initial cooling rate of the visualization test section to be below 5 K / min to prevent thermal stress damage to the viewing window and seals caused by sudden cooling, until the temperature inside the pipe drops below 200 K.

[0014] 2.3 Deep Pre-cooling: Switch the inlet pipe to the liquid phase side of the fluid source, maintain the pressure at approximately 400 kPa, and set the pressure regulating valve opening to 5%. Continue for 20 minutes to allow deep cooling of the piping and visualization window until the temperature drops to approximately 110 K, meeting the experimental operating conditions.

[0015] Step 3: The low-temperature fluid inside the coil heat exchanger tank partially vaporizes, and the discharge flow rate is adjusted by a valve. After being reheated by a rethermator, it is discharged. Simultaneously, a pressurized gas passes through a liquid nitrogen subcooler and becomes low-temperature pressurized gas, which enters the gas-liquid separator. Together with the fluid discharge port in the gas-liquid separator, this process regulates the fluid pressure within the separator. Specifically, this includes:

[0016] 3.1 Maintain dynamic pressure balance in the system operating conditions. Adjust the pressure regulating valve opening to 25%, allowing a small flow of low-temperature fluid to pass through the pipeline. After a half-minute pause, gradually increase the opening until the pipeline before throttling is full of liquid and free of air bubbles. Control the subcooling by adjusting the liquid level in the coil heat exchanger. If the subcooling is too large, continue to increase the pressure regulating valve opening until the subcooling is maintained. At this point, activate the high-speed camera to capture cavitation images and simultaneously record the thermodynamic test parameters.

[0017] 3.2 Set the system data acquisition frequency to quickly capture changes in pressure, temperature, and flow rate before and after throttling. Set the pressure regulating valve opening to 25% to accommodate fluid impact, and adjust it to 50% after half a minute. Observe whether the fluid reaches a fully liquid state before throttling. Continuously increase the valve opening until the saturation temperature corresponding to the pressure before throttling is lower than the measured temperature, i.e., when tiny bubbles appear in the fluid, it is determined to be a critical condition. Stop the experiment and record the data.

[0018] Technical effect

[0019] This invention integrates a fluid pretreatment structure with pressure stabilization and a two-phase outlet. A gas-liquid separator with an internal pressure regulation system is installed upstream of the throttling device, with a top gas phase outlet and a bottom liquid phase outlet. Through an active radiation protection and precise subcooling control structure design, a coil heat exchanger is connected in series in the fluid pipeline, and a copper cold shield with welded fluid cooling pipes is wrapped around the outside of the visual throttling test section. Using an ultra-low temperature quick-change seal adapted to the liquid hydrogen temperature range, the core observation area of ​​the visual throttling device adopts a double-sealed structure of indium wire and knife-edge flange, with VCR connectors at both ends connecting to the fluid pipeline. Temperature sensors and pressure taps with ferrule seals are installed immediately before and after the throttling element. A high-speed camera and light source are respectively installed close to the upper and lower visualization windows of the test chamber.

[0020] Compared with existing technologies, this invention uses a gas-liquid separator to not only eliminate pressure fluctuations in the upstream fluid and achieve stable flow, but its dual-outlet design also allows a single device to meet the needs of both low-temperature pure gas phase experiments (such as nitrogen flow testing) and pure liquid phase experiments (such as liquid gas cavitation testing), as well as the switching between the two. The internal pressure regulating system ensures precise control of the experimental operating pressure. The coil heat exchanger achieves precise adjustment of the subcooling of the ultra-low temperature fluid, and through specific pipeline cooperation, directs the low-temperature waste flash vapor generated by the phase change inside the coil heat exchanger and the gas-liquid separator into the internal flow channel of the copper screen outside the visualization test section. This structure converts the exhaust waste cooling capacity into a driving source for the active radiation-proof cold screen, maximizing the shielding of ambient temperature radiation heat leakage. These two components work together to ensure that the fluid remains in a pure single-phase state before entering the visualization throttling section, eliminating the interference of premature boiling of the pipe wall on the actual throttling phase change parameters.

[0021] This invention addresses the issue of brittle fracture in transparent observation tubes in the liquid hydrogen temperature range. The visualization test section abandons traditional end-face compression sealing and instead constructs an asymmetric adaptive sealing unit consisting of a knife-edge flange, indium wire, a brittle transparent observation tube, and external limiting rods. The metal knife edge, under flange preload, deeply embeds itself in the highly ductile indium wire, absorbing the significant radial and axial shear differences generated between the metal component and the brittle transparent tube during cryogenic contraction. Simultaneously, the external rods and supporting flange form a rigid framework, transmitting the vibrational stress generated by cavitation impacts along the outer edge of the metal. This structure enables dynamic compensation of material deformation under extreme cold contraction conditions, thoroughly protecting the internal brittle transparent test piece from damage. Furthermore, this sealing method allows for low-cost replacement of the test piece without reprocessing. The front and rear ferrule-sealed pressure-sensing tubes and temperature probes achieve spatial synchronization of local transient temperature, pressure drop data, and phase change processes. Attached Figure Description

[0022] Figure 1 This is a schematic diagram of the system structure of the present invention;

[0023] Figure 2 This is a schematic diagram of the internal structure of the test cavity of the present invention;

[0024] Figure 3 This is a schematic diagram of the gas-liquid separator structure of the present invention;

[0025] Figure 4 This is a schematic cross-sectional view of the throttling visualization test section of the present invention;

[0026] Figure 5 This is a schematic diagram illustrating the effect of the example;

[0027] In the diagram: 1. Vacuum filling chamber; 2. Vacuum gauge; 3. Vacuum unit; 4. Control module; 5. Data acquisition instrument; 6. Cryogenic fluid source; 7. High-pressure gas source; 8. Pressure regulating valve; 9. Liquid nitrogen subcooler; 10. Coil heat exchanger; 11. Gas-liquid separator; 1101. Fluid inlet; 1102. Pressure tap ferrule; 1103. Exhaust port; 1104. Flange; 1105. Support; 1106. Liquid phase outlet; 1107. Gas phase outlet; 1108. Penetration fitting; 1109. High-pressure gas inlet; 12. High-speed camera; 13. Temperature sensor; 14. Pressure sensor. 15 Throttling test piece, 1501 Inlet / outlet VCR connector, 1502 Temperature sensor ferrule, 1503 Pressure tap ferrule, 1504 Stainless steel connector, 1505 Flange knife edge, 1506 Indium wire sealing ring, 1507 High-transparency quartz test piece, 1508 Screw support, 16 Test chamber, 1601 Flange, 1602 Optical window, 1603 Cavity, 1604 Copper screen, 1605 Copper screen cryogenic pipeline, 1606 Epoxy support, 17 Light source, 18 Fluid discharge outlet, 19 Flow meter, 20 Recryotherm. Detailed Implementation

[0028] like Figure 1 As shown, this embodiment relates to a cryogenic fluid throttling characteristic visualization experimental device, including: a filling vacuum chamber 1 and a test chamber 16 connected thereto, a cryogenic fluid filling and discharging module, a data acquisition system, and a cryogenic visualization module. The filling vacuum chamber 1 is equipped with a coil heat exchanger 10 and a gas-liquid separator 11 for regulating the subcooling and stability of the fluid before throttling. The cryogenic visualization module is located in the test chamber 16, and the throttling test section 15 to be tested is located in the test chamber 16. The cryogenic fluid filling and discharging module and the data acquisition system are respectively connected to the filling vacuum chamber 1 and the test chamber 16.

[0029] The cryogenic fluid filling and discharging module includes: a cryogenic fluid source 6, a high-pressure gas source 7 and its liquid nitrogen subcooler 9, a rewarmer 20 and its discharge outlet 19, all connected to the filling vacuum chamber 1. The discharge end of the cryogenic fluid source 6 is connected to the inlet vacuum sleeve of the filling vacuum chamber 1. The discharge end of the high-pressure gas source 7 is connected to the coil inlet end of the liquid nitrogen subcooler 9. The outlet end of the liquid nitrogen subcooler 9 is connected to the inlet vacuum sleeve of the filling vacuum chamber 1. The inlet end of the rewarmer 20 is connected to the outlet of the throttling test section 15, the outlet of the copper screen cryogenic pipeline 1606, the coil heat exchanger 10 and the outlet section of the gas-liquid separator 11.

[0030] The cryogenic fluid source 6 is connected to the tank 1003 and the copper coil 1004 in the coil heat exchanger.

[0031] The reheater 20 achieves reheating by exchanging heat between the low-temperature fluid in the pipeline and the hot water, and then discharges it centrally.

[0032] The liquid nitrogen subcooler 9 includes a liquid nitrogen storage tank and a copper coil, used to cool pressurized gas to a low temperature.

[0033] The high-pressure gas source 7 is introduced into the liquid nitrogen subcooler 9 after the pressure is adjusted by the pressure regulating valve 8, and then connected to the gas-liquid separator 11 to regulate and stabilize the pressure of the experimental working medium.

[0034] The data acquisition system includes: a data acquisition instrument 5 and its control module 4, a vacuum gauge 2, nine temperature sensors 13, four pressure sensors 14, and a flow meter 19. The input data terminal of the data acquisition instrument 5 is connected to a pre-reserved passageway on the side of the vacuum filling chamber; the output data terminal of the data acquisition instrument 5 is connected to the input terminal of the control module 4; the vacuum gauge 2 is connected to the vacuum filling chamber 1; the inlet and outlet terminals of the flow meter 19 are connected to the outlet terminal of the rewarmer 20 and the fluid discharge outlet 18, respectively. Three temperature sensors 13 are located in the throttling test section; three temperature sensors 13 are equidistantly located on the temperature measuring rod inside the gas-liquid separator; two temperature sensors 13 are located on the temperature measuring rod inside the coil heat exchanger; and one temperature sensor 13 is located in the pipe section after the rewarmer. One pressure sensor 14 is located inside the top flange of the coil heat exchanger; one pressure sensor 14 is located inside the top flange of the gas-liquid separator; and two pressure sensors 14 are located in the throttling test section.

[0035] The pressure sensors 14 are all set in a normal temperature environment and transmit pressure data through stainless steel capillary tubes as pressure input tubes.

[0036] The low-temperature visualization module includes a high-speed camera 12, a throttling test piece 15, and a cold light source 17, which are arranged vertically on the upper and lower sides of the test chamber 16 to form a visual path and realize the recording of images of the fluid flow characteristics inside the throttling test section.

[0037] The filling vacuum chamber 1 is connected to the vacuum unit 3 to provide a vacuum environment for the system.

[0038] like Figure 3 As shown, the gas-liquid separator 11 adopts a gravity phase separation and top gas phase pressure stabilization cooperative structure, specifically including: fluid inlet 1101, pressure tapping tube sleeve 1102, exhaust port 1103, flange 1104, support 1105, liquid phase outlet 1106, gas phase outlet 1107, through-chamber component 1108, and high-pressure gas inlet 1109. Among them, exhaust port 1103, gas phase outlet 1107 and high-pressure gas inlet 1109 are centrally connected through the top flange of the gas-liquid separator, while liquid phase outlet 1106 is separately located at the bottom of the gas-liquid separator. This space structure utilizes the high-pressure gas inlet 1109 to introduce pressure-stabilizing gas, forming a gas-phase pressure-stabilizing layer in the upper part of the separator. This not only effectively eliminates the severe pressure pulsation caused by the phase change of the low-temperature fluid in the upstream pipeline, but also enables targeted delivery of pure subcooled liquid phase (through the liquid phase outlet) or pure gas phase (through the gas phase outlet) without entrained bubbles to the downstream test section through the gravity separation effect of the physical height difference. This achieves stable supply and flexible switching of different phase fluids on a single experimental platform.

[0039] like Figure 2As shown, the test chamber 16 and the filling vacuum chamber 1 are connected and placed in parallel by a bellows and a flange. The test chamber 16 includes: a flange end cap 1601 with a transparent viewing window 1602, a stainless steel cavity 1603, and a copper screen 1604, a copper screen cryogenic pipe 1605, and an epoxy support 1606 arranged sequentially in the stainless steel cavity 1603. The external high vacuum insulation is maintained by the stainless steel cavity 1603, and the internally active cooling copper screen 1604 is suspended by the epoxy support 1606 with extremely low thermal conductivity. Thus, a double nested insulation barrier is constructed inside the test chamber 16 to provide an insulation boundary, a visualization window for photothermal decoupling, and a highly stable cryogenic background environment for the cryogenic throttling test section.

[0040] The copper screen 1604 has holes of the same size at the same position in the light path channel of the transparent glass window 1602 to form a smooth observation light path.

[0041] The inlet end of the low-temperature pipeline 1605 on the copper screen surface is connected to the outlet end 1012 of the copper screen cold source of the coil heat exchanger, and the outlet end is connected to the fluid discharge port 18.

[0042] The epoxy support 1606 is made of epoxy resin and is used to support the weight of the entire copper screen and the throttling test section while minimizing heat conduction.

[0043] like Figure 2 and Figure 4As shown, the throttling test section 15 includes: inlet / outlet VCR connectors 1501, temperature sensor sleeves 1502, pressure tap sleeves 1503, stainless steel connectors 1504, knife-edge flanges 1505, indium wire sealing rings 1506, high-transparency quartz test pieces 1507, and screw supports 1508. The high-transparency quartz test piece 1507 is located in the core observation area, with its two end faces respectively abutting against the flexible indium wire sealing rings 1506. The knife-edge flanges 1505 are disposed on the indium wire sealing rings 1506. On the outside of section 06, the screw support 1508 is axially connected to the knife-edge flanges 1505 at both ends. By applying an adjustable axial preload, the sharp edge of the knife-edge flange 1505 is deeply embedded in the indium wire sealing ring 1506, completing the rigid-flexible synergistic anti-cold brittle stress release structure with the high-transparency quartz test piece 1507. The stainless steel connector 1504 is connected in sequence to the pressure tapping tube sleeve 1503 and the temperature sensor sleeve 1502. The outermost end is connected to the external pipeline through the inlet and outlet VCR connectors 1501. In particular, thanks to the mechanical clamping structure of the screw support 1508 and the standardized quick-release design of the VCR connectors 1501 at both ends, the entire throttling test section 15 constitutes a highly modular independent unit. Different internal visual flow channels can be easily disassembled and replaced simply by loosening the screw support 1508. This structure utilizes a screw support 1508 to construct a rigid force-transmitting frame, which transmits the extremely high-frequency oscillating stress generated by cavitation collapse during throttling along the external stainless steel components. Simultaneously, the strong plastic deformation of the indium wire absorbs the significant thermal expansion and contraction displacement difference between the brittle quartz glass and the metal tube in the cryogenic temperature range. Temperature measurement before and after throttling is achieved through a ferrule-sealed armored temperature sensor 15, and pressure measurement before and after throttling is achieved through a pressure tap and pressure sensor 16, working in conjunction with a high-speed camera 12 and a light source 17.

[0044] The outlet of the throttling test section 15 is equipped with a bellows and connected to the fluid discharge port 18.

[0045] Through practical application experiments, using liquid nitrogen as the test medium, and through programming to acquire and control temperature, pressure, and flow rate data (hardware and software), the cryogenic fluid throttling characteristic visualization experimental device of this invention was operated by controlling the pressure before throttling and the subcooling degree separately. This ensured that the fluid entering the visualized throttling device remained in a single-phase subcooled state, with a gas volume fraction of 0 in the inlet section. This device exhibited excellent thermal insulation and stability performance. The specific enthalpy error before and after actual throttling was controlled only between 0.2% and 1%. Based on the measured pressure, data from the National Institute of Standards and Technology (NIST) was consulted to obtain the corresponding saturation temperature. The measured temperature and saturation temperature values ​​were almost identical, with a maximum absolute error of only 0.09 K, proving that the test chamber of this invention achieved a near-ideal thermal insulation state. By visualizing the sealed setting in the cryogenic temperature region, the traditional black-box state of cryogenic temperature region measurement was broken, enabling high-speed cameras to capture the microscopic transient evolution processes such as cloud-like cavitation and sheet-like cavitation induced by ultra-low temperature pure liquid fluid at high frequency.

[0046] Compared with existing technologies, this invention ensures that the cryogenic fluid entering the visualization throttling device is in a stable single-phase subcooled liquid state, eliminating the interference of premature vaporization caused by pipe wall heat leakage or sealing failure in conventional devices on the observation of the real throttling cavitation flow field. Through the double sealing of indium wire and knife-edge flange, and the structure of rigid-flexible synergy with screw to resist cold brittle stress release, it overcomes the cold brittle leakage problem of conventional sealing methods such as rubber / PTFE in the liquid hydrogen and liquid nitrogen temperature range. Moreover, this sealing method can improve the system pressure resistance and pressure holding capacity in extremely low temperature environments to the level of engineering applications. Through the gravity phase separation of the dual-outlet gas-liquid separator and the top gas phase pressure stabilization structure, in conjunction with the copper screen design of the exhaust gas circulation of the coil heat exchanger, it not only realizes that the same set of devices can seamlessly switch and accurately control the test conditions of pure liquid phase and pure gas phase, but also, with the sunken window structure, it significantly shortens the observation optical path, significantly improves the imaging clarity of the micro flow field, and greatly expands the research boundary of the hydraulic characteristics and micro phase change evolution law of cryogenic fluids.

[0047] The above-described specific implementations can be partially adjusted by those skilled in the art in different ways without departing from the principles and purpose of the present invention. The scope of protection of the present invention is defined by the claims and is not limited to the above-described specific implementations. All implementation schemes within the scope of the claims are bound by the present invention.

Claims

1. A visualization experimental device for the throttling characteristics of cryogenic fluids, characterized in that, include: The system includes a vacuum filling chamber and a connected test chamber, a cryogenic visualization module, a cryogenic fluid filling and discharging module, and a data acquisition system. The vacuum filling chamber is equipped with a coil heat exchanger and a gas-liquid separator for regulating the subcooling and stability of the fluid before throttling. The cryogenic visualization module is located on the test chamber, and the throttling test section to be tested is located inside the test chamber. The cryogenic fluid filling and discharging module and the data acquisition system are connected to the vacuum filling chamber and the test chamber, respectively.

2. The experimental apparatus for visualizing the throttling characteristics of cryogenic fluids according to claim 1, characterized in that, The cryogenic fluid filling and discharging module includes: a cryogenic fluid source, a high-pressure gas source and its liquid nitrogen subcooler, a rewarmer and its discharge outlet, all connected to the filling vacuum chamber. The discharge end of the cryogenic fluid source is connected to the inlet vacuum sleeve of the filling vacuum chamber, the discharge end of the high-pressure gas source is connected to the coil inlet end of the liquid nitrogen subcooler, the outlet end of the liquid nitrogen subcooler is connected to the inlet vacuum sleeve of the filling vacuum chamber, and the inlet end of the rewarmer is connected to the outlet of the throttling test section, the outlet of the copper screen cryogenic pipeline, the coil heat exchanger and the outlet section of the gas-liquid separator. The cryogenic fluid source is connected to the tank and the copper coil in the coil heat exchanger, respectively. The aforementioned reheater reheats the fluid by exchanging heat between the low-temperature fluid and hot water in the pipeline, and then discharges it centrally. The liquid nitrogen subcooler includes a liquid nitrogen storage tank and a copper coil, used to cool pressurized gas to a low temperature; The high-pressure gas source is fed into a liquid nitrogen subcooler after the pressure is adjusted by a pressure regulating valve, and then connected to a gas-liquid separator to regulate and stabilize the pressure of the experimental working medium.

3. The experimental apparatus for visualizing the throttling characteristics of cryogenic fluids according to claim 1, characterized in that, The data acquisition system includes: a data acquisition instrument and its control module, a vacuum gauge, nine temperature sensors, one pressure sensor, and a flow meter. Specifically: the input data terminal of the data acquisition instrument is connected to a pre-reserved passageway on the side of the vacuum filling chamber; the output data terminal of the data acquisition instrument is connected to the input terminal of the control module; the vacuum gauge is connected to the vacuum filling chamber; the inlet and outlet terminals of the flow meter are connected to the outlet terminal of the rewarmer and the fluid discharge outlet, respectively; three temperature sensors are located in the throttling test section; three temperature sensors are equidistantly located on the temperature measuring rod inside the gas-liquid separator; two temperature sensors are located on the temperature measuring rod inside the coil heat exchanger; one temperature sensor is located in the pipe section after the rewarmer; one pressure sensor is located inside the top flange of the coil heat exchanger; one pressure sensor is located inside the top flange of the gas-liquid separator; and two pressure sensors are located in the throttling test section. The pressure sensors described are all installed in a normal temperature environment and transmit pressure data through stainless steel capillary tubes as pressure input tubes.

4. The experimental apparatus for visualizing the throttling characteristics of cryogenic fluids according to claim 1, characterized in that, The low-temperature visualization module includes a high-speed camera, a throttling test piece, and a cold light source, which are arranged vertically to form a visual path and record images of the fluid flow characteristics inside the throttling test section.

5. The experimental apparatus for visualizing the throttling characteristics of cryogenic fluids according to claim 1, characterized in that, The gas-liquid separator employs a combined structure of gravity phase separation and top gas phase pressure stabilization. Specifically, it includes: a fluid inlet, a pressure tapping tube sleeve, an exhaust port, a flange, a support, a liquid phase outlet, a gas phase outlet, a transom, and a high-pressure gas inlet. The exhaust port, gas phase outlet, and high-pressure gas inlet are centrally located at the top flange of the gas-liquid separator, while the liquid phase outlet is separately located at the bottom of the gas-liquid separator. This spatial structure utilizes the high-pressure gas inlet to introduce pressure-stabilizing gas, forming a gas phase pressure-stabilizing layer in the upper part of the separator. This not only effectively eliminates the severe pressure pulsation caused by the phase change of the low-temperature fluid in the upstream pipeline, but also, through the gravity separation effect of the physical height difference, can target and deliver pure subcooled liquid or pure gas phase without entrained bubbles to the downstream test section, realizing a stable supply and flexible switching of different phase fluids on a single experimental platform.

6. The experimental apparatus for visualizing the throttling characteristics of cryogenic fluids according to claim 1, characterized in that, The test chamber and the filling vacuum chamber are connected and placed in parallel by a bellows and a flange. The test chamber includes: a flange end cap with a transparent window, a stainless steel cavity, and a copper screen, a copper screen low-temperature pipe, and an epoxy support arranged sequentially in the stainless steel cavity. The external high vacuum insulation is maintained by the stainless steel cavity, and the internal active cooling copper screen is suspended by the epoxy support with extremely low thermal conductivity. Thus, a double nested insulation barrier is constructed inside the test chamber to provide an insulation boundary, a visualization window for photothermal decoupling, and a highly stable cryogenic background environment for the low-temperature throttling test section. The copper screen has holes of the same size at the same positions as the light path channel of the transparent glass window to ensure a smooth observation light path. The inlet end of the low-temperature pipeline on the copper screen surface is connected to the outlet end of the copper screen cold source of the coil heat exchanger, and the outlet end is connected to the fluid discharge port. The epoxy support is made of epoxy resin and is used to support the weight of the entire copper screen and the throttling test section while minimizing heat conduction.

7. The experimental apparatus for visualizing the throttling characteristics of cryogenic fluids according to claim 1, characterized in that, The throttling test section includes: inlet and outlet VCR connectors, temperature sensor ferrules, pressure tapping tube ferrules, stainless steel connectors, knife-edge flanges, indium wire seals, high-transparency quartz test pieces, and screw supports. The high-transparency quartz test piece is located in the core observation area, with its two end faces abutting against the flexible indium wire seals. Knife-edge flanges are positioned outside the indium wire seals. The screw supports axially connect the knife-edge flanges at both ends. By applying an adjustable axial preload, the sharp edges of the knife-edge flanges are deeply embedded in the indium wire seals, completing a rigid-flexible synergistic anti-cold brittle stress release structure with the high-transparency quartz test piece. The stainless steel connectors are sequentially connected to the pressure tapping tube ferrules and temperature sensor ferrules, with the outermost end connected to external pipelines via the inlet and outlet VCR connectors. The outlet of the throttling test section is equipped with a bellows, which is connected to the fluid discharge port.

8. A method for visualizing the cryogenic fluid throttling characteristics of the device according to any one of claims 1-7, characterized in that, include: Step 1: Wrap the pipes inside the filling vacuum chamber and the test chamber with multilayer insulation material (MLI) to further reduce heat leakage on the basis of vacuum; wrap the pipes outside the vacuum chamber with rubber and plastic cotton, then complete the vacuum chamber sealing and testing. Before the formal experiment begins, turn on the vacuum unit to extract the air in the vacuum chamber to achieve a certain vacuum level to ensure the insulation of the chamber. Step 2: Open the valve, introduce the cryogenic fluid, purge the air from the pipeline and pre-cool it. After pre-cooling for a period of time, the cryogenic fluid is divided into two streams from the cryogenic source and injected into the coil heat exchanger tank and the copper coil channel inside. The fluid that has undergone heat exchange flows out from the copper coil to the gas-liquid separator. The separated gaseous or liquid fluid enters the throttling test piece in the test chamber, and after throttling, it is discharged after passing through the reheater and the flow meter. Step 3: The low-temperature fluid inside the coil heat exchanger tank partially vaporizes, and the discharge flow rate is adjusted by the valve. After being reheated by the rethermator, it is discharged. Simultaneously, a pressurized gas path passes through a liquid nitrogen subcooler, transforming into a low-temperature pressurized gas that enters the gas-liquid separator. This gas, along with the fluid discharge port during gas-liquid separation, helps regulate the fluid pressure within the separator. Specifically, this includes: 3.1 To maintain the dynamic balance of system operating pressure, adjust the opening of the pressure regulating valve to 25%, allowing a small flow of low-temperature fluid to flow through the pipeline. After a half-minute stop, gradually increase the opening until the pipeline before throttling is full of liquid and free of air bubbles. Control the subcooling by adjusting the liquid level in the coil heat exchanger. If the subcooling is too large, continue to increase the opening of the pressure regulating valve until the subcooling is maintained. At this time, turn on the high-speed camera to capture cavitation images and record the thermodynamic test parameters simultaneously. 3.2 Set the system data acquisition frequency to quickly capture the pressure, temperature and flow changes before and after throttling. Set the pressure regulating valve opening to 25% to adapt to fluid impact. After half a minute, adjust it to 50% and observe whether the fluid reaches a full liquid state before throttling. Continue to increase the valve opening until the saturation temperature corresponding to the pressure before throttling is lower than the measured temperature, that is, when tiny bubbles appear in the fluid, it is determined to be a critical condition. Stop the experiment and record the data.

9. The experimental method for visualizing the throttling characteristics of cryogenic fluids according to claim 8, characterized in that, Step 1 specifically includes: 1.1 After connecting all the pipes inside the vacuum chamber, wrap them with multiple layers of insulation material. Wrap the pipes outside the vacuum chamber with rubber and plastic cotton and seal all the flanges of the vacuum chamber. 1.2 Connect the filling vacuum chamber to a helium mass spectrometer leak detector to check the leak rate inside the vacuum chamber. After passing the test, connect the vacuum pump unit. 1.3 First, use the mechanical pump in the vacuum pump unit to perform initial evacuation of the vacuum chamber, reducing the pressure inside the outer vacuum chamber to the level of 1 Pa. Then, use the molecular pump to perform high-vacuum evacuation of the vacuum chamber. When the vacuum level displayed on the ionization gauge reaches... Then, confirm that the experimental apparatus has a good vacuum seal and complete the vacuuming operation.

10. The experimental method for visualizing the throttling characteristics of cryogenic fluids according to claim 8, characterized in that, Step 2 specifically includes: 2.1 Drying stage: Introduce dry compressed air to purge the coil heat exchanger and test pipeline, use a hot air gun to heat to accelerate the evaporation of moisture at the bottom, turn on the light source to observe and visualize the test pipeline section, continue purging for 20 minutes until there are no liquid droplets or misty water vapor, and the pipeline drying is completed. 2.2 Preliminary precooling: Keep the coil heat exchanger free of liquid phase fluid, connect the inlet pipe to the gas phase side of the fluid source, keep the pressure inside the pipe at about 400 kPa, set the opening of the system low temperature pressure regulating valve to 10%, so that the gas preferentially cools the front pipeline, control the initial cooling rate of the visualization test section to be less than 5 K / min, and prevent the sudden cooling from causing thermal stress damage to the window and seals, until the temperature inside the pipe is lower than 200 K; 2.3 Deep precooling: Switch the inlet pipe to the liquid phase side of the fluid source, maintain the pressure at about 400 kPa, set the pressure regulating valve opening to 5%, and wait for 20 minutes to perform deep cooling of the pipeline and visualization window until the temperature drops to about 110 K, which meets the experimental operating conditions.

Images