Visualized experimental device for controlling nitrogen droplet as tracer particle under cryogenic environment

Abstract

The application discloses a kind of visualization experimental devices of control nitrogen droplet as tracer particle under deep low temperature environment, belong to flow field measurement technical field, comprising: high-temperature high-pressure nitrogen source, cryogenic liquid nitrogen source, heat exchanger, Laval nozzle, vacuum tank, first connector, second connector and control module;High-temperature high-pressure nitrogen source and cryogenic liquid nitrogen source are connected with the heat exchanger respectively;One end of first connector is connected with heat exchanger, the other end passes through one end of vacuum tank and is connected with the air inlet end of Laval nozzle, and the air outlet end of Laval nozzle passes through the other end of vacuum tank through second connector;Control module is used to change the temperature and pressure of the air inlet end of the Laval nozzle and the internal cryogenic high-pressure nitrogen by adjusting the flow of high-temperature high-pressure nitrogen and cryogenic liquid nitrogen, control the particle size and concentration of nitrogen droplet when spontaneous condensation is in the range set.The device of the application realizes the visualization of spontaneous condensation of nitrogen droplet and the control method research of nitrogen droplet as tracer particle in low-temperature wind tunnel experiment in deep low temperature environment.

Description

Technical Field

[0001] This invention belongs to the field of flow field measurement technology, and more specifically, relates to a visual experimental device for controlling nitrogen droplets as tracer particles in a cryogenic environment. Background Technology

[0002] Non-contact flow field measurement technology utilizes tracer particles to reflect the surface flow velocity of a flow field. By capturing externally applied tracer particles, it indirectly reflects the flow field velocity and is commonly used in wind tunnel experiments. Under cryogenic conditions, the aggregation effect of tracer particles needs to be considered. Therefore, currently, the most commonly used tracer particles in cryogenic wind tunnel flow field tracing research are only oil-based particles and ice particles.

[0003] However, oil droplets can adhere to and penetrate the insulation material, making them difficult to remove completely. This can affect the internal insulation material and components of the wind tunnel, causing permanent damage. Therefore, oil-based particles are not entirely suitable for continuous internal insulation cryogenic wind tunnels. Ice particle accumulation is significantly enhanced at extremely low temperatures, leading to excessively large particle sizes, uneven particle distribution, and poorer flow field tracking. Furthermore, the large amount of water vapor formed by ice particles remaining in the tunnel during rewarming is absorbed by the internal insulation material and released upon subsequent cooling, causing severe frost contamination on the test model surface and affecting the accuracy of the test data.

[0004] Meanwhile, in transonic cryogenic wind tunnel experiments, the construction of a deep cryogenic environment is one of the challenges. How to construct a deep cryogenic environment and achieve flow visualization within it, as well as how to study other tracer particles besides oil-based particles and ice particles in a deep cryogenic environment, are of great research significance. Summary of the Invention

[0005] To address the shortcomings and improvement needs of existing technologies, this invention provides a visual experimental device for controlling nitrogen droplets as tracer particles in a cryogenic environment. The purpose is to realize the use of nitrogen droplets as tracer particles in cryogenic wind tunnel experiments in a cryogenic environment, clarify the nucleation and growth characteristics of nitrogen droplets in the non-equilibrium condensation process, and promote the application of non-contact flow field measurement technology in transonic cryogenic wind tunnels.

[0006] To achieve the above objectives, according to one aspect of the present invention, a visual experimental apparatus for controlling nitrogen droplets as tracer particles in a cryogenic environment is provided, comprising:

[0007] High-temperature and high-pressure nitrogen source, low-temperature liquid nitrogen source, heat exchanger, Laval nozzle, vacuum tank, first connecting pipe, second connecting pipe and control module;

[0008] The high-temperature, high-pressure nitrogen source and the low-temperature liquid nitrogen source are respectively connected to the heat exchanger to generate low-temperature, high-pressure nitrogen. The inlet end of the Laval nozzle passes through one end of the vacuum tank via the first connecting pipe, and the outlet end passes through the other end of the vacuum tank via the second connecting pipe, so that the Laval nozzle is located inside the vacuum tank and does not contact the inner wall of the vacuum tank. The heat exchanger is connected to the first connecting pipe passing through one end of the vacuum tank, and the Laval nozzle is used to realize the spontaneous condensation of the low-temperature, high-pressure nitrogen.

[0009] The control module is used to change the temperature and pressure of the inlet end of the Laval nozzle and the internal low-temperature high-pressure nitrogen by adjusting the flow rates of high-temperature high-pressure nitrogen and low-temperature liquid nitrogen, thereby controlling the particle size and concentration of nitrogen droplets during spontaneous condensation within a set range.

[0010] Furthermore, a mass flow meter is provided at the inlet end of the heat exchanger;

[0011] Temperature and pressure sensors are equidistantly distributed along the air inlet end and lower wall surface of the Laval nozzle.

[0012] The spontaneous condensation process within the Laval nozzle is captured using optical measurement devices.

[0013] Furthermore, it also includes a nitrogen droplet flow rate measurement and control module, used to introduce oil-based particles or ice particles into the first connecting pipe, measure the flow rate of low-temperature high-pressure nitrogen gas in the Laval nozzle by particle image velocimetry, and control the flow rates of high-temperature high-pressure nitrogen gas and low-temperature liquid nitrogen so that the flow rate of nitrogen droplets in the Laval nozzle and the flow rate of low-temperature high-pressure nitrogen gas are within a set error range.

[0014] Furthermore, the spontaneous condensation process of the low-temperature, high-pressure nitrogen gas in the Laval nozzle is captured by optical measuring devices, and the flow rate of nitrogen droplets in the Laval nozzle is obtained by particle image velocimetry.

[0015] Furthermore, it also includes a droplet separation mechanism and a droplet collection tank;

[0016] The droplet separation mechanism is disposed between the gas outlet end of the Laval nozzle and the second connecting pipe, and is used to separate nitrogen droplets from the gas-liquid two-phase flow flowing out of the Laval nozzle;

[0017] The droplet collection vessel is used to collect the nitrogen droplets.

[0018] Furthermore, the droplet separation mechanism has a three-way structure, including a two-phase flow inlet pipe, a liquid outlet pipe, and a gas outlet pipe;

[0019] One end of the two-phase flow inlet pipe is connected to the outlet end of the Laval nozzle, and there is a downward first included angle θ1 along the flow direction of the Laval nozzle, 0<θ1≤20°;

[0020] One end of the gas outlet pipe is connected to the second connecting pipe, and there is an upward second included angle θ2 along the travel direction of the Laval nozzle, 0 < θ2 ≤ 20°;

[0021] The liquid outlet pipe is connected to the other end of the two-phase flow inlet pipe and the other end of the gas outlet pipe, and has a downward third included angle along the direction of the Laval nozzle.

[0022] Furthermore, the first included angle θ1 and the second included angle θ2 satisfy:

[0023] 5°≤θ1≤20°, and 5°≤θ2≤20°.

[0024] Furthermore, a frustum-shaped perforated plate is provided at the junction of the liquid outlet pipe and the gas outlet pipe.

[0025] Furthermore, a corrugated pipe is provided between the droplet separation mechanism and the second connecting pipe.

[0026] Furthermore, it also includes a buffer tank connected to the outlet end of the second pipe, and a pressure sensor and a solenoid valve are installed at the outlet of the buffer tank.

[0027] In summary, the above-described technical solutions conceived in this invention can achieve the following beneficial effects:

[0028] (1) The device of the present invention generates low-temperature high-pressure nitrogen gas through high-temperature high-pressure nitrogen gas and low-temperature liquid nitrogen gas, and can also pre-cool the entire device through low-temperature liquid nitrogen gas. Combined with the non-contact design between the Laval nozzle and the vacuum tank wall, a deep low-temperature environment is constructed. In the deep low-temperature environment, nitrogen gas will condense. By adjusting the flow rate of high-temperature high-pressure nitrogen gas and low-temperature liquid nitrogen gas, the gas phase parameters of low-temperature high-pressure nitrogen gas in the non-equilibrium condensation process in the Laval nozzle are changed, thereby controlling the concentration distribution and growth rate of nitrogen droplets in the Laval nozzle, so that nitrogen droplets can be used as tracer particles in low-temperature wind tunnel experiments.

[0029] (2) The temperature and pressure changes of the working fluid during the condensation process are measured by temperature and pressure sensors set on the lower wall of the Laval nozzle. The starting point and condensation time of condensation are then calculated to obtain the condensation law of nitrogen droplets. The appropriate temperature and pressure for nitrogen droplets as tracer particles are determined. The spontaneous condensation process is captured by optical measurement devices to realize the visualization of spontaneous condensation of nitrogen droplets. The particle size and concentration of nitrogen droplets during spontaneous condensation are determined to judge the growth rate of nitrogen droplets.

[0030] (3) As a preferred option, the flow rate of nitrogen droplets in the Laval nozzle is measured by the designed nitrogen droplet flow rate measurement and control module based on oil-based particles or ice particles commonly used in existing low-temperature wind tunnel experiments. The flow rate of nitrogen droplets is controlled by changing the flow rate of high-temperature and high-pressure nitrogen and low-temperature liquid nitrogen, and the flow characteristics of nitrogen droplets are adjusted so that nitrogen droplets can accurately reflect the true flow rate of gas, thereby further promoting nitrogen droplets as tracer particles in wind tunnel experiments.

[0031] (4) The spontaneous condensation process of low-temperature and high-pressure nitrogen in the Laval nozzle is captured by optical measurement devices. While realizing the visualization of the spontaneous condensation process of nitrogen droplets, the flow velocity of nitrogen droplets in the Laval nozzle is measured by particle image velocimetry.

[0032] (5) Preferably, nitrogen droplets are separated from the gas-liquid two-phase flow flowing out of the Laval nozzle by a designed droplet separation mechanism and droplet collection tank, which facilitates the measurement of liquid phase parameters, controls the holding time of nitrogen droplets, and meets the existence time requirement of nitrogen droplets as tracer particles.

[0033] (6) The droplet separation mechanism designed in this invention, with its position and angle design of the two-phase flow inlet pipe and gas outlet pipe, can avoid the two-phase flow from splitting into small droplets due to excessive liquid flow when entering the droplet separation mechanism, thus affecting the separation effect. It can also avoid the problem of excessive pressure drop causing the fluid temperature to drop, which would lead to further liquefaction of the two-phase flow and reduce the measurement accuracy of liquid phase parameters. At the same time, it can ensure that all components between the vacuum tank inlet and outlet are rigidly connected on the axis, avoiding contact between the components and the vacuum tank wall and thus damaging the constructed deep cryogenic environment.

[0034] (7) Preferably, the first included angle θ1 and the second included angle θ2 are within the preferred range, and the separation of droplets can be achieved by utilizing the inertial acceleration of the two-phase flow.

[0035] (8) Preferably, a frustum-shaped perforated plate is provided at the junction of the liquid outlet pipe and the gas outlet pipe, which can increase the probability of the droplets being impacted by the two-phase flow and improve the efficiency of droplet separation.

[0036] (9) Preferably, a bellows is provided between the droplet separation mechanism and the second connecting pipe to balance the axial shrinkage of the experimental pipe in the deep cryogenic environment and to balance the installation error.

[0037] (10) Preferably, a buffer tank is connected to the outlet end of the second pipe. The pressure is monitored by a pressure sensor and the flow rate of the released gas is controlled by a solenoid valve, so as to keep the pressure in the buffer tank relatively stable, that is, to keep the outlet pressure of the Laval nozzle stable, and the back pressure of the Laval nozzle can be controlled by the gas release flow orifice. Attached Figure Description

[0038] Figure 1 This is a schematic diagram of a device for controlling nitrogen droplets as tracer particles in a cryogenic environment according to an embodiment of the present invention.

[0039] Figure 2 This is a three-dimensional exploded view of the vacuum tank in an embodiment of the present invention.

[0040] Figure 3 This is a three-dimensional schematic diagram of the Laval nozzle in an embodiment of the present invention.

[0041] Figure 4 This is a schematic diagram of the liquid phase parameter measurement module in the device according to an embodiment of the present invention.

[0042] Figure 5 This is a schematic diagram of the liquid phase parameter measurement module in an embodiment of the present invention.

[0043] In all the accompanying drawings, the same reference numerals are used to denote the same elements or structures, wherein:

[0044] 1-Nitrogen cylinder assembly; 2-Pressure regulating valve; 3-Mass flow meter; 4-Vacuum tank; 5-Heat exchanger; 6-Liquid nitrogen tank; 7-Laval nozzle; 8-Buffer tank; 9-Solenoid valve; 10-Optical measuring device; 11-Laser generator; 12-Synchronizer; 13-Data acquisition instrument; 14-Computer; 15-Vacuum pump; 16-Illumination instrument; 17-Upper observation window; 18-Front observation window; 20-Vacuum pump connection pipe; 21-Aviation connector; 23-Rear observation window ; 24-Laval nozzle channel; 25-Bracket; 26-Sensor connection hole; 27-Quartz glass; 28-First connecting pipe; 29-Second connecting pipe; 30-Cryogenic plug; 31-Laval nozzle front adapter section; 32-Laval nozzle rear adapter section; 33-Droplet separation mechanism; 31-Droplet collection tank; 35-Bellbell; 36-Temperature and pressure sensor; 37-Two-phase flow inlet pipe; 38-Frustoconical pore plate; 39-Liquid outlet pipe; 40-Gas outlet pipe. Detailed Implementation

[0045] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention. Furthermore, the technical features involved in the various embodiments of this invention described below can be combined with each other as long as they do not conflict with each other.

[0046] In this invention, the terms "first," "second," etc., used in the invention and accompanying drawings are used to distinguish similar objects and are not necessarily used to describe a specific order or sequence.

[0047] like Figure 1 As shown, the device for controlling nitrogen droplets as tracer particles in a cryogenic environment according to the present invention mainly includes: a nitrogen cylinder group 1, a liquid nitrogen tank 6, a pressure regulating valve 2, a mass flow meter 3, a heat exchanger 5, a Laval nozzle 7, a vacuum tank 4, a first connecting pipe 28, a second connecting pipe 29, a temperature and pressure sensor 36, a data acquisition module, and a control module.

[0048] A high-temperature, high-pressure nitrogen source is connected to a heat exchanger 5 via a pressure regulating valve 2, and a low-temperature liquid nitrogen source is connected to a heat exchanger 5 via a pressure regulating valve 2. A mass flow meter 3 is installed at the inlet of the heat exchanger 5 to measure the flow rate of the high-temperature, high-pressure nitrogen flowing through the heat exchanger 5.

[0049] The inlet end of the Laval nozzle 7 passes through one end of the vacuum tank 4 via the first connecting pipe 28, and the outlet end of the Laval nozzle 7 passes through the other end of the vacuum tank 4 via the second connecting pipe 29, so that the Laval nozzle 7 is located inside the vacuum tank 4 and does not contact the inner wall of the vacuum tank 4; the first connecting pipe 28 is connected to the heat exchanger 5; temperature and pressure sensors 36 are evenly distributed along the inlet end of the Laval nozzle 7 and the lower wall of the Laval nozzle.

[0050] During operation, firstly, the cryogenic liquid nitrogen supplied by the cryogenic liquid nitrogen source is pressure-regulated by the pressure regulating valve 2 and then input into the Laval nozzle 7 through the heat exchanger 5 to pre-cool the Laval nozzle 7; secondly, the high-temperature and high-pressure nitrogen supplied by the high-temperature and high-pressure nitrogen source is pressure-regulated by the pressure regulating valve 2 and then input into the heat exchanger 5. In the heat exchanger 5, the cryogenic liquid nitrogen and the high-temperature nitrogen exchange heat and are converted into cryogenic and high-pressure nitrogen.

[0051] The low-temperature, high-pressure nitrogen gas output from heat exchanger 5 is fed into Laval nozzle 7 through first pipe 28 and undergoes spontaneous condensation within Laval nozzle 7.

[0052] The data acquisition module is used to collect data from the corresponding temperature and pressure sensors and transmit it to the control module.

[0053] The control module is used to change the temperature and pressure of the low-temperature high-pressure nitrogen at the inlet of the Laval nozzle 7 by adjusting the flow rate of high-temperature high-pressure nitrogen and low-temperature liquid nitrogen, and to change the pressure and temperature distribution of the low-temperature high-pressure nitrogen along the path inside the Laval nozzle. The temperature and pressure changes of the working fluid during the condensation process are measured by temperature and pressure sensors 36 that are equidistantly distributed along the lower wall of the Laval nozzle. The starting point and condensation time of condensation are then calculated to obtain the condensation law of nitrogen droplets. The appropriate temperature and pressure for nitrogen droplets as tracer particles can be determined.

[0054] Based on this condensation law, the growth rate of nitrogen droplets is controlled so that the particle size and concentration of nitrogen droplets are within a set range when low-temperature, high-pressure nitrogen gas spontaneously condenses in the Laval nozzle 7. Within this set range, the nitrogen droplets are visible and maintain good flow capability, so that the nitrogen droplets can be used as tracer particles in low-temperature wind tunnel experiments.

[0055] Specifically, during the nitrogen droplet condensation process, the starting point of condensation is determined by the pressure change on the lower wall of the Laval nozzle, and the subcooling of the internal gas is calculated by the temperature change, thereby determining the condensation time; the remaining gas is finally discharged through the second pipe 29. In this embodiment of the invention, the spontaneous condensation process of low-temperature, high-pressure nitrogen gas in the Laval nozzle is captured by an optical measuring device 10 (such as a camera), and the parameters of the condensed droplets, including droplet size, concentration, and distribution characteristics, are measured online using the extinction method.

[0056] While generating cryogenic high-pressure nitrogen using high-temperature, high-pressure nitrogen and cryogenic liquid nitrogen, the entire device can also be cooled by cryogenic liquid nitrogen. Combined with the fact that the Laval nozzle 7 and the wall of the vacuum tank 4 are not in contact, a deep cryogenic environment is constructed. Within this environment, by adjusting the flow rates of the high-temperature, high-pressure nitrogen and cryogenic liquid nitrogen, the gas phase parameters (temperature and pressure of the cryogenic high-pressure nitrogen) during the non-equilibrium condensation process of the cryogenic high-pressure nitrogen in the Laval nozzle are changed. The condensation initiation point and condensation time are calculated, yielding the condensation law of nitrogen droplets. Based on this condensation law, the growth rate of nitrogen droplets is controlled, ensuring that the particle size and concentration of nitrogen droplets are within a set range, so that the nitrogen droplets can be used as tracer particles in cryogenic wind tunnel experiments. Preferably, the device of this invention also includes a buffer tank 8, which is connected to the outlet end of the second connecting pipe 29. The remaining gas discharged from the Laval nozzle 7 is transferred to the buffer tank 8 through the second connecting pipe 29, stabilized, and then discharged into the environment. A pressure sensor and a solenoid valve 9 are installed at the outlet of the buffer tank 8. The pressure sensor monitors the pressure, and the solenoid valve 9 controls the flow rate of the released gas, thereby maintaining a relatively stable pressure in the buffer tank 8, that is, maintaining a stable outlet pressure of the Laval nozzle 7. The back pressure of the Laval nozzle 7 can also be adjusted by controlling the flow rate of the released gas through the solenoid valve 9.

[0057] In this embodiment of the invention, the pressure regulating valve 2 is a self-regulating pressure regulating valve. The high-temperature, high-pressure nitrogen source is a high-pressure, high-purity nitrogen cylinder group 1, with the nitrogen cylinders connected by steel cylinder connectors; the nitrogen cylinder group 1 is preferably a 16MPa four-gas high-purity nitrogen cylinder group. The low-temperature liquid nitrogen source is a liquid nitrogen tank 6.

[0058] Specifically, such as Figure 2As shown, the vacuum tank 4 is equipped with a vacuum pump connection pipe 20 for connecting to the vacuum pump 15. The vacuum pump 15 establishes a vacuum environment within the vacuum tank 4 to achieve insulation. A vacuum observation window is provided on the vacuum tank 4, connected to the main body of the vacuum tank via a flange. Preferably, the vacuum tank 4 has an upper observation window 17, a front observation window 18, and a rear observation window 23. The oppositely arranged front observation window 18 and rear observation window 23 are used to capture the spontaneous condensation process of cryogenic high-pressure nitrogen in the Laval nozzle using an optical measuring device 10 (e.g., a camera), and to provide supplementary lighting for the Laval nozzle 7 using a supplementary lighting instrument 16. The upper observation window 17 is used by the laser generator 11 to emit a light curtain into the Laval nozzle, facilitating the measurement of the flow rate of nitrogen droplets or cryogenic high-pressure nitrogen in the Laval nozzle using particle image velocimetry (PIV). The direction of the light curtain emitted by the laser generator 11 is perpendicular to the flow direction of the Laval nozzle.

[0059] like Figure 3 As shown, the Laval nozzle provides visibility, facilitating the capture of the spontaneous condensation process of cryogenic, high-pressure nitrogen within the nozzle and PIV experiments. Inside the Laval nozzle is a Laval nozzle channel 24. In this embodiment, the bottom of the Laval nozzle is supported by a stainless steel bracket 25, while the other surfaces are made of quartz glass 27. The quartz glass 27 is connected to the stainless steel bracket 25 via a stainless steel glass clamp. A sensor connection hole 26 is provided at the bottom of the bracket 25 for mounting temperature and pressure sensors 36.

[0060] The temperature sensor is bonded to the inner wall of the visible Laval nozzle using low-temperature thermally conductive grease, while the pressure sensor is connected to the outside of the Laval nozzle via a 304 stainless steel condenser tube. The measured pressure and temperature data are then transmitted to a data acquisition module outside the vacuum tank via an aviation connector 21 located on the outer wall for data acquisition and processing. After processing, the data is transmitted to the control module for corresponding control. In this embodiment, the control module consists of a synchronous controller 12 and a computer 14; the data acquisition module uses a multi-channel data acquisition instrument 13. A platinum resistance temperature sensor is preferred for the temperature sensor, and a high-performance pressure transmitter is selected for the pressure sensor.

[0061] As a further design of the invention, a nitrogen droplet flow rate measurement and control module is also included. This module is used to introduce oil-based particles or ice particles into the inlet end of the vacuum tank (i.e., the first connecting pipe). The flow rate of the cryogenic high-pressure nitrogen gas inside the Laval nozzle is measured using particle image velocimetry (PIV). This flow rate is compared with the flow rate of nitrogen droplets inside the Laval nozzle. At the same time, the flow rates of high-temperature high-pressure nitrogen gas and cryogenic liquid nitrogen are controlled so that the flow rates are within a set error range, further promoting the use of nitrogen droplets as tracer particles in wind tunnel experiments. The flow rate of nitrogen droplets inside the Laval nozzle is captured by an optical measurement device 10 (e.g., a camera) during the spontaneous condensation process of cryogenic high-pressure nitrogen gas inside the Laval nozzle and measured using particle image velocimetry (PIV).

[0062] As a further design of the present invention, a liquid phase parameter measurement module is also included, which is used to measure the mass fraction and surface tension of the liquid in the gas-liquid two-phase flow flowing out from the inside of the Laval nozzle. Specifically, it includes a droplet separation mechanism 33 and a droplet collection tank 31. The droplet separation mechanism 33 is disposed between the gas outlet end of the Laval nozzle 7 and the second connecting pipe 29, and is used to separate droplets by controlling the size and direction of the gas-liquid two-phase flow flowing out from the inside of the Laval nozzle.

[0063] like Figure 4 , Figure 5 As shown, the droplet separation mechanism 33 has a three-way structure, including a two-phase flow inlet pipe 37, a liquid outlet pipe 39 and a gas outlet pipe 40;

[0064] One end of the two-phase flow inlet pipe 37 is connected to the outlet end of the Laval nozzle 7, and there is a downward first included angle θ1 between it and the Laval nozzle 7 along the flow direction, where 0 < θ1 ≤ 20°. Under the designed first included angle, the two-phase flow can stably enter the two-phase flow inlet pipe 37, avoiding the splitting into small droplets due to excessive liquid flow during entry, which would affect the separation effect; at the same time, it avoids excessive pressure drop causing a decrease in fluid temperature, which would lead to further liquefaction of the two-phase flow and reduce the measurement accuracy of liquid phase parameters. Preferably, 5° ≤ θ1 ≤ 20°, within this range, the inertial acceleration of the two-phase flow can be used to achieve droplet separation.

[0065] One end of the gas outlet pipe 40 is connected to the second connecting pipe 29, and there is an upward second included angle θ2 between it and the Laval nozzle 7 along the flow direction, where 0 < θ2 ≤ 20°. The two-phase flow inlet pipe 37 and the gas outlet pipe 40 are symmetrically distributed about the vertical direction (y-axis) of the Laval nozzle 7, so that all components between the vacuum tank inlet and outlet are on the same axis, ensuring that each component is rigidly connected on the axis and avoiding contact between the components and the vacuum tank wall, which would damage the constructed cryogenic environment. At the same time, it allows the gas in the two-phase flow to pass smoothly through the three-way pipe. When the first included angle θ1 satisfies 5° ≤ θ1 ≤ 20°, the second included angle θ2 satisfies: 5° ≤ θ2 ≤ 20°.

[0066] The liquid outlet pipe 39 is connected to the other end of the two-phase flow inlet pipe 37 and the other end of the gas outlet pipe 40, forming a three-way structure. The liquid outlet pipe 39 and the Laval nozzle 7 form a downward third angle along the flow direction, preferably 45°. Preferably, the diameter of the liquid outlet pipe 39 is smaller than the diameters of the two-phase flow inlet pipe 37 and the gas outlet pipe 40, which can increase the liquid flow rate, reduce the interaction force of the separated droplets, and improve the separation efficiency.

[0067] The droplet collection tank 31 is used to collect droplets flowing out of the liquid outlet pipe 39 for measuring the mass fraction and surface tension of the liquid.

[0068] At the junction of the liquid outlet pipe 39 and the gas outlet pipe 40, a frustum-shaped perforated plate 38 is provided. The perforated plates are arranged in a closely staggered manner to increase the probability of the droplets being impacted by the two-phase flow and improve the efficiency of droplet separation.

[0069] By measuring the liquid phase parameters, the control module controls the holding time of the nitrogen droplets by adjusting the flow rates of high-temperature, high-pressure nitrogen and low-temperature liquid nitrogen, thereby controlling the time during which the nitrogen droplets act as tracer particles.

[0070] Preferably, a cryogenic plug 30 is provided at the position where the first connecting pipe 28 and the second connecting pipe 29 connect to the inside of the vacuum tank 4, in order to reduce the heat conduction between the first connecting pipe 28 and the second connecting pipe 29 and the main body of the vacuum tank 4; the non-visible metal part inside the vacuum tank is wrapped with multiple layers of heat insulation material; the heat exchanger shell is wrapped with heat insulation material to reduce heat loss during the heat exchange process between liquid nitrogen and nitrogen gas.

[0071] Preferably, a Laval nozzle front adapter section 31 is provided between the first connector 28 and the air inlet end of the Laval nozzle 7, and a Laval nozzle rear adapter section 32 is provided between the air outlet end of the Laval nozzle 7 and the droplet separation mechanism 33, for the purpose of connecting the interfaces.

[0072] Preferably, a bellows 35 is provided between the droplet separation mechanism 33 and the second connecting pipe 29 to balance the axial contraction of the experimental pipe in the deep cryogenic environment and to balance installation errors.

[0073] This invention utilizes high-precision, non-contact flow field measurement to ingeniously study the condensation phenomenon of nitrogen gas in a deep cryogenic environment. By controlling the growth rate of nitrogen droplets and changing their particle size and concentration, the invention achieves the study of the droplet size and concentration distribution control law of nitrogen droplets as tracer particles in cryogenic wind tunnels.

[0074] Simultaneously, this process also enabled the visualization and tracer characteristics study of the spontaneous condensation process of cryogenic nitrogen droplets. Furthermore, using the device of this invention, spontaneous condensation of cryogenic transonic nitrogen droplets was directly achieved within the Laval nozzle without the need for an additional cold source. This revealed the formation law of nitrogen droplets in high-speed cryogenic airflow, and allowed control over the particle size, concentration, traceability time, and flow field following properties of spontaneously condensed nitrogen droplets, enabling the use of nitrogen droplets as tracer particles in cryogenic wind tunnel experiments.

[0075] Those skilled in the art will readily understand that the above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A visual experimental device for controlling nitrogen droplets as tracer particles in a cryogenic environment, characterized in that, include: High-temperature and high-pressure nitrogen source, low-temperature liquid nitrogen source, heat exchanger (5), Laval nozzle (7), vacuum tank (4), first connecting pipe (28), second connecting pipe (29) and control module; The high-temperature high-pressure nitrogen source and the low-temperature liquid nitrogen source are respectively connected to the heat exchanger (5) to generate low-temperature high-pressure nitrogen; the inlet end of the Laval nozzle (7) passes through one end of the vacuum tank (4) through the first connecting pipe (28), and the outlet end passes through the other end of the vacuum tank (4) through the second connecting pipe (29), so that the Laval nozzle (7) is set inside the vacuum tank (4) and does not contact the inner wall of the vacuum tank (4); the heat exchanger (5) is connected to the first connecting pipe (28) passing through one end of the vacuum tank (4), and the Laval nozzle (7) is used to realize the spontaneous condensation of the low-temperature high-pressure nitrogen; The control module is used to change the temperature and pressure of the inlet end of the Laval nozzle (7) and the internal low-temperature high-pressure nitrogen by adjusting the flow rates of high-temperature high-pressure nitrogen and low-temperature liquid nitrogen, so as to control the particle size and concentration of nitrogen droplets during spontaneous condensation within a set range.

2. The apparatus according to claim 1, characterized in that, A mass flow meter (3) is provided at the inlet end of the heat exchanger (5); Temperature and pressure sensors (36) are equidistantly distributed along the air inlet end and lower wall surface of the Laval nozzle (7); The spontaneous condensation process within the Laval nozzle (7) is captured by an optical measuring device (10).

3. The apparatus according to claim 1, characterized in that, It also includes a nitrogen droplet flow rate measurement and control module, which is used to introduce oil-based particles or ice particles into the first connector (28), measure the flow rate of low-temperature high-pressure nitrogen in the Laval nozzle (7) by particle image velocimetry, and control the flow rates of high-temperature high-pressure nitrogen and low-temperature liquid nitrogen so that the flow rate of nitrogen droplets in the Laval nozzle (7) and the flow rate of low-temperature high-pressure nitrogen are within a set error range.

4. The apparatus according to claim 3, characterized in that, The spontaneous condensation process of the low-temperature high-pressure nitrogen gas in the Laval nozzle (7) is captured by an optical measuring device (10), and the flow rate of the nitrogen droplets in the Laval nozzle (7) is obtained by particle image velocimetry.

5. The apparatus according to claim 1, characterized in that, It also includes a droplet separation mechanism (33) and a droplet collection tank (31); The droplet separation mechanism (33) is disposed between the gas outlet end of the Laval nozzle (7) and the second connecting pipe (29) for separating nitrogen droplets from the gas-liquid two-phase flow flowing out of the Laval nozzle (7); The droplet collection tank (31) is used to collect the nitrogen droplets.

6. The apparatus according to claim 5, characterized in that, The droplet separation mechanism (33) has a three-way structure, including a two-phase flow inlet pipe (37), a liquid outlet pipe (39) and a gas outlet pipe (40); One end of the two-phase flow inlet pipe (37) is connected to the outlet end of the Laval nozzle (7), and there is a downward first included angle θ1 along the flow direction of the Laval nozzle (7), where 0 < θ1 ≤ 20°; One end of the gas outlet pipe (40) is connected to the second connecting pipe (29), and there is an upward second included angle θ2 along the direction of the Laval nozzle (7), where 0 < θ2 ≤ 20°; The liquid outlet pipe (39) is connected to the other end of the two-phase flow inlet pipe (37) and the other end of the gas outlet pipe (40), and has a downward third included angle along the direction of the Laval nozzle (7).

7. The apparatus according to claim 6, characterized in that, The first included angle θ1 and the second included angle θ2 satisfy: 5°≤θ1≤20°, and 5°≤θ2≤20°.

8. The apparatus according to claim 6 or 7, characterized in that, At the junction of the liquid outlet pipe (39) and the gas outlet pipe (40), a frustum-shaped perforated plate (38) is provided.

9. The apparatus according to any one of claims 5-7, characterized in that, A bellows (35) is also provided between the droplet separation mechanism (33) and the second connecting pipe (29).

10. The apparatus according to any one of claims 1-4, characterized in that, It also includes a buffer tank (8) connected to the outlet end of the second pipe (29), and a pressure sensor and a solenoid valve (9) are installed at the outlet of the buffer tank (8).

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