A liquid nitrogen subcooling system with jet flash evaporation impingement cooling

Abstract

The application discloses a kind of liquid nitrogen supercooling systems of jet flash evaporation impact cooling, it is related to low-temperature refrigeration and cooling technical field, including liquid nitrogen storage tank, supercooling box, heater, vacuum pump, cryogenic pump and to be cooled superconducting equipment, supercooling box includes end cover, cylinder, supercooling heat exchanger and turbulence device, supercooling heat exchanger is equipped with hollow cold plate, vacuum pump maintains the pressure in it in rough vacuum state, part liquid nitrogen in supercooling box enters hollow layer in cold plate through jet orifice opened in cold plate wall surface and occurs flash evaporation and forms low-temperature two-phase jet impact cooling cold plate, and then cool liquid nitrogen in supercooling box, turbulence device enhances heat exchange, liquid nitrogen in supercooling box is cooled to supercooling state by supercooling heat exchanger and is pressurized into to be cooled superconducting equipment by cryogenic pump after heat absorption, and return to supercooling box, liquid nitrogen storage tank replenishes liquid nitrogen consumed by supercooling heat exchanger, the application realizes jet impact cooling and turbulence enhanced heat transfer in supercooling box, and supercooling system has the advantages of high heat exchange efficiency and good heat exchange effect.

Description

Technical Field

[0001] This invention relates to the field of cryogenic refrigeration and cooling technology, specifically to a liquid nitrogen subcooling system using jet flash impingement cooling. Background Technology

[0002] With the rapid development of superconducting technology, high-temperature superconducting materials are widely used in equipment such as superconducting cables, superconducting magnets, and superconducting motors. Superconductors only exhibit superconducting properties below their critical temperature. However, superconducting equipment often faces heat loads due to factors such as AC losses, eddy current losses, and environmental heat leakage. Therefore, superconducting equipment must be equipped with corresponding cryogenic refrigeration and cooling systems. Furthermore, based on the temperature characteristics of superconductors, further reducing the operating temperature below the critical temperature can improve the current-carrying capacity of the superconductor.

[0003] Open-loop liquid nitrogen depressurization refrigeration systems are widely used in the refrigeration systems of superconducting devices due to their simplicity, reliability, low equipment and operating costs, and the safety and pollution-free nature of liquid nitrogen. Open-loop refrigeration utilizes the latent heat of the working fluid to absorb heat. Based on the principle of depressurization and cooling, reducing the vaporization pressure further lowers the refrigeration temperature. Liquid nitrogen has a boiling point of approximately 77K at atmospheric pressure, and when the pressure is reduced to 17kPa, the refrigeration temperature can reach 65K, which can meet the cooling temperature requirements of common high-temperature superconducting materials.

[0004] Open-loop liquid nitrogen vacuum refrigeration commonly uses two types of subcooling boxes: flooded and non-flooded. In a flooded subcooling box, high-pressure subcooled liquid nitrogen flows inside the subcooling heat exchanger, while low-pressure saturated liquid nitrogen flows outside. Due to the pressure gradient caused by gravity, there is a temperature difference between the upper and lower layers of saturated liquid nitrogen, with the lower layer having a higher temperature, resulting in lower cooling efficiency. In a non-flooded subcooling box, low-pressure saturated liquid nitrogen flows inside the subcooling heat exchanger, while high-pressure subcooled liquid nitrogen flows outside. To achieve a larger heat exchange area in a limited space, coiled or tube-bundled subcooling heat exchangers typically use smaller tube diameters. The two-phase flow pressure loss inside the subcooling heat exchanger tubes is significant, and the higher temperature along the front of the flow path leads to low heat exchange efficiency. Furthermore, the heat exchange of the subcooled liquid nitrogen outside the heat exchanger tubes is mainly through natural convection, which has a relatively low heat exchange intensity.

[0005] Based on the above technical background, in order to solve the problems of low heat exchange efficiency and poor heat exchange effect of liquid nitrogen subcooling box in open-loop liquid nitrogen depressurization refrigeration, the present invention proposes a liquid nitrogen subcooling system with jet flash evaporation impact cooling. Summary of the Invention

[0006] The purpose of this invention is to provide a liquid nitrogen subcooling system with jet flash impingement cooling, so as to solve the problems of low heat exchange efficiency and poor heat exchange effect of liquid nitrogen subcooling box in open-loop liquid nitrogen depressurization refrigeration mentioned in the background art.

[0007] To achieve the above objectives, the present invention provides the following technical solution: a liquid nitrogen subcooling system using jet flash impingement cooling, comprising a liquid nitrogen storage tank, a subcooling chamber, a heater, a vacuum pump, a cryogenic pump, and a superconducting device to be cooled. The subcooling chamber includes an end cap, a cylinder, a subcooling heat exchanger, and a turbulence device. The end cap is fixedly connected to the upper end of the cylinder, the subcooling heat exchanger is fixedly installed inside the subcooling chamber, and the end cap is fixedly connected to a liquid nitrogen inlet channel for introducing liquid nitrogen into the cylinder. The lower end of the cylinder is fixedly connected to a liquid nitrogen outlet for discharging liquid nitrogen. The liquid nitrogen outlet channel is fixedly connected to a cryogenic pump at one end. The subcooling heat exchanger has an annular cavity with a nitrogen outlet channel at the top. The end cap has a subcooling heat exchanger mounting hole, and the nitrogen outlet channel is fixedly inserted into the subcooling heat exchanger mounting hole. The lower end of the subcooling heat exchanger is fixedly connected to multiple hollow cold plates. The cold plates are interconnected with the annular cavity of the subcooling heat exchanger. The interior of the cold plates has a hollow layer, and the surface of the cold plates has jet holes. The jet holes are interconnected with the interior hollow layer of the cold plates and the main cavity of the subcooling chamber.

[0008] Liquid nitrogen, the cooling medium, is cooled to a supercooled state by a supercooling heat exchanger in the supercooling box. It then flows out through the liquid nitrogen outlet channel at the bottom of the cylinder and is pressurized by a cryogenic pump before entering the superconducting equipment to be cooled. After that, the liquid nitrogen returns to the supercooling box through the liquid nitrogen inlet channel on the end cover to form a cycle. The liquid nitrogen storage tank is replenished with liquid nitrogen consumed by the supercooling heat exchanger during the cycle through the liquid nitrogen inlet channel.

[0009] The subcooled heat exchanger operates under a low-pressure, rough vacuum state, typically around 17 kPa. At this pressure, liquid nitrogen has a lower saturation temperature compared to atmospheric pressure. The main chamber of the subcooled chamber is at atmospheric pressure, and the liquid nitrogen level within it is maintained between the upper and lower surfaces of the annular cavity of the subcooled heat exchanger. Driven by the pressure difference, the liquid nitrogen in the main chamber is injected through jet holes on the cold plate into the hollow layer of the lower cold plate of the subcooled heat exchanger. Under low pressure, the liquid nitrogen undergoes flash evaporation, forming a low-temperature two-phase jet that cools the cold plate wall and subsequently the liquid nitrogen in the main chamber. The turbulence device in the subcooled chamber enhances heat exchange between the liquid nitrogen in the main chamber and the outer wall of the cold plate. The nitrogen gas produced by the vaporization of liquid nitrogen in the subcooled heat exchanger flows out through the nitrogen outlet channel. After being heated to the allowable temperature of the vacuum pump in the heater, it enters the vacuum pump, which maintains the pressure of the subcooled heat exchanger at a rough vacuum state.

[0010] In a further embodiment, the turbulence device includes a turbulence impeller, a bearing, and a drive motor. The turbulence impeller has multiple uniformly distributed blades to enhance the circumferential flow of liquid nitrogen in the main cavity of the subcooling box. The turbulence impeller is rotatably installed in the main cavity of the subcooling box. The surrounding impeller is located inside the subcooling heat exchanger. The axis of the turbulence impeller coincides with the axis of the annular cavity of the subcooling heat exchanger. The outer diameter of the surrounding impeller is smaller than the inner diameter of the annular cavity of the subcooling heat exchanger.

[0011] A drive motor mounting plate for mounting the drive motor is fixedly installed on the upper end of the end cover. The end cover has bearing mounting holes for mounting bearings. A rotating shaft is fixedly connected to the upper end of the turbulence impeller. The rotating shaft passes through the bearing and the bearing mounting holes and is fixedly connected to one end of the output shaft of the drive motor. The drive motor drives the turbulence impeller to rotate. The bearing and drive motor are located outside the subcooling chamber, avoiding the low-temperature environment to ensure the normal operation of the bearing and drive motor.

[0012] In a further embodiment, the end caps and interior of the subcooling chamber are provided with high-vacuum insulation layers with excellent thermal insulation properties. These high-vacuum insulation layers enhance the thermal insulation performance of the end caps and interior of the subcooling chamber. Maintaining a certain distance between the liquid nitrogen level in the main cavity of the subcooling chamber and the end caps helps reduce heat leakage from the end caps.

[0013] In a further embodiment, the upper end of the end cap is provided with a sheathed thermocouple mounting hole for installing a sheathed thermocouple. By adjusting the length of the sheathed thermocouple, temperature information at different depths of the subcooled box can be obtained.

[0014] In a further embodiment, the upper end of the end cap has a level gauge mounting hole for mounting a level gauge measuring rod. The level gauge measuring rod is installed in the level gauge mounting hole to measure the liquid nitrogen level in the supercooled box.

[0015] In a further embodiment, multiple cold plates are evenly distributed in the lower part of the annular cavity of the subcooled heat exchanger, and a flow channel with a gradually decreasing cross-sectional area is formed between two adjacent cold plates. Through the gradually decreasing flow channel, the liquid nitrogen is accelerated in the flow channel, which is conducive to heat exchange with the outer wall of the cold plate.

[0016] In a further embodiment, the jet holes on the cold plate are evenly distributed on the front and rear walls of the cold plate, respectively. The projections of the jet hole axes on the front and rear walls onto the plate plane are staggered. Thus, the two-phase jet formed by the jet holes on the front wall of the cold plate impacts and cools the rear wall, and the two-phase jet formed by the jet holes on the rear wall impacts and cools the front wall.

[0017] In a further embodiment, the cross-sectional area of ​​the jet holes on the cold plate gradually decreases along the jet direction, and the diameter of the jet holes must satisfy the following relationship formula:

[0018]

[0019] Wherein, d1 is the diameter of the jet hole on the outer wall of the cold plate, d2 is the diameter of the jet hole on the inner wall of the cold plate, p1 is the pressure inside the main cavity of the subcooling box, p2 is the pressure inside the subcooling heat exchanger, N is the total number of jet holes, ρ is the density of liquid nitrogen, r is the latent heat of vaporization of liquid nitrogen at pressure p2, and Q is the system heat load, including the heat generated by the superconducting equipment to be cooled and other heat leakage.

[0020] In a further embodiment, the outlet diameter of the jet hole on the inner wall of the cold plate is such that the Reynolds number at the outlet position is not less than the critical Reynolds number of 2300, which is the transition from laminar to turbulent flow in the pipe. That is, the outlet diameter d2 must satisfy the following relationship formula:

[0021]

[0022] Where d2 is the outlet diameter of the jet hole on the inner sidewall of the cold plate, Q is the system heat load, N is the total number of jet holes, r is the latent heat of vaporization of liquid nitrogen under working conditions, and μ is the dynamic viscosity of liquid nitrogen under working conditions.

[0023] Compared with the prior art, the beneficial effects of the present invention are:

[0024] This invention relates to a liquid nitrogen subcooling system using jet flash evaporation and impact cooling. The subcooling heat exchanger operates under a low-pressure, rough vacuum state, with the internal pressure maintained at approximately 17 kPa. The main chamber of the subcooling box is at atmospheric pressure. The liquid nitrogen level in the main chamber is maintained between the upper and lower surfaces of the annular cavity of the subcooling heat exchanger. Driven by the pressure difference, the liquid nitrogen in the main chamber is injected through jet holes on the cold plate into the hollow layer of the lower cold plate of the subcooling heat exchanger. Under low pressure, the liquid nitrogen flashes, forming a low-temperature two-phase jet that cools the cold plate wall and subsequently the liquid nitrogen in the main chamber of the subcooling box. The turbulence device in the subcooling box enhances the heat exchange between the liquid nitrogen in the main chamber and the outer wall of the cold plate, solving the problems of low heat exchange efficiency and poor heat exchange effect in traditional liquid nitrogen subcooling boxes used in open-loop liquid nitrogen depressurization refrigeration. Attached Figure Description

[0025] Figure 1 This is a schematic diagram of the overall scheme of a liquid nitrogen subcooling system using jet flash impingement cooling according to an embodiment of the present invention;

[0026] Figure 2 This is a three-dimensional model diagram of the subcooling chamber according to an embodiment of the present invention;

[0027] Figure 3 This is a two-dimensional cross-sectional view of the subcooling chamber according to an embodiment of the present invention;

[0028] Figure 4 This is a schematic diagram showing the arrangement of the jet holes on the cold plate of the subcooled heat exchanger according to an embodiment of the present invention.

[0029] Figure 5 This is a bottom view of the subcooling heat exchanger according to an embodiment of the present invention;

[0030] Figure 6 This is a cross-sectional view of the jet holes in the cold plate of the subcooled heat exchanger according to an embodiment of the present invention;

[0031] Figure 7This is a three-dimensional model diagram of the impeller of the turbulence device according to an embodiment of the present invention.

[0032] In the diagram: 1. Liquid nitrogen storage tank; 2. Subcooling box; 21. Turbulence device; 3. Heater; 4. Vacuum pump; 5. Cryogenic pump; 6. Superconducting equipment to be cooled; 7. End cap; 71. Liquid nitrogen inlet channel; 72. Subcooling heat exchanger mounting hole; 73. Armored thermocouple mounting hole; 74. Drive motor mounting plate; 75. Bearing mounting hole; 76. Liquid level gauge mounting hole; 8. Cylinder; 81. Liquid nitrogen outlet channel; 9. Subcooling heat exchanger; 91. Nitrogen outlet channel; 92. Cold plate; 10. Turbulence impeller; 11. Bearing; 12. Drive motor. Detailed Implementation

[0033] The technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0034] Example

[0035] Please see Figure 1-7 This embodiment provides a liquid nitrogen subcooling system with jet flash impingement cooling, including a liquid nitrogen storage tank 1, a subcooling box 2, a heater 3, a vacuum pump 4, a cryogenic pump 5, and a superconducting device to be cooled 6.

[0036] Among them, liquid nitrogen storage tank 1 is connected to subcooling box 2 by pipeline, subcooling box 2 is connected to heater 3 by pipeline, heater 3 is connected to vacuum pump 4 by pipeline, cryogenic pump 5 is connected to subcooling box 2 by pipeline, cryogenic pump 5 is connected to superconducting device 6 to be cooled by pipeline, and superconducting device 6 to be cooled is connected to subcooling box 2 by pipeline.

[0037] The subcooling box 2 includes an end cover 7, a cylinder 8, a subcooling heat exchanger 9, and a turbulence device 21. The end cover 7 is fixedly installed on the upper end of the cylinder 8, and the cylinder 8 is closed by the end cover 7. The subcooling heat exchanger 9 is located in the main cavity of the subcooling box 2.

[0038] The end cap 7 is fixedly connected to a liquid nitrogen inlet channel 71 at its upper end, and the cylinder 8 is fixedly connected to a liquid nitrogen outlet channel 81 at its lower end. The end cap 7 is connected to the superconducting device 6 to be cooled through the liquid nitrogen inlet channel 71, and the cylinder 8 is connected to the cryogenic pump 5 through the liquid nitrogen outlet channel 81.

[0039] The subcooling heat exchanger 9 has an annular cavity. A nitrogen outlet channel 91 is fixedly connected to the upper end of the annular cavity of the subcooling heat exchanger 9. The nitrogen outlet channel 91 is interconnected with the annular cavity of the subcooling heat exchanger 9. A subcooling heat exchanger mounting hole 72 is opened at the upper end of the end cover 7. The nitrogen outlet channel 91 is fixedly inserted into the subcooling heat exchanger mounting hole 72. The subcooling heat exchanger 9 is then fixed inside the cylinder 8 through the nitrogen outlet channel 91. One end of the nitrogen outlet channel 91 is fixedly connected to the heater 3. The heater 3 is interconnected with the subcooling heat exchanger 9 through the nitrogen outlet channel 91.

[0040] Specifically, the liquid nitrogen storage tank 1 in this embodiment can be a self-pressurizing type or a type where liquid nitrogen is driven by high-pressure nitrogen. The heater 3 in this embodiment can be an electric heating type, or a water bath heating type, or an air heating type, or other types. The vacuum pump 4 in this embodiment can be an oil-free rotary vane pump or an oil-type vacuum pump with anti-backflow measures. The superconducting device 6 to be cooled in this embodiment can be a superconducting motor, a superconducting transformer, a superconducting current limiter, or other superconducting devices that require cooling.

[0041] To facilitate the cooling of liquid nitrogen in the subcooling chamber, a cold plate 92 is installed below the annular cavity of the subcooling heat exchanger 9.

[0042] The cold plate 92 is configured as a plurality of cold plates, which are evenly distributed in a ring array below the annular chamber of the subcooling heat exchanger 9. The cold plate 92 has a hollow layer inside, which is connected to the annular chamber of the subcooling heat exchanger 9. The cold plate 92 has jet holes, which are preferably configured as a plurality of jet holes, so that the main cavity of the subcooling box 2 is connected to the hollow layer of the cold plate 92 through the jet holes.

[0043] The subcooled heat exchanger 9 is in a low-pressure rough vacuum state, while the subcooled box 2 is in a normal pressure state. The liquid nitrogen level in the subcooled box 2 is maintained between the upper and lower surfaces of the annular cavity of the subcooled heat exchanger 9. Driven by the pressure difference, the liquid nitrogen in the subcooled box 2 is injected into the hollow layer of the lower cold plate 92 of the subcooled heat exchanger 9 through the jet holes on the cold plate 92. The liquid nitrogen flashes under low pressure to form a low-temperature two-phase jet, which cools the plate wall of the cold plate 92 and then cools the liquid nitrogen in the cylinder 8.

[0044] Nitrogen gas generated by the vaporization of liquid nitrogen in the subcooling heat exchanger 9 flows out through the nitrogen outlet channel 91 and enters the heater 3. In the heater 3, it is heated to the allowable temperature of the vacuum pump 4, and the pressure in the subcooling heat exchanger 9 is maintained in a rough vacuum state by the vacuum pump 4.

[0045] Preferably, a flow channel with a gradually decreasing cross-sectional area is formed between two adjacent cold plates 92, so that the liquid nitrogen accelerates in the flow channel, which is conducive to heat exchange with the outer wall of the cold plate 92.

[0046] Preferably, the jet holes on the cold plate 92 are evenly distributed on both walls of the cold plate 92, and the projections of the jet hole axes on the plate wall plane are staggered. Thus, the two-phase jet formed by the jet holes on the front wall of the cold plate 92 impacts and cools the rear wall, and the two-phase jet formed by the jet holes on the rear wall impacts and cools the front wall.

[0047] Preferably, the cross-sectional area of ​​the jet holes on the cold plate 92 gradually decreases along the jet direction, causing the fluid to gradually accelerate in the jet hole channel, which is beneficial for the formation of a spray flow. The orifice diameter of the jet holes needs to satisfy the following relationship formula:

[0048]

[0049] Wherein, d1 is the diameter of the jet hole on the outer side wall of the cold plate 92, d2 is the diameter of the jet hole on the inner side wall of the cold plate 92, p1 is the pressure inside the main cavity of the subcooling box 2, p2 is the pressure inside the subcooling heat exchanger 9, N is the total number of jet holes, ρ is the density of liquid nitrogen, r is the latent heat of vaporization of liquid nitrogen at pressure p2, and Q is the system heat load, including the heat generated by the superconducting device 6 to be cooled and other heat leakage.

[0050] Preferably, the outlet diameter of the jet orifice on the inner wall of the cold plate 92 needs to ensure that the Reynolds number at the outlet position is not less than the critical Reynolds number of 2300, which is the transition from laminar to turbulent flow in the pipe, so that the jet can easily form a spray flow. That is, the outlet orifice diameter d2 needs to satisfy the following formula:

[0051]

[0052] Where d2 is the diameter of the jet hole on the inner wall of the cold plate 92, Q is the system heat load, N is the total number of jet holes, r is the latent heat of vaporization of liquid nitrogen under working conditions, and μ is the dynamic viscosity of liquid nitrogen under working conditions.

[0053] In order to enhance the heat exchange between liquid nitrogen in the main cavity of the supercooled box 2 and the outer wall of the cold plate 92, the turbulence device 21 includes a turbulence impeller 10, a bearing 11 and a drive motor 12.

[0054] The end cover 7 has a bearing mounting hole 75, and the bearing 11 is fixedly installed in the bearing mounting hole 75. The upper end of the end cover 7 is fixedly installed with a drive motor mounting plate 74, and the drive motor 12 is fixedly installed on one side of the drive motor mounting plate 74. The drive motor 12 is fixed by the drive motor mounting plate 74, and the output shaft of the drive motor 12 faces downward. The turbulence impeller 10 is rotatably installed in the subcooling heat exchanger 9. The axis of the turbulence impeller 10 coincides with the axis of the annular cavity of the subcooling heat exchanger 9, and the turbulence impeller 10 is located between multiple cold plates 92.

[0055] A rotating shaft is fixedly connected to the upper end of the turbulence impeller 10. The rotating shaft passes through the subcooled heat exchanger 9, the bearing mounting hole 75, and the bearing 11 and is fixedly connected to one end of the output shaft of the drive motor 12. By starting the drive motor 12, the rotating shaft is driven to rotate, which in turn drives the turbulence impeller 10 to rotate. Blades are fixedly connected to the outer wall of the turbulence impeller 10. Multiple blades are arranged in a ring array and evenly distributed on the outer wall of the turbulence impeller 10. The rotation of the turbulence impeller 10 enhances the circumferential flow of liquid nitrogen in the cylinder 8.

[0056] Specifically, the connection between the output shaft of the drive motor 12 and the upper rotating shaft of the turbulence impeller 10 can be achieved by key connection, coupling connection or other feasible connection methods.

[0057] Liquid nitrogen, the cooling medium, is cooled to a subcooled state by the subcooling heat exchanger 9 in the subcooling box 2. It then flows out through the liquid nitrogen outlet channel 81 at the bottom of the cylinder 8 and is pressurized by the cryogenic pump 5 before entering the superconducting device 6 to be cooled. After that, the liquid nitrogen returns to the subcooling box 2 through the liquid nitrogen inlet channel 71 on the end cover 7 to form a cycle. The liquid nitrogen storage tank 1 is replenished with liquid nitrogen consumed during the cycle through the liquid nitrogen inlet channel 71.

[0058] In order to ensure that the subcooling box 2 has good thermal insulation performance, the end cover 7 and the inside of the cylinder 8 of the subcooling box 2 are equipped with high vacuum thermal insulation jacket.

[0059] To maintain a certain distance between the liquid nitrogen level and the end cap 7, which helps reduce heat leakage from the end cap 7, a level gauge is installed on the end cap 7.

[0060] The end cap 7 has a liquid level gauge mounting hole 76. The measuring rod of the liquid level gauge is threadedly connected to the liquid level gauge mounting hole 76, so that the measuring rod of the liquid level gauge extends into the cylinder 8 to measure the liquid nitrogen level in the cylinder 8.

[0061] Specifically, the level gauge can be a capacitive level gauge or a level gauge suitable for cryogenic fluids. The sealing method of the level gauge mounting hole 76 can be a polytetrafluoroethylene gasket end face seal, a plug seal, or other sealing methods effective at low temperatures.

[0062] To facilitate the installation of armored thermocouples on the subcooling box 2, an armored thermocouple mounting hole 73 is provided at the upper end of the end cover 7.

[0063] The armored thermocouple is fixedly installed in the armored thermocouple mounting hole 73 through a movable ferrule connector. By adjusting the length of the armored thermocouple, temperature information at different depths of the subcooled box 2 can be obtained.

[0064] Specifically, the armored thermocouple can be a T-type thermocouple or other low-temperature thermocouple with a temperature measurement range that can reach the liquid nitrogen temperature range. The sealing method of the armored thermocouple mounting hole 73 can be a polytetrafluoroethylene gasket end face seal, a plug seal, or other effective sealing methods at low temperatures.

[0065] The working process of the jet flash impingement cooling liquid nitrogen subcooling system is explained below:

[0066] A small portion of the liquid nitrogen in the subcooling chamber 2 enters the subcooling heat exchanger 9, where it vaporizes and absorbs heat. The subcooling heat exchanger 9 is under a low-pressure rough vacuum, while the subcooling chamber 2 is under atmospheric pressure. The liquid nitrogen level in the subcooling chamber 2 is maintained between the upper and lower surfaces of the annular cavity of the subcooling heat exchanger 9. Driven by the pressure difference, the liquid nitrogen in the subcooling chamber 2 is injected through the jet holes on the cold plate 92 into the hollow layer of the lower cold plate 92 of the subcooling heat exchanger 9. Under low pressure, the liquid nitrogen undergoes flash evaporation, forming a low-temperature two-phase jet that cools the plate wall of the cold plate 92 and then cools the liquid nitrogen inside the cylinder 8. The nitrogen gas generated by the vaporization of the liquid nitrogen in the subcooling heat exchanger 9 flows out through the nitrogen outlet channel 91 and enters the heater 3. In the heater 3, it is heated to the allowable temperature of the vacuum pump 4, which maintains the pressure inside the subcooling heat exchanger 9 under a rough vacuum state.

[0067] When the subcooled heat exchanger 9 maintains its internal absolute pressure at 17 kPa through the vacuum pump 4, the temperature of the low-temperature liquid nitrogen two-phase jet in the hollow layer of the cold plate 92 is about 65 K, and the outlet temperature of the nitrogen outlet channel 91 is also about 65 K. It is heated in the heater 3 to the allowable temperature of the vacuum pump 4, typically 20 to 80 °C.

[0068] Most of the liquid nitrogen in the subcooling box 2 is cooled to a subcooled state by the subcooling heat exchanger 9. It flows out through the liquid nitrogen outlet channel 81 at the bottom of the cylinder 8 and is then pressurized by the cryogenic pump 5 before entering the superconducting device 6 to be cooled. After that, the liquid nitrogen returns to the subcooling box 2 through the liquid nitrogen inlet channel 71 on the end cover 7 to form a cycle. The liquid nitrogen storage tank 1 is replenished with liquid nitrogen consumed by the subcooling heat exchanger 9 during the cycle through the liquid nitrogen inlet channel 71.

[0069] Due to the extremely high heat transfer coefficients of phase change heat transfer and shock cooling, and the presence of a flow-around device in the subcooling chamber 2 to heat the circumferential flow and enhance heat transfer between liquid nitrogen and the cold plate 92, a small heat transfer temperature difference can be achieved. The temperature of the liquid nitrogen cooled by the cold heat exchanger 9 can reach below 68K. The subcooled liquid nitrogen output from the subcooling chamber 2 is pressurized by the cryogenic pump 5 and transported to the superconducting device 6 to be cooled to absorb heat. To avoid local heat transfer deterioration in the superconducting device 6 due to liquid nitrogen vaporization, which could lead to equipment damage, the temperature of the liquid nitrogen at the outlet of the superconducting device 6 should be controlled below the liquid nitrogen saturation temperature corresponding to the operating pressure. For example, when the operating pressure is 200 kPa, the temperature of the liquid nitrogen at the outlet of the superconducting device 6 should be less than 83K.

[0070] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A liquid nitrogen subcooling system with fluidic flash evaporation impingement cooling, characterized by: The application relates to a superconducting device cooling device, which comprises a liquid nitrogen storage tank (1), a subcooling box (2), a heater (3), a vacuum pump (4), a low-temperature pump (5) and a superconducting device (6) to be cooled, the subcooling box (2) comprises an end cover (7), a cylinder body (8), a subcooling heat exchanger (9) and a flow disturbing device (21), the liquid nitrogen storage tank (1) and the low-temperature pump (5) are respectively connected with two ends of the subcooling box (2) through pipelines, two ends of the superconducting device (6) to be cooled are respectively connected with the subcooling box (2) and the low-temperature pump (5) through pipelines, and two ends of the heater (3) are respectively connected with the subcooling box (2) and the vacuum pump (4) through pipelines. The end cover (7) is fixedly connected with the upper end of the cylinder body (8), the subcooling heat exchanger (9) is fixedly installed in the subcooling box (2), the end cover (7) is fixedly connected with a liquid nitrogen inlet channel (71) for guiding liquid nitrogen into the cylinder body (8), the lower end of the cylinder body (8) is fixedly connected with a liquid nitrogen outlet channel (81) for guiding liquid nitrogen out, one end of the liquid nitrogen outlet channel (81) is fixedly connected with the low-temperature pump (5), the subcooling heat exchanger (9) has an annular cavity, the annular cavity is provided with a nitrogen outlet channel (91) at the upper portion, the end cover (7) is provided with a subcooling heat exchanger mounting hole (72), and the nitrogen outlet channel (91) is fixedly inserted into the subcooling heat exchanger mounting hole (72). The annular cavity of the subcooling heat exchanger (9) is provided with a plurality of hollow cold plates (92) at the lower portion, the cold plates (92) are in communication with the annular cavity of the subcooling heat exchanger (9), the inside of the cold plates (92) is provided with a hollow layer, the surface of the cold plates (92) is provided with jet holes, and the jet holes are in communication with the hollow layer in the cold plates (92) and the main cavity of the subcooling box (2).

2. The liquid nitrogen subcooling system with fluidic flash evaporation and impingement cooling of claim 1, wherein: The flow disturbing device (21) comprises a flow disturbing impeller (10), a bearing (11) and a driving motor (12), the flow disturbing impeller (10) has a plurality of uniformly distributed blades, the flow disturbing impeller (10) is rotatably installed in the main cavity of the subcooling box (2), the flow disturbing impeller (10) is located at the inner side of the subcooling heat exchanger (9), the shaft center of the flow disturbing impeller (10) is coincident with the shaft center of the annular cavity of the subcooling heat exchanger (9), and the outer diameter of the flow disturbing impeller (10) is smaller than the inner diameter of the annular cavity of the subcooling heat exchanger (9). The upper end of the end cover (7) is fixedly installed with a driving motor mounting plate (74) for mounting the driving motor (12), the end cover (7) is provided with a bearing mounting hole (75) for mounting the bearing (11), the upper end of the flow disturbing impeller (10) is fixedly connected with a rotating shaft, and the rotating shaft penetrates through the bearing (11) and the bearing mounting hole (75) and is fixedly connected with one end of the output shaft of the driving motor (12).

3. The liquid nitrogen subcooling system of flash evaporation impingement cooling according to claim 1, characterized in that: The end cover (7) and the inside of the cylinder body (8) of the subcooling box (2) are both provided with high-vacuum heat insulation interlayers.

4. The liquid nitrogen subcooling system of flash evaporation impingement cooling according to claim 3, characterized in that: The upper end of the end cover (7) is provided with an armored thermocouple mounting hole (73) for mounting an armored thermocouple.

5. A liquid nitrogen subcooling system with fluidic flash evaporation impingement cooling according to claim 4, characterized in that: The upper end of the end cover (7) is provided with a liquid level meter mounting hole (76) for mounting a liquid level meter measuring rod.

6. The liquid nitrogen subcooling system of flash evaporation impingement cooling according to claim 1, characterized in that: The cold plates (92) are uniformly distributed in the lower part of the annular cavity of the supercooling heat exchanger (9), and the flow channel with gradually decreasing cross-sectional area is formed between two adjacent cold plates (92).

7. A liquid nitrogen subcooling system with fluidic flash evaporation impingement cooling according to claim 6, characterized in that: The jet holes on the cold plate (92) are uniformly distributed on the front plate wall and the rear plate wall of the cold plate (92), respectively, and the projections of the jet hole axes on the plate wall plane are staggered.

8. A liquid nitrogen subcooling system with fluidic flash evaporation impingement cooling according to claim 7, characterized in that: The jet holes on the cold plate (92) gradually decrease in cross-sectional area along the jet direction, and the hole diameter of the jet hole needs to satisfy the following relationship formula: Wherein, d1 is the jet hole diameter at the outer side wall of the cold plate (92), d2 is the jet hole diameter at the inner side wall of the cold plate (92), p1 is the pressure in the main cavity of the supercooling tank (2), p2 is the pressure in the supercooling heat exchanger (9), N is the total number of jet holes, ρ is the density of liquid nitrogen, r is the latent heat of vaporization of liquid nitrogen under the pressure p2, Q is the system heat load, including the heat generation of the superconducting device (6) to be cooled and other heat leakage.

9. A liquid nitrogen subcooling system with fluidic flash evaporation impingement cooling according to claim 8, characterized in that: The outlet diameter of the jet hole at the inner side wall of the cold plate (92) makes the Reynolds number at the outlet position not less than the critical Reynolds number 2300 of the transition from laminar flow to turbulent flow in the pipe, that is, the outlet diameter d2 needs to satisfy the following relationship formula: Wherein, d2 is the outlet diameter of the jet hole at the inner side wall of the cold plate (92), Q is the system heat load, N is the total number of jet holes, r is the latent heat of vaporization of liquid nitrogen under working condition, and μ is the dynamic viscosity of liquid nitrogen under working condition.

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