Research On The Mechanism Of Cryogenic Cooling Of Self-pressurized System In A Point Layout Cold-shield System

With the rapid development of modern technology,the emergence of hypersonic vehicles provides a strong guarantee for national defence security,while more and more countries are joining the research on hypersonic defence systems and surprise defence technology.Infrared stealth technology uses space cold screen system to implement cold shielding effect on the surface of the vehicle,reducing the surface temperature and infrared radiation intensity of the vehicle to achieve stealth purposes.However,there is little research on infrared suppression technology for liquid nitrogen cold screen systems at home and abroad.The system combines self-pressurized technology,throttling cooling technology and phase change cooling technology to effectively cool down the surface of the vehicle so that its infrared radiation intensity meets the stealth requirements.This paper will focus on the research related to this system,as follows:(1)Starting from the energy conservation equation and mass conservation equation,the mathematical model of heat transfer between the self-pressurized liquid nitrogen tank and the wall of the cold screen is constructed,the equations of mass and heat transfer and flow resistance of the liquid nitrogen tank,booster tube and vapouriser are derived,and the calculation methods are analysed,so as to determine the matching relationship between the selfpressurized liquid nitrogen tank and the point layout type cold screen system.(2)The numerical simulation method is used to construct a simulation model of the throttling process of the throttling valve with different structures and different working conditions of nitrogen,and to select a suitable turbulence model and physical parameters for the numerical simulation of the nitrogen throttling refrigeration process of the self-pressurized system;the distribution of the temperature field,velocity field,pressure field and density field after the throttling of nitrogen is compared and analysed,and the distribution of each parameter of nitrogen along a certain flow line is analysed;the unit refrigeration capacity and refrigeration power of the throttling process are calculated according to the average distribution of each parameter at the outlet of the throttling valve,and the throttling refrigeration effect of different structures of the throttling valve is compared and analysed through data charts.(3)The point layout cold-shield system unit system model with staggered and side-by-side arrangement is constructed and the response surface method is applied to carry out a quadratic response surface analysis of the space cold screen system.The maximum temperature on the surface of cold-shield system is used as the response index,and the diameter of the refrigeration area of the self-pressurized liquid nitrogen vessel,the axis distance,the temperature of the refrigeration area and the thickness of the wall of the cold-shield system are used as response factors to obtain the quadratic polynomial relationship between the maximum temperature and the best combination of structural parameters of cold-shield system.The above research shows that the two types of cold-shield system structures proposed in this paper can meet the requirements of the infrared radiation intensity index on the surface of the aircraft,which can solve the infrared stealth problem of the aircraft and provide a reference for the design and research of the cold-shield system structures of the aircraft.Through numerical simulation,it is found that different structures of throttle valves throttle the same working condition of nitrogen will obtain different flow field distribution,the calculation of different structures of throttle valves to compare the cooling capacity and cooling power,it is found that the best cooling effect when the throttle nitrogen working condition is 0.2MPa,and appropriate increase throttle valve wall shaft angle,shaft cross-section diameter can enhance the cooling effect and save the use of refrigerant,indicating that the throttle valve can ensure the normal operation of the point layout space cold-shield system.According to the experimental data,the obtained quadratic polynomial for the highest temperature of the coldshield system surface and the best combination of parameter structure are more accurate,and the average error between the predicted and simulated values on the experimental values are within 15%,which can be used to optimise the layout of the point layout cold-shield system;comparison shows that the cooling effect of the staggered layout cold-shield system is better than that of the side-by-side layout cold-shield system,and the cold-shield system with different layouts can all reduce the surface temperature of the aircraft to 100 K to meet its surface infrared radiation intensity requirements and achieve infrared stealth.