Development Of Visual System For Cryogenic Throttling Experiment And Parametric Study Of Liquid Nitrogen Throttling

Along with the development of deep space missions,especially manned mission to the moon,Mars,cryogenic fluids(liquid hydrogen,liquid oxygen,liquid methane,etc.)will gradually replace conventional propellants as the preferred propellants of new launch vehicles due to their advantages of higher specific impulse,non-toxicity,and cleanliness,so as to meet the requirements of future deep space missions.However,cryogenic propellants have the properties of low latent heat of vaporization and low boiling point,and is subject to various factors such as microgravity conditions and thermal radiation in outer space during in-orbit storage,which inevitably leads to the problem of evaporation boost.The technology of thermodynamic vent system is considered to be one of the feasible solutions for long-term in-orbit storage of cryogenic propellant.It introduces the concept of throttling,makes full use of the latent heat and sensible heat after throttling a small part of the discharge fluid,cools the rest of cryogenic propellant in the tank,and effectively controls the pressure within a reasonable range.The discharged fluid changes into a low pressure gas after heat exchange,so as to realize only exhaust but not drain.The throttling part is the core part of the thermodynamic vent system.When cryogenic fluid flows through the throttling part,there are pressure changes and corresponding gas-liquid changes.The throttling of liquid or gas-liquid two-phase cryogenic fluid is different from that of conventional gas and refrigerant.In order to accurately design and calculate the throttling parts required for applications such as thermodynamic vent system,it is necessary to carry out research on the flow characteristics and thermodynamic characteristics of cryogenic fluids in throttling process.In particular,relevant experimental apparatuses are set up to measure thermal parameters and analyze high-speed visual images,so as to reveal the interaction law between structural parameters of throttling parts,physical parameters of cryogenic fluids and throttling effect.Specifically,this paper carries out research work in the following aspects:(1)Based on Bernoulli equation and continuity equation,the relationship between the volume flow and the pressure,density before and after throttling is deduced.With liquid nitrogen as the working medium,the relation curves between subcooling degree before throttling and the mass fraction of gas phase,unit mass refrigerating capacity after throttling are plotted.the relation curves between the mass fraction of gas phase before throttling and the mass fraction of gas phase,unit mass refrigerating capacity after throttling are plotted.Combined with the first law of thermodynamics and differential equations,theoretical analysis is made.(2)An experimental apparatus is designed and constructed to study the visualization of gas-liquid two-phase flow in cryogenic fluid throttling.An experimental vacuum chamber and transparent throttling parts for cryogenic liquid throttling are designed.The subcooling device,heat sink structure and adiabatic measures are designed to ensure that the liquid nitrogen before throttling has a certain degree of subcooling.The stable control of pressure before throttling can be realized by using PLC to adjust the opening of cryogenic pressure regulating valve.The high speed camera and the coaxial but different side lighting can be used to take pictures of the flow pattern,the length of cavitation zone and the periodic shedding of bubbles after throttling.With the small orifice throttling parts and liquid nitrogen as the research objects,the apparatus is used to measure the physical parameters of the throttling parts with different subcooling degrees and pressures before throttling and take video pictures of the flowing structure.Based on the theoretical analysis in chapter 2,the relationships between the mass fraction of gas phase after throttling and the maximum temperature drop,cavitation number and the pressure ratio before and after throttling are revealed.The effects of different subcooling degrees and pressures before throttling on temperature drop,mass fraction of gas phase and unit mass refrigerating capacity after throttling were analyzed.(3)The simulation model of cryogenic fluid throttling process is constructed,and the numerical simulation of cryogenic fluid throttling process is carried out by selecting appropriate cavitation model,turbulence model,physical parameters,boundary conditions and other calculation settings.The distribution of pressure field,temperature field and mass fraction of gas phase of liquid nitrogen after throttling were obtained,and the results were compared with the measured experimental datas which shows that there is some error,and the reason of error is analyzed.To sum up,this paper establishes an experimental apparatus for visualization of gas-liquid two-phase flow in cryogenic throttling process which can control the throttling pressure and subcooling degree before throttling,and also establishes a numerical simulation model for throttling process.The relationships between the mass fraction of gas phase after throttling and the maximum temperature drop,cavitation number and the pressure ratio before and after throttling are revealed experimentally and theoretically.The effects of throttling pressure and subcooling degree before throttling of supercooled liquid nitrogen on temperature drop,pressure drop,mass fraction of gas phase and unit mass refrigerating capacity are clarified.The obtained results will be beneficial to optimize the design of throttling parts for aerospace cryogenic propellant storage applications so as to reduce propellant emission loss.