Study On The Flow And Heat Transfer Characteristics Of Cryogenic Propellant Filling Process Under Special Conditions

Cryogenic propellants based on liquid hydrogen and liquid oxgen which own the advantages of high specific impulse as well as no pollution is the main fuel of China’s large thrust rocket in the future,and they are the important guarantee for China to “explore the vast universe,develop the space industry,and construct a space power”.In-depth study of the basic problems of heat and mass transfer of cryogenic propellants during the refueling process is conducive to the safe,stable,and efficient filling of cryogenic propellants,and is more conducive to the realization of China’s space dream.This thesis mainly studies the flow and heat transfer problems encountered by cryogenic fluids during the filling process of cryogenic propellants under special conditions such as vibration,microgravity and supercritical,including the flow boiling characteristics of the cryogenic propellant in vibration pipeline under terrestrial gravity condition of the external rocket filling system,the cryogenic propellants flow boiling characterstics under microgravity condtion of the on-orbit filling system,and the natural convective heat transfer characteristics of the supercritical helium in the pressurerization system of the internal rocket.Aiming at the external refilling process,a liquid hydrogen flow boiling model coupled with vibration,flow and heat transfer was constructed based on the Rensselaer Polytechnic Institute boiling model and coupled level-set and VOF model which describe the liquid hydrogen flow boiling in the tube,a simple harmonic boundary condition was introduced to the wall in the form of dynamic mesh with user defined functions.Then,the liquid hydrogen flow boiling heat transfer,pressure drop characteristics as well as flow state in the tube at different flow velocities,vibration amplitudes,and frequencies were analyzed and discussed.For the on-orbit refilling process,the bubble department mechanical model of liquid hydrogen as well as the numerical flow boililing model which considers the temperature rise of the vapor phase were established.The influence of gravity on the liquid hydrogen flow boiling was analyzed from three aspect including the individual hydrogen bubble department characteristic in microscopic,flow boiling heat transfer characteristics and pressure drop characteristics in macroscopic as well as the crtical heat flux condition.The mechanism of gravity influence on liquid hydrogen bubble department,flow boiling heat transfer and crtical heat flux condition was revealed.For the refilling process within the rocket,an experimental platform was buit in order to study the cryogenic supercritical helium natural convection heat transfer in confined space.The dynamic temperature characteristics and pressure characteristics during the natural convection of the pressurized gas in a spherical cavity under different initial pressure and heating power were analyzed.In addition,an experimentally verified natural convection model of cryogenic supercritical helium was adopted and the natural convection heat transfer characteristics of cryogenic supercritical helium in the spherical cavity under different pressure conditions were simulated.And a new correlation with higher precision of cryogenic supercritical helium natural convection heat transfer was proposed.Based on the above study,the main conclusions of this thesis were as follows:(1)The flow boiling state of liquid hydrogen changes greatly under vibration condition.Tube vibration significantly enhances the heat transfer between the wall and the cryogec fluid wtihin the tube,and makes the pressure drop inside the tube fluctuates sharply;also it destroys the stable vapor-liquid interface during flow boiling;the relative heat transfer coefficient and pressure drop show periodic changes corresponding to the vibration velocity,and its value is positively correlated with the relative velocity.Also,the heat transfer enhancement caused by vibration is more obvious at low flow velocities.(2)The liquid hydrogen flow boiling flow pattern under microgravity demonstrtes a special law and deteriorates the heat transfer intensity.Under microgravity condition,the bubble department diameter increases.At the same time,the hydrogen bubble with low thermal conductivity which adheres to the heating wall surface suppress heat transfer and also reduce the flow resistance.However,at high flow velocities,flow inertia becomes the main drving force of the hydrogen bubbles department,thus the heat flux changes little with the variation of gravity;(3)Gravity is an important factor that affects the condition and location of critical heat flux.The wall heat flux required to reach the critical heat flux state under microgravity is smaller and the critical heat flux location moves upstream under microgravity conditions,that is.At the same time,the maximum wall temperature at critical heat flux location under microgravity is significantly larger than that under terrestrial gravity;Under micrograivity condition,the maximum wall temperature at critical heat flux location increases with lareger inlet subcooling,while the change of the inlet subcooling under terrestrial gravity hardly affects the temperature at critical heat flux location.(4)The increase in pressure in the spherical cavity is conducive to enhancing the natural convection intensity and heat transfer performance of supercritical helium.For the supercritical helium pressurization system,increasing the initial pressure in the cavity significantly improve the pressurization efficiency.At the same time,through the numerical study on the natural convective heat transfer characteristics of supercritical helium in confined space.The positive correlation between the natural convection heat transfer intensity of supercritical helium in a confined space and the pressure in the spherical cavity is obtained.Finally,the improved correlation of natural convective heat transfer can better reflect the natural convection phenomenon for cryogenic supercritical helium in confined space.In this thesis,the study of the flow boiling law of liquid hydrogen under vibration is helpful to improve the fiiling quality of cryogenic propellants and prevent filling accidents.The study of liquid hydrogen flow boiling under microgravity can provide a theoretical basis for the design of cryogenic propellant in-orbit refilling system and the optimization of the control strategy;The heat transfer characteristic and correlation of cryogenic suepercritical helium natural convenction in the spherical cavity can provide a theoretical support for the realization of efficient pressurization.