Research On Influence Of Gravity On Flow Instability Of In-tube Flow Boiling

At present,there are few research results about flow boiling instability in tubes under hypergravity environment.In order to meet the application requirements of high-mobility fighters and aerospace fields,this paper conducts experimental and simulation studies of flow boiling instability under different gravity conditions.In this paper,an experimental system for the influence of gravity on the instability of in-tube flow boiling is successfully established,including a hypergravity simulation platform,a fluid circulatory system,a test section,and a data acquisition system.The working fluid is R134a,and the test section is a copper tube with an inner diameter of 2.01 mm and a length of 20.03 cm.The experimental conditions are as follows:gravity levels of 1.00 g-3.16 g,heat fluxes of 79.2 k W/m~2 and118.8 k W/m~2,saturation pressures of 0.65 MPa and 0.7 MPa,inlet subcooling of 10-13℃,and mass fluxes of 250-3900 kg/m~2s.The experiments on the instability of in-tube flow boiling were carried out under the gravity range from 1.00 g to 3.16 g.The flow characteristic curves under different experimental parameters are obtained.The effects of saturation pressure,heat flux and inlet subcooling on the flow characteristic curve are analyzed.As the saturation pressure decreases,or the heat flux increases,the curve as a whole moves up and the corresponding mass flux at the negative portion is greater.With the decrease of inlet subcooling,the curve moves up and the negative portion shifts to the right.Under larger heat flux,the effect of inlet subcooling is more obvious.When other conditions remain unchanged,the increase of gravity causes the slope of the flow characteristic curve increases,and the negative portion shifts to the position with larger mass flux and pressure drop.In addition,the increase of gravity will suppress the influence of saturation pressure.The pressure drop oscillation and density wave oscillation under different gravity conditions are obtained.As gravity increases,the frequency of pressure drop oscillation increases,and the oscillation occurs at locations where the mass flux and pressure drop are greater.While the frequency of density wave oscillation decreases with the increasing gravity.The simulation research on the instability of in-tube flow boiling under different gravity conditions is conducted.Based on the mathematical model,a simulation system is designed based on Matlab/Simulink platform,and a large number of simulation results are calculated.The results are consistent with theoretical and experimental laws.In addition,under special conditions,the sudden increase of gravity may cause the system to change from stable to unstable.