Research On Flow Instability And Friction Pressure Drop Of In-tube Flow Boiling Under Hypergravity

The phenomenon of flow boiling instability in the tube is widespread in the two-phase flow system,which adversely affects both the control system and the heat transfer capacity.With the increasing application of two-phase flow systems on modern high-performance fighters and the increase in maneuver overload of them,there is a requirement for thorough research on flow instability and friction pressure drop of in-tube flow boiling under hypergravity.According to the requirement,this paper conducts experimental and simulation research on the flow boiling instability in the tube under hypergravity environment.The main work and results are as follows:(1)The experimental system for flow instability and friction pressure drop of in-tube flow boiling under hypergravity was designed and set up,including a centrifugal acceleration machine,a fluid circulatory system,and a data acquisition and control system.The centrifugal accelerator was used to generate the hypergravity condition.The working fluid was R134a,and the test section was a copper tube with an inner diameter of 2.168 mm,which was perpendicular to the direction of centrifugal acceleration.The experimental work was done under saturation pressure of 0.5 MPa and 0.6 MPa,heat flux density of 73.4 k W/m~2 and 110.1 k W/m~2,6-9 of inlet subcooling,mass flux of 1000-4000kg/m~2s,and hyper-gravitational acceleration of 1.00–3.16 g.(2)Experimental research on the flow boiling instability in the tube was conducted under the normal gravity environment and hypergravity environment.The flow characteristic curve under hypergravity environment.was obtained first time,and was compared with the flow characteristic curve under the normal gravity environment.According to the flow characteristic curve,the effects of gravity,inlet subcooling,saturation pressure and heat flux density on the flow characteristic curve were analyzed.The gravity increases lead to the slope of the flow characteristic curve increases,and the right shifts position of the negative portion of the curve.The increase in inlet subcooling causes the flow characteristic curve to shift down-ward while its negative portion shifts to the left.The effect of saturation pressure on the flow characteristic curve is especially obvious near the negative portion.The decrease of the saturation pressure causes the negative portion to shift to the right and upward.Compared with the normal gravity environment,the influence of saturation pressure on the flow characteristics under hypergravity environment is weakened.(3)The density wave oscillation and pressure drop oscillation under normal gravity environment and hypergravity environment were obtained,and the influence of hyper-gravitational acceleration on them was analyzed.The frequency of the density wave oscillation decreases significantly with the increase of the hypergravity,while that of pressure drop oscillation increases slightly,and its position shifts to the right.(4)In order to find out the existing frictional pressure drop calculation model with good adaptability under normal gravity,the calculated values of the models were compared with the normal gravity experimental data.Comparing the calculated values of the existing pressure drop calculation models with the experimental data under hypergravity environment,it was found that they cannot adapt to the hypergravity environment.A new frictional pressure drop calculation model that can be used under hypergravity environment was proposed,which provided a basis for subsequent simulation calculations.(5)Based on the new frictional pressure drop calculation model,the calculation models of pressure drop oscillation and density wave oscillation were proposed.The calculation program was designed based on Matlab/Simulink platform for simulation.The calculated results are basically identical with the experimental ones.It is proved that the calculation models of pressure drop oscillation and density wave oscillation are reasonable,and the new frictional pressure drop calculation model is accurate.