Investigation On The Phase Change Heat Transfer Process In Mini-channels For Regenerative Cooling Of Liquid Rocket Engine

In this paper,experimental research and theoretical analysis are used to study the phase-change heat transfe process in mini-channels for the regenerative cooling of the liquid rocket engine.Three pressure fluctuation types of phase-change heat transfer in mini-channels are found and the internal relationship between the fluctuation frequency and the bubbly/annular periodic alternating flow pattern in phase-change heat transfer process of water is clarified.The mechanism of phase-change heat transfer of liquid nitrogen under subcritical pressure in mini-channel is rerealed,and the modified correlation of phase-change heat transfer based on experimental data is proposed.Besides,it is shown that the self-sustaining fluctuation in the pipeline of liquid nitrogen cryogenic system is a kind of thermal unstable flow phenomenon.A one-dimension calculation model of subcritical phase-change heat transfer is established and the calculation method of regenerative cooling heat transfer considering subcritical phase-change heat transfer process is proposed.Based on the flow pattern analysis,the phase-change flow and heat transfer characteristics and mechanism of water in mini-channels are studied.Three types of pressure oscillation of phase-change heat transfer in mini-channels are determined,i.e.low-frequency oscillation(f<2.0Hz),medium-frequency oscillation(2.0Hz<f<10.0Hz)and high-frequency oscillation(f>10.0Hz).The periodic vapour catapult phenomenon was found during the low-frequency fluctuation,which was inferred to be caused by the bubbly/annular alternating flow pattern transition.It was found that the pressure drop and the wall temperature had the approximate fluctuation period with the periodic vapour catapult process.This consistency reflects the influence of bubbly/annular alternating flow pattern transition on the fluctuation of wall temperature and pressure drop.The medium-frequency fluctuation and the high-frequency fluctuation are related to the passage of the elongated bubbles with different periods.In superimposed medium and high frequency fluctuation,the amplitude of the pressure drop fluctuation is relatively low and no synchronous fluctuation was observed between wall temperature and pressure drop profiles.The flow boiling heat transfer of nitrogen at higher pressure conditions in a single vertical mini-channel was experimentally investigated.Effects of the heat flux,inlet pressure and mass flux on the boiling curve and the heat transfer coefficient were discussed and analyzed to better understand the heat transfer mechanism of nitrogen flow boiling under high saturation pressures in vertical mini-channel.The increasing inlet pressure increases the heat transfer coefficient over a wide range of vapour quality until the partial dry-out inception.The lower surface tension and lower latent heat of evaporation enhance the nucleate boiling for higher inlet pressures.At the same time,the increase of the inlet pressure leads to the earlier occurrence of the partial dry-out,the reason is related to the unsteady liquid film of the annular flow with a lower surface tension under high inlet pressure conditions.It was found that the Klimenko and Tran correlation have better prediction accuracies among the five selected correlations.A modified correlation(MAE=19.3%)was proposed on the basis of the Tran correlation considering both the nucleate boiling and the partial dry out heat transfer mechanism,which is suitable for nitrogen flow boiling in mini-channel at high pressure conditions.Based on the analysis of the flow instability in the cryogenic fluids experimental system,the method of suppressing the flow instability in the system pipeline was obtained.It is considered that the instability of liquid nitrogen pipeline of experimental system is a kind of thermal instability.It is triggered by the periodic film boiling of liquid nitrogen in the evaporation section.The large amount of vapor produced in the evaporation section will affect the pressure drop and mass flow rate of the system.When the pressure drop,the mass flow rate and the heat flux in the pipeline match,the above process will be repeated,forming self-sustaining instability.It is found that in the experimental section,the front venturi could effectively suppress the flow instability from the downstream evaporation section.Since the venturi tube is installed between the experimental section and the nitrogen tank,once the cavitation occurs in the venturi tube,it could effectively suppress the propagation of downstream pressure perturbation.The pressure drop of the liquid nitrogen tank and venturi tube is basically constant,ensuring the mass flow rate in the experimental pipeline is constant and suppress the flow instability in the evaporation section.In view of the insufficient cooling capacity of liquid oxygen/methane expansion cycle variable thrust liquid rocket engine under low working conditions,the evaluations heat flux of the gas side wall is estimated and coolant temperature rise is calculated at different conditions of the combustion chamber pressure and the coolant mass flow rate.The design and validation method of regenative cooling channel with phase-change is proposed.The influence of the height-width ratio and the fuel mass flow rate on the heat transfer characteristics of the cooling channel under low operation conditons is studied in detail.It is found that incrasing the aspect ratio of the cooling channel could reduce the maximum temperature of the gas side wall,which is beneficial to the regenerative cooling of the thrust chamber wall to a certain extent.The maximum temperature of the gas side wall decreases with the increase of the chamber pressure(mass flow rate).Due to the decreasing of the methane mass flow rate and the rise of wall temperature under low operation conditions,other cooling methods such as film cooling and radiation cooling should be considered comprehensively.