Experimental Study On Hydrodynamic Characteristics Of Cyclohexane In Horizontal Tubes At Trans-and Supercritical Pressures

Due to the harsh working environment of the scramjet engine,the thermal load of the engine surface is high and uneven,and there are serious thermal barrier phenomena.At present,the regenerative cooling thermal protection structure using hydrocarbon as coolant is a most effective cooling method for the scramjet thermal management and has been widely studied.In the multi-channel regenerative cooling structure,the flow distribution of the fluid and the flow stability will directly affect the heat transfer process and efficiency of the coolant in the small channel.As the hydrodynamic characteristics is a necessary condition for the static instability in the fluid flow process,it is of great significance to study the hydrodynamic characteristics of the fluid in the small channels for the design of regenerative cooling active thermal protection structure.Because of the fuel in the regenerative cooling structure is the supercritical state,and the viewpoints of hydrodynamic characteristics on the supercritical fluid have not attained a consensus.Based on a combination of analytical model and experimental study methods,this paper use cyclohexane as the research object,to study its hydrodynamic characteristics under the supercritical condition in horizontal channels.Firstly,an analytical model is established for the hydrodynamic characteristics of supercritical fluids in the horizontal channel.The model proves that the multi-valued hydrodynamic characteristics exists at the supercritical pressures and the existence conditions are obtained for supercritical fluids.Next,to confirm the validity of the qualitative analysis results of the analytical model,the hydrodynamic characteristics of cyclohexane at trans-and supercritical pressures in a horizontal channel is studied experimentally.The experimental results show that the multi-valued hydrodynamics characteristics exists in the supercritical condition and it is caused by the rapid density variation near the pseudo-critical point at the supercritical pressures.Increasing the system pressure and inlet temperature,and decreasing the heat flux can weaken the multi-valued characteristics.The corresponding mass flow rate at the onset of flow instability(OFI)moves a higher mass flow rate when the system pressure decreases and the inlet temperature and heat flux increase.The two dimensionless formulas of the onset and the offset boundaries of the negative slope area on the hydrodynamic characteristic curves are developed.Finally,experimental investigations on the effect of tube length and tube diameter on the hydrodynamic characteristics of supercritical cyclohexane are perfomed by considering the geometrical factors of the small channel.The results indicate that the multi-valued characteristics is weakened with the decrease in tube length and the increase in tube diameter.Increasing the tube length and tube diameter make the corresponding mass flow rate at OFI move a higher value.Based on all the experimental data,the method of dimensional analysis is used to obtain a new dimensionless correlation predicting the OFI for supercritical fluids.