Research On Unsteady Flow And Heat Transfer Characteristics For Regenerative Cooling System Under External Vibration Condition

The regenerative cooling taking the endothermic hydrocarbon fuel as a coolant could effectively solve the thermal protection problem of scramjet engine.The differential pressure flowmeter measuring flow rate and cooling channels for wall cooling are important part of regenerative cooling system.However,the unstable flow and heat transfer phenomena occur in the vibration condition and pseudo-critical regions,which cause serious harm to operation of cooling system.In order to ensure the cooling effect and structural safety of the engine,it is urgent to develop the mechanism of flow instability,the developing process from stability to instability and heat tranfer dynamic characteristics of flow instability,which is important support for related designs and methods.This thesis focuses on the above mentioned issues:Because the hydrocarbon fuel is at a normal temperature in the differential pressure flowmeter and at a high temperature in the cooling channel,the “cold throttling channel” represents the differential pressure flowmeter,and the “hot channel” represents the cooling channel.The experimental study shows that the flow instability occur when hydrocarbon fuel flow in the oscillating throttle channe l and in the heating channel.The problem is classified according to the leading factors: external vibration induced case and internal heat flux induced case.In order to understand the mechanism of flow instability in the external vibration induced case,,a three-dimensional model of the throttle passage with hydrocarbon fuel was established.After validation of the present mo del with experimental results,a detailed discussion has been performed to study the hydrodynamic characteristics of flow instability in throttle passage subject to vibration.The behaviors of steady flow fields and unsteady flow fields under different conditions were investigated.The results show that the axial passage vibration leads to flow oscillation.It is because that vibratin g passage causes change of fluid velocity field.The recirculatory separation region and mainstream region change periodically.The amplitude of vibration velocity and the inlet Reynolds number of fuel have a great influence on flow dynamic behavior.For the internal heat flux induced case,the flow instability occurs during the heating process of hydrocarbon fuel flowing in the cooling channels.For small flow condition,boiling oscillations can induce axial motion of the boiling space under subcritical pressure conditions.Near critical pressure,the temperature of the wall of the thermoacoustic oscillations will drop significantly without fluctuations,and the mainstream temperature fluctuations will increase linearly,leading the heat transfer coefficient of the process fluctuates greatly.The unstable flow will strengthen the heat transfer under supercritical pressure.The devoloping process from stability to instability is affected by many parameters,such as mass flow rate,pressure,heat flux,the inlet fluid temperature,the increase rate of the heating power,and the ratio of fluid mass over metal mass.The interaction of each parameter on the dynamic process is a kind of nonlinear relationship.In our opinion,this relationship is prior to the pressure fluctuations change.There is a time delay involved between the formation of nonlinear relationship and pressure oscillation.The time that obtained with wavelet entropy method and recurrence network method from pressure time series shows the time delay.It is found that the instability developing process can be detected before the amplitude change of the pressure time serie s by using wavelet entropy method.The instability developing region,stable region and unstable region are analyzed by recurrence complex network method.The heat transfer characteristics of fuel flow in the heating channel under external vibration condition and internal flow instability condition are discussed.The experimental results show that the external vibration could enhance heat transfer under different working condition.The greater the system pressure,the better the heat transfer enhancement effect.The external vibration has no effect on heat transfer when system pressure is less than critical pressure.The heat transfer enhancement occurs then the temperature of inner wall exceeds the critical temperature of the hydrocarbon fuel.The greater the heat flux density,the better the effect of vibration-induced heat transfer.When the overall temperature in the tube is less than the critical temperature,the vibration does not contribute to heat transfer.The enhancement effect of heat transfer decreases with increasing of the flow rate.The vibration parameters have a great influence on the effect of heat transfer enhancement.The enhancement effect of heat transfer increases with increasing of the vibration frequency and amplitude.The heat transfer deteriorates when instability flow caused by internal heat flux.At last we find that external vibration can suppress instability flow caused by internal heat flux and improve the heat transfer.