Research On Heat Transfer Characteristics Of N-Decane With Fuel Pyrolysis Under Vibration Condition

Due to the wide application of ultra-high velocity aircraft in aerospace and military fields,the research and development of scramjet engine has been paid attention to by researchers in many countries.Scramjet engine is faced with severe thermal protection problem because of its special working environment.At present,active regenerative cooling is one of the most effective ways to solve the problem of engine thermal protection.In the actual flight,the engine vibrates with the flight of the aircraft,and the vibration problem of the cooling channel is unavoidable.Therefore,to study the cracking and heat transfer characteristics of hydrocarbon fuel under vibration conditions is the theoretical and technical basis for realizing efficient thermal protection of scramjet engines.Therefore,this paper conducted the following numerical simulation research on the cracking heat transfer characteristics of hydrocarbon fuel under vibration conditions:Using hydrocarbon fuel n-decane as the flow medium,the physical property parameters of ndecane in supercritical state were obtained by polynomial fitting of the NIST(National Institute of Standards and Technology)correlation physical property data,aiming at the properties of ndecane near the pseudo-critical point,as well as the pyrolysis and heat transfer phenomena of ndecane under supercritical pressure.Detailed variations and distributions of temperature,density,n-decane pyrolysis rate,thermal conductivity,etc.are obtained by numerical calculations.Compared with the existing literature,it is found that the two are in good agreement,which verifies the accuracy and effectiveness of the established heat transfer model of n-decane cracking under supercritical pressure.This topic explores the influence of different vibration frequencies,vibration amplitude and vibration direction on n-decane temperature field,n-decane cracking endothermic reaction,wall convection heat transfer coefficient and turbulent kinetic energy.The results show that,compared with the static channel,the fluid temperature and turbulent kinetic energy increase with the increase of vibration frequency and amplitude,and the increase of vibration frequency and amplitude makes the temperature distribution of n-decane in the pipeline more uniform,and the conversion rate of n-decane cracking decreases with the increase of vibration frequency and amplitude.The heat transfer coefficient increases with the increase of vibration frequency and amplitude.Under the same vibration intensity A×f,axial vibration has A more significant effect on fluid temperature improvement than radial vibration,especially when the vibration intensity is low,i.e.A×f = 50(mm·Hz),the wall temperature under axial vibration is 17 K lower than that under radial vibration.The average fluid temperature increases by about 20 K compared with that under radial vibration,and the n-decane cracking rate under radial vibration is always higher than that under axial vibration.Finally,the effects of heat flux,inlet velocity and outlet pressure on pyrolysis heat transfer of ndecane under the same vibration condition were investigated.The results show that the influence of outlet pressure on temperature change is not obvious.When the outlet pressure of cooling channel decreases towards the direction of n-decane critical pressure,the convective heat transfer deteriorates more seriously.The lower the inlet flow velocity,the higher the average fluid temperature,the higher the average cracking rate,and the heat transfer capacity decreases first and then increases with the increase of flow velocity.Increasing the heat flux and strengthening the heat transfer capacity,the ndecane fluid absorbs more heat,and the n-decane fluid temperature increases.N-decane can easily obtain the activation energy required for thermal cracking in the channel,and the cracking rate increases accordingly.