Cooling And Drag Reduction Characteristics Of Hydrocarbon Fueled Film For Scramjet Engines

Regenerative cooling with endothermic hydrocarbon fuel used as coolant is considered to be the best way to cool the scramjet engines with extremely high heat load.However,using regenerative cooling only can not meet the increasing cooling requirement of hydrocarbon fueled scramjet engines when the flight Mach number keeps increasing and with the requirement of engine reusability.Supersonic film cooling,especially combines with regenerative cooling,is a promising way to achieve better protection for scramjet combustors.For hydrocarbon fueled regenerative cooling/film cooling,part of the high temperature fuel which has finished its job to regeneratively cool the engine wall as film coolant.In this way,both the fuel's ability to absorb and insult heat can be used and the skin friction of the combustor may also be reduced.However,in the case of hydrocarbon fueled film coolant,the possibility of combustion of high temperature gaseous hydrocarbon fuel must be taken into consideration.The combustion reaction brings new characteristics and challenges to the supersonic film flow and heat transfer process and its cooling characteristics and skin friction distribution characteristics,and finally affects the engine performance.This paper focuses on the above problems:In order to reveal the effects of non-equilibrium combustion reaction on the flow and heat transfer mechanism of hydrocarbon fueled supersonic film cooling,a numerical model of gaseous hydrocarbon fueled supersonic film cooling in terms of the coolant combustion has been developed.In the numerical model,both endothermic pyrolysis chemical reaction and endothermic/exothermic oxidation pyrolysis chemical reaction are considered.On the basis of the model,the development of the combustion reaction in the mixing process between supersonic film and mainstream are analyzed.The results indicate that the combustion of the coolant film occurs in the boundary layer.The combustion flame is formed apart from the wall and restricted near the boundary layer edge.The combustion of supersonic film coolant may release heat and increase the wall temperature which is not beneficial for film cooling.Based on the development characteristics of combustion reaction,the cooling characteristics are further analyzed.It is found that the combustion first reduces and then increases the wall temperature rather than only increases the wall temperature significantly as expected,which brings double effects on film cooling.Due to the high density and long molecular chain of hydrocarbon fuel,the hydrocarbon fueled supersonic film has the characteristics of low speed and low momentum injection,coexistence of endothermic/exothermic chemical reaction,and non-equilibrium.Therefore,the combustion reduces the wall temperature due to low-temperature endothermic chemical reactions occur and dominate the near wall region over a lon g distance after the film injection.It is worth mentioning that,the increase of mainstream Mach number significantly increases the portion of combustion reaction beneficial effect on cooling characteristics.Furthermore,after the combustion flame occurs,the skin friction is reduced.It is found that the combsution reaction has a zonal effect on the skin friction reduction characteristics.The flow field is divided into endothermic region and exothermic region according to the characteristics of chemical react ion.The skin friction reduction by combustion in exothermic region is attributed to the combined effects of Reynolds stress reduction and the viscosity stress reduction.Considering the cooling characteristics and skin friction reduction characteristics,it is found that combustion reaction brings favorable conditions for the coupling of thermal protection and friction reduction.Because the endothermic region is close to the film inlet,the coupling performance of thermal protection and friction reduction of the supersonic film without chemical reaction is good,therefore,it is more significant to improve the coupling performance in the exothermic region.In the exothermic region,there is a larger film cooling equivalence ratio,which makes the coupling performance of thermal protection and drag reduction the best.Finally,based on the cooling characteristics and skin friction reduction characteristics of supersonic film with combustion reaction,the influence of hydrocarbon fueled regeneration cooling/ film cooling on the engine performance is evaluated.When the mass flow rate of fuel used for film cooling is 20% of that for regenerative cooling,compared with single regenerative cooling,regenerative cooling/film cooling can broaden the thermal boundary and improve the thrust performance of the engine significantly.The results show that the length diameter ratio of combustor has a significant effect on the thermal boundary.The smaller the length diameter ratio of combustor is,the larger the expansion range of engine thermal boundary is.When the ratio is 7.5,the combined cooling can widen the thermal boundary by 1.35,and the combustion reaction can further widen the thermal boundary by 0.07.When the engine carries an additional 20% mass flow rate of the fuel and the hydrocarbon fueled film combustion efficiency is 0.5,the regeneration cooling/ film cooling can broaden the engine thermal boundary and improve the specific impulse and thrust of the engine by the joint effect the wall skin friction reduction and the film combustion.