Study On Mixing,Ignition And Combustion Stabilization Mechanism Of Wall-Injection Fuel Jet In Supersonic Flows

Scramjet combustor is regarded as a key component of air-breathing propulsion system,which can determine the overall property of hypersonic vehicles.Based on some critical issues that are involved in the operation of one-sided divergent scramjet combustor,this paper addresses the fuel mixing,ignition and flame stabilization mechanism in supersonic flows by the high-precision large eddy simulation and the advanced combustion diagnosis technology.The conserved governing equations of large eddy simulation for compressible reactive flows in the curvilinear coordinate are introduced,and the temporal and spatial derivatives as well as the chemical source term are provided in detail.For supersonic problems,the 6th-order adaptive central-upwind WENO scheme are adopted to capture the shocks,thus Fourier analysis and some inviscid test cases are used to demonstrate its property of high precision and low numerical dissipation.Finally,the reliability of the reactive solver is validated by the DLR strut-stabilized combustion problem.Aiming at the experiment of a transverse jet injection into the Mach 1.6 supersonic crossflow,large eddy simulations are conducted.The grid independence analysis is first performed,and the reliability of numerical results is verified by comparing with the experimental data.On this basis,the effect of wall expansion on the flowfields generated by a transverse jet injection is investigated,and it is found that the conversion of internal energy to kinetic energy plays an important role in mixing.In comparison with the jet injection through flat plate,the expansion case has a lower penetration depth,a smaller mixing efficiency,and a larger total pressure loss.Some details such as shocks and recirculation zones are compared between air and hydrogen jet injection.Although there is little difference in jet penetration depth and mixing mechanism,the scale of large-scale recirculation regions behind the injector is larger in the air jet injection.The characteristics of a lifted hydrogen jet flame and its stabilization mechanism in high-enthalpy supersonic flows are revealed.The results indicate that the lifted height depends on the ignition delay time of hydrogen,and the chemical reaction rate in the induction region is much larger than diffusion rate.As a result,autoignition-dominated flame base should be responsible for achieving stable combustion,and the reacting zone downstream behaves obvious diffusion in character.Based on the transverse jet combustion experiments in Mach 8 flight condition,large eddy simulations reproduce two combustion modes,namely the near-wall combustion and the jet windward shear-layer mode.In addition,it is also found that although the autoignition process is vigorous,the amount of heat released is not enough so that the local temperature does not increase obviously.On the basis of understanding the mixing characteristics in a cavity combustor,the ethylene ignition inside a one-sided divergent combustor is studied by high-precision large eddy simulations,especially for the establishment of unsteady reactive process from ignition to flame stabilization.Subjected to the reflux in the cavity,the initial fire kernel moves upstream after ignition.With the flame base is gradually established at the leading edge of cavity,the adjacent premixed gas is constantly ignited,and eventually the premixed flame spread to the entire cavity.The stable ethylene jet flame is partially-premixed in character and the intensity of chemical reactions downstream is relatively weak.In comparison with non-reactive flows,the reaction exothermic heat causes the local temperature to rise,and the jet wake is pushed into the deeper region.Utilizing the advanced combustion diagnose technology,the hydrogen jet flame in the one-sided divergent combustor is observed,and it is found that the flameholding ability of cavity decreases when the fuel jet is located far away from the cavity;the flame region becomes large and transports upstream along the cavity shear layer with the increase of equivalence ratio.Thus,the characteristics of cavity-stabilized flame are studied for the conditions with a global equivalence ratio of 0.124.It indicates that diffusion flame governs the entire reacting zone,especially in the downstream region.Turbulent fluctuations can promote the mixing of fuel and air,however,a large scalar dissipation rate may also lead to the decrease of local temperature.Finally,the interaction between jet wakes in the parallel injection scheme with three orifices are discussed,and then its advantages are summarized.Despite the decrease of jet penetration depth,the parallel strategy has a higher combustion efficiency and does not lead to redundant total pressure loss.