Investigation On The Mixing,distribution And Ignition Processes Of Kerosene In A Supersonic Crossflow

The present study focuses on the gas flow,mixing and combustion in the scramjet combustor.Combining with experimental research,numerical simulation and theoretical analysis,the fuel distribution,ignition and combustion processes of kerosene in supersonic flow are studied.Firstly,kerosene distribution characteristics around the cavity before ignition are measured using kerosene-PLIF.For far-field injection,the majority of kerosene is present in the cavity shear layer as well as its upper region.Large scale structures are evident on the windward side of kerosene.Kerosene extends gradually into the cavity,almost,at a constant angle.The injection position has less importance on kerosene distribution.The characteristics of the flow field,especially the cavity shear layer,play an important role in determining the kerosene distribution and its entrainment into the cavity.The forepart of cavity is in a fuel lean environment,and the rear part of cavity is in a fuel rich environment.Thus,the middle part of cavity is the most suitable location for ignition as a result of a favorable local equivalent ratio.For near-field injection,large amounts of liquid kerosene droplets above the cavity shear layer can be observed.The successional large scale structures are hardly visible on the windward side of the kerosene.There are no intense fluorescence signal in the cavity,indicating kerosene droplets entrained into the cavity are evaporated completely.The kerosene distribution in the cavity recirculation region is relatively uniform,and the forepart of cavity is the most suitable location for ignition.Secondly,the laser-induced plasma ignition of ethylene and kerosene are performed in scramjet engine.Using CH/OH chemiluminescence imaging,high speed camera and schlieren,the flame structure and propagation during transient ignition processes are captured.The spatio-temporal evolution of CH and the dominating parameters for flame kernel formation and flame propagation are analyzed.In quiescent air,the laser breakdown processes include the formation and decay of plasma,the detachment and propagation of shock wave and the evolution of hot gas.The shock wave takes away 80% energy of the plasma,while the energy left for ignition is only 20% of the plasma.The entire ignition process of kerosene can be divided into five stages,which are referred as turbulent dissipation stage,quasi-stable state,combustion enhancement stage,reverting stage and combustion stabilization stage.A local closed loop of propagations of the burning mixtures from the shear layer into the recirculation zone of cavity is important for flame propagation,where the large-scale eddy in the shear layer plays a key role in this process.Thirdly,the ignition characteristics of kerosene are investigated in a single-cavity combustor.For near-field injection,when injection pressure is in a low and high range,the cavity recirculation region has a favorable local equivalent ratio and ignition is achieved.When injection pressure is in a middle range,the cavity recirculation region is fuel rich and ignition is failed.For far-field injection,when injection pressure is in a low and high range,the cavity recirculation region is fuel lean and ignition is failed.When injection pressure is in a middle range,the cavity recirculation region has a favorable local equivalent ratio and ignition is achieved.Finally,the paper presents an ignition enhancement scheme based on dual-cavity.Whether the flame can propagate from the cavity recirculation region to mainflow depends on the turbulence intensity of the incoming flow,while the equivalent ratio and fuel distribution has minor effect.In a dual-cavity combustor,the large-scale eddies in the shear layer of upstream cavity shed off at cavity rear wall and enter into the boundary layer after the cavity.The turbulence intensity in the boundary layer after the cavity increases.So the high-temperature products and radicals in cavity recirculation region can be transported into mainflow quickly,inducing ignition of the fuel in mainflow.Thus,the flame can propagate from the cavity recirculation region to mainflow.If the equivalent ratio is high,the high pressure resulted from combustion can separated the boundary layer upstream of cavity,the flame can propagate from the downstream cavity to the upstream cavity and get stabilized.The upstream flame propagation occurs at the side wall of the combustor,because the thicker boundary layer at the side wall provides a favorable environment for combustion and flame stabilization.The upstream flame propagation is accompanied by movement of the pre-combustion shock train.The high pressure resulted from combustion pushes the shock train upstream,and the low-speed separation zone moves upstream along with the shock train.So the flame in separation zone can propagate against the incoming flow.The shock wave interacts with the flame and a close coupling between combustion and flow exists during the flame propagation process.