Investigation Of The Mechanism Of Laser Induced Plasma Ignition In Supersonic Flows Fueled By Gaseous Hydrocarbon Fuel

Reliable ignition is a precondition for the normal operation of scramjet engines.However,successful ignition in scramjet engines is a challenging work at low flight Mach number because of the limited gas residence time,inhomogeneous fuel-air mixing and high turbulence intensity.Therefore,proposing advanced ignition schemes and revealing the mechanism of igniton are important for promoting the possibility of successful ignition.This dissertation presents experimental studies of the mechanism of laser induced plasma(LIP)ignition in supersonic flows fueled by gaseous hydrocarbon fuel.High-speed photography,Schlieren visualization and CH~*-OH~*chemiluminescence imaging were employed to capture the evolutions of the flame kernels.The impacts of ignition energy,position and the time interval between successive laser pulses on ignition processes were investigated.Various ignition techniques involving LIP ignition,laser ablation ignition and spark discharge ignition were compared in detail.The evolutions of single-pulse and dual-pulse LIPs in quiescent air at atmospheric pressure were investigated firstly.The results reveal that the energy absorption of single-pulse LIP increases as the incident laser energy increases,and reaches the maximum at a laser energy of 200 mJ;however,the shock wave energy invariably accouts for approximately 80%of the absorbed laser energy.Dual-pulse LIP with a pulse interval shorter than 200 ns is inferior to single-pulse LIP with the same total energy in terms of energy absorption and shock loss,but the advantages of a larger initial plasma volume and lower energy dissipation can compensate for this deficiency.When the pulse interval is longer than 200 ns,the second laser pulse cannot be absorbed effectively,and the second shock wave promotes the turbulent intensity of the hot plume,which significantly enhances the energy and radical dissipation.Secondly,the ignition processes at different ignition energies and positions were studied by single-pulse LIP ignition.The ignition time can be reduced,and the possibility of successful ignition can also be promoted by increasing ignition energy.In streamwise,the middle of the cavity is preferred for ignition,as it keeps a good balance between the strength of initial flame kernel and the impacts of strain rate in recirculation flow.In the direction that is vertical to the cavity bottom,positions which are close to the cavity bottom are suggested for ignition because of the high strain rate in cavity shear layer.Thirdly,the impacts of pulse interval on dual-pulse LIP ignition were assessed.There is no significant difference between dual-pulse LIP ignition at pulse interval shorter than 40μs and single-pulse LIP ignition with the same total energy in terms of ignition process and ignition time.Therefore,dual-pulse LIP ignition is a promising alternative to single-pulse LIP ignition.However,a pulse interval~100μs should be avoid for dual-pulse LIP ignition,because the flame kernel induced by the first laser pulse is surpressed by energy and radical dissipation resulting from high strain rate before releasing the second laser pulse.Finally,the comparison of different ignition techniques indicates that the ignition processes of laser ablation ignition and LIP ignition are similar,when the ignition erergies and positions are the same.The spark discharge ignition is inferior to LIP ignition,because the energy liberation of a spark plug is much slower than a laser system.