Investigation On The Ignition And Combustion Characteristics Of Vaporized Kerosene In A Supersonic Flow

In this dissertation,the working process of the hydrocarbon fuel regenerative cooling scramjet engine is used as the research background.The supersonic transverse jet,ignition process in cavity and combustion organization of vaporized RP-3 aviation kerosene are experimentally investigated,accompanied with theoretical analysis and numerical simulation.The comparison study of room temperature liquid kerosene is carried out as well,and the detailed working characteristics of vaporized kerosene in the scramjet under the influences of injection temperature,flow disturbance and unsteady injection are obtained.The transverse injection of vaporized kerosene is accompanied with a sharp phase transition.The jet plume distributions under different injection temperatures are obtained by high-speed schlieren and pulsed laser sheet technology.With transverse fuel injection upstream,a kerosene liquid film will be formed on the side wall of the cavity.The liquid film evaporates fastly under the influence of the downstream back pressure increasing,which is closely relative to the cavity recirculation zone.The evaporation rate in the jet plume is effected by the fuel temperature and local velocity,which determined the inhomogeneous distribution of jet phase on the vertical direction.The existence of the cavity does not affect the evaporation rate in the jet wake,but accelerates the evaporation at lower edge of jet plume.The interaction between the jet and the cavity during near-field injection causes the change of jet distribution,which deflects toward the main flow at the rear edge of the cavity.Vaporized kerosene is compressible,and the phase transition of fuel has decisive effect on the rate of injection pressure change.According to the formation and development of the initial fire core of vaporized kerosene,the cavity ignition is divided into four modes.The distribution law of the ignition mode under various injection temperatures and pressures is summarized in different cavity configurations.The time required from ignition to the entire cavity flame of critical state kerosene is longer than that of normal temperature kerosene.This is mainly due to the rapid evaporation of vaporized kerosene,which reduces the static temperature and increases equivalence ratio in cavity recirculation zone,resulting in the deterioration of reaction environment in the cavity.The fore recirculation zone of the cavity plays a key role in stabilization and development of the initial fire core.A deep cavity is against the formation of a reliable fore recirculation zone,and is difficult to maintain the initial fire growing.In the tandem dual-cavity,the ignition process in the deeper cavity downstream has threshold characteristics.The far-field cavity downstream with large depth is more difficult to form a local resident flame.Once the flame is formed,it will rapidly develop into a global cavity flame and stabilized in the shear layer.The flame oscillation in the downstream cavity is more obvious.Three mixing enhancement methods,including vortex generator,auxiliary cavity and shock wave generator,are used to study the combustion characteristics of vaporized kerosene under the effect of flow control.The upstream vortex generator has a certain promotion effect on the mixing and combustion of vaporized kerosene,and it becomes more obvious with the distance decrease.Single generator in the near field causes cold kerosene ignition failure,which can be solved using the spanwise parallel scheme to improve the uniformity of disturbance.The upstream arrangement of auxiliary cavity can also produce a similar mixing enhancement,but the multiple cavities are easier to lead the combustion pressure to spreading forward.The distance of pressure spread become large with the auxiliary cavity size increasing.Under the condition of low equivalence ratio,the effect of mixing enhancement on combustion becomes more obvious.The wall slope is difficult to form shock waves in the subsonic combustion field,which mainly affects the combustion by choking the mainflow.However,the separation shock wave,expansion wave,and reattachment shock wave formed by the slope significantly affect the jet wake distribution before ignition.The expansion wave generated by the middle posited slope compresses the cavity recirculation zone and cause the failure of supercritical kerosene ignition.The uplift of the jet plume caused by the shock wave generated by the upstream slope causes the ignition failure of cold kerosene.The dissertation designed two unsteady injections to simulate the dynamic process of scramjet starting and working condition transition.The injection pressure of vaporized kerosene changes continuously and slowly due to the filling process in the pipeline before injector.The change of jet penetration depth affects the instability of cavity flame,inducing a different variation between the flame light intensity and injection pressure.The flow field need a time to establish during the combustion state transition.The stabilization process of cavity pressure may cause delaying characteristics in flame evolution.Cold kerosene is sensitive to the fuel state before injection,which is usually accompanied with strong pressure oscillations in pipeline,causing the engine flameout in a suitable overall equivalent ratio condition.Because of the strong compressibility,vaporized kerosene can maintain a flame for a long time after a sharply change of injection condition,which reduces the risk of flame blowout in the injection variation during the engine condition transformation.