Investigation Of Flameholding Mechanisms And Methods For Flameholding Enhancement In A Rocket-based Combined Cycle Engine Operating In Ramjet/scramjet Mode

To achieve robust combustion in a rocket-based combined cycle(RBCC)engine operating in ramjet mode and scramjet mode,the present study assesses the ability of the cavity recirculation flow and the recirculation flow at the rocket base to stabilize the flame.Two methods for flame stabilization enhancement are proposed and tested.In the non-reacting flowfield,the flowfield distortion resulting from the rocket exerts a considerable influence on the cavity flow.The cavity shear layer reattaches at the cavity floor when most of the cavity is exposed to the expansion fan.Consequently,the size of the cavity recirculation flow decreases significantly.The cavity fails in flame stabilization.When the reattachment shock wave impinges on the cavity shear layer,the cavity shear layer is lifted because of the adverse pressure gradient.The cavity with a small length-todepth ratio is less sensitive to the negative impacts of the expansion fan.Cavity ignition and flame stabilization involve complex interaction between the flowfield and the chemical reaction.In the early stage of cavity ignition,the flowfield dominates the propagation of the flame kernel.The flame kernel generated by the spark plug is carried to the fore part of the cavity by the recirculation flow.When the locally stabilized flame is vigorous enough,the positive feedback between the cavity shear layer flame and the flame base accelerates the propagation of the flame.The heat release of the flame is intense enough to change the flowfield when the flame spreads to the mainstream.Driven by the interaction between the reaction zone and the flowfield upstream of the reaction zone,the flowfield gradually settles into the final condition.There are three typical combustion modes in the cavity-based combustor.The combustor operates in the cavity stabilized scramjet mode when only the rear of the cavity is exposed to the expansion fan.The flame chemiluminescence intensity oscillates at a frequency between 300 and 500 Hz because the cavity shear layer impinges on the cavity ramp.Jet-wake stabilized scramjet mode is witnessed in the combustor when the reattachment shock wave impinges on the cavity flow.The shock-flame feedback loop leads to the oscillation of the flame chemiluminescence intensity(～100 Hz).In the combustor with a cavity downstream of the reattachment shock wave,the expansion fan reduces the static pressure of the mainstream upstream of the cavity.The reattachment shock wave makes the boundary layer more sensitive to adverse pressure gradient.The intense heat release induces a large-scale separation region in the combustor.As a consequence,the combustor is choked and operates in the ramjet mode.The unsteady thermal throat induces the combustion oscillation with a frequency of approximately 800 Hz.The investigation of cluster analysis suggests that the auto-encoder based on convolutional neural networks can accurately distinguish different combustion modes according to flame chemiluminescence images and schlieren images.In the combustor that employs a cavity and a rocket base as the flameholders,the reaction zone stabilized by the cavity extends the recirculation flow at the rocket base and the wake of the rocket.A large subsonic zone that is suitable for ignition and flame stabilization is established.Compared to the combustor that only employs a cavity flameholder,this combusor shows a higher combustion efficiency and a reacting flow field that is more satble.When the cross-sectional areas of the rockets are the same,the recirculation flow just downstream of the rocket with a circular cross section is larger than its counterpart downstream of the rocket with a rectangular cross section.Therefore,the rocket with a circular cross section is superior to that with a rectangular cross section in terms of flame stabilization.However,the recirculation flow downstream of the rocket is too small to stabilize the ethylene or kerosene flame.Shock wave generators(SWGs)is employed to extend the operation envelope of the combustor.In the non-reacting flowfield,the oblique shock waves generated by the SWGs have no significant impact on the recirculation flow just downstream of the rocket.However,in the reacting flowfield,the adverse pressure gradient resulting from the oblique shock waves extend the size of the recirculation flow.The oblique shock waves also create a region with a high temperature and a high pressure,which significantly enhances the chemical reaction.Robust ethylene flame is achieved in the combustor.Based on the precondition that the SWGs do not choke the combustor,the chemical reaction is more vigorous in the combustor equipped with high blockage ratio SWGs.Along with the increase of the distance between the rocket and the SWGs,the size of the recirculation flow successively experiences an increase and a decrease.The optimal distance between the rocket and the SWGs increases with the increase in blockage ratio.Microjet arrays(MJAs)have the potential to replace the conventional SWGs in terms of flame stabilization enhancement.The experimental results indicate that the shock waves generated by the MJAs and the SWGs are quite similar.The combustors that employ these two methods share the same combuston mode.