Supersonic Flow Mode And Flame Stabilization Mechanism Of A Rearwall-Expansion Cavity Flameholder

How to organize robust and efficient combustion under high-speed flows in the supersonic combustor is one of the key issue of the scramjet technology.Conventional cavity flameholder show some significant disadvantages for wide-ranged flight Mach number and high equivalence ratio,while a new configuration of cavity flameholder,the rearwall-expansion cavity,is thus proposed in recent years.The present study,focusing on supersonic flow mode and flame stabilization mechanism of the rearwall-expansion cavity for scramjet combustor application,carried out numerical simulation,experimental observation and mathematical modeling to investigate the flow,mixing and combustion process in the supersonic combustor with a rearwall-expansion cavity.Based on the hybrid RANS/LES turbulence model,a set of high-precision numerical methods suitable for supersonic turbulent flow and mixing problems are established.A high-order nonlinear filter based shock-capturing scheme with high resolution and strong robustness was constructed,and in order to generate the inflow boundary layer,an improved synthetic eddies method was proposed.Furthermore,the numerical models and methods for LES of supersonic turbulent combustion are established.By decoupling the calculation of flow and combustion process,a simplified algorithm is given for the transport coefficients of multi-component mixtures,and the reduced reaction mechanism for gaseous fuel and air are further improved.Based on the bi-model quadrature method,a partially stirred reactor/scale similarity(PaSR-SS)turbulence/chemical reaction interaction closure model was proposed.While the turbulent Schmidt number is calculated via a simplified linear variation model.The flow characteristics in a rearwall-expansion cavity and the cavity geometry effect on flow structures were numerically investigated.It was found that the cavity rearwall-expansion effect will significantly weaken the shock waves around the cavity.The vortical structures in the recirculation zone were greatly compressed and split.A lower cavity rearwall height is proven to have lower flow resistance and this effect shows a nonlinear trend with the change of the cavity rearwall height.The flow mode of the rearwall-expansion cavity under the supersonic flow was analyzed,and a mathematical model of the shear layer growth rate associated with the flow parameters inside and outside the cavity was established.The mixing process of the fuel jet upstream of the cavity was further studied with different cavity configurations and injection momentum ratios.The results showed that the cavity rearwall-expansion effect helps to reduce the total pressure loss along the flow path,while it decreased the mixing efficiency between jet and inflow and reduced the entrainment of fuel jet into the cavity.The combustion characteristics of the rearwall-expansion cavity with the upstream fuel injection were numerically and experimentally studied.The results showed that,under a relatively low global equivalence ratio,the reacting regions were mainly concentrated around the cavity,while the wake flame was stabilized near the bottom wall downstream of the cavity.It was found that in the cavity recirculation zone,the local combustion heat release is highly relevant to the local turbulence intensity.The rearwallexpansion configuration tended to weaken the flame stabilization ability of the cavity,and an excessive ratio of rearwall offset would likely result in flame blowout.The simulation results revealed that the possible reason for the initial flame blowout in a rearwallexpansion cavity is the impact of the high-speed airflow above the cavity recirculation zone and along the upstream shear layer,which would prevent the formation and stabilization of the flame base in the shear layer or recirculation zone.The concept of geometry limit for flame stabilization of a rearwall-expansion is proposed,and it helped to illustrate the phenomena that a minor change in cavity geometry may cause the qualitative change in flame stabilization ability.The influence of the cavity rearwall height on flame structure was further analyzed.The results showed that,compared to the conventional non-expansion cavity,the flame in a rearwallexpansion cavity is closer to the combustor bottom wall and seemed more compressed by the main stream.Meanwhile the flame downstream of the cavity becomes weaker.The rearwall-expansion effect tended to reduce the combustion efficiency and weaken the concentrated heat release in the center region along the spanwise direction.And it prevented the formation of side-wall flame under high equivalence conditions.By changing the cavity configuration and the global equivalence ratio,three combustion stabilization modes of the rearwall-expansion cavity were found,including the cavity shear-layer stabilized mode,the expanded recirculation zone/lifted shear-layer stabilized mode and the cavity-assisted jet wake stabilized mode.The positions of the flame bases were different in the three modes,and the corresponding flame stabilization mechanisms were also significantly different.Experimental studies showed that the cavity rearwall-expansion effect tended to delay the transition from a weaker heat-releasing mode to a stronger one,which helped to prevent the thermal choke in the combustor under high equivalence ratio conditions.Furthermore,a preliminary criterion for the design of the rearwall-expansion cavity configuration are given.The flame stabilization boundary,or the flame blowout limit for a rearwallexpansion cavity was mathematically modeled based on the experimental observation and simulation results.Firstly,an empirical model of the transverse jet mixing process in a supersonic crossflow was proposed,which predicts the jet center trajectory,the penetration height and the spanwise distribution.With the improvement of the models for all relevant sub-processes,the rich and lean blowout limit for the conventional nonexpansion cavity were mathematical modeled by correlating the effective equivalence ratio and the Damokhler number.While for the rearwall-expansion cavity,the jet diffusion model,the calculation method of the effective equivalent ratio,the cavity characteristic flow time and other relevant sub-process models are modified according to the reduction of the rearwall height.The mathematics model of the lean boundary of the rearwallexpansion cavity was thus established.The overall effect of the cavity rearwall-expansion configuration on combustor performance was summarized.