Investigation On The Flame Ignition And Stabilization Processes In A Cavity-Based Scramjet Combustor With A Rear-Wall-Expansion Geometry

To investigate the flow and combustion processes in a scramjet combustor with a rear-wall-expansion geometry,this study conducts systemic investigations on the fuel transport,mixing,forced ignition and flame stabilization processes both experimentally and numerically.By employing the CH*/OH* chemiluminescence,flame luminosity and high-speed schlieren optical diagnoses,ignition processes by both spark igniter and LIP(Laser-induced Plasma)have been visualized.PSI system is also utilized to provide quantitative information about the flame stabilization.In addition,numerical simulations are performed to study the flow field both prior and during the combustion process based on the OpenFOAM.The foundational mechanism regarding the supersonic flow and combustion issues have been further revealed.Firstly,a supersonic turbulent combustion LES(Large Eddy Simulation)method is introduced based on the OpenFOAM.The density-based compressible multi-species gas-combustion solver scramjetFoam is given in detail as well as the turbulence/combustion model PaSR(Partially Stirred Reactor).scramjetFoam has been demonstrated favorable for simulating supersonic flow and combustion issues by conducting a numerical validation against the classical DLR case.In this study,these newly developed numerical methods provide practical ways to investigate the supersonic flow and combustion processes both preciously and comprehensively.Then,effects of cavity fueling scheme and cavity geometry on the fuel transport and mixing processes in the scramjet combustor with a rear-wall-expansion geometry have been analyzed by LES results.It shows that the fuel transport process will be affected greatly with different cavity upstream fueling distances,however,the mixing fields are less sensible to the cavity upstream fueling distance.By applying the fueling scheme with cavity upstream cascaded injectors,fuel distributes more in the core supersonic region,and mixing enhancement will be achieved in the combustor by the impingement-interaction between the fuel jets.On the contrary,fuel distributes more in the subsonic region within the cavity and a local fuel-rich region will be formed in the rearward of the cavity by applying the fueling scheme with cavity upstream parallel injectors.With increasing expansion ratio of the cavity geometry the vertical flow velocity in the front of the cavity is enhanced,which results in an enhanced fuel entrainment into the cavity.Due to the enhanced fuel entrainment into the cavity at lower rear wall height,the mixture inside the cavity becomes more fuel-rich.It is indicated that the optimized cavity expansion ratio for the maximum fuel entrainment is around 0.40.Subsequently,the forced ignition process in a scramjet combustor with a rear-wall-expansion geometry has been investigated experimentally.The initial flame propagation routines have been visualized and effects of the cavity geometry on the ignition process are then discussed.It implies that the fueling scheme with cavity upstream parallel injectors is favorable for the ignition process and the rear cavity region also provides optimized ignition locations.The LIP ignition process could be classified into four stages,an initial plasma ignition stage,followed by the plasma quenching stage,the re-ignition stage and finally the stable flame stage.It is observed that the initial flame will first present a slow growing period within the cavity corresponding to the plasma quenching stage,then shows a main heat-release zones flashback phenomenon after the initial flame spreads out of the cavity corresponding to the re-ignition and stable flame stages which will be more obviously at high equivalence ratios.It is revealed that the cavity recirculation zone is crucially important for the ignition process.Both increasing the ignition energy and the equivalence ratio are practical ways to achieve successful ignitions.In addition,two ignition modes could be identified in a cavity-based supersonic combustor: weak ignition mode and intense ignition mode.Low equivalence ratios lead to a weak ignition mode assisted by the cavity angular recirculation zone.Relatively high equivalence ratios produce an intense ignition mode characterized by robust initial flame and quick propagation.The transitional ignition process is established at a condition between weak ignition mode and the intense ignition mode,which can be recognized at moderate equivalence ratios.The numerical results also show that a more successful LIP ignition in a cavity-based scramjet combustor can be achieved inside the subsonic zones and in the rear side of the cavity.Compared to the high-temperature thermal environment,it has been revealed numerically that the induced-activation chemical environment plays a dominant role in affecting the ignition process.Finally,the flame stabilization process has been investigated both experimentally and numerically in a scramjet combustor with a rear-wall-expansion geometry.The advantage of thermal chocking prevention of the rear-wall-expansion geometry is clarified firstly.Effects of cavity geometry on the flame stabilization process are then distinguished.It shows that the cavity shear-layer stabilized combustion is the most common flame stabilization mode,and the combined cavity shear-layer/recirculation stabilized combustion will occurred with increasing the equivalence ratio.However,the cavity assisted jet-wake stabilized combustion is rarely observed in the scramjet combustor with a rear-wall-expansion geometry.It has been demonstrated that the fueling scheme with cavity upstream cascaded injectors is favorable for flame stabilization processes.The fundamental mechanism of the combustion enhancement by spark-plasma is also revealed.