Mechanism Research On Flame Instability In Supersonic Combustor With Dual Cavities

In this dissertation,the combustion flow in a rectangular scramjet combustor with dual parallel cavities is studied.The transverse injection and mixing processes of gaseous ethylene upstream the cavity as well as flame stabilization and propagation processes were investigated through experiments,numerical simulation and theoretical analysis.The low-frequency flame oscillation phenomenon appears under moderate or relatively low overall equivalence ratios in Ma 2.92 airflow.This dissertation explored the flame instability character and its main influencing factors.A possible flame oscillation mechanism is proposed based on the combustion theory.The effects of fuel/air equivalence ratio and the fueling scheme on the flame oscillation were examined systematically.Experimental results show that the main flow in the combustor keeps supersonic under moderate or lower equivalence ratio.And the shock train moves upstream due to the back pressure caused by heat release.During the shock train movement,the shock train deviates from the center of the flow path and induces the turbulence boundary layer to separate forming asymmetric separation region.This asymmetric structure of flowfield changes the jet injection and mixing state on both sides.The jet penetration depth on the side of the large separation zone is deeper,and the flame is stabilized in cavity-assisted jet-wake stabilization mode.On the other side,the detached flow reattaches to the wall.The jet is exposed to the high speed supersonic main flow and the flame is in cavity stabilized mode.As the flow and combustion unsteadiness couples under this condition,the flame oscillates between cavity stabilized mode and jet-wake stabilized mode intermittently.The fuel jet in the large separation zone forms large vortex structure under the shearing action of the inflow.As the vortex entrains the air into the jet,the fuel is partially mixed with the air and generates partially premixed flame.The flame on large separated region side is broken and unstable while it is continuous on the other side and stabilizes in cavity stabilization mode.The flow is subsonic in separation region and the jet with high momentum almost penetrates into the incoming flow perpendicularly.The jet wake is lifted up due to the combustion back pressure which weakened the interaction between the jet plume and the cavity.At the same time,the fuel and the air near the jet are not fully mixed.And the flame temperature is not high because of the incomplete combustion.The strong turbulent dissipation and the large shear strain rate in the windward surface of the jet make the flame difficult to stabilize.All above factors lead the distributed partially premixed flame easy to be blown downstream.The flame blow-out causes the combustion intensity to be weakened,and the separation zone shrinks and moves downstream.While the jet penetration depth decreases,the interaction between the jet and the cavity is enhanced.The mixing state of the fuel is improved.After a period of time,the combustion intensity of the flame in cavity shear layer increases gradually to make the separation zone develops and it can propagate upstream again.This blow-off and flash-back process of the flame could repeat intermittently in the combustion chamber to form a low-frequency flame oscillation process.The back pressure caused by heat release affects the shock train movement and the separation region.And the mixing process as well as the turbulence brings perturbation to the flame stabilization and propagation.Flame is essentially unstable under the strong coupling of combustion and flow.The characteristic time of flow and reaction is close in supersonic flow that the flame instability in scramjet is stronger than that in ramjet.