Investigation Of Gliding Arc Plasma-assisted Ignition And Combustion In A Supersonic Flow

In this thesis,due to the problematic ignition and flameholding in the scramjet combustor,a gliding arc plasma(GA)is utilized to strengthen the ignition ability and the combustion stability in the supersonic flow.Experimental investigations of plasmaassisted ignition and combustion in the supersonic flow are mainly conducted in this thesis using advanced laser techniques.Combined with the zero-dimensional plasma calculation and the three-dimensional flow field simulation,characteristics of the gliding arc discharge,ignition,combustion enhancement,and flame stabilization in the supersonic flow are systematically studied,and underlying mechanisms of the plasmaassisted ignition and combustion are revealed.Firstly,the mode transition mechanisms of the gliding arc plasma in a supersonic air flow are revealed,and the high-frequency(1.4～2.1 MHz)discharge mode transition occurs between the spark-type and the glow-type modes.The small-scale turbulent eddies and the shock wave structure periodically distributed are the main reason for the highfrequency discharge mode transitions in the supersonic air flow.In the supersonic reacting flow,due to the flame-induced high temperature around the plasma,the reduced electric field strength of the gliding arc is smaller than that of the supersonic air flow,and the gliding arc tends to become the glow-type discharge.The ignition characteristics of the gliding arc plasma in the scramjet combustor are studied.The critical factors of the ignition performance and the ignition mode of the gliding arc plasma in the scramjet combustor are illustrated.The Ma2.92 inflow has a total temperature of 1650 K.Fuel injections upstream of a cavity flameholder are utilized to stabilize the flame in the experiments.The spark-type discharge of the gliding arc discharge is found to be the key factor to ignite the flame because the flame kernels are generally initiated at the power spikes of the spark-type discharges.If the flame kernel contains more discharge energy,the area of the initial flame becomes larger,which is easier to form the global flame.The direct ignition mode and the re-ignition mode in the scramjet combustor can be identified.In the direct ignition mode,the initial flame can be directly ignited by the gliding arc plasma.In the re-ignition mode,the flame kernel is unable to form an initial flame in the early stage of the ignition process,which can be found from the quenching CH* emission of the flame kernel.The CH* emission can be seen again,although its quenching lasts for ～700 μs and the re-ignition of the initial flame can be developed to further establish a cavity-stabilized flame.The re-ignition mode is more likely to be observed near the fuel-lean limit condition and exhibits a longer flame propagation time and a smaller flame kernel than those of the direct ignition mode.The repeatable ignition with wide equivalence ratios in the ethylene-fueled and the kerosene-fueled scramjet combustor is achieved by the multi-channel gliding arc(MCGA)plasma.In the ethylene-fueled ignition process,several flame kernels produced simultaneously by the power spikes of the MCGA can be merged in the propagation process,resulting in a faster flame propagation(48% reduction in the flame propagation time)and a shorter ignition time(61% reduction in the ignition time)compared to the ignition by a single-channel gliding arc.For the kerosene-fueled ignition process,the MCGA is more likely to produce a large area of the flame kernel in the spark-type discharge with long arc columns and overlaps of each channel gliding arc.The formation of the flame kernel mainly depends on the discharge characteristics of the MCGA plasma,whereas the flame propagation process is mainly determined by the cavity flow.The resident flame is hard to spread to the back of the cavity along the shear layer due to the large velocity gradient with a relatively low temperature.However,the resident flame is formed several times by the MCGA plasma ignition,which increases the possibility of forming the global flame.The MCGA plasma is utilized to enhance the combustion stability significantly,which suppresses the combustion mode transitions and maintains the unstable flame in the scramjet combustor.When the MCGA is off,the flame frequently oscillates between a cavity shear-layer mode and a cavity-stabilized mode at a low global equivalence ratio,whereas the flame is more likely to be kept in the cavity-stabilized mode when the plasma is on.The combustion frequently oscillates between a cavity-stabilized mode and a jetwake stabilized mode at a high global equivalence ratio,but the mode transitions can be suppressed significantly in the presence of the MCGA.A plausible explanation of the suppression of the combustion mode transitions can be mainly related to the temperature rises in the entire cavity by the MCGA plasma with higher backpressure,leading to more intense combustion and the blocking in the mainstream.The MCGA plasma can sustain the oscillation flame which is nearly blown out.After the plasma is off,the flame is unable to be held when the fuel injection is located near the cavity ramp.When the plasma is on,the unstable combustion is more likely to be kept.The averaged flameholding time is increased by 51% when the plasma is added.The main reason for the flameholding by the plasma is that the MCGA plasma significantly reduces the ignition delay time and increases the residence time.The MCGA plasma is used to suppress the flame flashback phenomenon in the scramjet combustor and control the behavior of the heat release region in the jet-wake stabilized combustion.At relatively high equivalence ratios,the scramjet combustor exhibits a ramjet mode,and the large flame oscillations occur with the evident flame flashback phenomenon.When the plasma is added,the flame flashback is suppressed,and the flame leading edge is located in the vicinity of the plasma.The re-ignition and the attached characteristics of the MCGA plasma contribute to the suppression of the flame flashback.At relatively low equivalence ratios,the flame in the scramjet combustor is mainly located near the cavity leading edge after the global flame is ignited,which exhibits the jet-wake stabilized mode.When the plasma is added,the heat release region of the flame spreads from the cavity leading edge to the plasma region and is kept there.Once the plasma is turned off,the combustion reverts to its original state.The mechanisms of the MCGA plasma controlling the heat release region are due to the thermal and kinetical effects of the plasma.Intense combustion near the plasma is formed with higher pressure,which further pushes the separation positions of the boundary layer closer to the fuel jet.The flame moves upstream and the heat release region propagates forward to the plasma region.In this thesis,the MCGA plasma is used to achieve the repeatable ignition with wide equivalence ratios in the scramjet combustor.The combustion heat release is controlled by the MCGA plasma and the combustion stability is significantly enhanced.Meanwhile,the mechanism of the gliding arc plasma-assisted ignition and combustion in the supersonic flow is revealed,which has essential academic and engineering significance for the repeatable ignition and flameholding in the air-breathing hypersonic vehicle.