Investigation Of Flame Structure In A Supersonic Combustor Using Planar Laser-induced Fluorescence

Hypersonic vehicle attracts much attention due to its fast speed and strong penetrating capability.Its propulsion system has always been one of the key focus in researches,and the scramjet engine is usually used as the propulsion system of hypersonic vehicle.In a supersonic incoming flow,the scramjet usually uses a cavity to stabilize a flame and obtain stable combustion,and then generate the thrust.The investigation of the cavity-stabilized flame structure can provide reliable experimental validation for the design of the scramjet combustor and for the study of the flame stabilization mechanism.In this thesis,planar laser-induced fluorescence(PLIF)technique is used to visualize the flame structure of cavity-stabilized combustion in a supersonic flow.In the supersonic incoming flow,the OH-PLIF technique was used to observe the reaction zone structure of the cavity-stabilized flame,and the CH-PLIF technique was used to measure the heat-release zone structure of the cavity-stabilized flame.The OH-PLIF experiment was carried out on the direct-connected supersonic test facility at the National University of Defense Technology.Under the incoming flow with a mass rate of 1 kg/s,a Maher number of 2.92,and a total temperature of 1530 K,the Nd:YAG-pumped dye laser with a wavelength of ?283 nm,a pulsen energy of 12 mJ and a frequency of 30 Hz is used to generate a laser sheet for PLIF measurements.Under the conditions of an equivalent ratio of 0.29 and 0.52,the multi-sectional spatial distribution and the development trend of the cavity-stabilized reaction zone structure with three streamwise sections and two spanwise sections in the combustor were obtained.When the equivalence ratio is 0.29,the OH radicals are distributed in the strip structure is symmetrical to the central axis and develops along the flow direction,and the strip structure converges at the rear edge of the cavity;when the equivalence ratio is 0.52,OH distributes in the sheet structure close to the side wall,and the sheet structure converges in the downstream of the cavity.The CH-PLIF experiment uses a tunable Alexandrite solid-state laser with a wavelength of ?387 nm,a pulse energy of 15 mJ and a frequency of 10 Hz.The heat-release zone structure of the methane/air premixed flame with different Reynolds numbers was observed by using the CH-PLIF technique.The CH-PLIF optical system was optimized on-line in a burner for more complex diagnostic experiments in a supersonic combustor.In the 1 kg/s direct-connected supersonic test facility,the CH-PLIF technique was used to image the heat-release zone structure of the cavity-stabilized flame.It is found that the heat-release zone with a thickness of 0.5-6.5 mm in the cavity-stabilized scramjet combustor become highly distorted and wrinkled.Meanwhile,the separation of the heat-release zones can be observed.In this thesis,the OH-PLIF technique was used to recognize the multi-sectional distribution of the cavity-stabilized flame structure in a supersonic combustor,which is helpful to understand the development and change of the cavity-stabilized flame in space.The CH-PLIF technique was employed to image the heat-release zone structure of the cavity-stabilized flame in the supersonic flow.The distribution and development of the heat release zone in the cavity were obtained.The results are beneficial to understand the structure of the reaction zone and heat-release zone in a cavity-stablized flame in the supersonic inflow.The presented results can offer reliable experimental validation for the simulation results,and provide some references for the design and improvement of a scramjet combustor.