Investigation On The Ignition And Flame Stabilization Mechanism Using The Hot Gas In Supersonic Flow

The air-breathing ramjet and its combined engines are one of the best engines to realize supersonic flight in the future.For the air-breathing ramjet with a wide speed range and a wide airspace,there is a complex flow in the combustor,accompanied by shock waves.It is difficult to achieve ignition,flame propagation and flame stabilization.In this study,the ignition and flame stabilization using hot gas in the combustor of the propulsion system were taken as the research background.The research object was the unsteady high temperature gas coaxial supersonic jet flame model burner and the model engine combustor.With the high-precision experiment,refined large eddy simulation and theoretical analysis as the research methods,the ignition using hot gas in supersonic flow and model engine combustor were carried out.The main work and conclusions in this paper are as follows:(1)The ignition process using hot lean gas in supersonic flow can be divided into three stages:(I)fuel and high-temperature gas are mixed in the shear layer and chain reaction occurs,(II)ignition kernel is formed in the reaction mixture layer,(III)the ignition kernels develop into a continuous flame.The time and location of the first occurrence of the ignition kernel are greatly affected by the flow time scale and the chemical time scale.As the flow time scale decreases,the time of the ignition kernel occurrence remains unchanged,but the location moves downstream;as the chemical time scale decreases,the time of the ignition kernel occurrence shortens,and the location moves upstream.The formation of the ignition kernel is dominated by the high temperature and low speed region,while the development of the ignition kernel and the flame propagation are dominated by the supersonic flow.In the initial stage of ignition kernel formation,diffusion combustion is the main part of the kernel,while in the condition of large ratio of molecular diffusion time to zero-dimensional ignition delay time,premixed combustion is the main part of the kernel.With the development of the ignition kernel,the proportion of diffusion combustion decreases gradually,and the proportion of premixed combustion increases.With the increase of fuel jet velocity,the flame state transits from "transition flame" to "lifted flame",and the flame stabilization mechanism transits from flame propagation to auto-ignition.When the fuel jet velocity is constant,with the increase of hot gas temperature,the flame position moves upstream,and the flame stabilization mechanism is autoignition.(2)The ignition process using hot rich gas in supersonic flow is divided into three stages:(I)supersonic jet fuel and hot rich gas are mixed;(II)hot mixture reacts with the air;(III)flame is localized extinct when it mixed with the cold air downstream;(IV)flame reaches a stable state.There are three states of supersonic jet flame under the action of hot rich gas,namely‘stable’,‘unstable’ and ‘blow-off’.The mechanism of flame stabilization is the ignition due to hot rich gas.When the equivalence ratio of co-flow is constant,the high speed jet flame state changes from stable to unstable with the increase of injection pressure of fuel jet,and finally blow-off.With the increase of equivalence ratio of co-flow,the temperature of hot gas decreases,the range of stable flame state becomes smaller,and the range of unstable flame state becomes larger.For the flame in supersonic flow,the stable,unstable and extinction of the flame are controlled by the chemical and flow.The stable flame is caused by the chemical time scale being continuously smaller than the flow time scale.The unstable flame is caused by periodic changes of chemical time scale and flow time scale.Extinction is caused by the chemical time scale continuously larger than the flow time scale.(3)In the RBCC model engine combustor containing the strut/cavity and rocket jet,rocket jet plays the role of ignition in flame stabilization,the main role of strut is to promote the mixing of fuel and air flow,and the main role of cavity is that its thermodynamic characteristics make the momentum and energy exchange of hot gas and fuel in the shear layer reduce the chemical time scale of fuel.The flame stabilization mechanism is defined as the hot gas and cavity shear layer ignition mode.In the strut/cavity combustor without rocket jet,the recirculation zone of the strut and the cavity makes the fresh mixture enter into the recirculation zone and which reacts with the hot gas due to its chemical kinetic characteristics.The recirculation zone extends the residence time of the mixture.The flame stabilization mechanism is defined as the strutcavity recirculation zone flame stabilization mode.In the strut combustor with rocket jet,rocket jet also plays the role of ignition in flame stabilization.The flame stabilization mechanism is defined as hot gas ignition mode.For this type of flame stabilization mode,when the rocket is shut down,the chemical time scale increases rapidly and continues to be larger than the flow time scale,resulting in flameout.