The Influence Of Low Flight Pressure On Supersonic Combustion Properties Within Scramjet

As a new type of aircraft,hypersonic vehicles are required to achieve ultra-high-speed flight in large airspace,with dynamic pressure decreasing sharply.As the best optimal power unit,the stability and efficiency of supersonic combustion within scramjet is facing a great chanllenge.On the one hand,as the dynamic flight pressure decreasing,the environment pressure drops drastically,which will cause the pressure within the combustor to drop,resulting in the chemical reaction rate to decrease.On the other hand,the reduction of dynamic pressure increases the the inlet velocity of the combustor,leading to the decrease of residence time of fuel within the combustor,which makes it more difficult to mix,ignite and realise the stabilization of supersonic combustion in the combustor with a finite length.Proper temperature and pressure conditions and mixing of fuel with oxidant on the molecular scale are two prerequisites for combustion.Based on that,the research work about the effect of flight dynamic pressure changs on supersonic combustion is carried out from two aspects,the chemical reaction kinetics and the fuel mixing.The main research contents are as follows:Firstly,a zero-dimensional performance evaluation model is used to analyze the effects of flight dynamic pressure,combustion efficiency and fuel equivalent ratio changes on the performance of the ramjet.The average temperature and pressure within combustor were obtained to have a further study about the effects of temperature and pressure on the chemical reaction kinetics.Secondly,the influence of the average temperature and pressure of combustor on the ignition delay time,reaction time,laminar flame speed and the flame thickness was studied by Chemkin/Cantera based on the premixed laminar flame combustion model.Then the premixed turbulent flame combustion mode is judged according to the premixed laminar flame characteristic parameters and the basic theory of turbulence.Under the high fight Mach number with low flight dynamic pressure conditions,the results is shown that the Damkohler is much larger than 1 and Karlovitz is much lower than 1,with combustion being in a fast chemical reaction state.The turbulent combustion mode is mainly located in the thin reaction flame mode zone and the wrinkle flame mode zone,implying the fuel mixing is the main factor affecting combusiton.Thirdly,based on the assumption of rapid chemical reaction,two-dimensional numerical simulations within a ‘equal-expansion-equal’ central strut supersonic combustor was carried out with the commercial software Fluent.To some extent,the fuel mixing efficiency can be relected by the kerosene diffusion boundary.Therefore,the influence of dynamic pressure changes on the diffusion boundary of kerosene is analyzed.The results show that the kerosene diffusion boundary first rises and then decreases as the flight pressure decreases,resulting in the same trendency of combustion efficiency.It reveals that better fuel mixing efficiency better combustion efficiency.Finally,as one of the significant parametes affecting supersonic combustion,the influence of combutor expansion ratios on the performance of combustor was studied,under the low flight dynamic pressure conditions.The results show that,comparing with high flight dynamic pressure conditions,the performance of the combustor is more susceptible to the changes of geometry of combustor under low flight dynamic pressure conditions.As combustor expansion ratios increasing,the combustion efficiencies decline continuously.At the same combustor expansion ratio,the combustion efficiency increases with the increase of the flight dynamic pressure,causing the combustor thrust to increase.