Investigation On The Mechanisms Of Combustion Dynamics Under Supersonic Inflow Conditions

Air-breathing engines,such as ramjets,scramjets,and combined-cycle engines have laid the foundation of hypersonic flight above the Mach number of 5.They are among the optimized propulsion systems for future large scale and reusable airplanes,near-space flight vehicles and hypersonic missiles.The unsteady characteristics of turbulent combustion in such high-speed propulsion systems are inherent features when concerning their operating in a large range of speed,a broad range of trajectory,and a variety of combustion modes.The variation of inflow condition,the adjustment of flow-path geometry,and the regulation of fuel injecting strategy all have strong coupling effects with the turbulent mixing,the combustion heat release,and the shock movement in the combustor,which leads to the remarkable unsteady characteristics of the combustion process.Examples for the most important questions in the refined design of scramjet or scramjet-based combined-cycle engines are listed but not limited to the following:(a).The identification and distribution of supersonic and subsonic combustion modes under supersonic combustor inlet conditions.(b).The quantification and distribution of premixed and diffusion combustion modes under partially-premixed fuel injection conditions.(c).The distribution law and time evolution of spatial heat release under the co-existing of different combustion modes.(d).The fuel ignition,flame propagation,flame stabilization,and flame extinction in high-speed flows under unsteady combustion conditions,etc.High accuracy numerical simulation has been an important support for the national strategy of the incorporation of aeronautics and astronautics,which can accelerate the development of high-speed,broad-range propulsion systems such as scramjet engines significantly.In the present work,the unsteady combustion features intrinsically in high-speed propulsion systems are investigated numerically.Two basic model combustors,one with a wedge-shaped strut for fuel injection and flame stabilization,and the other with a hybrid of one strut and two wall cavities,are investigated by Large Eddy Simulations(LESs)under different combustor inlet conditions and/or fuel equivalence ratios.Combining with the results of LES,theoretical analyses and data mining methods are applied to the fundamental research of the following two scientific problems: the distribution of combustion modes and the dynamic evolution of heat release under supersonic inflow conditions,and the flame propagation characteristics and flame stabilization mechanisms in high-speed flows.The research results are in favor of the organization and controlling of combustion under multiple combustion modes,the designing and improving of flame stabilization devices that operate in a wide range,and the guiding of the development of turbulence-combustion subgrid models.The main contents and important conclusions of this research are summarized as follows:(1)Based on the flow Mach numbers of the gas when combustion takes place,three methods are proposed for the identification of supersonic and subsonic combustion modes.Based on the mixing regimes of fuel and oxidizer when combustion takes place,another three methods in quantifying the extent to which combustion takes place in premixed or diffusion flames are proposed and compared.The Heat Release Rate(HRR)is introduced as a progress variable to properly separate the gas in flowing state from the gas that really burns.The reasonability and necessity of introducing such a progress variable that represents the reaction process for the accurate identification of different combustion modes is well verified with validation cases.(2)The mixing rates between fuel and oxidizer under different flow regimes are dominated by different transport and/or diffusion mechanisms.The baroclinic effect is more in favor of the vorticity transport in subsonic combustion.However,the dilatational effect is better for the convection of vorticity in supersonic combustion.For the design of a high-speed combustor,mixing can be enhanced by increasing the streamwise vorticity and/or forcing the transversal pressure gradients.Example measures include but are not limited to,e.g.,the design of vortex generators and the use of swirling fuel injections,etc.Particularly,different measures can be taken at different part of the combustor.For example,baroclinic effect can be enhanced in the recirculation zone of flame stabilization devices and in the low-speed zone after fuel injection through increased ratios of total pressure,momentum,and density of the fuel jet to that of the main flow.Dilatational effect in the supersonic main flow can be enhanced through dispersed and multiple stages of fuel injection.(3)A statistical analysis method to construct the spatial Probability Density Function(PDF)of different combustion modes,i.e.supersonic and subsonic combustion,premixed and diffusion combustion is developed based on the idea of variable filtering.The transfer function of the statistical uncertainty by using limited sample space to represent the infinite ensemble space is derived.Subgrid scale combustion models can be analogically constructed based on the PDFs of different combustion modes on the resolved scale in LES,which will own a wide range of modeling ability,i.e.from subsonic combustion to supersonic combustion,and from diffusion combustion to premixed combustion.(4)A quantitative calculation method to evaluate two kinds of time scales and two kinds of energy scales concerning with the recirculation zone and control the flame propagation and flame stabilization in the combustor is developed.The flame stabilization mechanisms of two typical flame stabilization devices that are widely used in high-speed combustors,i.e.the strut and the combination of strut/cavity are investigated.At low stagnation temperatures,the flame stabilization in the whole combustor is achieved through the large-scale flame propagation to downstream,which is continuously ignited in the recirculation zone.At moderate stagnation temperatures,flame in the whole combustor is not only stabilized by the ignition effect of the recirculation zone,but also enhanced by the intersection of the two oblique shocks reflected on the combustor walls.At high stagnation temperatures and high inflow Mach numbers,the flow field is severely modulated by the flame stabilization devices.A localized zone with very high temperature due to the intersection of shocks that exceeds the auto-ignition temperature of the gas mixture is severed as an ignition source for the downstream gas.The rates of energy generation and dissipation,each at a high level,are balanced dynamically and statistically in the recirculation zone when the flame is well stabilized.(5)A quasi-steady state quantitative analysis method that applicable to the ignition and flame extinction process in very short time duration is developed based on the energy balance in the recirculation zone.The ignition and flame extinction process in typical cases are investigated.For the cases in which flame fails to be finally stabilized,local flame extinction firstly occurs in the recirculation zone,where the rate of energy generation is continuously lower than that of energy dissipation.Flame in the recirculation zone itself cannot be well sustained,leading to the blow out of flame and the form of a typical lift-off flame.For the cases in which flame builds up,the local auto-ignition hot spots firstly occur near the aft wall of the cavity as well as in the intersection region of the oblique shocks originated due to the modulation effect that the cavity acts on the main flow.When this local flame propagates downwards,meanwhile,to a small extent,it also propagates upstream.Therefore,the wall cavity is filled with flame after a certain long time,which then steadily becomes an ignition source for the downstream gas.A positive feedback between the accumulation of energy in the recirculation zone and the heat released from chemical reactions can be found in this process,which effectively promotes the establishment and propagation of the flame in the whole combustor.