Turbulence Modeling For Supersonic Combustion

The research on Supersonic Combustion Ramjet(Scramjet) has been widely concerned in recent years.As the vast investment requirement of experimental research as well as the restrictions on the measuring and testing technique,numerical simulation method has become an important research tool. In this paper,turbulence modeling,one of the key issues in numerical simulations,has been deeply studied.The major research works and progresses are:developed several compressibility modification RANS turbulence models with a higher simulation accuracy;research and development of hybrid RANS/LES model to simulate complex unsteady turbulent flows;deeply research on a variety of complex supersonic flows and supersonic combustion problems relevant;proposed a technology to enhance supersonic combustion utilizing shunting.Traditional compressibility modification for RANS turbulence models generally focus on the dilatational compressibility modifications.Based on the latest primary research findings abroad,other parts of the generalized compressibility modifications were introduced and developed,viz.structure compressibility modification and shock unsteadiness modification.In the framework of standard k-εturbulence model,four generalized compressibility modified k-εturbulence models were proposed coupling with the traditional dilatational compressibility modifications.On the basis of compressible mixing layers,structure compressibility modeling was developed in the two first models.It can depress the overlarge turbulent kinetic energy,and can predict the depressed spreading rate of mixing layers very well.From a different point of view,shock unsteadiness modeling was developed in the other two models coupled with well developed dilatational compressibility models.The excessively production of turbulent kinetic energy can be depressed in the shock regions.As these two factors were considered together in the later two models,viz.the compressibility effect in free developed mixing layers and the unsteadiness effect of strong shocks,the numerical accuracies were further improved,especially for the prediction of fluid flow separation in high speed flows.Primary results show that one of them performs better than the widely used k-ωSST model in engineering applications for the flow separation prediction.The hybrid RANS/LES method demonstrates advantages in more accurate predictions than RANS method and much fewer computation costs than LES,and thus a very potential methodology.Focus on the kind of hybrid method in which the zonal computations are controlled by the model parameters, various hybrid methods were studied with different form.The theoretical deduction shows that these models utilizing the turbulence integral length scale as the reference length scale are essentially the same,and is also the same with the most popular Smagorinsky Subgird-Scale model in LES.But those utilizing the Kolmogorov length scale as reference length scale,can evolve from the RANS calculation directly to DNS simulation dependent on the simulation conditions.A new hybrid method was proposed applying an exponential form.The classic bluff body stabilized flows and supersonic mixing layers flows were studied accordingly.The four modified k-εturbulence models were evaluated,and applied in the studies on complex supersonic flows and supersonic combustion problems.The supersonic flow cases are supersonic mixing layers,the supersonic backward facing flows,the transonic flow over an arc bump,supersonic flow interacted with transverse injection etc.The supersonic combustion problems concerned here are supersonic wall-bounded reactive mixing layers,supersonic combustion with transverse fuel injection etc.The four modified models were validated and compared.The compressibility effects(including the dilatational compressibility,structure compressibility and shock unsteadiness effect) were studied on the depression of the mixing layers and enhancement of fluid flow separation,and these effects on the supersonic combustion performance were also investigated.The wall injector is the most basic fuel injector in supersonic combustor.The relevant flow field was studied.Based on the flow structure,a supersonic combustion enhancement technology was proposed utilizing shunting that is to connect the upstream region and the downstream region of fuel injection slot through a channel.Higher pressure fluid upstream partly shunts into downstream region and the flow field was changed correspondingly,hence the combustion was enhanced.The mechanics and discipline of shunting effects on combustion enhancement were investigated under different injection pressures both in the external and internal supersonic flows.