Direct Numerical Simulations Of Compressible Two-phase Turbulent Flow And Combustion

The supersonic combustion ramjet(scramjet)is a crucial engine to provide thrust and overcome drag for the hypersonic vehicles,which have revolutionary applications in national security,space exploration,and commercial aviation.Compressible two-phase turbulent flow and combustion is the common phenomenon in scramjets with the liquid hydrocarbon fuel and involves the multiphysics and multiscale coupling among compressible turbulence,droplets/particles and flame.The design and improvement of the scramjet requires the clear and deep understandings of this problem.However,the fundamental researches on the compressible two-phase turbulent flow and combustion remain widely open because of the complicated and strongly nonlinear interactions.Therefore,based on the Eulerian-Lagrangian point-source method,the present thesis develops a massively parallel computing platform on the high-fidelity direct numerical simulations of the compressible two-phase turbulent flow and spray combustion.With this computing platform,we separately investigate the particle-laden compressible isotropic turbulence,the compressible mixing layer laden with particles and the spray combustion in a supersonic mixing layer.Besides,the analysis system with the transport equations and the self-similar equations is constructed,which reveals the underlying physical mechanisms of the complicated interaction among compressible turbulence,droplets/particles and flame and provides the theoretical instructions for the practical application of scramjets.For the particle-laden compressible isotropic turbulence,the thesis conducts a systematic investigation of the interaction between particles and turbulence.The numerical results show that particles tend to accumulate in the regions with high density and low vorticity,and such a tendency becomes more obvious when the Stokes number of particles is close to 1.Besides,small particles(St₀?0.5)augment turbulent kinetic energy and the rotational motion of fluid,medium particles(St₀?1.0)enhance the rotational motion of fluid,and large particles(St₀?5.0)attenuate turbulent kinetic energy and the rotational motion of fluid.However,the compressibility of the turbulence field is suppressed for all the cases,and the suppression is more significant if the particle Stokes number is close to 1.Through analyzing the transport equation of dilatation,the underlying mechanism responsible for the suppression of the compressibility is revealed,which is attributed to the preferential concentration and the inertia of particles.For the compressible mixing layer laden with particles,we set up the self-similar equations and the Reynolds stress transport equations of the particle-laden spatially developing compressible mixing layer and reveal the physical mechanisms of the interaction between particles and the anisotropic turbulence in a three-dimensional spatially developing compressible mixing layer.The particles tend to accumulate in the peripheries of the vortical structures with high density,low vorticity,and high strain rate inside the mixing layer,as well as the high-density regions behind the shocklets outside the mixing layer.Besides,large particles attenuate the mixing layer asymmetry and growth rate,while small particles augment the asymmetry and growth rate of the mixing layer.However,the existence of particles augments turbulence anisotropy of the shear layer,and the augmentation becomes greater as the particle inertia increases.For the spray combustion in the supersonic mixing layer,the thesis discusses the coupling among compressible turbulence,droplet evaporation and flame.The investigation displays that the ignition occurs at the positions of hairpin vortex heads in the high-temperature layer and induces three strong shock waves on the two sides outside the mixing layer.The flame tends to extend towards the lower fuel side,which makes the droplets gradually spread downward and enhances the droplet evaporation and the collection in the regions with low temperature and high density.Besides,the chemical reaction also augments the mixing layer thickness,the asymmetry and the turbulence anisotropy.The Reynolds stresses are attenuated while the fluctuations of temperature,density and the oxygen mass fraction are enhanced.