Investigation On Droplet Collision Dynamics And Laminar Premixed Flame Dynamics

Engine spray combustion is a multi-phase,multi-scale and strongly nonlinear problem which contains several physical phenomena of droplet dynamics and flame dynamics.Deep understanding of these phenomena,from the fundamental level,is an important theoretical basis for the realization of high efficiency and clean combustion.In the present thesis,droplet collision is numerically studied,and a diffuse interface method for simulating premixed flames is further proposed.Only laminar flow is considered in the present study so as to simplify the problem.The lattice Boltzmann method is employed as the computing platform,and its widely-applied pseudopotential multiphase model is evaluated and analyzed firstly.The results show that,both numerical stability and coexistence densities are related to the relaxation time,and the spurious current around the interface is rather high.On this basis,the Lee model which was proposed in the framework of the phase-field method,is further modified through using multiple-relaxation-time collision operator and conducting cylindrical coordinate transformation.As a result,the improved model is applicable to model axisymmetric two-phase flow problems with high density ratio and low fluid viscosity,while keeping the spurious current at a low level.Based on the improved multiphase model,numerical investigation on droplet collision is conducted.The results show that droplet collision is accompanied by strong flow strain,and the Weber number(We)and the diameter ratio are the controlling parameters on droplet deformation as well as collision outcome.On the other hand,while the Ohnesorge number(Oh)only has a quantitative adjustment effect on the droplet deformation and outcome,it can significantly affect the mixing characteristics,i.e.mixing is directly related to viscous dissipation.At high Oh number,the small droplet gathers at the side of the large droplet;at medium Oh number,the small droplet spreads on the surface of large droplet;at low Oh number,the small droplet penetrates into the large droplet,forming two types of internal jet either dominated by surface tension(We number approaching 0)or by inertia force(finite We number).Recognizing the existence of strong flow strain and the dominant effect of viscous dissipation on mixing,non-Newtonian fluids,whose viscosities vary with the shear rate,are further taken into consideration.It is found that,for low We number droplet coalescence,shear-thinning effect can significantly facilitate the formation of internal jet,and the small droplet is more critical;for finite We number droplet collision,the imbalanced viscous dissipation caused by different rheology of the droplets is able to break the symmetry of droplet collision,leading to improved mixing,and the improvement in mixing increases with the rheology disparity.Apart from the better understanding towards droplet collision phenomenon in engine spray combustion,the above results also suggest that,in the next-generation gelled hypergolic rocket engines,the ignition stability could be improved through breaking the symmetry of either droplet size(different nozzle diameters for fuel/oxidizer injectors)or droplet rheology(fuel/oxidizer gelled differently).By further realizing that premixed flames and two phase flows are highly similar in nature,i.e.fluid density transits continuously across the interface but keeps incompressible in the bulk flow,a diffuse interface method for simulating premixed flames is then proposed.The flame is treated as a diffuse moving front with its propagation automatically captured by the convection-diffusion equation of the progress variable.The diffusion and reaction terms are constructed using the flame speed and flame thickness,and the flame speed is taken as a function of the local stretch rate to incorporate the stretch effect.To test the performance of the method,simulations including 1-D flame propagation,2-D Darrieus-Landau instability,2-D cylindrical flame propagation with stretch effect,and 2D flame-vortex interaction are conducted,and the consequent results are in good agreement with analytical solutions.A new method for simulating premixed flames is therefore developed and verified.