Numerical Research On Subcritical/Supercritical Fuel Spray Characteristics And Cavitation

Spray and atomization of fuel jet have significance influence on the efficiency and pollution emissions of direct injection diesel engine.In this thesis,the numerical simulation of subcritical/supercritical sprays,flow in nozzle and cavitation phenomenon are carried out to study the mechanism of spray in subcritical/supercritical environment and the effect of cavitation phenomenon under high injection pressure on atomization.The open source software OpenFOAM based on the finite volume method is used to solve the conservation equations.Commercial software ICEM is used to generate structured grids to improve the efficiency of numerical simulation.An approach of large eddy simulation is used to simulate turbulent flow and transport in the spray and cavitation phenomenon,and the VOF method is used for capturing the interface of fuel and gas.In supercritical environment,the P-R equation of sate is used for the liquid and the ideal gas equation of state is used for the gas.Thermophysical parameters are determined by polynomial fitting.The post-processing software paraView is used to obtain the distributions of volume fraction,temperature,velocity,turbulent kinetic energy,isosurface and isoline of dodecane.The following conclusions are drawn from the computational results:(1)In the subcritical environment,the shape of the jet is a cylinderical in the initial stage of injection;under the action of disturbance waves,surface tension and turbulence,the liquid ligaments are striped off from the jet surface in the split stage,forming filaments and finally spliting into smaller droplets.The large eddy turbulence model can accurately simulate the asymmetric structure of the spray.(2)In the supercritical environment,the fuel is injected into the combustion chamber at a higher speed,the surface temperature of the jet gradually increases,and the fuel volume fraction gradually decreases.In the high temperature environment,the radius of the fuel increases and in the radial direction,there is a large temperature gradient and density gradient.The density gradient and temperature gradient in radial indicate the presence of gas-liquid mixing layer.The jet structure is composed of the liquid core,gas-liquid mixing layer and high temperature gas.The jet surface is pleated and separated from the surrounding fluid,and the surface of the subcritical spray exhibits a helical structure.In the supercritical environment,the penetration of the spray under high back pressure is less than that of low back pressure.(3)In the open source software OpenFOAM,a compressible solver to simulate cavitation phenomenon is developed which is based on the incompressible two-phase flow solver to calculate phase change phenomenon and the compressible two-phase flow.By analyzing the distribution of volume fraction,velocity,pressure and the mass flow rate inside the nozzle in the numerical results and comparing with the experiment of cavitation in nozzle under low injection pressure,the model of cavitaion is verified.