Numerical Research On CNG Direct-injection Mixing Process

Natural gas is a clean alternative to burn.While the intake manifold injection severly decreases the power of tranditional compressed natural gas engine,direct-injection can eliminate power loss effectively.And it can still make the fuel-air mixture well-stratified,and improve the power and exhaust performance potentially by optimizing combustion mode.Therefore,it is necessary to learn the law of the mixing process before researching on CNG direct-injection.In this research,numerical simulation was applied to analyze the grid scale near nozzle exit which influenced the CH₄ mass fraction and 1/16 was considered.Two different boundry conditions were compared and the result conduced to a conclusion that boundry condition influenced the jet penetration due to the total enthalpy at nozzle exit.Combustion bomb system was utilized to check the simulation by comparing the jet length and the length after collising the chamber wall.Finally,the variants were studied influenced on mixing process of different injection parameter,chamber shape,swirl/tumble ratio,nozzle number and nozzle angle.Excessively large or small Nozzle diameter led to dense mixture accumulating near combustion chamber wall.Ecessively large nozzle influenced the gas momentum which slowed down the mixing near chamber wall and limited the flammable mixture growth.Long injection duration conduced to a high diffusion but a dense mixture area near piston wall and chamber wall,while ecessively long duration influenced the jet structure and fluactuated the flammable fraction growth.Delaying injection starting timing conduced to a high flammable fraction growth velocity,but the flammable fraction was limited by mixing timing.Chamber shape severly influenced CH₄ diffusion which made CH₄ accumulated at part area.Open chamb er drew the dense CH₄ close to axis.The contracted was still closer than the open did.While the tub barricaded the diffusion and made the dense area near chamber wall.Swirl could slow down the diffusion at the radial direction and the flammable growth to accumulate the dense near axis.Tumble stimulated the flammable growth and it could trip the dense at chamber centre area while the ratio was high enough.Nozzle number and angle influenced mixing process severly.Increasing the number would decrease the area of each nozzle and the jet might interfere mutually at a certain range of angle.Properly optimizing nozzle angle could increase the area of the interface between CH₄ and air by controlling the jet direction and the flow to the top of chamber and piston which conduced to a high flammable growth velocity and formed a dense area near the chamber centre.