Multi-dimensional Numerical Modeling Of Spray Mixing Process In Diesel Engines Based On CFD

A theoretic and experimental study on fuel spray mixing process in diesel engine hasbeen performed. The main content is focused on the analyses of the mechanism of spray mix-ing process and the construction of models. Then, the spray mixing processes are modeledbased on the multi-dimensional spraymodel constructed in this work. For the purpose of further research on fuel atomization mechanism, this work extendedthe problem to spray upstream, namely the internal flow of fuel injector nozzle. Researchedon the internal flow in nozzle using CFD method with combined two phase mixture modeland cavitation model, and supplied the detail boundary condition for fuel spray modeling.The computed results show that the flow condition in ordinary D.I diesel engine injector noz-zle is fully cavitating flow. The fully cavitating flow consists of two distinct zones, ahigh-speed and mostly liquid one and a second zone with low velocity, density and momen-tum near the wall. The distribution of two zones at the nozzle exit result in distinct breakupmode on sprayjet, and benefit to understand fuel atomization mechanism. Based on the analysis of the present atomization theories, the experimental conclusionon the spray structure near nozzle in recent year and the analysis of the flow structure in in-jector nozzle, a new spray concept model is put forward in this thesis. It suggested that cavi-tation, turbulence and aerodynamic interactions are the most dominant mechanisms for at-omization under the like diesel engine injection condition. According to the acting position ofthis factors, spray atomization processes is divided into primary breakup and secondarybreakup. From the point of energy, the author firstly set up a new cavitation and turbulenceinduced primary breakup model. The model describes the transition from the flow inside theinjector hole to the dense spray near nozzle and reflects the dominant influence of the cavita-tion and turbulence on the 3D spray. The new primary breakup model provides all necessarystarting conditions for the calculation of secondary breakup like drop sizes, location and ve-locity components. In addition, different locations and sizes of both liquid and cavitationzones in nozzle can be taken into account and make the calculation of asymmetric sprayspossible. From the point of the theories of unstable waves on droplet surface due to aerody-namic interaction, a secondary breakup model is set up, which combined KH (Kelvin-Helmhotlz) and RT (Rayleigh-Taylor) two unstable waves theories and agree well to modelall kinds of drop breakup mode in diesel spray. The whole spray breakup model was imple- IIImented in the 3D CFD code KIVA3V and verified successfully by comparison to experimen-tal data. It shows that the effect of the cavitation and turbulent nozzle flow on the primarybreakup is accuratelymodeled. After reading the experimental references on drop collision, more collision outcomes aretaken into accounted in new collision model, such as bounced, coalescence, reflexive separa-tion and stretching separation. The new collision model solved the error of larger droplet atspray tip due to old O' Rourke model. In addition, a concept of parcel radius is introduced todrop number density in computing parcels, which is enlarged with parcel moving downward.Mesh independence is also obtained using this method. The numerical convergence of spray model based on DDM (discrete droplet model) isdiscussed systemically. It shows that the computed result is mesh dependence. The reason is acomputational mesh contradiction between the distribution of source terms and the gaseousphase field, and the contradiction can not be resolved under DDM framework. The contradiction is relieved by correcting the relative velocity between drops and gase-ous phase near nozzle and improving the couple method between drops and gaseous phase inthis thesis. The different fuels sp...