CFD Simulation Study On Comubustion Process Of DME Engine

Dimethyl ether(DME) is one kind of the new type and the clean alternative fuels for diesel engine. The research that DME is universal used on diesel engine is processing. So studying the burning characteristics of DME is propitious to manufacture and optimize DME engine. In this thesis, the mathematics methods and numerical methods for three dimensional in-cylinder process simulations were introduced. Simulation works were carried out on DME engine by AVL FIRE. The in-cylinder gas fluid, spray and combustion process were predicted. The detail of in-cylinder gas fluid and spray development and temperature field and density field were showed on.Because there is no intake stroke and exhaust stroke, details like intake ports, ex-haust ports, valve are not considered. Asimplified geometry is used to reduce the effort for mesh generation. In mesh generation process, triangular combustion chamber from UG was imported. To get the correct compression ratio in the model without changing the characteristics of the flow field in the bowl (squish flow), a compensation volume is added at the outer part of the *.stl surface model. This volume accounts for geometrical details which cannot be considered in this segment approach. Because of relatively complex structure, combustion chamber is meshed with hexahedron unstructured grids by automatic mesh generating order. The ground of the bowl should be meshed with at least one continuous layer for a proper calculation of the heat transfer through the piston wall. The cylinder mesh at the different crank angle has been generated by moving grid maker in this thesis. Because of movement characteristic of the piston, the cylinder is meshed with hexahedron grid which short the calculation time more. As the piston moves up and grids of cylinder are compressed, grids of the cylinder field are meshed by the Redimension .The mesh density should have approximately ten times the diameter of the nozzle hole. The parameters in the model were adjusted to ensure the indicated diagram resulting from the simulation accorded with the data from the testing.The influence has been studied in this thesis which the injection advance angle is 8℃A11℃A and the intake temperature 25℃, 40℃ on combustion of DME engine. The ignition delay period is controlled by the fuel injection advance angle which is an important parameter. If the advance angle of injection is adjusted earlier, the density of the com- bustion chamber is small and its temperature is low, when the fuel is injected into the combustion chamber. So it needs long time to atomization. SMD of the spray is bigger which indicates that the gasification surface of the spray is small. The fuel is atomized worse. And the W_e of the fuel is large, it dedicates the spray is unstable and has bigger inertia force than the surface tension. It will be broken up until reach the critical We. In a word, if the advance angle of injection is adjusted earlier, the ignition delay period will become long and there will be more mixture in the premixed period. When the combustion starts at this condition, the maximum combustion temoper -ature, pressure, the maximum pressure increaser ratio all increased. The increase of NO_x emissions is resulted by the high temperature. But the energy released in the diffusion period decreased. If the advance angle of injection is adjusted too earlier, knock phenomenon will been show in the combustion process.Combustion process will be improved by increasing charge air. Combustion temperature will be lower by cooling the intake temperature. The formation of the NO_x will be restrained by this method and emissions are improved. The study provides some advice for optimizing the DME engine.At last, The optimization design on improving combustion characteristics of DME engine is considered. Form the combustion mechanism and the requirement of combustion chambers' sound shape, the dimension shape, and orienting postion of combustion chamber , in addition ,the connection of inter-cooperation among fuel, air and combustion chamberes are discussed.