Effects of grid alignment on modeling the spray and mixing process in direct injection diesel engines under non-reacting operating conditions

In the present study, the Lagrangian-Eulerian approach was applied in simulation of non-reacting high pressure diesel fuel injection process under different modes of engine operating conditions. Investigations were shown that if coarse commonly used polar Cartesian grids were applied in the engine calculations, highly deflected spray morphology would be resulted, leading to subsequent erroneous air-fuel mixing predictions. Interestingly, more accurate and grid independent results cannot necessarily be achieved by further grid refinement for the aforementioned polar Cartesian grid. This study focuses on the effects of the computational grid alignment and its essential role in regions near to the injector nozzle where the coupling of the Lagrangian liquid and the Eulerian gas flow approaches is critical. Enhanced and more realistic spray penetration, air-fuel mixing, equivalence ratio and scalar dissipation rate distributions were resulted by discretizing the computational grid aligned to the direction of the spray. With this regard, a novel conical grid alignment was introduced and applied in liquid fuel spray simulations of an optically accessible direct injection diesel engine under different operating conditions. Comparisons in terms of liquid and gaseous fuel distributions were made with conventional polar Cartesian grids and spray-oriented grid with the experimental data. It has been also shown that radial alignment of grids in the conical grid can effectively reduce unexpected numerical diffusions ensuring well-captured spray morphology, evolution and air-fuel mixing.