Lagrangian CFD Modeling of Impinging Diesel Sprays for DI HCCI

Sammanfattning: The homogeneous charge compression ignition (HCCI) concept has been acknowledged as a potential combustion concept for engines, due to low NOx and soot emissions and high efficiency, especially at part-load.Early direct-injection (DI) during the compression stroke is an option when Diesel fuel is used in HCCI. This implies that the risk for wall impingement increases, due to the decreasing in-cylinder density. The fuel sprays has to be well dispersed in order to avoid wall impingement.Specially designed impinging nozzles providing a collision of the Diesel sprays in the vicinity of the orifice exits have experimentally been verified to yield well dispersed sprays and the desired benefits of HCCI under various conditions.The purpose of this work is to use Computational Fluid Dynamics (CFD) as a tool to simulate and evaluate non-impinging and impinging nozzles with respect to mixture formation in direct-injected HCCI. Three different nozzles are considered: one non-impinging and two impinging nozzles with 30 and 60 degree collision angle respectively.Lagrangian CFD simulations of impinging sprays using the traditional collision model of O’Rourke is not sufficient in order obtain the correct spray properties of impinging sprays. This work proposes an enhanced collision model, which is an extension of the O’Rourke model with respect to collision frequency, post collisional velocities and collision induced break-up. The enhanced model is referred to as the EORIS model (Enhanced O’Rourke model for Impinging Sprays).The initial drop size distribution at orifice and break-up time constant of the standard Wave model is calibrated and calculated wall impingement (piston and liner) is compared with combustion efficiency, smoke, HC and CO emissions as a function of injection timing. A set of model parameters were selected for further evaluation. These model parameters and the EORIS collision model were applied to non-impinging and impinging nozzles under low- and high load conditions.The EORIS model and the selected model parameters are able to predict wall impingement in agreement with experimental measurements of combustion efficiency and smoke emissions under low- and high load conditions for the investigated nozzles. A benefit is that one set of model parameters can be used to predict mixture formation, and there is no need for additional model calibration when, for instance, the injection timing or nozzle geometry is changed.In general, experiments and simulations indicate that impinging nozzles are recommended for early injection timing in the compression stroke. This is due to the shorter penetration which leads to a reduced risk for wall impingement. The non-impinging nozzles are, however, beneficial for later injection timing in the compression stroke. During these injection conditions the impinging nozzles have a more stratified charge and under some conditions poor mixture quality is achieved.