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Recent experimental results have shown that the penetration length of the liquid phase in a Diesel spray under normal operating conditions is relatively short compared to the penetration length of the overall jet. In addition, the results indicate that, for a significant fraction of the injection duration, the mass and volume of the injected fuel that is in the liquid phase is relatively small compared to the total volume and mass of fuel injected. Based on these considerations, a Virtual Liquid Source (VLS) model for Diesel sprays has been developed which treats the liquid region of the spray as a source of mass, momentum and energy without directly computing the liquid phase. The penetration length of the liquid phase along the axis of injection is obtained from recent measurements.

Interactions between the jets in a multi-hole injector and between the jet and the wall may affect the fuel-air mixing processes in a direct-injection Diesel engine. These interactions are the subject of the investigation in this work. It is known that in the case of free jets, for a given total mass and momentum flow rate, increasing the number of holes would result in an increase in the mixing rate. In the case of a multi-hole injector in an engine, however, if the number of holes are increased beyond an optimum value, the interaction between the jets themselves may result in a reduced mixing. In the limit of increasing the number of holes, a hollow-cone jet would result. The fuel-air mixing in the hollow-cone jet is shown to be slower than in a multi-hole injector with an optimum number of holes.

A simplified model for multicomponent droplet vaporization is developed and implemented in a multidimensional model for flows, sprays and combustion in engines. The model is applied to study the vaporization characteristics of a multicomponent droplet under Diesel conditions, the distribution of the vapor components in a Diesel spray and the distribution of the components in a Diesel engine. It is shown that for typical warm Diesel engine operating conditions, the droplets vaporize sufficiently rapidly that the stratification of the different components in the spray is not significant. However, under engine starting conditions and, in particular, cold starting conditions, there is a significant stratification of the different components of the fuel. When the species are stratified, the heavier and slower vaporizing components are predicted to be on the periphery of the spray envelope. However, these components also take longer to reach there.

Results are presented of 3-D computations of direct injection of gaseous methane and of liquid tetradecane through a multi-hole injector into a Diesel engine. The study focusses on the distribution of fuel/air ratio within the resulting gas and spray jets under typical Diesel conditions prior to ignition. It is shown that for a significant time after start of injection, the fraction of the vapor fuel which is in richer-than-flammable mixtures is greater in gas jets than in sprays. For methane injection, it is also shown that changing some of the flow conditions in the engine or going to a poppet-type injector, does not result in improved mixing. An explanation of these results is provided also through an analysis of the self-similar gas jet and 2-D computations of gas and spray jets into constant pressure gas. A scaling for time and axial distance in the self-similar gas jet also clarifies the results.

Computations of combustion in a stratified-charge rotary engine are presented. A three-dimensional model for flows, sprays and combustion which includes the ignition cavity of the engine is used to make these computations. The geometric complexity of the cavity and its coupling with the main chamber is handled by using an unsteady generalized curvilinear coordinate system. The grid is generated using an algebraic grid generator in the main chamber and by solving an elliptic equation in the cavity. Computations of the flows in the cavity are presented for different arrangements of the pilot injector and spark plug and for different timings and fuel injection rates from pilot and main injectors. The dominant feature of the flowfield in the cavity is shown to be the presence of a vortex, induced by the flow in the main chamber, which controls the distribution of the fuel and also the burning rate in the cavity.