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A computational model is proposed and analysis is carried out to study the atomization processes of hollow-cone fuel sprays from pressure-swirl injectors for a Gasoline Direct-Injection (GDI) Spark Ignition (SI) engine. The flow field inside a swirl injector is numerically analyzed, and characteristics of the liquid sheet at the nozzle exit are predicted. The intact length (i.e., breakup length) of the sheet is calculated from a semi-empirical correlation and a Sauter Mean Diameter (SMD) at the breakup location is estimated based on the classical wave instability theory. The spray dynamics that address the interactions between liquid drops and surrounding gas phase are simulated using FIRE code with modified spray models. The objective is to understand the effects of nozzle geometry and engine operating conditions on spray characteristics so that the spray structure can be optimized through the injector design to meet the fundamental requirements of GDI engines.

Gasoline direct injection requires that the injection time may be very short in duration, indicating that transient flow effects can have a strong influence on the flow behavior and on the spray properties. Consequently, a computational analysis of the dynamic flow in a high pressure swirl injector was conducted. In order to perform the flow simulation during a complete injection event, movement of the needle that controls the amount of fluid to be discharged has been considered and deduced from experimental data. To validate the computational model, the predicted dynamic flow rate, temporal cone angle and instantaneous mass flow rate were compared to experimental data. The calculated results were found to be consistent with measurements. The dynamic calculations allow a better understanding of the complex transient flow during one injection event and may be divided into four different stages where characteristics of the liquid emerging from the nozzle are completely different.

Computational analysis of flow field inside a high pressure swirl injector is carried out. The effects of injection pressure and internal geometry on velocity field inside the nozzle and especially at the injector exit are studied in detail. From the velocity distribution at the exit plane, methods to determine the discharge coefficient and liquid sheet cone angle are given. To validate the computational model, the spray cone angles in the immediate vicinity of the nozzle exit were measured from photographs over the injection pressure differential range of 3.5 to 10.3 MPa. Static flow rates were measured using a flow meter over the same pressure range. The calculated results are found to be consistent with the experimental measurements. Extensive calculations were then conducted to examine the influence of swirl inlet port area and orifice diameter on discharge coefficient and spray cone angle.