Effect of Cavitation Regimes on Stepped Diesel Injectors
Sykes, David Michael
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The objective of this work was to determine the effects of different cavitating flow regimes on a stepped orifice geometry relevant to diesel injectors. Conventional diesel orifice geometries consist of a single tapered or straight circular passage. The geometry used in this work had two circular passages in series: a first orifice that was similar to conventional geometries and a second larger orifice immediately downstream that induced cavitation. The large orifice served to stabilize the flow, allowing for different flow regimes to exist at the exit plane of the large orifice (single-phase, intermittent bubbly, bubbly, annular, etc.). A series of experiments was conducted at room temperature with injection pressures up to 8.5 megapascal (MPa), ambient pressures of up to 798 kilopascal (kPa), and using both water and dodecane as working fluids. Internal and external visualizations were conducted along with measurements of discharge coefficient, spray angle, and representative droplet sizes for both conventional and stepped geometries. It was discovered through experiments that the different types of two phase flows at the exit plane gave rise to four distinct spray regimes for the stepped geometries and that the operating regime was controlled primarily by the cavitation index. Regime 1 (single phase) provided almost no atomization, near zero spray angle. Regime 2 (intermittent bubbly) provided spray angles up to 55°, and very large droplet sizes. Regime 3 (bubbly flow) provided a decreasing spray angle and droplet size. Regime 4 (annular flow) provided a consistent spray angle of 14° and smaller droplet sizes in the range tested. Spray angles for the conventional geometry were measured consistently at 7°. Droplet sizes for the conventional geometry were also much more consistent over the range of parameters tested than for the stepped geometries. It was shown that the stabilization owed to the large orifice effected the spray parameters dramatically depending on regime. The experimental measurements were compared to computational calculations using established models for two phase flow, phase interaction, and cavitation. It was calculated that the increase in spray angle in Regime 2 and decrease in spray angle in Regime 3 corresponded to an increase followed by a decrease in turbulence intensity. Additionally, it was calculated that the initial rapid decrease followed by asymptotic decay in droplet size corresponded to the initial rapid increase followed by asymptotic increase in turbulent kinetic energy at the exit plane. It was concluded that the changing bubble density and phase distribution created by using the stepped geometry led to changing turbulence levels at the exit plane and ultimately dictated spray angle and droplet size. It was acknowledged that this turbulence augmentation worked in conjunction with other factors such as aerodynamic, surface tension, and velocity profile relaxation effects to ultimately determine spray parameters; however, the induced turbulence from the different flow regimes appeared to be the most significant driving force.