SUMMARY

For direct-injection engines, fuel injection and atomization are known to influence combustion and emissions formation. In light of increasingly stringent emissions regulations, accurate modeling of these spray physics is therefore key to enable simulation-based design of high-efficiency clean combustion engines. However, the physical mechanisms governing the primary breakup of the liquid fuel spray into droplets are still unknown. The most widely employed spray models assume that breakup is dominated by aerodynamic shear-induced instabilities at the surface of the liquid fuel jet. However, recent experimental images have shed light on the importance of liquid turbulence in the spray breakup process, particularly for fuel injection into atmospheric conditions. While conventional injection conditions are characterized by dense ambient environments, many advanced combustion strategies utilize advanced fuel timing to achieve low temperature combustion; control strategies such as this result in injection conditions that approach atmospheric densities. These results suggest that turbulence may play a more influential role under engine-relevant conditions than is assumed in the majority of models employed in today’s engine simulation packages.

The hypothesis of this thesis is that liquid turbulence contributes to the breakup of fuel sprays, especially under fuel-injection conditions of relevance to modern engines, and should be included in the formulation of spray breakup models for engine CFD codes. To examine this hypothesis, a joint experimental-computational approach is taken to improve current understanding of the processes governing spray breakup. Towards this goal, laser extinction measurements were conducted in a high-pressure spray chamber and compared to existing x-ray radiography measurements in order to quantify spray breakup outcomes, such as droplet size and number density, over a range of engine-relevant conditions. Spray breakup was then simulated in the computational fluid dynamics code CONVERGE using a variety of currently-accepted models to evaluate their physical accuracy. The key contributions of this thesis are to determine fuel injection regimes where liquid turbulence plays a role in spray breakup, and to identify current or improved spray model formulations that accurately predict spray structure across relevant modern-engine operating conditions.