Modeling the influence of nozzle-generated turbulence on diesel sprays
Magnotti, Gina Maureen
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Predictive engine simulations are key for rapidly exploring and optimizing the design of cleaner burning and more fuel efficient engines. Injection strategies in advanced engine concepts are resulting in the injection and atomization of fuel under a wide range of operating conditions in order to meet stringent emission regulations. However, the physics governing the breakup of an injected liquid fuel jet into droplets under these conditions have not been well studied or experimentally characterized to date. In the sprays literature, three agents have been proposed as the likely mechanisms contributing to primary atomization in diesel sprays, namely the aerodynamic growth of waves on the fuel jet surface, turbulence generated in the injector nozzle, and cavitation. If computational design tools are to be used to guide the use of direct injection strategies for cleaner and more fuel efficient engines, the physics underpinning the role of these primary atomization mechanisms must be better understood to ensure the development of predictive simulations of fuel-air mixing and vaporization within the engine. Thus, the central aim of this thesis is to improve the physical representation of spray breakup physics within today's engine simulation packages. The work presented in this thesis investigates the role of the proposed physical mechanisms on the primary atomization process in diesel sprays. In order to advance current understanding of spray breakup, the dynamic and geometric factors contributing to cavitation were suppressed so that primary atomization due to aerodynamics and nozzle-generated turbulence could be studied in isolation. In the absence of sufficiently resolved images to visualize the primary atomization process under diesel-relevant conditions, droplet sizing spray measurements are needed to characterize the outcomes of the spray breakup process. Therefore, a new experimental methodology, called the scattering absorption measurement ratio technique, was developed and applied in a high-pressure spray chamber to characterize the average size of droplets formed from the spray breakup process. This experimental data, in conjunction with x-ray measurements from the Advanced Photon Source at Argonne National Laboratory, have been used to assess predictions from existing spray breakup models. Comparison between predicted and measured drop size distributions revealed that a spray atomization model, premised on droplet formation from the growth of aerodynamically induced instabilities, could capture experimentally observed sensitivities and features in the measured droplet size distributions under conventional diesel engine conditions. However, for injection into relatively lower ambient density environments, aerodynamic breakup models could not accurately predict the initial rate of droplet size decrease in the near-nozzle region, suggesting that other mechanisms, such as turbulence generated inside the nozzle, likely augment and enhance the primary breakup process. Evaluation of newly available droplet sizing measurements under low ambient density conditions allowed for the turbulence-induced breakup process to be studied, while minimizing the influence of aerodynamic inertial forces on the spray. Although several turbulence-induced breakup models have been proposed in the literature, the scaling of droplet sizes with the integral length scale, assumed in the majority of turbulent breakup models, was found to be inconsistent with the experimentally observed trends in droplet size along the spray centerline. However, empirical correlations describing droplets formed from eddies within the inertial sub-range of the turbulence spectrum were better able to capture the measured sensitivities in droplet size to changes in ambient and injection conditions. These findings informed recommendations for an improved hybrid spray breakup model, capable of representing both aerodynamic and turbulent breakup mechanisms in the atomization of non-cavitating diesel sprays.