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Document Type

Open Access Dissertation

Degree Name

Doctor of Philosophy (PhD)

Degree Program

Mechanical Engineering

Year Degree Awarded

2018

Month Degree Awarded

September

First Advisor

David P. Schmidt

Second Advisor

Blair Perot

Third Advisor

Dimitrios Maroudas

Subject Categories

Computer-Aided Engineering and Design | Heat Transfer, Combustion | Other Mechanical Engineering

Abstract

Gasoline engines operating under the principle of direct injection are susceptible to flash-boiling due to superheated nature of the fuel and the sub-atmospheric in-cylinder pressures during injection. A review of the literature on flash-boiling sprays shows that a majority of the studies have focused on the far-field regions of the spray, with limited attention given to understanding the influences of the injector geometry and the near-nozzle regions of the spray. Modeling the internal nozzle flow and the primary atomization, on which the far-field spray depends, is a challenge. This thesis, therefore, is aimed at understanding the complex flow through a fuel injector nozzle and the nature of the spray in the near-nozzle region, with the help of computer simulations under flash-boiling and non-flash-boiling conditions. In the current study, the simulations were performed using an in-house Eulerian CFD solver called HRMFoam. Improvements to the solver's near-nozzle spray modeling capability are discussed. These improvements include the implementation of a liquid-gas interface-area-density transport equation to model the primary atomization process. The simulations of direct injection of gasoline and gasoline-like sprays were performed on single-hole and multi-hole injectors, for a wide range of operating conditions. Spray characteristics such as the nozzle's coefficient of discharge and the mean droplet diameter in the dense region of the spray were seen to be captured adequately well with the help of a 2D axi-symmetry assumption in the case of single-hole injectors. A novel approach to identify the near-nozzle spray plume boundary in CFD simulations is presented and validated against experimental measurements for a single-hole asymmetric injector. Case studies on single-hole asymmetric injectors revealed a direct correlation between the drill angle of the nozzle and near-nozzle spray plume angle. A hypothesis of the similarity between a stepped-hole two-phase nozzle and a conventional single-phase converging-diverging nozzle is presented. Furthermore, it was observed that flash-boiling jets behave as underexpanded jets, and therefore, are wider. Whereas, non-flash-boiling behave as overexpanded jets, and thus are narrower. Through the case studies on multi-hole injectors, the collapse of the spray or lack thereof was qualitatively and quantitatively characterized. In this process, a resemblance between the experimentally and computationally identified spray collapse mechanism was established. The application of LES modeling to internal and near-nozzle GDI sprays was explored in a pilot study, and the results were qualitatively validated against the experimentally available near-nozzle X-ray radiography measurements. Finally, in another pilot study, an attempt to model the interphase slip velocity is discussed.

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