Effects of Fuel Physical Properties and Breakup Model Constants on Large-Eddy Simulation of Diesel Sprays

The Kelvin-Helmholtz/Rayleigh-Taylor (KH-RT) wave breakup model is a commonly used model in predicting primary and secondary atomization and breakup processes in Lagrangian-Eulerian Diesel spray simulations. Droplet sizes predicted by this model are dependent on several parameters. The parameters include fuel physical properties, such as density, viscosity, and surface tension, and a number of adjustable model constants, such as KH and RT time constants, KH and RT size constants, and the breakup length constant. The purpose of this study is to investigate the effects of these parameters on predicting spray motions using large-eddy simulation with the dynamic structure sub-grid stress model. The code used in this study is OpenFOAM. This study has three major parts. Firstly, effects of the model constants on the prediction of momentum exchange process were examined by comparing liquid and gas momentum fluxes. Drag Forces exerted on liquid spray by gas phase can be determined from the slopes of gas and liquid momentum fluxes plotted against axial distance. We found that the prediction of momentum exchange between gas and liquid is most sensitive to the KH time constant, B₁, among the other model constants. Secondly, effects of fuel physical properties were investigated by using four different fuels in the simulations of non-vaporizing and vaporizing sprays. The four fuels used were n-dodecane, F76 fuel, n-hexadecane, and methyl tetradecanoate. The F76 fuel is a multi-component fuel containing twenty-one hydrocarbons. Global spray quantities such as liquid and vapor penetrations, Sauter mean diameter, total liquid mass, number of parcels, and breakup model quantities such as Ohnesorge number and KH wave speed were compared. The key finding is that not all of these quantities monotonically increase or decrease with fuel molecular weight. Lastly, effects of fuel physical properties on sensitivities of the breakup model constants were studied. We compared liquid penetration and vapor penetration for each fuel using different values of the model constants. We found that the prediction of vapor penetrations is more sensitive to the KH time constant B₁ when a fuel with lighter molecular weight was used, and the prediction of liquid penetrations is sensitive to the breakup length constant, C_b, in all of the four fuels. The computational investigations in this study reveal some limitations of the current spray breakup model, and motivate us to develop more advanced models to overcome these limitations.