Numerical analysis of the influence of the jet breakup model formulation on diesel engine combustion computations

The multidimensional simulation methods available today for spray motion predictions solve the spray equations (including the mass, momentum, and energy changes due to the interaction between the drops and the gas), and also consider drop collision and coalescence phenomena The most-used breakup spray models irt CFD computations are based on an analysis of the instability of a liquid column injected unbroken from the nozzle orifice tin the following WAVE model), or in an analogy between a damped spring-mass system and a liquid column (TAB model). Both models require some empirical constants. Considering also that the mechanism that controls atomization is not get well understood, further calculations and experimental comparisons over a range of injection conditions may be useful to improve the prediction capability of these models. In previous work, an analysis was performed to determine the influence of spray breakup model constants setting on the spray tip penetration, using the KIVA II code. The mesh size adopted was quite coarse, but typical of that used in computations of diesel engine combustion. It was outlined that both the TAB and the WAVE models are sensitive mainly to the breakup time constant value; the influence of the other model constants on the tip penetration results is minimal. In spite of the fact that the physics of the two models is very different, the best setting of the constants falls in the same range. In the present article a further analysis of spray patterns is reported, particularly related to the spray breakup phenomenon. After a brief description of the break-up models, a sensitivity analysis of the main spray features to the model constants is presented. bt addition, the numerical data of jet penetration, computed with both the TAB and WAVE models, are compared with literature data for vaporizing and nonvaporizing conditions. In order to improve the numerical predictions, a "hybrid" model is proposed, based on both the TAB and WAVE models. Finally, because the overall goal of the spray computations is to obtain a reliable simulation of the overall diesel combustion process, the influence on combustion computations of breakup modeling is also evaluated and discussed.