NUMERICAL-ANALYSIS OF THE INFLUENCE OF THE JET BREAKUP MODEL FORMULATION ON DIESEL-ENGINE COMBUSTION COMPUTATIONS

The multidimensional simulation methods available today for spray moti on 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 m ost-used breakup spray models irt CFD computations are based on an ana lysis of the instability of a liquid column injected unbroken from the nozzle orifice tin the following WAVE model), or in an analogy betwee n a damped spring-mass system and a liquid column (TAB model). Both mo dels require some empirical constants. Considering also that the mecha nism that controls atomization is not get well understood, further cal culations and experimental comparisons over a range of injection condi tions may be useful to improve the prediction capability of these mode ls. In previous work, an analysis was performed to determine the influ ence of spray breakup model constants setting on the spray tip penetra tion, 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 ma inly to the breakup time constant value; the influence of the other mo del constants on the tip penetration results is minimal. In spite of t he 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 arti cle a further analysis of spray patterns is reported, particularly rel ated 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 compa red with literature data for vaporizing and nonvaporizing conditions. In order to improve the numerical predictions, a ''hybrid'' model is p roposed, based on both the TAB and WAVE models. Finally, because the o verall goal of the spray computations is to obtain a reliable simulati on of the overall diesel combustion process, the influence on combusti on computations of breakup modeling is also evaluated and discussed.