Modeling Engine Spray/Wall Impingement 880107

A computer model was used to study the impingement of sprays on walls. The spray model accounts for the effects of drop breakup, drop collision and coalescence, and the effect of drops on the gas turbulence. These effects have been shown to be important in high-pressure sprays where breakup of the liquid yields a core region near the nozzle containing large drops. A new submodel was developed to describe the spray/wall interaction process. The model uses an analogy with the oblique impact on a wall of liquid jets. Following impact, the trajectory of a drop is specified to be tangent to the wall surface. The computations were compared with recent endoscope pictures of engine sprays impinging on a piston bowl and also with constant-volume-bomb measurements of spray shape and penetration. Predictions of the effect of engine swirl, ambient gas pressure (density), wall inclination angle and the distance from the nozzle to the wall, were in good qualitative agreement with the experiments. The computations showed that small drops approaching the wall are deflected away by the stagnation flowfield which is set up in the gas near the wall; large drops strike the wall and then flow along the wall surface. The effect of gas swirl is to spread the liquid layer over the surface leading to higher vaporization and fuel-air mixing rates.


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