Modeling of Film Boiling and Film Vaporization on Engine Piston Tops

Engine-out HC emissions resulting from liquid fuel, which escapes from the combustion process, provides the motivation to better understand the film vaporization in a combustion chamber. Previous work theorized that the removal of liquid fuel from the combustion cycle was a result of the increase in film vaporization time due to the Leidenfrost phenomenon. Currently, KIVA 3V predicts a continuous decrease in vaporization time for piston top films. The objective of this work is to improve the KIVA 3V film vaporization model through the inclusion of established boiling correlations, and thus, the Leidenfrost phenomenon. Experimental results have been reviewed from which expressions encompassing high acceleration effects for the nucleate boiling regime and the film boiling regime were investigated, implemented, and validated. Validation was conducted using published experimental data sets for boiling heat flux. As a result of the implementation, a noticeable increase in heat flux occurred due to high accelerations for films in saturated film boiling in both nucleate and film boiling. Computational simulations were conducted using a semi-infinite plate and a direct-injection spark-ignition engine. The semi-infinite plate provided a controlled environment which could separate the effects of pressure and acceleration on film boiling heat flux, film vaporization rates, and film vaporization times. The effect of decreased film vaporization rates, during the Leidenfrost phenomenon, was observed to decrease with increasing acceleration. Finally, the engine computations were used to provide the first film boiling and film vaporization rates for engine fuel films at temperatures above saturation temperature. As a result of this work, a film vaporization model capable of improved prediction of vaporization rates of piston top films in saturated boiling conditions has been created.