Some Limitations to the Spherical Flame Assumption Used in Phenomenological Engine Models

A two-dimensional axisymmetric numerical model is used to illustrate how the presence of walls can significantly influence the shape of a spark-ignited premixed gas flame, even when wall boundary layers are neglected. This in-viscid model is based on tracking the flame interface as it interacts with the combustion-generated flow field. Comparisons made with a quasi-dimensional phenomenological model show that the assumption of a spherical flame surface held centered at the ignition location can lead to a large underprediction of the flame area. This occurs because the spherical flame is prematurely truncated by the chamber walls, and because the spherical assumption constrains the flame surface-to-volume ratio to be the absolute minimum. In contrast, results from the two-dimensional model show that the flame slows down as it approaches a wall since the velocity induced by volume expansion must be zero normal to the boundary. Truncation of the flame area by the wall is therefore delayed, and as the flame moves further from the ignition site its interactions with the combustion-generated flow field result in a more highly curved flame of significantly greater area.