A numerical assessment of the novel concept of crevice containment in a rapid compression machine

Rapid compression machines (RCMs) typically incorporate creviced pistons to suppress the formation of the roll-up vortex. The use of a creviced piston, however, can enhance other multi-dimensional effects inside the RCM due to the crevice zone being at lower temperature than the main reaction chamber. In this work, such undesirable effects of a creviced piston are highlighted through computational fluid dynamics simulations of n-heptane ignition in RCM. Specifically, the results show that in an RCM with a creviced piston, additional flow of mass takes place from the main combustion chamber to the crevice zone during the first-stage of the two-stage ignition. This phenomenon is not captured by the zero-dimensional modeling approaches that are currently adopted. Consequently, a novel approach of ‘crevice containment’ is introduced and computationally evaluated in this paper. In order to avoid the undesirable effects of creviced piston, the crevice zone is separated from the main reaction chamber at the end of compression. The results with ‘crevice containment’ show significant improvement in the fidelity of zero-dimensional modeling in terms of predicting the overall ignition delay and pressure rise in the first-stage of ignition. Although the implementation of ‘crevice containment’ requires a modification in RCM design, in practice there are significant advantages to be gained through a reduction in the rate of pressure drop in the RCM combustion chamber and a quantitative improvement in the data obtained from the species sampling experiments.