Computational assessment of an approach for implementing crevice containment in rapid compression machines

Abstract

In Rapid compression machines (RCMs), creviced pistons are often employed to suppress the formation of the roll-up vortex. The use of a creviced piston, however, can enhance other multi-dimensional effects especially during the first-stage of the two-stage ignition. Such multi-dimensional effects can be avoided by using ‘crevice containment’ in which the crevice zone is separated from the main reaction chamber at the end of compression. In this work, an approach for the implementation of ‘crevice containment’ in RCMs is assessed through reactive and nonreactive CFD simulations for a stepped combustion chamber geometry in which ‘crevice containment’ is achieved by using a seal between the mating taper surfaces of the piston and the reaction chamber at the end of compression. The results with the optimized chamber geometry show that the roll-up vortex is largely suppressed with ‘crevice containment’. The reactive CFD simulations of the optimized geometry are done for ignition of n-heptane/oxidizer mixtures to assess the validity of zero-dimensional modeling. The CFD results are in excellent agreement with the zero-dimensional modeling in terms of predicting the first-stage and total ignition delays. The agreement suggests absence of detrimental chemical kinetic coupling due to boundary layer and vortex during two-stage ignition. The approach developed here is expected to be easy for implementation in existing RCMs and will also yield significant quantitative improvement in the data obtained from the species sampling experiments.