CFD simulations of Rapid Compression Machines using detailed chemistry: Evaluation of the ‘crevice containment’ concept

Abstract

The use of creviced pistons in Rapid Compression Machines (RCMs) has proven to be very efficient in making the temperature homogeneous inside the reaction chamber but has the disadvantage of inducing an unintended mass transfer from the reaction chamber to the crevice, especially during the preliminary heat release of two-stage ignition processes. Aiming to mitigate this mass transfer, the technique of ‘crevice containment’ (CC) has been proposed. It consists of a physical separation between the reaction chamber and the crevice region that is engaged at the end of the compression, physically preventing any mass transfer between both parts of the geometry. In order to numerically assess this novel design concept across a broader range of conditions than previously investigated, reactive simulations using detailed chemical kinetic mechanisms are performed for n-heptane and iso-octane mixtures. It is found that for compressed temperatures outside of the NTC (negative temperature coefficient) region, the CC approach is very effective in suppressing the influence of the crevice mass transfer and thus increases the validity of the widely-used 0-D model based on the adiabatic core assumption. Still, for most of the temperature cases inside the NTC region, ignition appears to be prematurely initiated in the residual vortex region formed after the seal engagement, possibly inducing very significant differences with the 0-D model. The benefits of eliminating the post-compression crevice mass transfer appear to be counter-balanced by effects that have not been previously identified.