The Effects of Non-Uniform Boundary Temperatures in Ignition Delay Time Measurements from Heated Rapid Compression Machine Experiments

Abstract

Computational fluid dynamics (CFD) simulations are used to investigate the role that non-uniform boundary temperatures have on ignition delay time predictions and temporal compressed gas temperature field development in rapid compression machine (RCM) experiments that use either creviced or flat pistons. Five boundary temperature cases are investigated at a compressed pressure of 7 bar for five different compression ratios. For each compression ratio, a uniform boundary temperature case was used as a baseline for comparison to the other cases which represent non-uniform boundary temperatures. Initial gas temperature fields were based on steady state simulations which account for the effect that non-uniform boundary temperatures have on the gas temperature field prior to the RCM compression stroke. The steady state simulations were used as the initial temperature fields in transient RCM simulations of the compression stroke and the post-compression period. These simulations reveal that non-uniform boundary temperatures significantly influence the development of the compressed temperature field, but their impact on ignition delay times depends on the experimental conditions. The thermal stratification in the compressed gas only becomes consequential to ignition delay time if the controlling chemistry occurs in the region affected by the non-uniformities. This is particularly relevant for measurements made in the negative temperature coefficient region.