Autoignition of reacting mixtures at engine-relevant conditions using confined spherically expanding flames

Abstract

Propagation of a confined spherically expanding flame induces isentropic compression that can culminate in autoignition and/or detonation under conducive thermodynamic conditions. This relatively simple technique measures a distinct ‘characteristic ignition delay time’ and complements other established approaches such as the rapid compression machine and shock tube. The present study details this methodology by examining the autoignition characteristics of dimethyl-ether/oxygen/nitrogen/helium reactive mixtures for equivalence ratios of 0.6 and 0.9, an initial temperature of 468 K, and initial pressures of 3 to 6 atm. The experimental results display the classic two-stage ignition typical of dimethyl-ether oxidation at low-temperatures with first-stage ignition occurring at approximately 3.6 times the initial pressure. To aid in the interpretation of the experimental results, two numerical models were used: a zero-dimensional batch reactor model, which accepts experimental pressure-time history and calculates the sensitivities of characteristic ignition delay times to kinetics, and a low Mach number, Lagrangian one-dimensional code that was developed to model both flame propagation and end-gas autoignition. Simulation results were shown to adequately capture the physics of unsteady flame propagation, end-gas autoignition, and the controlling reactions of the latter. It was found also that under certain conditions the behavior of first and second ignition stages could be modified by unsteady pressure effects.