Controlled autoignition in stratified mixtures

Abstract

The shockless explosion combustion (SEC) is a recently proposed concept aiming for pressure gain combustion through an unsteady process of multiple, distributed autoignitions occurring simultaneously. For this, a stratified fuel profile of dimethyl ether is injected into a continuous air flow. This profile is tailored such that the mixture residence time and ignition delay time are matched, allowing multiple ignition kernels to initiate simultaneously leading to an aerodynamic confinement during heat release. This work first presents an injection strategy for injecting a defined mixture profile into a convection air flow to control the local equivalence ratio throughout the combustor. Line-of-sight measurements are applied to visualize the concentration profile and subsequently used to develop a one-dimensional tool to predict the local equivalence ratio before ignition. Next, an extremum seeking control algorithm is applied to an existing SEC test rig to control the cycle averaged formation of different autoignition modes by optimizing the fuel supply. Pressure and ionization probe data indicate the successful initiation of specific modes of flame propagation by adjusting the fuel injection trajectory. The previously developed simulation tool is applied to the injection trajectories optimized by the controller. Correlating the fuel concentration distribution and the obtained autoignition modes reveal that the ignition process can be very well controlled by the fuel injection trajectory. Lastly, single representative ignition cycles are further investigated by applying optical measurement techniques to obtain OH* and CH* chemiluminescence. The results reveal a complex interaction between heat release and pressure waves influenced by temperature-dependent ignition behavior of the applied fuel. As a conclusion, four different flame propagation modes are identified, namely: turbulent deflagration, subsonic autoignition, supersonic autoignition and aerodynamic confinement by multiple simultaneous autoignition fronts.