DNS of multi-stage ignition in a simplified gas turbine premixer

With the increasing need for fuel-flexibility, mitigation of auto-ignition inside gas turbine (GT) premixers has become crucial. GT premixers must be designed to ensure sufficient fuel-air mixing while avoiding the potential occurrence of auto-ignition, a challenge that requires a detailed understanding of turbulent mixing and chemistry interaction. Here, direct numerical simulations of a downscaled and simplified version of a GT premixer are presented. The configuration consists of multiple hot air jets in a fuel-rich crossflow (Rej=4000) at high pressure. A fuel blend of methane and dimethyl ether (DME) is chosen with a recent reduced chemical kinetics model validated in GT conditions. The addition of DME drastically increases the explosivity of the fuel mixture and causes a multi-stage ignition to occur, each stage resulting in a corresponding flame. For each stage we track the temporal evolution of individual ignition kernels in both physical- and phase-space, and assess their propagation mode through chemical explosive mode analysis. The observed ignition events differ for the different stages, with large ignition patches for low temperature ignition and quickly growing small kernels for high temperature ignition. In all cases, both diffusion and auto-ignition contribute significantly to the subsequent flame stabilization.