Premixed flame ignition in high-speed flows over a backward facing step

Abstract

Ignition plays a major role in combustion science and the ignition sequences are a crucial element in many engines. The current study focuses on ignition in subsonic, high-speed flows of premixed methane/air mixture. The Minimum Ignition Energy (MIE) is first evaluated for quiescent and constant speed flows to analyze its dependency on the flow speed and the value of this speed beyond which ignition can not be achieved anymore. To be able to obtain flame ignition and stabilization at higher flow velocities, ignition by a spark placed in the recirculation zone of a backward facing step is studied. For a range of inlet velocities, Direct Numerical Simulation (DNS) is used to measure the energy required for flame stabilization and analyze ignition sequences. Results show that a flame kernel is first created in the low-speed recirculation zone for all speeds. However, the subsequent flame propagation away from the recirculation zone depends on the flow speed: while low-speed cases produce a global ignition, high-speed cases can lead to quenching when the flame tries to leave the recirculation zone and enter the high-speed region, leading to global ignition failure. DNS also allows to propose and verify criteria for ignition in terms of flow speed and spark energies: following scaling arguments by Shanbhogue et al. (2009) for flame stabilization over obstacles, a simple Damkohler number is shown to control the success of ignition. Finally, DNS results are used to investigate mechanisms controlling the success or failure of the ignition sequence: they show that heat losses to the combustor walls play a limited role while flame stretch on the flame elements leaving the recirculation zone have a major influence.