Swirl flame boundary layer flashback at elevated pressure: Modes of propagation and effect of hydrogen addition

Abstract

Operating lean-premixed gas turbine burners with fuels containing large amounts of hydrogen severely increases the risk of flame flashback. Enabling fuel-flexible burners, i.e. mitigating that risk for a wide range of hydrogen admixed to natural gas, for instance, is particularly challenging. An improved understanding of the responsible mechanisms and their impact on flashback limits is required to aid the development of such fuel-flexible and yet flashback-resistant burners. Only few detailed and systematic investigations on flame flashback have been conducted at elevated pressure and preheat temperature. In the current experimental study, boundary layer flashback of methane-hydrogen air flames has been investigated in an optically accessible swirl burner installed in a high-pressure test rig. High-speed imaging and laser diagnostics have been applied to characterize the flame propagation pathway and flame shapes. The current findings show that the flame propagation pathway, which determines the relevant flow-flame interaction for flashback, differs compared to previous swirl flame studies. The characteristics of flashback in the current configuration are found to be comparable to flashback in non-swirling boundary layers. Flashback limits have been measured at 2.5 bar and 200∘C preheat temperature to quantify the increase in flashback propensity as the amount of hydrogen is increased from 50% to 100% by volume. Counterflow premixed flame simulations have been performed to investigate the increase in flashback propensity with hydrogen addition. The results suggest that the investigated flames are close to being critically stretched when flashback occurs in the turbulent boundary layer, which allows deriving a Karlovitz number based criterion for an improved prediction of flashback limits.