Experimental Investigation on the Effect of Boundary Layer Fluid Injection on the Flashback Propensity of Premixed Hydrogen-Air Flames

Abstract

Premixed combustion systems show potential to meet future regulations on nitrogen oxide emissions. However, premixed systems always involve the risk of flame flashback into the premixing section. From a gas turbine manufacturer’s point of view it is desirable to broaden the safe operating range, in particular with respect to flame flashback. It has been reported in the literature that flashback along the wall boundary layer represents the most critical failure mechanism for many burner configurations using hydrogen-rich fuels. This paper focuses on the effect of fluid injection into the wall boundary layer on the flashback propensity of a hydrogen-air jet burner. For this purpose, an experiment with a tube burner was designed, where the flame is anchored in the free atmosphere at the burner exit. Pure air was injected through an annular gap at three streamwise locations upstream of the stable flame position and at two different angles — perpendicular and at 45° to the main flow direction, respectively. The turbulent flashback limits for fully premixed hydrogen-air mixtures at atmospheric conditions were measured for a variety of equivalence ratios and different amounts of air injection. It turned out that there is a significant increase in flashback stability with injection devices close to the burner exit. The main reason for this behavior is the dilution of the mixture in the near-wall region and the resulting reduction of the flame speed. The positive effect diminishes quickly with increasing distance between flame and injection location due to considerable mixing of injected flow and main flow. This has been verified by RANS simulations. The simulations also showed that the momentum generated by the injection into the boundary layer has negligible influence on the flashback limits. It was also found that the fluid injection is not capable of stopping the upstream flame propagation once the flame has entered the tube. A probable explanation for this effect is the change from open flame holding at the burner exit to the confined flame situation inside the tube during flashback. The latter is known to substantially increase the flashback propensity, which cannot be counteracted by air injection.