Flashback Resistance and Fuel-Air Mixing in Lean Premixed Hydrogen Combustion

To prevent flame flashback in swirl-stabilized, lean premixed combustion, a nonswirling air jet is introduced on the central axis of the radial swirl generator. This axial injection of air alters the flowfield as required for flashback-resistant combustion of premixed hydrogen. This study evaluates the impact of axial injection on the isothermal flowfield in a water tunnel by particle image velocimetry. Atmospheric reacting tests with hydrogen for inlet temperatures up to 620 K and up to stoichiometric conditions show a substantial increase in flashback resistance for the burner setup with axial injection. To verify the increase in flashback resistance to not be achieved at the expense of increased NOx emissions, fuel-air mixing is evaluated in a water tunnel by planar laser-induced fluorescence of a fuel tracer. Even for high axial injection rates, the recorded unmixedness and NOx emissions are uncorrelated. This results in single-digit NOx emissions below adiabatic flame temperatures of 2000 K. Consequently, axial injection significantly extends the operational range of the combustor without increasing the NOx emissions.