Investigating Boundary Layer Flashback of a High Turbulence Intensity Jet Flame at Gas Turbine Conditions

Abstract

Hydrogen derived from non-fossil sources is an attractive candidate to replace carbon based fuels in gas turbines, as it is inherently carbon free. Yet the unusual combustion properties of hydrogen requires some care to successfully use it in gas turbines. To attain the lowest NOx emissions, uniformly low reaction temperatures must be assured thus the reactants must be well mixed. This is accomplished in low emission gas turbines by mixing the reactants within a pre-mixer section prior to entry into the combustor. With the addition of hydrogen into the fuel, certain issues arise such as higher flame speeds compared to carbon based fuels. Flashback is a phenomena that occurs when the flame no longer propagates beyond the exit of the premixer/injector but instead retracts and propagates upstream towards, and ultimately into the pre-mixer, causing significant damage due to such high temperatures. Flashback occurs when the flame speed exceeds either the local or bulk flow velocity. In practice, the question arises regarding the impact of turbulence levels. While an increase in turbulence intensity may help improve mixing, it also known to increase turbulent burning velocity. In the present work, the influence of bulk turbulence intensity of the flow on boundary layer flashback is investigated. Data are acquired for a different turbulence intensities at pressures from 3 to 8 bar with preheated reactants up to 750 deg. K. Various mixtures of hydrogen and methane are evaluated. The results show that even with significantly different bulk flow turbulence intensities (based on the ratio of flow centerline turbulence to centerline axial velocity) boundary layer flashback is not strongly affected. This is attributed to the role of the quenching distance in connection with damping within the boundary layer. It is noted that core flow flashback or other flashback mechanisms may be affected differently.