Effect of fuel-blend ratio in methane-hydrogen reheat flames

A recent study on the operation of a reheat burner (RB) in Ansaldo's GT36 engine with H2-enriched natural gas found that for a H2 fraction >70%, the engine must be derated to achieve the desired combustion characteristics. This was due to a greater contribution of the flame propagation as opposed to the design intended autoignition. In this study, 2D direct numerical simulations are performed in a simplified RB to investigate the effect of blending ratio in CH4-H2-air premixed flames. The geometry consists of a rectangular mixing duct that undergoes a sudden expansion into a combustion chamber. Three fuel blends comprised of CH4 and H2 at a ratio 0.5:0.5, 0.3:0.7 and 0:1 by volume are considered. At 5 bar the thermo-chemical conditions are chosen to achieve a constant ignition delay time. The results show flame stabilization by autoignition in the bulk flow and by flame propagation in the shear layers immediately downstream of the sudden area expansion. The flame rapidly oscillates in the combustion chamber for pure H2, which is suppressed with addition of CH4. With an increase in CH4 fraction, the flame broadens with a more volumetrically distributed heat release rate. The quantification of the fuel consumption by the two combustion modes based on CEMA will also be presented.