Experimental and computational study on stability characteristics of hydrogen-enriched oxy-methane premixed flames

The combustion and stability characteristics of premixed swirl-stabilized H-2-enriched CH4-O-2-CO2 flames were investigated experimentally and computationally in a model gas turbine combustor adopting lean premixed combustion technology. The stability map was obtained by identifying the lean blowout and flashback limits over ranges of hydrogen fraction (from 0 to 75%) and equivalence ratio (from 0.3 to 1.0) at a fixed inlet bulk velocity of 5.2 m/s and fixed volumetric oxygen fraction of 30%. Large Eddy Simulation methodology was implemented to model the premixed turbulent flames. The results indicate an increase in the heat release factor and a wider operability range with hydrogen enrichment. Reaction rates are enhanced, and the flames tend to be more compact and intense with the increase in hydrogen percentage, until flashback is observed. The results indicate that the role of the outer recirculation zone on flame stabilization is significant at lower hydrogen fraction and diminishes at higher hydrogen fraction, At 20% hydrogen fraction, the flow inside the inner recirculation zone is characterized by having a counter-clockwise spinning primary eddy and a secondary eddy. Further increase in hydrogen fraction resulted in breakdown in the inner recirculation zone to have two secondary eddies spinning opposite to each other in addition to the primary eddy. The CO emission at the combustor exit increased from 46 ppm with pure methane to 378 ppm at 50% hydrogen fraction. Hydrogen enrichment was found to increase the critical rate of strain and diminish the possibility of flame quenching. Flames of higher hydrogen fraction are characterized by having higher Damkohler number and lower Karlovitz number.