Interaction mechanisms of fuel momentum with flashback limits in lean-premixed combustion of hydrogen

The impact of fuel momentum on the combustor flow field is studied experimentally in a swirl-stabilized, technically premixed hydrogen flame. The volumetric heating value of hydrogen is about 3.5 times lower compared to natural gas, which leads to significantly higher volumetric fuel flow rates at the same power level. This additional fuel momentum significantly alters the combustor flow field. Therefore, the fuel momentum also affects the combustor stability limits. Previous studies were mostly conducted at perfectly premixed conditions, where the fuel momentum does not alter the combustor flow field. In the current study, non-reacting and reacting combustor flow fields of a technically premixed model combustor injecting fuel in axial direction are recorded. Results reveal a strong impact of fuel momentum on axial velocity distribution at the mixing tube outlet and, thus, on the stability limits. Additionally, OH-PLIF recordings for different flow rates, air preheat temperatures, and equivalence ratios show that the axial location of the upstream flame front, xf, poses a telling estimator for flashback resistance. No flashback was observed when the upstream flame front was located downstream of the mixing tube. However, the flame tip always located upstream of the mixing tube outlet prior to flashback. A high value of xf was, thus, identified as a sufficient condition for flashback resistance. At the investigated conditions, xf is shifted downstream with increasing equivalence ratio due to the added momentum of the fuel flow, thereby superseding any parallel augmentation in the turbulent flame speed. This has been identified as the driving mechanism affecting the combustor stability limit.