Effect of pressure on counterflow H2–air partially premixed flames

A computational investigation of high-pressure hydrogen–air partially premixed flames (PPFs) is reported to characterize the effect of pressure on the flame structure, and the relevance of reaction limits for these flames. The flames are computed using the Mueller mechanism consisting of 19 elementary reactions and 9 species. Although the mechanism has been validated during previous investigations, additional validations are provided at high pressure. The PPF structure is characterized by two spatially distinct reaction zones, namely a rich premixed zone on the fuel side and a nonpremixed zone on the air side. In both reaction zones, consumption of reactants occurs primarily through reactions H+O2↔OH+O (R1), H2+O↔OH+H (R2), H2+OH↔H2O+H (R3), and H+O2+M↔HO2+M (R9). As pressure increases, it decreases the physical separation between the two reaction zones. This can be attributed to the effects of pressure on (i) flame speed associated with the rich premixed zone, which moves this zone further downstream and (ii) mass diffusivity which moves the nonpremixed zone further upstream (toward the fuel nozzle). At higher pressures, however, these effects are significantly reduced, and the flame maintains its twin-flame structure even at very high pressures. Three reaction limits are identified for these flames. While the chemical structure of the nonpremixed zone is characterized by the first reaction limit in the range of pressure investigated (p=1 to 40 atm), that of the rich premixed zone is characterized by transition from first to second limit, and then from second to third limit, as pressure is increased. This implies that H2–air PPFs can exploit the advantages of the two reaction zones; each dominated by different reaction limits or chain reactions. Thermal radiation is found to have a negligible effect on the flame structure, while the Soret effect is found to cause transition between the reaction limits at lower pressure.