Flame propagation and NO formation characteristics of hydrogen-air mixture combustion due to dilution effect

Hydrogen, abundant and clean with zero carbon emissions, possesses high calorific value, rapid flame propagation, and excellent detonation resistance, positioning it as a prime contender for zero-carbon vehicle propulsion systems. This study explored the effects of dilution effect and equivalence ratio on premixed hydrogen/air combustion through numerical simulations, examining the thermal, chemical, and dilution impacts of H2O, N2, and exhaust gas recirculation (H2O + N2) on laminar flame speed, structure, radical profiles, and Nitric Oxide formation by employing virtual substances (FH2O, FN2, and FEGR). Results indicated that dilution gases primarily affect flame speed through physical mechanisms, with H2O exerting strong chemical effects, followed by exhaust gas recirculation and N2. The laminar flame speed decreases significantly with the increase of dilution rate. Under the same equivalent ratio, when the dilution ratio increases from 0 % to 30 %, the laminar flame speed decreases by more than 50 % and the diminishing trend of laminar flame speed gradually stabilizes. The chemical effect of dilution gas can affect the concentration of free radicals, but the free formation rate is mainly affected by physical effects. The mole fraction of Nitric Oxide exhibits an initial increase followed by a decline as the equivalence ratio increases, peaking near an equivalence ratio of 1. The application of exhaust gas recirculation could reduce Nitric Oxide generation, during equivalent combustion, increasing the dilution ratio from 0 % to 30 % results in a Nitric Oxide reduction of over 90 % with both H2O and exhaust gas recirculation dilution. However, excessively high exhaust gas recirculation rate will lead to unstable combustion. An exhaust gas recirculation rate of 20 %, coupled with an equivalence ratio slightly above 1, emerged as an effective strategy for controlling Nitric Oxide emissions while maintaining a substantial flame velocity. These findings provide theoretical guidance for hydrogen engine combustion and emissions optimization, particularly through controlled exhaust gas recirculation rates for Nitric Oxide mitigation.