NO emission characteristics in partially-premixed turbulent combustion of methane/hydrogen fuel at high pressures

• Proposed a modified weight factor for handling differential diffusion in LES. • Evaluated separate NO formations from four different reaction mechanisms. • Analyzed pressure and hydrogen addition effects on combustion and NO emissions. • Thermal and N 2 O NO grow with both increased hydrogen addition and pressure. • Prompt NO has opposite variations with increased hydrogen addition and pressure. NO emission characteristics have been comprehensively studied in high-pressure partially-premixed turbulent combustion of the hydrogen/methane fuel, using the large eddy simulation (LES) approach and flamelet-generated manifold (FGM) model. A modified weight factor appropriate for LES is proposed and incorporated in the linear differential diffusion (LDD) model to treat the unequal molecular diffusion of H 2 and H species. The total NO emission and, in particular, NO emissions from different reaction mechanisms (pathways) are evaluated separately and quantitatively. Results indicate that the total NO emission grows with both hydrogen enrichment (from 0 to 60 % in volume) and pressure increase (from 5 to 15 bar), dictated by the thermal, prompt, and N 2 O reaction pathways. NO formations from the thermal and N 2 O pathways grow with both enriched hydrogen and increased pressure. Their shares in the total NO emission index (EINO) reach more than 70 % and 10 %, respectively, in the partially-premixed turbulent combustion with 60 % hydrogen addition at a high pressure of 15 bar. The prompt NO formation weakly decreases with enriched hydrogen but weakly increases with increased pressure. Overall, its share in the total EINO declines to less than 16 % with 60 % hydrogen enrichment at 15 bar. The amount of NO formation from the NNH pathway is very small, < 5 % in the total EINO, in each studied case. Results would help gain fundamental and comprehensive understating of NO formation in high-pressure turbulent combustion of the hydrogen/hydrocarbon fuel.