Experimental and numerical study on the effect of diluents on laminar burning velocities of near-limit hydrogen flames at normal and reduced pressures

In this study, the effects of diluents (CO₂, N₂, AR and HE) and pressures on laminar burning velocity (LBV) of near-limit H₂/Air/diluent flames were investigated experimentally and computationally. The LBVs of H₂/Air at CO₂ and N₂ dilution at varying equivalence ratios (ϕ) and dilution ratios (up to 52 % by volume) under normal and reduced pressures were measured using a constant volume vessel. Several H₂ kinetic models (FFCM-1, NUIG 2013, Davis model, Sun model, Dryer model and Li model) were compared and validated against experimental data. The fictitious dilution gas method was applied to investigate the chemical, physical, and global effects of the four diluents. Corresponding kinetic analysis was conducted to gain deep insight of the combustion process of near-limit H₂ flames. The results show that the measured minimum LBV for H₂/Air/diluent flames is yield below 2 cm/s and the models prediction results are significantly lower than measured values. Inhibition effect on LBV is CO₂ > N₂ > AR > HE. CO₂ exists the strongest chemical effect to suppress LBV. HE exists the weakest inhibition effect, benefiting from its highest thermal diffusivity, as well as its low specific heat capacity. A reversal of sensitivity was observed at 40 % CO₂ dilution with ϕ = 0.8. The sensitivity of the reaction H + O₂ + M = HO₂ + M increases with increasing dilution ratio, and the diluent as the third body of the system, M, controls the rate of the reaction. The difference in rate of this reaction may be responsible for the discrepancy between model predictions and experimental values.