The effects of water dilution on hydrogen, syngas, and ethylene flames at elevated pressure

This work investigates experimentally and numerically the kinetic effects of water vapor addition on the burning rates of H₂, H₂/CO mixtures, and C₂H₄ from 1 atm to 10 atm at flame temperatures between 1600 K and 1800 K. Burning rates were measured using outwardly propagating spherical flames in a nearly constant pressure chamber. Results show good agreement with newly updated kinetic models for H₂ flames. However, there is considerable disagreement between simulations and measurements for H₂/CO and C₂H₄ flames at high pressure and high water vapor dilution. Both experiments and simulations show that water vapor addition causes a monotonic decrease in mass burning rate and the inhibitory effect increases with pressure. For hydrogen flames, water vapor addition reduces the critical pressure above which a negative pressure dependence of the burning rate is observed. However, for C₂H ₄ flames, the burning rate always increases with pressure. The results also show that water vapor addition has the same effect as a pressure increase for H₂ and H₂/CO flames, shifting the reaction zone into a narrower window at higher temperatures. For all fuels, water vapor addition increases OH formation via H₂O + O while reducing the overall active radical pool for hydrogen flames. For C₂H₄, the additional HO₂ production pathway through HCO results in a dramatic difference in pressure dependence of the burning rate from that observed for hydrogen. The present work provides important additions to the experimental database for syngas and C₀-C₂ high pressure kinetic model validations.