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 H-2, H-2/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 H-2 flames. However, there is considerable disagreement between simulations and measurements for H-2/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 C2H4 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 H-2 and H-2/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 C-0-C-2 high pressure kinetic model validations. (C) 2012 The Combustion Institute. Published by Elsevier Inc. All rights reserved.