Experimental and kinetic studies of acetylene flames at elevated pressures

The kinetic effects of CO₂ and H₂O dilution on the laminar flame speed of acetylene at elevated pressure has been investigated experimentally using outwardly propagating spherical flames from 1 - 20 atm. A high pressure kinetic model is assembled by including pressure dependent elementary reaction rates determined by recent theoretical calculations and experimental measurements. It is found that, unlike other hydrocarbons, the kinetic inhibition caused by CO₂ dilution on acetylene flame speeds via CO₂+H=CO+OH is reduced at both fuel rich and lean conditions due to the existence of direct CO₂ formation pathways (HCCO+O₂ and CH₂+O₂) in acetylene oxidation. On the other hand, the inhibiting kinetic effects of water dilution on acetylene flame speed are promoted because the shifting equilibrium of the H₂ O+O=OH+OH reaction inhibits production of O radicals needed for the O+C₂ H₂=HCCO+H chain propagation reaction. Detailed analysis on the combustion chemistry of acetylene reveals that C₂ H₂+O, HCCO+O₂, HCO+O₂, CH₃+HO₂, H+C₂ H₃, CO+OH, CH₂(S)+C₂ H₂, and HCO decomposition are among the most important reactions for predicting the laminar flame speed, especially at high pressures. The results show that the new high pressure model (HP-Mech) improved prediction of flame speeds of acetylene with CO₂ and H₂O dilution.