Experimental and kinetic study of laminar flame characteristics of H_2/O_2/diluent flame under elevated pressure

摘要

Laminar burning velocity, Markstein length, and critical flame radius of an H-2/O-2 flame with different diluents, He, Ar, N-2 and CO2, were measured under elevated pressure with different diluent concentrations. The effects of pressures, diluents, and dilution and equivalence ratios were studied by comparing calculated and experimental results. The laminar burning velocity showed non-monotonic behavior with pressure when the dilution ratio was low. The reason is the radical pool reduced with increasing pressure and leads to the decrease of overall reaction order from larger than 2 to smaller than 2, and further leads to this non-monotonic phenomenon. A modified empirical equation was presented to capture the relationship between active radicals and laminar burning velocity. Critical radii and Markstein lengths both decrease with initial pressure and increase with equivalence ratio and dilution ratio. The calculated critical radii indicate that the Peclet number and flame thickness control the change of R-cr. It can be found that Le eff has a significant influence on Peclet number and leads to the decrease of critical flame radii of Ar, N-2, and CO2 diluted mixture. Interestingly, the CO2 diluted mixture has the lowest Markstein length under stoichiometric conditions and a high value under fuel-rich conditions, consistent as the flame instability observed on the flame images. The reason is that the Le(eff )of CO2 diluted mixture increased rapidly with the equivalence ratio. (C) 2020 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.