Effects of radiation absorption on spherical flame propagation and radiation-induced uncertainty in laminar flame speed measurement

Outwardly propagating spherical flames are popularly used to measure the laminar flame speed, especially for high pressure conditions. Since radiation always exists in spherical flame experiments, the accuracy of laminar flame speed measurement is inherently affected by radiation. In this study, the radiation-induced uncertainty in laminar flame speed measurement was investigated numerically. We focused on CO2 diluted mixtures in which the radiation absorption effects are important. The outwardly propagating spherical flames of different CO2 diluted mixtures at a broad range of pressure up to 25 atm were simulated. Different fuels (hydrogen, methane, dimethyl ether and iso-octane) with different amounts of CO2 dilution were considered and detailed chemistry was included in simulation. Two radiation models were used: one is the optically thin model considering only radiation emission and the other is the statistical narrow band model considering both radiation emission and absorption. The effects of radiation absorption on spherical flame propagation and radiation-induced uncertainty in laminar flame speed measurement were quantified through comparison among results predicted by these two radiation models. It was found that for CO2 diluted mixtures, radiation absorption has great impact on spherical flame propagation: it greatly reduces the radiation-induced thermal and flow effects. The influence of radiation absorption was show to be stronger at higher pressure. When only radiation emission is considered and radiation absorption is neglected, the radiation-induced uncertainty in laminar flame speed measurement is substantially over-predicted for CO2 diluted mixtures. When radiation absorption is included, the radiation-induced uncertainty in laminar flame speed measurement is nearly negligible (within 2.5%) for all the CO2 diluted mixtures considered in this study. (C) 2016 by The Combustion Institute. Published by Elsevier Inc.