Extrapolations of laminar flame speeds from expanding spherical flames based on the finite-structure stretched flames

The modified models MLC (modified linear curvature model) and MNQ (modified nonlinear quasi-steady model) for the extrapolation of laminar flame speeds and Markstein lengths based on the expansion of the finite-structure stretched flames were derived from the mass conservation equation, which consider effects of finite flame thickness and finite species distributions on flame propagation. These modified extrapolation models were verified on determining the hydrogen/air laminar flame speeds and Markstein lengths from the experimentally measured instantaneous stretched spherical flame speeds. The experiment results show that, for lean hydrogen/air flames, the modified models have stronger nonlinearity. Thus, the extrapolated laminar flame speeds from modified models have lower values, which are closer to the calculation results compared to the values by using unmodified models. For rich hydrogen/air flames, the uncertainty of unmodified models can exceed 10% for the mixtures with equivalence ratio of 4.0 under 0.5 atm, while the modified models can reduce the uncertainty to 3%. When the pressure exceeds 1.5 atm, both unmodified and modified models have uncertainties below 2%, and the consideration of the finite flame structure is no longer significant. Additionally, the Markstein lengths obtained from the modified models are relatively accurate compared to the theoretical values both for lean and rich mixtures. Besides the hydrogen/air flames, for other flames such as ethylene/air and syngas/air flames, modified extrapolation models also show improved accuracy. (c) 2020 The Combustion Institute. Published by Elsevier Inc. All rights reserved.