Determination of burning velocities from spherically expanding H2/air flames

Laminar burning velocities of hydrogen/air mixtures can show discrepancies up to 30%, making chemical mechanism validation and improvement difficult. The source of uncertainties may come from different factors influencing at each processing and post-processing steps the final value. Considering a spherically expanding flame configuration, reflection on the accuracy of the formulations, used to derive the desired quantity, is proposed. Starting from the exact definition of the laminar burning velocity, two formulations - direct and indirect flame speeds formulations - are derived for spherical flames. Each single source of uncertainty involved in the formulations is pointed out. The emphasis is focused on a specific mixture at an equivalence ratio of 0.50, atmospheric pressure, and an initial temperature of 300 K. This point represents the best tradeoff between low ratio of flame velocity and recording sampling rate and the occurrence of cellular flames (Le < 1). An extensive experimental and numerical study (1D spherically expanding flames) of this mixture is carried out. As a result, the experimental laminar burning velocities determined by using the direct flame speed or the indirect flame speed formulae depict different values. However, when numerically determined, both formulae yield the same value. This paves the way to understand and identify the experimental error sources. Stretch and Lewis numbers effects (super-adiabatic temperatures) as well as radiation processes (burned gas motion) are studied. Nonetheless, they do not show to be the main source of uncertainty. The extrapolation procedure (linear or non-linear) according to the limited number of experimental points (rapid apparition of cellular structure) appears as the main factor influencing the discrepancy in laminar burning velocities.