Optimizing Mixture Properties for Accurate Laminar Flame Speed Measurement from Spherically Expanding Flame: Application to H2/O2/N2/He Mixtures

The uncertainty on the laminar flame speed extracted from spherically expanding flames can be minimized by using large flame radius data for the extrapolation to zero stretch-rate. However, at large radii, the hydrodynamic and thermo-diffusive instabilities induce the formation of a complex cellular flame front and limit the range of usable data. In the present study, we have employed the flame stability theory of Matalon to optimize the properties of the initial mixture so that transition to cellularity may occur at a pre-determined, large radius. This approach was employed to measure the laminar flame speeds of H2/O2/N2/He mixtures with equivalence ratios from 0.6 to 2.0, at pressures of 50/80/100 kPa and a temperature of 300 K. For all the performed experiments, the uncertainty related to the extrapolation to zero stretch-rate (performed with the linear curvature model) was below 2% as shown by the position of the data points in the (Lb/Rf;U, Lb/Rf;L) plan, where Lb is the burned Markstein length; and Rf;L and Rf;U are the flame radii at the lower and upper bounds of the extrapolation range. Comparison of the predictions of four chemical mechanisms with the present unstretched laminar flame speed data indicated an error below 10% for most conditions. In addition, unsteady 1-D simulations performed with A-SURF demonstrated that the flame dynamical response to stretch rate could not be captured by the mechanisms. The present work indicates that although the stability theory of Matalon provides a well defined framework to optimize the mixture properties for improved flame speed measurement, the uncertainty of some of the required parameters can result in largely over-estimated critical radius for cellularity onset which compromise the accuracy of the optimization procedure.