Authors: Andrew J. Smallbone, Toshiaki Kitagawa, and Tetsuo Oonishi

Measurements of the laminar burning velocity are fundamental to understanding those processes which occur in practical combustion systems such as the SI engine. In particular, these measurements are vital inputs to the turbulent combustion models/correlations adopted by the automotive industry to predict in-cylinder flame propagation rates. However, determination of the laminar burning velocity for mixtures with a Lewis number of less than unity and/or for increasing mixture pressures have proven difficult due to an increasingly unstable and perturbed flame front. In this study, a technique is outlined to determine unstretched laminar burning velocities from propagating unstable flames. Laminar burning velocities were obtained from spherically propagating propane-air and methane-air explosions carried out at 0.10, 0.25 and 0.50MPa at equivalence ratios of 0.8, 1.0 and 1.3 using a pre-mixed combustion vessel and high speed imaging. Estimates of the unstable flame front surface area were conducted by counting the number of visible cells. A “fractal” approach was then employed to estimate the unstretched laminar burning velocity. Laminar burning velocity correlations are presented in the range 0.10 to 1.0MPa and 300 to 475K.