Experimental limit on the validity of Markstein relation

A PIV-based system has been set-up for the simultaneous measurement of the local propagation velocity and the flame stretch of laminar jet premixed flames. The contributions of the flame front curvature and the strain rate of the fresh gas flow to the flame stretch are separately discussed. Laminar jet flames of methane-air lean mixtures have been studied with a secondary flame in a coflow configuration to avoid blow-off. In weakly curved flame tips (low inlet gas velocity), these measurements confirm the prediction of linear velocity-stretch dependence and allow the indirect determination of the Markstein length and planar flame propagation velocity, by extrapolating the experimental results to the zero-stretch condition. In moderately curved flame tips (large inlet gas velocity), experiments show a faster than linear increase of the flame velocity with the stretch. The transition between these two regimes occurs suddenly when a threshold value of the inlet gas velocity is exceeded.