Comparative Experimental Study of Methane and Propane Turbulent Bunsen Flames at Low Turbulence

Methane/air and propane/air turbulent premixed flames were investigated in a slot Bunsen burner at fuel-to-air equivalence ratios 0.7 ≤ φ≤ 1.3, inlet velocities 2.6 m⋅s−1 ≤ Uin ≤ 7.0 m⋅s−1, and nondimensional turbulence intensities 0.4 ≤ u’/SL ≤ 2.5. Experiments were performed at atmospheric pressure and included simultaneous planar laser-induced fluorescence (PLIF) of the CH radical and particle image velocimetry (PIV) for assessing the instantaneous flame front and the flow field, respectively. In methane flames with a fixed φ, the turbulent flame speeds ST/SL exhibited a nonmonotonic dependence on u’/SL when Uin increased with increasing u’/SL. The ST/SL initially increased, then attained a maximum at a specific value of u’/SL that depended on φ, and finally decreased with a further increase of u’/SL. This dependence was qualitatively different from a related one reported in the literature for stoichiometric methane/air Bunsen flames, where Uin was maintained constant as u’/SL increased. Furthermore, the present nonmonotonic dependence was observed for u’/SL as low as 0.55, values considerably lower than the u’/SL ≈3 in past literature. In propane flames with a fixed φ, the ST/SL displayed a monotonically increasing dependence on u’/SL when Uin was increasing with increasing u’/SL. Contrary to the methane flames, the propane ST/SL increased with increasing equivalence ratio from φ = 0.8 to 1.2 due to preferential diffusion effects. Moreover, in the propane flames and for 0.4 ≤ u’/SL ≤ 1.4, the ST/SL were mainly controlled by Uin rather than u’/SL. Analysis of the flame front curvature and the skewness of the curvature distributions attested the absence of Darrieus–Landau (DL) hydrodynamic instabilities. The increase in the propane turbulent burning velocities at rich stoichiometries was mainly attributed to an enhancement of the local burning rates rather than to an increase in flame surface wrinkling.