Consumption rate characteristics of a fine-scale unburnt mixture in a turbulent jet premixed flame by high repetition rate PLIF and SPIV

Abstract

A 10 kHz simultaneous measurement of OH-CH planar laser induced fluorescence (PLIF) and stereoscopic particle image velocimetry (SPIV) is applied to a methane-air turbulent jet premixed flame. The measurement of the flame tip for high Reynolds number conditions shows that isolated fine-scale unburnt mixtures, so-called unburnt mixture islands or reactant pockets, are frequently generated. POD analysis shows that the separation of unburnt mixture from the upstream main reactants is the characteristic flame structure. In our previous study (Johchi et al. 2015), the consumption rates of isolated unburnt mixtures are estimated from changes of area of unburnt region detected in OH and CH PLIF images under the assumptions that the isolated unburnt mixtures are spherical and pillar shapes. The most expected consumption rate conditioned by curvature of flame front is about 0.71 m/s, which is much higher than the laminar burning velocity of the corresponding reactants. The consumption rate increases with the decrease of the radius of the isolated reactants. The reason that the consumption rates of the fine-scale isolated unburnt mixtures are much higher than the laminar burning velocity is discussed based on heat conduction in the isolated unburnt mixture by assuming that the heat release from mass difference between going out and coming in the preheat zone increases the mean temperature of remained reactants to consider enhancement of the effect of heat conduction. From the analysis, characteristic scale of fine-scale unburnt mixture in which heat conduction effect is significant is discussed.