Flame stretch effects on partially premixed flames

Abstract

Flame stretch is an important characteristic of flame behavior that arises due to flow motion and nonuniformity, and flame curvature. Various investigators have obtained different expressions for flame stretch by employing dissimilar analytical approaches for diverse flame configurations. In this investigation, we consider two such formulations, one which is “exact” and the other that assumes a simplified unidirectional flow, and apply both relations to characterize the behavior of the rich premixed reaction zone in partially premixed flames (PPFs). Steady two-dimensional partially premixed methane–air slot burner flames are established by introducing a rich fuel–air mixture from the inner slot and air from the two outer slots. The flames contain two distinct spatially separated reaction zones. In order to characterize the flame stretch, the corresponding flame deformation must be first measured. The experimental diagnostics include the use of C ∗ 2-chemiluminescence images, holographic interferometry, and particle image velocimetry. The flame structure depends upon the heat release, temperature, and fluid velocity (therefore, momentum) associated with a particular burner or flame configuration. We focus attention on stretch effects on the inner premixed reaction zone of the PPFs, which exhibits a highly curved portion near its tip and planar portions along its sides. By applying the two formulations, we have inferred unstretched flame speeds and found these to be in good agreement with previously measured results. The methodology also allows us to infer “effective flame speeds” for rich methane–air mixtures with compositions that lie beyond the rich flammability limits for which no data is available in the literature. In general, both formulations provide satisfactory results, although the exact relation is a more accurate representation, as is to be expected. Unidirectional flow can be assumed for relatively larger flames in which the flow behind the flamefront is relatively unperturbed by the preheat zone. As we follow the flame curvature, both the sign and magnitude of the corresponding values of the stretch rate change in a systematic manner. This suggests that the dominating effect in these flames occurs more due to flame curvature and less due to flow nonuniformity effects. In the curved region, the deduced “unstretched” flame speed S uc o is greater than S u o . The measured values of Ma are of O(10 −1).