Lean Blowoff Of Bluff Body Stabilized Flames: Scaling And Dynamics

摘要

This paper overviews the dynamics of bluff body stabilized flames and describes the phenomenology of the blowoff process. The first section of the paper provides an overview of the fluid mechanics of the non-reacting and reacting bluff body wake flow. It highlights the key features of the flow (the boundary layer, separated shear layer, and wake), the flow instabilities that influence each of these features, and the influences of the flame on these instabilities. A key point from these studies is the large differences between the non-reacting wake (dominated by an absolutely unstable, sinuous instability associated with vortex shedding from the bluff body) and the reacting wake of high dilatation ratio flames. The latter are dominated by the lower intensity, convective instability of the shear layer. Very low dilatation ratio flames begin to approach the behavior of the non-reacting wake, as might be expected. Next, the paper presents a compilation of bluff body blowoff data from the literature and shows that the basic Damkohler correlations developed from prior studies are recovered, but without the need for semi-empirical fits or adjustable constants for chemical time estimation. The third section considers in detail the dynamics and phenomenology of near blowoff flames. It is shown that spatio/temporally localized extinction occurs sporadically on near blowoff flames, manifested as "holes" in the flame sheet that form and convect downstream. However, these extinction events are distinct from blowoff - in fact, under certain conditions the flame can persist indefinitely with certain levels of local extinction. The number of these events per unit time increase as blowoff is approached, eventually leading to large scale alteration of the wake. We hypothesize that simple Damkohler number correlations contain the essential physics describing the intial stage of blowoff; i.e., they are correlations for the conditions where local extinction on the flame begins, but do not fundamentally describe the ultimate blowoff condition itself. However, such correlations are reasonably successful in correlating blowoff limits because the ultimate blowoff event is related to the onset of this first stage. Key conclusions from this review are that blowoff occurs in multiple steps - local extinction along the flame sheet, large scale wake disruption, and a final blowoff whose ultimate "trigger" is associated with wake cooling and shrinking. A key challenge for future workers is understanding these latter processes that lead to ultimate blowoff of the flame.