Dynamics of Bluff Body Flames Near Blowoff

Abstract

This paper compiles a number of experimental and computational results on the dynamics of bluff body stabilized flames and proposes a phenomenological explanation of the blowoff process. This work is motivated by observations that flames near blowoff exhibit a variety of transient characteristics. We believe that this unsteadiness is not simply a result of the flame's proximity to blowoff, but also an important contributor to the processes ultimately leading to blowoff. As such, if this hypothesis is correct, then understanding and predicting blowoff requires a treatment of these dynamical processes. Moreover, it implies that classic blowoff explanations relying on purely steady state characteristics of the flow, such as a characteristic residence time and chemical time, are necessarily incomplete. We propose that the two key processes driving blowoff are the presence of vortex shedding in the wake of the bluff body and localized, strain induced extinction of the flame. The order in which these processes become dominant depends upon the gas expansion ratio across the flame, T b /T u . However, neither process by itself can fully describe the blowoff phenomenology. Localized extinction events, or in the flame, cause unsteadiness in the flame, but are not solely responsible for blowoff because re-circulating hot products serve as a torch to hold the flame at the anchoring point. On the other hand, vortex shedding leads to a sinuous wake that pulls cold reactants on one side of the bluff body toward the other side. In some cases, the recirculating flow is so strong that the reactants actually reverse direction before passing through the flame. As long as the flame stays connected and no holes appear in it, these cold reactants will pass through the flame front. If, however, the flame locally extinguishes, this creates a passage for cold reactants to shoot directly at the stabilization point of the flame branch. If this happens, the flame will not re-ignite in this low velocity region and begin to move downstream, which is blowoff.