Simultaneous OH and Schlieren visualization of premixed flames at the lean blow-out limit

Abstract

The structure of an air-propane premixed flame was studied experimentally at the lean flammability limit, using Schlieren photography synchronized with OH-imaging done with the Planar Laser Induced Fluorescence (PLIF) technique. The flame was studied in a wide range of fuel equivalence ratios. Various steps in the process of the flame destabilization were investigated, including partial lift-off, stable lift-off, and final blow-out conditions. The flame structure was visualized for each stage showing the transition from a flame held at the nozzle to a flame held by the flow structures. In order to study the latter conditions in more detail the flame was acoustically excited at the preferred mode frequency generating large, stable, coherent structures in the core region. The modified flame structure was visualized to understand the interaction between the flame and vortical flow dynamics.

It is shown that for the flow conditions when the flame cannot be stabilized at the nozzle, a new anchoring point is reached at the location of the initial vortex roll-up in the jet shear layer. At this point the flow reversal and transition to turbulence produce stagnation points with relatively low local velocities and velocity gradients where the flame can be stabilized. When the flame jet is being forced at the jet most unstable frequency, large coherent structures are formed and the flame is stabilized intermittently on these vortices.