Microgravity observations of premixed laminar flame dynamics

Abstract

Microgravity experiments have been performed to investigate the role of buoyancy in the structure and stability of premixed laminar flame fronts. Planar and spontaneously wrinkled (cellular) flames burning in propane-oxygen-nitrogen mixtures and propagating slowly downward through a 6 cm diameter tube against a flow of fresh gas were filmed under normal and low gravity conditions in the NASA Lewis 2.2 Second Drop Tower. Results show that downwardly-propagating flames that are planar in normal gravity become curved toward the unburned mixture in reduced gravity, forming a single cell nearly filling the tube. This behavior is consistent with theoretical predictions that flat flames are possible only when the stabilizing effect of buoyancy in this geometry balances the destabilizing effect of the hydrodynamic instability mechanism. Thus when gravity is reduced, this mechanism is unopposed and produces a curved flame. Behavior of cellular tube flames in microgravity is considerably more complex. Mixtures producing small cells in normal gravity produce flames with larger, irregular cells in low gravity. Often in the microgravity tests, one cell grows at the expense of the others, which move downstream and disappear. The remaining cell propagates through the tube as an oblique curved flame, while smaller cells form and disappear along the front. These observations are interpreted qualitatively in terms of the predicted effects of buoyancy on flame stability and cell sizes. It is concluded that while buoyancy is not the dominant mechanism in the formation of the cells, it contributes significantly to the overall flame stability and structure.