Experimental study on the effects of large centrifugal forces on step-stabilized flames

Results are presented from an experimental study that examines the effects of transverse accelerations as large as ±5000 g's on premixed combustion stabilized at a backward-facing step. Large centripetal accelerations are induced by flowing a premixed reactant stream - in this case a propane-air mixture - at various speeds through a curved channel. The resulting centrifugal forces act on the high-density reactants and low-density products and have profound effects on the reactant/product mixing, flame propagation mechanisms, and the resulting blowout limits. A backward-facing step placed on either the radially inner or outer boundary of the flow channel is used to stabilize the flame. The flow speed, equivalence ratio, step height and step position are systematically varied, and shadowgraph and chemiluminescence imaging are used to reveal the effects of the transverse acceleration. Negative accelerations, where the step is placed on the inner circumference of the flow path, act to inhibit the mixing of reactants into the recirculation zone and lead to incomplete combustion. Positive accelerations, where the step is placed on the outer circumference of the flow path, dramatically enhanced the mixing between reactants and products. In this case the flame propagation angle, which is driven primarily by "centrifugal pumping", is found to be independent of the flow velocity. In the non-ideal environment of the laboratory combustor, the blowout limits under a positive centrifugal field are shown to be largely comparable to those in the straight combustor, while a negative centrifugal field demonstrates significantly enhanced blowout limits.