Hyperacceleration effects on turbulent combustion in premixed step-stabilized flames

Experimental results are presented from an investigation of the effects of large transverse accelerations on flame propagation and blowout limits in premixed step-stabilized flames. The accelerations, which exceed ±10,000 g in the present study, induce large body forces on the high-density reactants and low-density products. These body forces can substantially alter the flame propagation mechanisms and dramatically increase the flame blowout limits. Sustained centripetal accelerations a_c = U²/R are created by flowing a premixed propane-air reactant stream with equivalence ratios 0.7 ≤ Φ ≤ 1.9 at various speeds U through a semicircular channel with radius R. A backward-facing step of height h on the radially outer (a _c > 0) or inner (a_c < 0) wall stabilizes the flame. For a_c > 0 the acceleration acts to force high-density reactants into the recirculation zone and low-density products into the reactant stream, while a_c < 0 forces hot products into the recirculation zone and impedes cold reactants from entering this zone. An otherwise identical straight channel provides corresponding baseline (a_c = 0) results for comparison. The flow speed U, equivalence ratio Φ, and step height h are systematically varied for a_c = 0, a_c > 0, and a _c < 0. Shadowgraph and chemiluminescence imaging show that as a_c → +∞ the propagation of the flame across the channel becomes independent of the flame burning velocity and instead is primarily due to large-scale "centrifugal pumping" driven by the induced body forces. For a_c →-∞ the body forces effectively segregate reactants and products to produce a nearly flat flame. In both cases, for large |a_c| values the resulting blowout limits can be substantially higher than those at a_c = 0.