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 ac ≡ 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 (ac > 0) or inner (ac < 0) wall stabilizes the flame. For ac > 0 the acceleration acts to force high-density reactants into the recirculation zone and low-density products into the reactant stream, while ac < 0 forces hot products into the recirculation zone and impedes cold reactants from entering this zone. An otherwise identical straight channel provides corresponding baseline (ac = 0) results for comparison. The flow speed U, equivalence ratio Φ, and step height h are systematically varied for ac = 0, ac > 0, and ac < 0. Shadowgraph and chemiluminescence imaging show that as ac→ +∞ 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 ac → −∞ the body forces effectively segregate reactants and products to produce a nearly flat flame. In both cases, for large |ac| values the resulting blowout limits can be substantially higher than those at ac = 0.