Quenching of a Boundary-Layer Laminar Diffusion Flame in Microgravity

Abstract

Radiative quenching of a nonpremixed, heavily sooting, laminar flame established in a shear boundary layer at very low strain rates in microgravity is investigated. The computations include detailed chemistry, transport, and radiation for a three-dimensional reacting flow. Radiative quenching is expected at long residence times, due to soot formation resulting from flame expansion downstream of the flame leading edge. The soot model is based on the formationoftwo-andthree-ringedaromaticspeciesandcorrectlyreproduces theexperimentaldatafromalaminar ethylene diffusion flame over a flat plate. The purpose of this study is to better understand the effects of a dimensionless volume coefficient, defined as CqVF=Vox (where VF is the fuel-injection velocity and Vox is the airstreamvelocity),on flamequenchinganditsstandoffinashearreactiveboundarylayer.Experimentshaveshown that a blue unstable flame (negligible radiative feedback) may change to a yellow shorter flame (significant radiative feedback) with anincrease ofCq.Thisexperimental trend isnumerically reproduced, showing thatanincrease ofCq results in a reduction in flame length that is significantly affected by increasing VF or decreasing Vox, favoring soot formation. The flame quenching at very low strain rates is a combination of radiative heat loss and combustion efficiency, depending on the fuel-zone geometry and oxygen concentration. Along a semifinite fuel zone, the ratio df =db betweenthe flamestandoffdistancedf andthe boundary-layerthicknessdb convergestowardaconstant value of 1.2. With reduction in fuel size, radiation loss causes the flame temperature and magnitude of the ratio df =db to decreaseuntilthe flamemigratestowardthe boundarylayer(df =db < 1)farawayfromthetrailingedge. Inallcases, the soot resides within the boundary layer far from the flame, despite the fuel-zone size and oxygen concentration. Thetwo-dimensional flowstructureisapproximateforCq below0.02,beyondwhichthethree-dimensionaleffectsare ofimportance,andthereactiveboundarylayerissignificantlyliftedabovethesurface.This flamebehaviorcannotbe described by the available asymptotical solution from a reactive boundary-layer model without taking into account radiation loss.