Quenching of a Boundary-Layer Laminar Diffusion Flame in Microgravity

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 formation of two- and three-ringed aromatic species and correctly reproduces the experimental data from a laminar 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 C-q = V-F/V-ox (where V-F is the fuel-injection velocity and V-ox is the airstream velocity), on flame quenching and its standoff in a shear reactive boundary layer. Experiments have shown that a blue unstable flame (negligible radiative feedback) may change to a yellow shorter flame (significant radiative feedback) with an increase of C-q. This experimental trend is numerically reproduced, showing that an increase of C-q results in a reduction in flame length that is significantly affected by increasing V-F or decreasing V-ox, 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 d(f)/d(b) between the flame standoff distance d(f) and the boundary-layer thickness d(b) converges toward a constant value of 1.2. With reduction in fuel size, radiation loss causes the flame temperature and magnitude of the ratio d(f)/d(b) to decrease until the flame migrates toward the boundary layer (d(f)/d(b) < 1) far away from the trailing edge. In all cases, the soot resides within the boundary layer far from the flame, despite the fuel-zone size and oxygen concentration. The two-dimensional flow structure is approximate for C-q below 0.02, beyond which the three-dimensional effects are of importance, and the reactive boundary layer is significantly lifted above the surface. This flame behavior cannot be described by the available asymptotical solution from a reactive boundary-layer model without taking into account radiation loss.