Gravity effects on steady two-dimensional partially premixed methane-air flames

Under normal-gravity conditions the flame heat release produces both flow d ilatation and buoyancy effects. While it may be possible to minimize gravit ational effects in a fully premixed flame by isolating buoyancy effects to the lower-density postflame region or plume, this cannot be accomplished in nonpremixed flames. It is known that partially premixed flames can contain two reaction zones, one with a premixed-like structure and the other consi sting of a transport-limited nonpremixed zone (in which mixing and entrainm ent effects are significant). For these reasons it is important to understa nd the fundamental interaction between flow dilatation and buoyancy effects in partially premixed flames. A detailed numerical study is conducted to c haracterize the effect of buoyancy on the structure of two-dimensional part ially premixed methane-air flames. The computational model is validated by comparison with the experimentally obtained chemiluminescent emission from excited-C-2* Gee radical species as well as with velocity vectors obtained using particle image velocimetry. Both the experiments and simulations indi cate the presence of two reaction zones that are synergistically coupled, w ith each region providing heat and/or chemical species for the other. While the inner premixed flame is only weakly affected by gravity, the outer fla me shows significant spatial differences for the two cases due to buoyancy- induced entrainment, since advection of air into the outer reaction zone in creases in the presence of gravity. The presence of gravity induces more co mpact flames, influences the velocity profiles in the post-inner flame regi on, and increases the normal strain rate. Although the spatial differences between the 0- and 1-g flames are more significant on the lean side, the st ate relationships in that region are relatively unaffected by gravity. On t he other hand, the inner (rich-side) reaction zone shifts toward less-rich locations in the presence of gravity, possibly due to the enhanced buoyant mixing. The 1-g flames exhibit a larger energy loss in the form of CO and H -2 emissions. (C) 1999 by The Combustion Institute.