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

Under normal-gravity conditions the flame heat release produces both flow dilatation and buoyancy effects. While it may be possible to minimize gravitational 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 consisting of a transport-limited nonpremixed zone (in which mixing and entrainment effects are significant). For these reasons it is important to understand the fundamental interaction between flow dilatation and buoyancy effects in partially premixed flames. A detailed numerical study is conducted to characterize the effect of buoyancy on the structure of two-dimensional partially premixed methane–air flames. The computational model is validated by comparison with the experimentally obtained chemiluminescent emission from excited-C∗2 free radical species as well as with velocity vectors obtained using particle image velocimetry. Both the experiments and simulations indicate the presence of two reaction zones that are synergistically coupled, with each region providing heat and/or chemical species for the other. While the inner premixed flame is only weakly affected by gravity, the outer flame shows significant spatial differences for the two cases due to buoyancy-induced entrainment, since advection of air into the outer reaction zone increases in the presence of gravity. The presence of gravity induces more compact flames, influences the velocity profiles in the post–inner flame region, 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 state relationships in that region are relatively unaffected by gravity. On the 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 H2 emissions.