Effects of buoyancy on premixed flame stabilization
The stabilization limits of v-flame and conical flames are investigated in normal gravity (+g) and reversed gravity (upside-down burner, -g) to compare with observations of flame stabilization during microgravity experiments. The results show that buoyancy has the most influence on the stabilization of laminar v-flames. Under turbulent conditions, the effects are less significant. For conical flames stabilized with a ring, the stabilization domain of the +g and -g cases are not significantly different. Under reversed gravity, both laminar v-flames and conical flames show flame behaviors that were also found in microgravity. The v-flames reattached to the rim and the conical flame assumed a top-hat shape. One of the special cases of -g conical flame is the buoyancy-stabilized laminar flat flame that is detached from the burner. This flame implies a balance between the flow momentum and buoyant forces. The stretch rates of these flames are sufficiently low (< 20 s-1) such that the displacement speeds SL are almost equal to the laminar burning speed SL0. An analysis based on evaluating the Richardson number is used to determine the relevant parameters that describe the buoyancy/momentum balance. A perfect balance i.e. Ri = 1 can be attained when the effect of heat loss from the flame zone is low. For the weaker lean cases, our assumption of adiabaticity tends to overestimate the real flame temperature. This interesting low-stretch laminar flame configuration can be useful for fundamental studies of combustion chemistry.