The coupling of conical wrinkled laminar flames with gravity

This work explores the influences that gravity has on conical premixed laminar and mildly turbulent flames (i.e., wrinkled laminar flames). The approach is to compare overall flame characteristics in normal (+g), reverse (−g), and micro-gravity (μg). Laser schlieren is the principal diagnostic for the μg experiments. Laboratory investigation of +g and −g flames also include two components laser doppler anemometry. The results obtained in a wide range of flow, mixture and turbulence conditions show that gravity has a profound effect on the lean stabilization limits, features of the flowfield, and mean flame heights. In +g, buoyancy driven instability causes flame flickering. Without this unstable interaction, flames in −g and μg do not flicker. Analysis of the flame flickering frequencies produces in an empirical relationship St*2/Ri = 0.0018 Re2/3 (where St*, Ri, and Re are, respectively, the Strouhal number normalized by the heat release ratio, the Richardson number, and the Reynolds number). This correlation would be useful for theoretical prediction of buoyancy induced flame instabilities. Comparison of mean flame heights shows that +g, −g, and μg flame properties do not converge with increased flow momentum. Velocity measurements in laminar flames show that in +g, the flow generated by the rising products plum is almost non-divergent, slightly turbulent and unstable. In −g, the flow becomes divergent, but is stable and non-turbulent in the region surrounding the flame cone. The change from a nondivergent to divergent flow field seem to account for the differences in the observed mean flame heights. The schlieren images and the velocity measurements in +g and −g also provide some insight into the overall flowfield features of μg flames.