Co-Flow Laminar Diffusion Flames of Monodisperse Sprays: Structure, Evaporation and Microgravity Effects

Abstract

An experimental investigation was performed on co-flow laminar diffusion flames of monodisperse sprays that were generated electrostatically. Self-sustained, axisymmetric, stable laminar flames were successfully stabilized in a co-flow of air, using heptane as the fuel. The overall flame appearance was similar to that of a candle flame. Rather than burning, the majority of the droplets simply evaporated at relatively low temperatures in the inner core of the flame, which appeared as a relatively dark region. This inner core was enveloped by a higher temperature region where soot was present. Outside of the sooty region, a flame characterized by a soft blue luminescence could be clearly seen in the lower half of the luminous combustion region. Phase Doppler anemometry and sizing were used to examine the droplet life-histories along the centerline trajectory of five flames that were operated at different fuel flow rates. Droplet evaporation was found to follow the d-square law, with evaporation coefficients ranging between 0.55 and 0.59 mm 2/S, values that are consistent with the evaporation of an isolated droplet in a convective environment of N2 at the prevailing temperature and that are a factor of two smaller than the evaporation coefficient of an isolated burning droplet. A comparison of the droplet evaporation time lo a characteristic diffusion time showed that the latter is much longer, which implies that these spray flames are still diffusion-controlled. Experiments under microgravity conditions showed that the flames are longer, wider and brighter than the normal-gravity counterparts, which can be attributed to an increase in residence time in the flames as buoyant acceleration of the gas is diminished. Measurement of the gas velocity field on the axis of the flame showed that the flame is under a mixed regime, with a momentum-controlled lower part and a buoyancy-controlled upper region.