High-pressure droplet burning experiments in microgravity

Abstract

High-pressure droplet burning characteristics of five fuels are investigated under normal and reduced gravity conditions. The reduced gravity experiments have been conducted by using the parabolic flights of the CNES Caravelle and the NASA KC-135 aircrafts. A fully automatic high-pressure droplet gasification facility has been developed for these experiments. Rapid videography is used to determine the time histories of burning droplets, from which average droplet burning rates are determined. For all experiments, the suspended droplet technique is used. Initial droplet diameters are about 1.5 mm. Subcritical and supercritical droplet burning regimes are explored. Droplet time histories are only determined for weakly sooting fuels, such as methanol. These results show that the D 2 law holds even under very high pressures and allows the estimation of an average droplet burning rate. For the heavily sooting n -alkane droplets, this result is used to deduce the average burning rate by measuring the droplet burning life-time. The experimental results for all fuels show that the droplet burning lifetime decrease strongly with increasing pressure in the subcritical regime. When the pressure is increased above the critical pressure of the pure liquid, the droplet burning life-time remain constant on the average. Therefore, the minimum burning time observed in the literature is not confirmed here. To take into account the residual gravity-enhanced natural convection effects, a correction based on the Grashof number is applied. This correction allows a comparison of various experiments under different gravity and pressure conditions and an estimate of the burning time without any convection effect. In the subcritical regime, this burning time varies as p −0.37 .