Effects of reduced gravity on methanol droplet combustion at high pressures

Abstract

An experimental study was carried out on the burning of methanol droplets in high-pressure air under the normal gravity and microgravity conditions which are available both during the parabolic flights of the CNES Airbus (g≈10−2g0) in France, and in the JAMIC drop shaft in Japan (g≈10−4g0). Methanol was chosen as the test fuel both for its lower sooting tendency and because it is considered to be one of the most promising candidates as an alternative fuel. The well-known suspended droplet technique was adopted: a methanol droplet was suspended at the tip of a fine quartz fiber and was ignited with an electrically heated kanthal wire in high-pressure air at room temperature. The results showed that the d2 law holds for all ambient pressures and gravitational accelerations, and that the burning rate constant increases monotonically with the ambient pressure up to 1.4 times the critical pressure of pure methanol. Under microgravity, the dependence of the burning rate constant on the ambient pressure becomes weaker with increasing ambient pressure above the critical pressure. The burning rate constant decreases with a decrease in gravitational acceleration. The gravitational acceleration of the order of 10−2g0 is found to induce an appreciable influence on the burning rate constant. The variation of the burning rate constant as Gr1/4 was confirmed for the first time both by varying the gravitational acceleration and the ambient pressure. An experimental data set was therefore established on the effects of high pressure on droplet burning rates free from natural convection effects.