Extinction of a free methanol droplet in microgravity

Abstract

The combustion of isolated, unsupported methanol droplets under microgravity conditions has been studied in the NASA-Lewis 2.2 second drop tower. The substitution of O2/He mixtures for O2/N2 mixtures results in a subtstantial increase in the oxygen index required for ignition, the rate of gasification, and the diameter at extinction. As a result, droplets with initial size larger than 1 mm can be produced, deployed, ignited, and burned to extinction, all under microgravity conditions in this facility. The droplet at extinction is sufficiently large (order of 0.3 mm) that photographic resolution and residual energy at extinction do not result in inaccuracies in determining the extinction diameter. A comprehensive time dependent numerical model incorporating detailed multicomponent molecular transport mechanisms and complex elementary chemical kinetics was developed and applied to simulate the entire transient droplet combustion processes (ignition/quasisteady burning/extinction). Model outputs consisting of droplet burning rates, flame stand-off ratios, flame temperatures, chemical species profiles, and temperature profiles at each time step show good agreement with the experimental measurements, without parameter adjustment. Analysis of modeling results shows that the thermal conductivity of the gas phase under combustion environments increased by a factor of two when the 50% O2/50% He mixture is used instead of air. The increased thermal conductivity accelerates the heat transfer from the flame, front to the droplet surface, resulting in a factor of two increase in the burning rate. The increased heat loss from the flame zone results in a factor of two increase in the extinction diameter. Comparison with d2-law results shows that property averaging rules which apply to droplet burning in air do not apply for helium mixture results.