Microgravity experiments of fuel droplet evaporation in sub- and supercritical environments

Droplet evaporation in sub- and supercritical environments has been studied experimentally under microgravity conditions. A single suspended droplet of n-hexadecane was employed for the experiments. The initial droplet diameter was 0.4 mm. A pair of alumina/silica fibers of 7 µm in diameter was applied to suspend a droplet. The ambient pressure was varied in the range of 1.0-3.0 MPa, and the ambient temperature was set at 773 K. Sequential backlit images of an evaporating droplet were recorded using a high-speed digital video camera. Temporal variations in the droplet diameter were measured using a self-made computer-aided image analyzer. Microgravity conditions were produced by a 50-m drop tower. Temporal variations in the droplet diameter were successfully obtained for droplet evaporations in the supercritical environments. The normalized droplet lifetime increased with the ambient pressure. The evaporation rate constant increased with the ambient pressure, reached the maximum value at an ambient pressure slightly above the critical pressure of the fuel, and then decreased. The initial heat-up period linearly increased with the ambient pressure, reached the maximum value at an ambient pressure of 2.0 MPa, and then decreased. The ratio of the initial heat-up period to droplet lifetime increased with the ambient pressure, reached the maximum value of about 0.6 at an ambient pressure of 2.0 MPa, and then decreased. The droplet evaporation lifetime increased with the ambient pressure at subcritical ambient pressures even though the evaporation rate constant increased because the increase in the initial heat-up period overtook the decrease in the quasi-steady evaporation period. It was found that, in the case of fuels with a high critical temperature, the initial heat-up period determines the ambient pressure dependence of the droplet evaporation lifetime in the environments around the critical point of the fuel.