Droplet combustion near a cold surface

Abstract

The combustion of a liquid droplet adjacent to a cold surface was studied experimentally. To isolate the effect of the proximity of the droplet to the surface, the ambient pressure (0.101 MPa), liquid composition (n-heptane), initial liquid volume (7 x 10-4 ml), surface material (quartz) and ambient temperature (20 ± 2°C) were held constant. A range of distances L from the surface were studied (1 mm < L < ∞). Both horizontal and vertical surface orientations were examined. A more limited set of experiments were carried out in a low gravity (i. e. low buoyancy) environment to provide a basis of comparison with relevant theoretical analyses. The flame shape, soot formation, fuel condensation, and droplet burning rate were all found to be strongly affected by the proximity of the droplet to the surface. For sufficiently large L the flame was observed to be closed around the droplet throughout burning. As L decreased, the flame was truncated. The droplet burning rate decreased as the droplet was brought progressively closer to the surface (in qualitative agreement with a relevant closed form potential flow solution to the analogous problem of a droplet burning adjacent to an adiabatic surface) and the burning rate of a droplet adjacent to a vertical surface was larger than for a horizontal surface. Surface orientation effects were observed to be absent for burning at low gravity. The extent of sooting as revealed by the flame colour was decreased, and fuel vapours condensed in a lens-like shape on the surface, as L was sufficiently reduced.