Combustion of monosized droplet streams in stationary self-supporting flames

Abstract

Free-burning flames on single streams of monosized oil droplets of diameters between 500 and 10 μm were produced by a new experimental technique. The effects of initial drop size, fuel characteristics, and the inter-drop distances on the progress and mechanism of combustion of the drops in stationary free-burning flames have been studied photographically using a high-speed flash technique. The plots of (initial diameter) 2 against burning times give straight lines for all the fuels investigated, but the slopes of the lines (termed as the burning constants) were about one-half of those obtained in experiments with single droplets. The explanation for the reduction of the burning constants is considered to be due to droplet interaction which leads to the oxygen depletion around the droplets. For a drop a particular initial diameter, the squares of diameters do not decrease linearly with time in an initial period during which drop diameter is reduced to 90% of its initial value. The results of the flame sizes of the drops show that the ratios of flame to drop diameter d f/d increase markedly during the progress of combustion. These results contradict the quasi-steady-state model of combustion and point to the fact that combustion is an unsteady-state diffusion-controlled process. The Nusselt numbers of the burning drops are reduced by 80%/90% compared with those reported for drops undergoing negligible mass transfer. Photographs of the stationary flames show that the luminous areas of kerosine flames have been found to be inversely proportional to a modified equivalence ratio S Av /D AV (1− N D ), where S Av is the average drop spacing, D Av the average drop diameter, and N D the number of drops within the flame.