The influence of initial diameter on the combustion of single drops of liquid fuel

Abstract

Previous studies of the combustion of single drops have been confined almost entirely to large drops (greater than 1 000μ) and application of the results to practical systems is hindered because the mean drop size in practical sprays is less than one tenth of this value. The influence of initial diameter on the combustion of single drops in the size range 150 to 2 000μ has therefore been measured. The drops were suspended on fine silica fibres in a controlled atmosphere, ignited by a spark discharge and photographed using a modified drum camera. The effect of initial diameter on the combustion characteristics was as follows: (i) At a given diameter during burning, the rate of combustion was lower for drops which were initially small. The results indicate that the burning rate of drops in sprays will be about 25 per cent less than the values predicted from studies of large single drops; (ii) The ratio of flame ‘radius’ to drop radius at a given drop diameter increased with a reduction in initial drop diameter. These effects were explained quantitatively by postulating that the larger drops burnt in an upward stream of gas induced by natural convection, the velocity of which was independent of drop size during burning, but was a function of initial drop diameter. The influence of the oxygen content of the ambient gas on burning rate was significantly greater for large drops than for small ones. Initial diameter did not appear to affect the influence of fuel type on burning rate.