Drop Size and Velocity Measurements in an Electrostatically Produced Hydrocarbon Spray

Abstract

Liquid hydrocarbons are difficult to atomize electrostatically at practical flow rates due to their high resistivities and low concentration of charge carriers. However special “charge injection” techniques have been used in this investigation to produce combustible sprays of oils. An experimental investigation of the drop size and velocity distributions within a charged kerosine spray is presented, using a PDA system and photographic methods. Bimodal size distributions are found with a central core of larger drops or ligament formations near the nozzle surrounded by a sheath of smaller drops. Because of the bimodal character the concept of average diameter for the spray is difficult to apply so that there is little practical use defining a relationship between mean drop diameter and mean specific charge without a knowledge of a relationship between charge and size of individual drops. Examination of the velocity component distributions showed the processes which control the two-zone characteristics of the spray. The larger drops have a high inertia and were less deflected by the space charge force within the spray and it is argued that the larger drops possess a smaller specific charge compared with the smaller drops which reinforces the tendency for the large drops to remain along the spray centreline. For the smaller drops the converse is the case, to the extent that at low flow rates their trajectories have a negligible axial velocity component and recirculation toward the earthed injector body is observed.