Characterization of a simple, high Reynolds number droplet generator for combustion studies

A new technique is described for generating droplets in the size range between 240 and 750 μm. The droplets are characterized by Reynolds numbers (in air at standard conditions) greater than 100, hence, with a liquid fuel as the droplet fluid, this technique has applicability in studies of convective droplet burning. The concept behind the technique is simple. A capillary tube is employed to deposit a droplet in the center of a flight tube. Gas flow through this tube detaches the droplet from the capillary, and the droplet accelerates down the tube. Droplets remain stable in the tube for tube diameters of 1.5 to 3 times the droplet diameter. Generation rate is dependent on the liquid flow rate through the capillary and can be varied sufficiently to produce droplets at a rate between 1 and 100 per second. Exit velocities are reported as a function of droplet size for a variety of flight tube lengths and diameters. Water was used in all experiments reported here, but no difficulty should arise in using the technique to produce liquid fuel droplets. The largest droplet generated was 750 μm with an exit velocity of 3.5 m/s yielding a Reynolds number of 175. The smallest droplet produced was 240 μm with an exit velocity of 11 m/s yielding a Reynolds number of 165. The highest Reynolds number achieved in this work, 371, was obtained with a droplet size of 390 μm and an exit velocity of 14.3 m/s. Decreasing the droplet size while increasing exit velocity should be possible.