LDV measurements of axial drop velocity in evaporating, dense sprays

Abstract

The axial component of the drop velocity was measured by laser Doppler velocimetry (LDV) within steady hexane-into-nitrogen sprays from a cylindrical nozzle at gas temperatures from 300 to 500 K. Also varied were the injection velocity and the gas-to-liquid density ratio. It was found that the gas temperature had no effect on the mean centerline velocity for distances smaller than 200 and greater than 650 nozzle diameters. At short distances, the core of the spray is not sensitive to the gas temperature; at long distances, the large amount of entrained gas renders inconsequential the degree of vaporization. Beyond 300 nozzle diameters, so much gas has been entrained that the mean axial velocity approaches the fully developed incompressible jet structure. The approach is accelerated by higher gas temperatures and gas-to-liquid density ratios. The axial drop velocity fluctuation amplitude responds to variations in gas temperature similarly to the mean axial drop velocity. Skewness and flatness indicate nearly Gaussian distributions near the axis and rapid increases beyond the half-radius and differ somewhat from those of incompressibl e jets, possibly because small drops vaporize fast and large ones do not follow the flow as closely as the small ones.