Experimental Study of Isothermal and Evaporative Sprays

Abstract

The present research focuses on studying an isothermal spray to understand the mechanism of interaction between droplets and turbulent air flow, and an evaporative spray to evaluate the group evaporation of droplets as opposed to single droplet evaporation. The thesis describes the development and application of two novel experimental techniques for simultaneous characterization of droplet and gaseous phases in isothermal and evaporative sprays respectively. Both approaches use the out-of-focus imaging technique, Interferometric Laser Imaging for Droplet Sizing (ILIDS), for planar measurements of droplet size and velocity. The in-focus imaging techniques Particle Image Velocimetry (PIV) and Planar Laser Induced Fluorescence (PLIF) are respectively combined with ILIDS for simultaneous measurement of gas flow characteristics in an isothermal spray and vapour concentration distribution in an evaporative spray. Combination of either of the two optical arrangements results in a discrepancy in the location of the centre of a droplet leading to erroneous identification of the droplets in the PIV/PLIF images. This issue has been addressed and a method is proposed to reduce the droplet positioning error. The coupling between the droplet and gas phases in the isothermal spray is explained by evaluating several statistical quantities, the most important being the spatial correlation coefficients of the droplet-gas velocity fluctuations obtained conditional on droplet size classes. The effect of anisotropy and gravity on the momentum transfer between the two phases are studied. The gas flow eddy structures are extracted by applying Proper Orthogonal Decomposition (POD) on the instantaneous gas velocity data and the selective influence of the large scale eddy structures of the gas phase flow on the droplet-gas flow interaction are examined. In order to study the effect of inter droplet spacing on the droplet evaporation rate, experiments are first performed for the mono-sized droplet stream. The smaller inter droplet spacing of the larger droplet sizes causes the vapour to surround the droplet stream leading to droplet group evaporation. The smaller magnitude of the mean group evaporation number, evaluated at different radial locations in the evaporative acetone spray, suggests the mode of evaporation is in the range of regimes of individual to group evaporation. It is shown that the assumption of uniform droplet spacing in the theoretical expressions for the evaluation of the group evaporation number always leads to overestimation of the group evaporation number.