Effect of atomizer scale and fluid properties on atomization mechanisms and spray characteristics
Creators: name: Waind, Travis
Publisher: name: University of Utah
Year: 2016
Subjects: atomization
energy
fluid dynamics
gasficiation
multiphase flow
scaling

description: Atomization is chaos. The breakup of liquid structures by a gas encompasses such a wide range of possible configurations that a definitive mechanism describing breakup in any and all situations is an impossibility. However, when focus is applied, trends can be teased out of experimental data that seem to appropriately describe the action undertaken. These studies sought to better understand atomization, specifically coaxial, two-stream, airblast (or air-assist) atomization in which a central liquid jet is broken up by an annular, high-velocity gas stream. The studies enclosed focused on identifying the effect of changing the atomizer's scale on atomization. While most (but not all) atomization studies only focus on the resulting far-field drop diameters, these studies placed the focus largely on the intermediate structures, in the form of the intact liquid jet (ILJ), while also quantifying the resulting drop diameters. The location and shape of the ILJ constantly change, and on its surface, wavelengths were seen to form and grow, which have been correlated to the resulting drop diameters in previous studies. The studies enclosed herein are unique in that they attempt to apply and explain exiting mechanism-based breakup mechanisms to regimes, fluids, and geometry changes not yet evaluated in the literature. Existing correlations were compared to the experimental data for a range of atomizer geometries, and when they were found lacking, Buckingham-\(\Pi\) theorem was used to develop new correlations for predicting behavior. Additionally, the method developed for the calculation of these parameters for other image sets is included, allowing for easy comparison and value verification. A small-scale, coaxial atomization system was used to atomize water and two silicone oils with air. The atomizers used in these studies had the same general geometry type, but had varying sizes, allowing for the effect of both scale and geometry to be evaluated. These studies quantified instability development and growth along with the resulting spray characteristics, allowing for correlations to be made between the two data sets as the more recent mechanism-based atomization models do. Existing mechanism-based models from the literature are compared to the experimental results, as these existing models have not been evaluated significantly with changing atomizer geometry, high-viscosity fluids, and high flow regimes as was done here. Additionally, two experimental campaigns were undertaken with atomizers used to operate the University of Utah's PDU-scale (process development unit) entrained flow gasifier. The first campaign showed the effect of gas velocity, atomizer load (total flow), and gas-liquid impingement angle on the qualitative cold-flow atomizer performance. These trends are then tied to behavior of the entrained flow gasifier, showing the existence of a minimum required degree of atomization to avoid substantial losses in fuel conversion and efficiency in a gasifier. The second campaign showed the effect of gas flow, liquid flow, and fluid on the quantitative cold-flow atomizer performance. While in the literature, changing fluid properties of Newtonian fluids are shown to have a relatively linear response on atomizer performance, the behavior of non-Newtonian fluids was shown to be much more complex and difficult to predict. The correlations developed for the small-scale atomizers are applied to the micro-hole atomizers and found to be erroneous for the change in atomizer geometry. Buckingham-\(\Pi\) theorem is then used to develop correlations to predict the spray Sauter mean diameter for the micro-hole atomizers. Slurry-fed combustion and gasification systems are but one industrial implementation of atomization. Atomization plays important roles in numerous other industries, and despite decades of study, it is not well understood. This document serves to shed some light on a few small, specific subsets of the topic.