Seeking Understanding of Acoustics and Spray Character in a Three-Stream Pulsating Transonic Airblast Injector

Abstract

Despite the staggering volume of work in the open literature on primary and secondary atomization, there is nothing known that addresses the mechanisms for, and injector geometry implications for, primary atomization within a self-sustained pulsating transonic three-stream injector. Thus, a computational effort involving 86 simulations, including multiple validation exercises, has been executed in order to develop a numerical foundation and then study the effects of nozzle geometry, numerical methodology, grid resolution, modeled domain extent, feed rates, feed flow modulation, feed flow swirl, feed materials, and operating conditions. This is the first undertaking ever reported to disclose the intense details of transonic pulsating flows within the three-stream injector. Metrics for assessment of acoustics and temporal spray character were numerous. Frequency responses among those metrics implied a common pulsation-driving mechanism. It has been discovered that liquid bridging with the production of a liquid fountain and shocklet-like structures in the retracted (pre-filming) zone, along with localized gas-liquid normal pressure gradients, are responsible for bulk pulsations. These findings were never reported in the literature, thus represent an important contribution of this study. Unexpectedly, a new trend for temporal mean droplet size, when normalized by distance from the nozzle, versus distance from the nozzle has been found, which took a common form among all geometries and feed materials tested. Therefore, there is some value to simulate air-water flows, first, to scope general parameters and characteristics, before modeling more computationally challenging slurry flows. This represents an additional contribution of this work not previously reported in the literature. Newly unveiled strong interactions between feed materials, geometry, and feed rate were discovered. Various combinations of inner nozzle retraction and slurry annular thickness were shown to be advantageous, depending on the goals of the injection system. The importance of either geometry variable for three-stream injectors has not been quantified until now. The predictive power of various modeling frameworks has been assessed for the first time. Axi-symmetric (AS) simulations can successfully predict absolute acoustic details; remarkably and surprisingly, AS simulations can also be used for directional indicators of bulk droplet size. This is an especially powerful revelation given the massive reduction in computational requirements for AS models. Reduced order 3-D models are required for better droplet size estimates. A relatively simple eddy-viscosity turbulent model seems to be adequate for predicting droplet sizes for three-stream injectors, in which the primary energy source is bulk pulsations. For larger two-stream systems (atomization energy is sourced in local shear layer instability development), however, a state-of-the-art hybrid model (newly implemented for this effort) appeared to be necessary to capture the resulting droplet scales. Lastly, droplet size and characteristic flow length scale predictions for two open literature non-Newtonian liquid atomizers were made available.