Investigating Ultrasonic Atomization as a Seeding Mechanism for PIV at High Pressures

Abstract

The goal of this thesis is to aid in the design of a Particle Image Velocimetry (PIV) system for the SuperTank, a high-pressure wind tunnel at Princeton University. Since most seeding systems use compressed air to produce droplets, they are inefficient for use in high-pressure systems. Another method, ultrasonic atomization, is investigated here. It consists of a vibrating piezoelectric element exciting a fluid boundary to produce droplets and is used commercially in the atomization of low viscosity fluids. Since certain high-viscosity fluids are preferred for PIV, experiments were done to establish a relationship between viscosity and the resulting distribution of droplet diameters made using a mesh atomizer. This distribution was bimodal, with a larger peak at smaller diameters and a smaller peak of larger particles. As the viscosity increased and the amplitude of the piezo vibrations decreased, the larger particles became more prevalent. Boundary curves were also made, delineating the minimum conditions at which atomization occurred. The data suggests a critical amplitude for the piezo to run at to produce droplets, which is based on the fluid parameters and the oscillation frequency. This, in turn, elicits a critical pressure difference across the piezo mesh independent of the static pressure. Future work would involve attempting to replicate this behavior for even higher viscosities, as well as in an increased pressure setting.