Jesi Ehrhorn

Date of Award

January 2012

Document Type

Thesis

Degree Name

Master of Science (MS)

Department

Mechanical Engineering

First Advisor

William Semke

Abstract

Existing literature on the operation of ultrasonic vibrating mesh nebulizers does not entirely explain the principles by which these devices atomize liquid medication. Many of the studies on this topic assume a spray or extrusion mode of droplet generation, but it can be demonstrated that the high frequency vibration of these devices is sufficient to produce appropriately-sized aerosol droplets. A sufficiently small volume or "thin film" of liquid that is vibrated under correct conditions will produce a fountain of atomized liquid droplets which are appropriately sized for transport and deposition deep into the lungs, which is necessary for inhalation therapy. The formation of standing waves on the surface of this sort of thin film have an oscillating frequency that is roughly half the driving frequency and a wavelength that is equal to a function of the ultrasonic driving frequency, fluid density, and interfacial surface tension. The standing wavelength in particular is shown to be approximately three times the mean droplet diameter that makes up the resulting spray. Also, several studies have shown that cavitation is likely to be present in vibrating films of water which destabilize the capillary waves and may alter the overall droplet diameter distribution of the resulting fountain.

This study validates these phenomena by relating existing concepts of liquid atomization to the operating parameters of known atomizing systems and the Omron Micro Air vibrating mesh nebulizer, along with numerically altering these parameters to show trends in response conditions. A CFD analysis is performed which assists in model verification and reveals that some critical configuration driving amplitude and liquid depth must be fulfilled in order for droplet kinetic energy to exceed fluid resistance energy so that the atomization process can initiate.

The Omron Micro Air operates at an ultrasonic frequency of approximately 180 kHz and is able to maintain a liquid film that is the correct thickness to generate capillary waves leading to droplet ejection. The vibrating mesh component is assumed to be largely responsible for maintaining this film thickness along with acting as a sizing screen to only release droplets that are 3 µm or smaller. The exact function of the vibrating mesh is not analyzed in detail during this study, as the primary focus is to verify and identify parameters of atomization of a thin film of water under the aforementioned operating conditions.

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