טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
Ph.D Thesis
Ph.D StudentMoshkovich Yahav
SubjectStudy of Liquid Atomization by Homogeneous Flash
Boiling Mechanism
DepartmentDepartment of Aerospace Engineering
Supervisors Professor Emeritus Yeshayahou Levy
Professor Eran Sher


Abstract

Over the years, different methods have been developed in order to obtain suitable sprays for different applications. The most important characteristics of a spray include the drops diameter, droplet size distribution, sprays shape, flow velocity and mass flux. Former studies show that the flash boiling method is one of the most efficient methods to obtain a spray with very small droplets, with a uniform distribution. This is relevant for many applications such as combustion systems, for which higher combustion efficiency and low pollution are important. Today, flash boiling sprays are widely used to generate fine sprays in air refreshers, insect fighting, painting and some pharmaceutical applications.

Flash boiling is obtained when a liquid at a high pressure is allowed to flow through an orifice towards a low ambient pressure, which is below its saturation pressure, so bubbles nucleation is likely to occur. In fuel atomization systems, the nucleation is followed by bubbles’ growth, leading to liquid jet atomization. Depending on the liquid initial temperature, its critical temperature, the ambient pressure and the rate of pressure drop, two major types of nucleation may occur; nucleation at the liquid/solid interface (heterogeneous nucleation), and nucleation at the liquid bulk (homogeneous nucleation).

The occurrence of flash boiling atomization involves kinetic stability problems that are characterized by two criteria that include a minimal initial liquid temperature and a minimal rate of pressure drop. In this work, we analyze, and suggest analytical expressions for those two criteria. Furthermore, this study presents experimental results of sprays that were generated by flash boiling mechanism in a variety of operation conditions. Those results support the findings of the theoretical analysis.