טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
Ph.D Thesis
Ph.D StudentMoti Levy
SubjectSpray Formation by Homogeneous Flash Boiling
DepartmentDepartment of Aerospace Engineering
Supervisors Full Professor Sher Eran
Professor Emeritus Levy Yeshayahou


Abstract

Injection systems are integrated in a variety of applications including fuel injection systems, painting and coating processes, medical inhalers and more. In some applications, and particularly in fuel injection systems, the size and size distribution of the droplets are extremely important properties. One of the more effective methods is to inject the fluid under suitable conditions to attain flash boiling atomization; i.e. injecting a pure liquid or a mixture under initial conditions that ensure an evaporation of the liquid or at least one of the components in a mixture during the rapid depressurization caused by the expansion in the orifice. The majority of the studies in this field deal with the most common case in which the bubbles are formed in preferred sites on the orifice walls or near contaminants (heterogeneous nucleation). Studies dealing with homogeneous nucleation, in which the bubbles are formed in the entire bulk, are quite rare. The current study deals with homogeneous flash boiling through a plane orifice.

At the initial stage, an attempt has been made to reach the desired conditions for homogeneous nucleation and a simple visualization technique has been used in order to determine the border between heterogeneous and homogeneous nucleation regimes for a simple one-component fluid flowing through a plane atomizer.

At the following stage a Phase Doppler Particle Analyzer (PDPA) has been used to characterize the properties of a spray that is generated from a simple one-component fluid (R-22) while flowing through a single-orifice and undergoes homogeneous nucleation. The initial pressure and temperature ranges were selected to ensure homogeneous nucleation at the atomizer orifice. The primary conclusions were that the homogeneous nucleation process is strongly affected by the initial liquid temperature; and that the initial pressure has only a minor extent on the nucleation process.

At the last experimental stage, the influence of adding a second component to the pure component at different propellant mass fractions has been examined. The main objective at this stage was to observe the spray characteristics dependence on the mass fractions of the two components. The experiment results indicate that the most dominant parameter that affects the spray characteristics is the mass fraction of the propellant.

At the final stage, an analytical model has been developed to estimate the characteristic droplet diameter in the spray for flash boiling atomization. Since the majority of the studies deal with the heterogeneous case, there are neither suitable correlations nor analytical estimations for the typical droplets' diameter under homogeneous nucleation regime. The model is based on the classic theory of nucleation and bubble growth calculations. The model quite predicts the droplets' characteristic diameter, comparing to the experimental stages; and by calibrating the model, it is possible to even predict the arithmetic mean diameter.

Finally, under homogeneous flash boiling, remarkably small and uniform droplets are achieved; and therefore by using this relatively simple method extremely fine sprays are obtained comparing to the other methods. Thus, it is potentially suitable for fuel injection systems in combustors and engines.