Date of Award

11-2014

Document Type

Thesis - Open Access

Degree Name

Master of Science in Mechanical Engineering

Department

Mechanical Engineering

Committee Chair

Shahrdad G. Sajjadi, Ph.D.

First Committee Member

Eric R. Perrell, Ph.D.

Second Committee Member

Laksh Narayanaswami, Ph.D.

Abstract

Fuel is a major cost item for civil transport airplanes. Minimizing fuel consumption of these crafts is a stream of economic opportunity and a stage for innovation, competition and exploration. Regardless of the fuel type, bio or crude oil based, the civil transport stakeholders would like to minimize fuel consumption of their airplanes. The mixing of fuel and air is a critical process for optimum combustion in gas turbine engines due to its high influence on the downstream combustion process. The need for minimizing fuel consumption coupled with the requirement for combustion systems to perform well at all flight conditions puts the spotlight on fuel spray systems. Consequently, there is a need to develop novel fuel spray systems to safely propel airplanes with least amount of fuel possible. The need for these new fuel spray systems equally necessitates new tools capable of modeling these systems. This study is about constructing analytical model capable of predicting the onset of fuel atomization for varied spray angles.

Two-dimensional stability analysis for laminar jet is performed numerically as a foundation for stability studies. An analytical investigation into the stability equation is also carried out using an asymptotic approach. This approach subdivides the jet into three layers; outer, critical and the inner layer. The core of the asymptotic approach is the use of inviscid solution to obtain the viscous solution through successive solving of ODE’s. The end result of successive solution of ODE’s are lengthy algebraic expressions for the solution to the stability equation.

A higher order analytical model, is proposed to predict the onset of instability of liquid sheet at inclined angles relative to the impinging air. In this approach, the instability process is explained by the formation of conical liquid surface at the tip of the nozzle. This liquid surface becomes unstable and ruptures due to instability. Mathematically, a coordinate transformation is made such that coordinate system aligns with the liquid film axis and then the balance of normal stresses at liquid-gas interface is carried out. With this model, it is observed that higher incidence angles results to higher growth rate curves. This means that the disturbances in the liquid sheet will amplify more and leads to faster liquid sheet disintegration and consequently to a rapid atomization. This approach can therefore be used to make a correlation between fuel incidence angle and droplet diameters.

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