An analysis of liquid jet instabilities formed from coaxial air-blast atomization of viscous fuels

Abstract

The disintegration of a liquid jet into a spray by a fast gas stream is a process known as air-blast atomization and is widely used for many practical processes. Details of the successive steps of atomiza- tion from the initial instabilities to the eventual formation of droplets remains the subject of debate. Measurements of instabilities are not widely available and limited work has been done on the inuence of viscosity on jet instability. The thesis will provide a comprehensive, experimental study into the statistics of instabilities from the nozzle exit to the liquid jet breakup. This includes an analysis of size and propagation rates of waves acting on the liquid jet and how they are distributed along the axial location. Measurements will also include probability density functions and averaged values of both wavelength, speed, amplitude and acceleration. This thesis demonstrates a novel quantitative approach in which jet instabilities can be measured from use of two-dimensional high speed microscopic backlit imaging. The accuracy of the image processing method was veri_ed and deemed suitable for calculating parameters relevant in liquid jet instability. It was determined from measurements that viscosity plays a critical role in how a wave is formed on the surface of the liquid jet and how it propagates downstream to jet breakup. Several key _ndings were established between the relationship of viscosity and primary atomiza- tion. The measurements suggest that the initial distribution of both wavelength and amplitude with respect to wave propagation veloc- ity is driven by the viscosity of the fuel. It was observed that for a the cases studied all measurements exhibited a similar distribution of both wavelength and amplitude against normalized velocity irrespec- tive of viscosity as the wave propagates to jet breakup. Also, it was evident that less viscous fuels resulted in jet instabilities with smaller wavelengths and larger amplitudes and vice versa.