Investigation of break-up process of liquids and downstream spray characteristics in air-blast atomisers

Abstract

The research of this thesis focuses on the study of sprays produced by twin fluid air-blast atomisers and the main objective is to study the liquid jet break-up mechanism and relate it to the downstream spray characteristics. Two different air-blast atomiser geometries are used; coaxial, where the liquid co-flows with the gas stream, and the liquid jet in a gaseous cross-flow. The thesis describes advanced and novel measurements to reveal the temporal and spatial development of the liquid flow and its interaction with the surrounding gas stream. Initially, the break-up process is studied by measuring the characteristics of the continuous liquid jet. Techniques such as electrical conductivity, high-speed shadowgraphy and optical connectivity were used to characterise the atomisation process. The latter is a novel laser-based technique used to illuminate internally the continuous liquid column by introducing a laser beam within the liquid nozzle, while a fluorescent dye in the liquid ensures that the whole volume of the liquid is visualised. The laser light propagates downstream while reflecting on the gas-liquid interface to be interrupted at the break-up position, where the light is scattered and diffuses widely. In the case of a jet in a cross-flow gas stream the fluorescent intensity images were recorded from two different angles to reveal the various features involved in the liquid jet structure. The study of the spatial and temporal characteristics of the instabilities and the developed surface waves on the liquid column can provide information on jet morphology and a better understanding of the physics that elicit the break-up phenomenon. For that purpose, Proper Orthogonal Decomposition (POD) is applied to reveal the various flow scales and elucidate the mechanism of transfer of momentum from the gas to the liquid flow. The most energetic modes are used to describe the jet interface dynamics that may well define the formation of the downstream droplet sizes. Interferometric Laser Imaging for Droplet Sizing (ILIDS) was also used for planar measurements of droplet sizes and velocities. ILIDS images the scattered light from droplets in an out-of-focus mode at different streamwise distances from the nozzle exit to obtain interference fringe patterns associated with each droplet. The spacing of each fringe pattern is proportional to the corresponding droplet diameter. Instantaneous droplet clustering is measured along with the primary atomisation process and the liquid jet break-up characteristics are correlated with the downstream droplet sizes. Several time delays are used between optical connectivity and ILIDS measurements to capture the various classes of droplet sizes that travel with different velocities from the break-up region to the downstream spray location. The small droplets travel faster and move with a velocity similar to the gas flow, in contrast to the larger droplets, which are conveyed to the size measuring region with a lower velocity and, therefore, higher time delays. A conditional correlation method was developed to reduce statistical uncertainties. Negative correlations were found between the break-up length of the liquid jet and downstream number of droplets, indicating that they are inversely proportional. The relation between the break-up length and droplet Arithmetic and Sauter Mean Diameters seems to be more complex since a sinusoidal relation was extracted. The estimated correlation coefficients varied with time delay and a repeatable trend was observed which exposed the coherent behaviour of the break-up process and its frequency, revealing that it is not a random phenomenon, but rather a multifaceted mechanism governed by physical laws.