Author Identifier
Date of Award
2026
Keywords
directional conditional filtering, spray atomization, flash evaporation, droplet dynamics, Large Eddy Simulation (LES), Particle Image Velocimetry (PIV), shadowgraph imaging, multiphase flow, aqueous sprays, desalination systems
Document Type
Thesis
Publisher
Edith Cowan University
Degree Name
Doctor of Philosophy
School
School of Engineering
First Supervisor
Yasir Al-Abdeli 
0000-0001-5672-9448
Second Supervisor
Mehdi Khiadani 0000-0003-1703-9342
Abstract
Spray atomisation plays a critical role in a wide range of engineering applications, including thermal desalination, spray cooling, combustion, and chemical processing, where efficient heat and mass transfer is required. Among these, spray-assisted flash desalination systems have gained increasing attention due to their potential to reduce energy consumption by utilising pressure-driven phase change mechanisms. However, the performance of such systems is fundamentally governed by droplet-scale characteristics, including droplet size, velocity, count, and temperature distribution. Despite extensive research on spray systems, most existing studies rely on ensemble-averaged measurements that do not explicitly account for droplet trajectory and residence time. This limitation obscures important directional effects, particularly in vertically oriented sprays where droplets may move both upward and downward within the same flow field. To address this gap, the present thesis introduces and systematically applies Directional Conditional Filtering as a novel analytical framework to resolve trajectory dependent droplet behaviour in aqueous spray systems under both atmospheric and sub atmospheric pressure conditions.
The research adopts a comprehensive experimental–computational approach combining Particle Image Velocimetry (PIV), Shadowgraph imaging, and Large Eddy Simulation (LES) coupled with the Discrete Phase Model (DPM). This integrated methodology enables detailed spatial and temporal characterisation of droplet dynamics while ensuring strong validation between experimental observations and numerical predictions. The study investigates full-cone pressure-swirl nozzles with spray angles of 65° and 75°, operating at flow rates of 1.75 L/min and 3.5 L/min under atmospheric pressure and reduced chamber pressures of 0.51 and 0.26 bar absolute. The feed water temperature was approximately 25–30°C and remained below the corresponding saturation temperatures at the reduced chamber pressures. Consequently, these cases represented sub-atmospheric or flash-pressure spray conditions rather than thermodynamically developed flash boiling. The effects of spray direction, spray angle, flow rate, ambient pressure and nozzle configuration on droplet behaviour were systematically examined.
The first major contribution of this work lies in demonstrating the significance of directional conditional filtering in resolving hidden spray characteristics. Conventional ensemble-averaged data were shown to mask critical differences between upward- and downward-moving droplets. By separating droplets based on their axial velocity direction, it was found that upward-moving droplets exhibit enhanced breakup, smaller sizes, and greater velocity decay and increased interaction with surrounding air. In contrast, Directional Conditional Filtering has shown that downward-moving droplets maintain higher velocities and larger sizes due to gravity-assisted motion and shorter residence times. These findings highlight the importance of trajectory resolved analysis in accurately characterising spray atomisation.
The second key contribution focuses on the influence of thermodynamic conditions, particularly pressure, on droplet dynamics. Under reduced pressure environments (−0.5 and −0.75 bar gauge), the study observed substantial increases in droplet count and reductions in droplet size due to flash-assisted atomisation and secondary breakup. From raw data, it has been found that droplet counts increased by up to 78% compared to atmospheric conditions, while droplet sizes decreased by up to 30%, indicating significantly enhanced atomisation efficiency. Additionally, upward droplet velocities increased under flash conditions due to rapid pressure-driven expansion, whereas downward droplets exhibited stronger recirculation and negative velocity regions. Because the feed water remained below the saturation temperature corresponding to each chamber pressure, these changes are attributed to the macroscopic effects of reduced ambient pressure, altered droplet–air interaction, evaporation and secondary breakup rather than classical flash-boiling atomisation driven by internal vapour nucleation. These findings demonstrate that ambient pressure plays a dominant role in governing spray behaviour, and that directional filtering is essential for correctly interpreting these effects.
The third major contribution is the systematic comparison between upward and downward spray configurations. The results show that spray direction fundamentally alters droplet transport mechanisms. Upward sprays are characterised by reduced axial velocity, increased residence time, enhanced breakup, and broader radial dispersion. Conversely, downward sprays exhibit higher velocities, more concentrated droplet distributions along the centreline, and reduced breakup intensity due to gravity-assisted motion. These differences become more pronounced with increasing axial distance from the nozzle, highlighting the coupled influence of gravity and aerodynamic forces on droplet evolution. The study also demonstrates that wider spray angles (75° compared to 65°) enhance radial dispersion and produce smaller droplets, while higher flow rates (3.5 L/min compared to 1.75 L/min) increase droplet momentum, count, and penetration length.
In addition to hydrodynamic effects, the fourth major contribution of this thesis was the investigation of spray temperature distribution using directional filtering. From Directional Conditional Filtering data, it has been found that upward-filtered droplets experience more significant temperature gradients than downward-filtered droplets. The interaction between droplet size, velocity, and temperature highlights the coupled nature of heat and mass transfer processes within spray systems. This finding is particularly relevant for applications such as spray cooling and desalination, where thermal performance is directly linked to droplet dynamics.
The fifth contribution extends the analysis to multi-nozzle configurations, which are commonly used in industrial applications. The study shows that nozzle spacing plays a critical role in governing plume interaction, droplet coalescence, and atomisation efficiency. Close nozzle spacing (e.g., 30 mm) leads to strong plume interaction and larger droplets due to coalescence, while wider spacing (e.g., 120 mm) results in largely independent spray plumes. An intermediate spacing (60 mm) was identified as optimal, providing balanced interaction and improved atomisation. Directional Conditional Filtering further revealed that conventional measurements results from non-Directional Conditional Filtered data are dominated by downward-moving droplets, reinforcing the necessity of trajectory-resolved analysis for accurate spray characterisation.
Overall, this thesis provides a unified and comprehensive understanding of aqueous spray behaviour by integrating hydrodynamic and thermodynamic effects within a trajectory resolved framework. The results demonstrate that droplet dynamics are governed by the complex interplay between gravity, aerodynamic forces, pressure-driven atomisation, and droplet–air interactions. The flash-pressure experiments establish a baseline for aqueous spray behaviour under sub-atmospheric conditions but are not presented as fully developed flash boiling experiments because the feed water was not thermodynamically superheated. By introducing and validating directional conditional filtering across multiple operating conditions and configurations, this work significantly advances the current understanding of spray systems and provides a robust methodology for future research.
The findings have important implications for the design and optimisation of spray-based technologies, particularly in desalination, thermal management, and energy systems. By enabling more accurate prediction and control of droplet behaviour, this research contributes to improving system efficiency, reducing energy consumption, and enhancing performance in practical engineering applications.
Access Note
Access to this thesis is embargoed until 7th November 2028
Recommended Citation
Hossain, F. (2026). An experimental and numerical study of aqueous sprays under atmospheric and sub-atmospheric environments. Edith Cowan University. https://doi.org/10.25958/2eyk-s171