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Internal flow studies for the characterisation and optimisation of an effervescent atomiser

Niland, Andrew 2017. Internal flow studies for the characterisation and optimisation of an effervescent atomiser. PhD Thesis, Cardiff University.
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Abstract

This thesis is concerned with the study of effervescent atomisation, a two-phase gas-liquid spray generation technique that offers many advantages over conventional atomisers. Following a thorough literature review, it was found that the effects of various parameters were disputed between studies or untested with many reports presenting findings without internal flow regime study – in fact, the quantification of gas injection at the aerator was completely unrepresented throughout the literature. Hence, two purpose-built transparent experiment systems were designed and commissioned at Cardiff School of Engineering to characterise the complete effervescent atomisation, from gas injection to spray generation, and to investigate the effect of various design and operating parameters on the internal two-phase flow. All investigations were performed from unbled start-up conditions, to best simulate industrial applications. The results of this work identified that the droplet size decreases with an increase in the mass ratio of input air to liquid (ALR) and a homogenous flow of bubbles within the mixing chamber (bubbly flow) generates a stable spray compared to alternative heterogeneous flow regimes, due to a regular and consistent atomisation process. Hence, an optimal effervescent atomiser configuration would enable a homogenous bubbly flow at the highest ALRs. Further work was performed to quantify the bubbly flow operating range for various independent parameters. It was determined that bubbling at the aerator was encouraged by the injection of an unstable gas-phase into a strong liquid cross-flow, suiting low ALRs, high liquid flow rates (e.g. large exit orifice diameters, high operating pressures), small aerator orifice diameters, high aeration areas and small mixing chamber diameters. However, a conventional flat-end aerator body design was found to be unsuitable for inside-out effervescent atomisation in a vertically downwards orientation, due to the formation of a gas void in the aerator wake – this was found to be a result of aerator bluff body recirculation and gas-phase buoyancy effects. Hence, bubbly flow was only enabled in a vertically upwards orientation or with a streamlined aerator body profile.

Item Type: Thesis (PhD)
Date Type: Completion
Status: Unpublished
Schools: Engineering
Uncontrolled Keywords: Effervescent Atomisation / Atomization; Gas Injection Regimes; Gas-Liquid Two-Phase Internal Flow Regimes; Spray Characterisation; High Speed Shadowography; Phase Doppler Anemometry (PDA).
Date of First Compliant Deposit: 31 May 2018
Last Modified: 31 May 2018 12:37
URI: http://orca-mwe.cf.ac.uk/id/eprint/111843

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