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Novel venturi technology for the purpose of gas-liquid mass transfer

Ryan, Paul 2013. Novel venturi technology for the purpose of gas-liquid mass transfer. PhD Thesis, Cardiff University.
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The introduction of a gas into a liquid occurs in many chemical and biological engineering processes which require a chemical or biological reaction to occur. In the case of aeration, air is introduced into water. The aim of this thesis is to investigate the use of a novel venturi technology, termed the insert that can alleviate the problems of existing technologies such as, restricted depth of use, mechanical wear and failure due to the moving parts and problems with clogging and fouling, whilst providing high aeration efficiency. The inserts tested comprise of a central hub surrounded by a number of aerofoil shaped vanes, which have air orifices located on their surfaces. The vanes create a number of discrete channels, which separate the flow and each channel is representative of a venturi. Three inserts and a regular venturi were tested. The inserts had different angles of attack and a blockage ratio of either 1.5 or 4. Three orientations of the air orifices with respect to the vanes were considered. All inserts were compared to a regular venturi of blockage ratio 1.5, which was made to British Standard 5167-4:2003. Two flow regimes were identified. The first is when a bubbly flow exists throughout the entire length of the downcomer. The second is when a large ventilated cavity forms at the point of air injection, which is typical at low water and higher air flow rates and was more prominent with the lower blockage ratio inserts. The ventilated cavity was seen to have a negative effect in terms of bubble size, specific power and mass transfer performance. The results show that the bubble size produced depends on the air and water flow rates, the flow regime and the insert design. The average bubble size at fixed flow rates is essentially the same (differences within ± 10 %), when a ventilated cavity is present. However, when a full bubbly flow is present throughout the downcomer there were smaller average bubble sizes. Also inducing a swirl in combination with a high blockage ratio resulted in coring of the air at the higher flow rates. Reducing the air to water velocity slip ratio at the throat, by increasing the blockage ratio and the amount of air orifices, reduces the length of a ventilated cavity. This study also examines the hydrodynamic and mass transfer characteristics of the inserts. The inserts were tested in a laboratory scale experimental setup within a 100 mm ID pipe, where they were located above the liquid surface. The results show that increasing the blockage ratio of the insert promoted a smaller mean bubble size, resulting in an increased mass transfer rate. However, the increased blockage ratio results in significantly higher specific power consumption. The effect of insert design on the volumetric mass transfer coefficient was measured using a dynamic method outlined in the ASCE standard ASCE/EWRI 2-06. The results confirmed that a reduced bubble size had a superior performance. The mass transfer coefficient is observed to be up to 50 % larger with a higher blockage ratio at higher flow rates. Computational fluid dynamic simulations are validated against the laboratory scale experimentation to Abstract iii determine the average bubble size, specific power consumption and the mass transfer coefficient, which were found to be within 6, 15 and 25 % of the laboratory scale values respectively. In addition to with these validated models, geometric scaling was investigated for the one of the inserts, where it was geometrically scaled from 100 mm to 190 mm ID. It was found that the geometrically scaled insert had an increased Sauter mean bubble size of 18.6 %, an increased pressure loss of 14.5 % and increased specific power consumption of 18.5 %. Along with the laboratory scale experimental work, hydrodynamic and mass transfer testing was conducted on high strength wastewater and clean water with an insert geometrically scaled up to 150 and 190 mm respectively. A number of key parameters were seen to affect the system performance, including the physical properties of the water, such as dissolved solids, the upstream geometry and mixture outlet. In conclusion it was found that the inserts in this experimental work have an improved aeration performance in comparison to the regular venturi, at the lower air and water flow rates. However, above these lower flow rates, the regular venturi has the best aeration performance. Areas for improvement have however been identified for the inserts, such as decreasing the air to water velocity slip ratio at the throat, where the performance of the redesigned insert can successfully be investigated using CFD simulations.

Item Type: Thesis (PhD)
Status: Unpublished
Schools: Engineering
Subjects: T Technology > TJ Mechanical engineering and machinery
Uncontrolled Keywords: Mass transfer; Two phase flow; Aeration; Ventilated cavity
Date of First Compliant Deposit: 30 March 2016
Last Modified: 10 Oct 2017 15:14

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