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Lagrangian studies of turbulent mixing in a vessel agitated by a rushron turbine: positron emission particle tracking (PEPT) and computational fluid dynamics (CFD)

Chiti, Fabio (2008)
Ph.D. thesis, University of Birmingham.

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Abstract

Stirred vessels are used in a wide variety of process industries such as fine chemicals, pharmaceuticals, polymers and foods. In order to design efficient mixing vessels, a deep understanding of the blending processes is required. In cases where the fluid is not completely transparent, traditional optical laser based techniques are ineffective. One of the most promising techniques to study opaque systems is based on the detection of a tracer that emits gamma rays. Positron Emission Particle Tracking (PEPT) has been developed at the University of Birmingham and has been used in a wide range of applications including stirred tanks. However, for agitated vessels, any attempt of validation of the PEPT technique versus other techniques cannot be found. Hence, this work aims to validate and explore the potential of Lagrangian data in a well known mixing system such as a standard baffled vessel stirred by a Rushton turbine. As part of the validation, comparison with Eulerian PIV/LDA data has been also undertaken and some underestimation of the high velocities in the system was found in the impeller region. By using a selective interpolation algorithm of the tracer locations, this problem was greatly reduced although a perfect match with optical technique is not feasible. As further contribution to Lagrangian studies of mixing processes, Computational Fluid Dynamics (CFD) simulations have been undertaken to give both Eulerian and Lagrangian velocities and particle paths. However, it has been shown that traditional approaches to Lagrangian numerical simulation are unable to produce good trajectories that can be compared to experimental data. A novel three-step approach was suggested and implemented in order to achieve good paths, which then have been compared to the experimental trajectories. Qualitative and quantitative analysis of experimental Lagrangian data showed that the trajectories are erratic and follow random paths; furthermore, frequency analysis applied to portions of trajectories does not reveal any dominant low frequency in the system. Finally, circulation studies were undertaken in order to characterise mixing processes. This focused on tracking the tracer every time it leaves and returns a control volume proving the value of analysing time and return length distributions, since it was possible to compare the circulation times achieved in PEPT with published work. The trajectography approach used in this work is the first attempt at using trajectories from PEPT as a tool to characterise mixing performance rather than only using the data to find Eulerian velocities and vector plots.

Type of Work:Ph.D. thesis.
Supervisor(s):Bakalis, Serafim and Bujalski, Waldemar and Nienow, A. W.
School/Faculty:Schools (1998 to 2008) > School of Engineering
Department:Department of Chemical Engineering
Subjects:TP Chemical technology
Institution:University of Birmingham
Library Catalogue:Check for printed version of this thesis
ID Code:1607
This unpublished thesis/dissertation is copyright of the author and/or third parties. The intellectual property rights of the author or third parties in respect of this work are as defined by The Copyright Designs and Patents Act 1988 or as modified by any successor legislation. Any use made of information contained in this thesis/dissertation must be in accordance with that legislation and must be properly acknowledged. Further distribution or reproduction in any format is prohibited without the permission of the copyright holder.
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