Senthil Kumar, Sundararaj (2009) Computations of bubble dynamics with heat transfer. PhD thesis, Dublin City University.
Abstract
Gas-liquid flows with heat transfer play an important role in many natural and
industrial processes such as combustion, petroleum refining. In particular, the heat transfer enhancement caused by air bubble motion is of practical interest in many industrial
applications ranging from boiling solar collectors to nuclear reactors. A bubble sliding
over a heated obstacle increases heat transfer by displacing liquid, particularly in the wake
region behind the bubble. This, in turn, increases heat transfer from the hot surface, by
continuously bringing cooler liquid into contact with the hot surface and removing hot
liquid from the surface. However, despite its industrial relevance, many important hydrodynamics and heat transfer phenomena associated with bubble flow, such as bubble
formation, bubble coalescence, bubble breakup and bubble wake effect on heat transfer
are still poorly understood.
The primary objective of this research is to develop a numerical tool to simulate
multi-fluid flow problems and assess its suitability to study the enhancement effect of an
ellipsoidal air bubble on heat transfer from a heated flat plate immersed in water, and
the resulting flow patterns.
The Volume-of-Fluid (VOF) method is adopted to model the multi-fluid interface dynamics, where the interface is tracked and advected by Young’s Piecewise Linear
Interface Construction (PLIC) Method. The mass, momentum, and energy conservation equations are solved on a fixed (Eulerian) Staggered Cartesian grid using the Finite
Volume formulation of Semi-Implicit Pressure Linked Equation (SIMPLE) method along
with Krylov subspace and iterative multigrid solvers. In order to consider wall adhesion
effects, while simulating a sliding bubble over an obstacle, the static contact angle model
is adopted.
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Numerous single-and multi-fluid flow problems have been computed and the results have been compared against published experimental, analytical and computational
information. For single phase fluid flow, the code has been validated with the benchmark
lid driven cavity problem, and for single phase heat transfer, buoyancy driven flow of air
with the Boussinesq approximation has been studied. However, convective heat transfer
in water cannot be modelled using the Boussinesq approximation, so a variable thermal
property model has been included and validated against published experimental results.
For multi-fluid flow, the code has been validated against published experimental results
of rising air bubbles of different diameters.
The problem considered is that of sliding bubbles over inclined heated and non
heated flat plates. The rising and sliding bubble shapes and velocity plots are presented
and discussed to study the fluid flow behavior, and to study the dependance of timeresolved surface temperature distribution on bubble dynamics are produced. In order to
investigate the suitability of a two-fluid flow model when the fluid interface is in contact
with a surface, simulations are carried out with three contact angles and assessments of
contact angle effects on bubble dynamics and on wall surface temperature are made. The
effects of plate inclination on heat transfer characteristics are also highlighted. Results
are analysed and discussed in order to gain an understanding of the relationship between
bubble wake interaction and heat transfer performance.
It is found that the rising velocity of an air bubble sliding along the inclined plate
increases monotonously as the inclination angle increases towards the vertical and that
bubbles lift off from the surface with larger plate inclination angles. It is also shown that
the bubble moving through the liquid phase strongly influences the heat transfer rates
occurring between the hot surface and the liquid phase. The most significant effect is
enhanced convection due to an increase in fluid agitation caused by bubble motion as the
bubble acts as a bluff body, displacing the liquid and disrupting the thermal boundary
layer at the hot surface and significantly promoting fluid mixing.
Comparison with experimental results is made in spite of the two dimensional limitation of the computational model. This is justified by the fact that the primary objective
of the study is to assess the suitability of the numerical modelling methods adopted to
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represent the main mechanisms affecting the dynamics behaviour of the sliding bubbles.
It is observed that the predicted temperature drop is more in the computations than in
the experiments. This can be explained by the fact that, in the computations, the calculations are carried out using a 2D model which cannot account for lateral mixing as the
bubble slides in the boundary layer. Conduction from the third direction might be effecting the experimental observations. This brings heat from the surrounding region of the
plate surface in that direction. This effect can not be considered in the 2D computational
model and this is a limitation of the present model. However, it gives an insight into the
underlying mechanism of mixing and vortex-shedding that are responsible for increases in
the heat transfer from the surface and has qualitative agreement with the experimental
results. It is worth mentioning here that it is difficult to gain a good insight into processes taking place in the thermal boundary layer and how the bubble interacts with it
through experiments. Computational results, on the other hand, help to understand the
mechanisms that are responsible for temperature reduction.
Metadata
Item Type: | Thesis (PhD) |
---|---|
Date of Award: | November 2009 |
Refereed: | No |
Supervisor(s): | Delauré, Yan |
Uncontrolled Keywords: | multifluid flow modelling; volume of fluid method; heat transfer; |
Subjects: | Engineering > Mechanical engineering Mathematics > Numerical analysis Mathematics > Applied Mathematics |
DCU Faculties and Centres: | Research Institutes and Centres > Scientific Computing and Complex Systems Modelling (Sci-Sym) DCU Faculties and Schools > Faculty of Engineering and Computing > School of Mechanical and Manufacturing Engineering |
Use License: | This item is licensed under a Creative Commons Attribution-NonCommercial-No Derivative Works 3.0 License. View License |
ID Code: | 14862 |
Deposited On: | 12 Nov 2009 14:06 by Yan Delaure . Last Modified 28 Nov 2023 10:14 |
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