This study addressed the numerical modelling of flow and diffusion in packed beds of mono-sized
spheres. Comprehensive research was conducted in order to implement various numerical
approaches in explicit1 and implicit2 simulations of flow through packed beds of uniform spheres.
It was noted from literature that the characterization of a packed bed using porosity as the only
geometrical parameter is inadequate (Van Antwerpen, 2009) and is still under much deliberation
due to the lack of understanding of different flow phenomena through packed beds. Explicit
simulations are not only able to give insight into this lack of understanding in fluid mechanics, but
can also be used to develop different flow correlations that can be implemented in implicit type
simulations.
The investigation into the modelling approach using STAR-CCM+®, presented a sound modelling
methodology, capable of producing accurate numerical results. A new contact treatment was
developed in this study that is able to model all the aspects of the contact geometry without
compromising the computational resources. This study also showed, for the first time, that the LES
(large eddy simulation) turbulence model was the only model capable of accurately predicting the
pressure drop for low Reynolds numbers in the transition regime. The adopted modelling approach
was partly validated in an extensive mesh independency test that showed an excellent agreement
between the simulation and the KTA (1981) and Eisfeld and Schnitzlein (2001) correlations'
predicted pressure drop values, deviating by between 0.54% and 3.45% respectively.
This study also showed that explicit simulations are able to accurately model enhanced diffusion
due to turbulent mixing, through packed beds. In the tortuosity study it was found that the tortuosity
calculations were independent of the Reynolds number, and that the newly developed tortuosity
tests were in good agreement with techniques used by Kim en Chen (2006), deviating by between
2.65% and 0.64%.
The results from the TMD (thermal mixing degree) tests showed that there appears to be no explicit
link between the porosity and mixing abilities of the packed beds tested, but this could be attributed
to relatively small bed sizes used and the positioning and size of the warm inlet. A multi-velocity test
showed that the TMD criterion is also independent of the Reynolds number. It was concluded that
the results from the TMD tests indicated that more elaborate packed beds were needed to derive
applicable conclusions from these type of mixing tests. The explicit BETS (braiding effect test section) simulation results confirmed the seemingly irregular
temperature trends that were observed in the experimental data, deviating by between 5.44% and
2.29%. From the detail computational fluid dynamics (CFD) results it was possible to attribute these
irregularities to the positioning of the thermocouples in high temperature gradient areas. The
validation results obtained in the effective thermal conductivity study were in good agreement with
the results of Kgame (2011) when the same fitting techniques were used, deviating by 5.1%. The
results also showed that this fitting technique is highly sensitive for values of the square of the
Pearson product moment correlation coefficient (RSQ) parameter and that the exclusion of the
symmetry planes improved the RSQ results. It was concluded that the introduction of the new
combined coefficient (CC) parameter is more suited for this type of fitting technique than using only
the RSQ parameter. / Thesis (M.Ing. (Mechanical Engineering))--North-West University, Potchefstroom Campus, 2012
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:nwu/oai:dspace.nwu.ac.za:10394/8499 |
| Date | January 2011 |
| Creators | Preller, Abraham Christoffel Naudé |
| Publisher | North-West University |
| Source Sets | South African National ETD Portal |
| Language | English |
| Detected Language | English |
| Type | Thesis |
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