Thesis (PhD)--Stellenbosch University, 2011. / ENGLISH ABSTRACT: Natural draught wet-cooling tower rain zone performance can be significantly
enhanced by reducing the mean drop size in the rain zone with the installation of
specially designed grids below the cooling tower fill. Drops enter the rain zone in the
form of a polydisperse drop distribution, dripping from below the cooling tower fill,
comprising relatively large drops. In order to design and optimize a grid for breaking
up these drops, the mechanisms of drop break-up after impingement on the grid
surface, referred to as splashing, straddling and dripping, need to be clearly
understood. Two of these mechanisms, splashing and straddling, are therefore
investigated experimentally using high speed video cameras to measure initial drop
sizes, mass fractions and drop size distributions after impingement on different
horizontal slats covered with a thin layer of water. The following parameters are
varied independently for these experiments: drop fall distance, initial drop size, slat
width and the water film thickness on the slats. Dripping from below the grid, is
investigated theoretically. The effect of drop interaction on the drop size distribution
in the rain zone is also investigated experimentally by measuring the drop distributions
at the top and bottom of rain zones with a height of approximately 7.05 m to 7.65 m
for different inlet distributions. The experimental drop break-up data, numerically
obtained splash drop trajectory data and drop interaction data found in literature are
used to develop a theoretical model of a purely counter flow cooling tower rain zone
with and without installed grids. The model is compared to experimental data and
theoretical data from literature and the predicted thermal and dynamic behaviour of the
rain zone are generally found to be in good agreement with these results. Ultimately,
this model is used for the optimization of the grid layout in terms of variables such as
distance between the grid and the fill, slat width, slat spacing and slat height. It is
found that the best drop break-up is achieved for grids comprising narrower slats with
lower grid porosities as opposed to grids comprising wider slats. For the determined
optimal grid layout it is found that a significant improvement in cooling tower
performance can be achieved. / AFRIKAANSE OPSOMMING: Nat-koeltoringreënsonevermoë kan aansienlik verhoog word deur die druppelgrootte
in hierdie gebied te verklein deur roosters, wat spesifiek vir hierdie doel ontwerp is,
onder die pakkingsmateriaal te installeer. Die inlaatdruppelverdeling aan die bokant
van die reënsone bestaan uit ‘n verdeling van relatief groot druppels wat drip van die
onderkant van die pakkingsmateriaal. Ten einde ‘n rooster te ontwerp en te optimeer
wat hierdie druppels kan opbreek moet die meganismes van druppelopbreking, bekend
as spatting, vurking en drip goed verstaan word. Spatting en vurking is om hierdie rede
eksperimenteel ondersoek, met behulp van hoëspoed videokameras. Die volgende
veranderlikes is onafhanklik verander tydens hierdie eksperimente: valafstand van die
druppel, aanvanklike druppelgrootte, latwydte en die dikte van die lagie water bo-op
die lat. Die dripmeganisme aan die onderkant van die rooster is slegs teoreties
ondersoek. Die effek wat druppelinteraksie in die reënsone het op die druppelgrootte is
ondersoek deur die druppelgroottes aan die bo- en onderkant van ‘n 7.05 m tot 7.65 m
reënsone te meet vir verskillende druppelinlaatverdelings. Die eksperimentele
druppeldata, sowel as numeries berekende data wat die snelheid en trajek van
spatdruppels beskryf, tesame met data vir druppelinteraksies wat uit die literatuur
verkry is word gebruik om ‘n teoretiese model te ontwikkel vir ‘n suiwer teenvloei
koeltoringreënsone met en sonder roosters. Hierdie model word vergelyk met
eksperimentele data en data wat uit die literatuur verkry is en daar is gevind dat daar
oor die algemeen ‘n goeie ooreenstemming is tussen die voorspelde en gemete
termiese en dinamiese gedrag van die reënsone. Uiteindelik word die model gebruik
vir die optimering van die rooster in terme van die volgende veranderlikes: afstand
tussen rooster en pakkingsmateriaal, latwydte, latspasiëring en lathoogte. Daar word
gevind dat beter druppelopbreking verkry word deur gebruik te maak van smaller latte
en ‘n laer roosterporeusiteit. Daar is gevind dat die bepaalde optimale roosteruitleg in
die reënsone van ‘n koeltoring ‘n wesenlike verbetering in koeltoringvermoë tot
gevolg kan hê.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:sun/oai:scholar.sun.ac.za:10019.1/17973 |
| Date | 12 1900 |
| Creators | Terblanche, Riaan |
| Contributors | Reuter, H. C. R., Stellenbosch University. Faculty of Engineering. Dept. of Mechanical and Mechatronic Engineering. |
| Publisher | Stellenbosch : Stellenbosch University |
| Source Sets | South African National ETD Portal |
| Language | en_ZA |
| Detected Language | Unknown |
| Type | Thesis |
| Format | 275 p. : ill. |
| Rights | Stellenbosch University |
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