Thesis (PhD) -- University of Stellenbosch, 2010. / ENGLISH ABSTRACT: In the design of a modern natural draught wet-cooling tower (NDWCT), structural
and performance characteristics must be considered. Air flow distortions and
resistances must be minimised to achieve optimal cooling which requires that the
cooling towers must be modelled two-dimensionally and ultimately threedimensionally
to be optimised. CFD models in literature are found to be limited to
counterflow cooling towers packed with film fill, which is porous in one direction
only and generally has a high pressure drop, as well as purely crossflow cooling
towers packed with splash fill. This simplifies the analysis considerably as the
effects of flow separation at the air inlet are minimised and fill performance is
determined using the method of analysis originally employed to determine the fill
performance characteristics from test data. Many counterflow cooling towers are,
however, packed with trickle and splash fills which have anisotropic flow
resistances, which means the fills are porous in all flow directions and thus air
flow can be oblique through the fill, particularly near the cooling tower air inlet.
This provides a challenge since available fill test facilities and subsequently fill
performance characteristics are limited to purely counter- and crossflow
configuration.
In this thesis, a CFD model is developed to predict the performance of NDWCTs
with any type of spray, fill and rain zone configuration, using the commercial
code FLUENT®. This model can be used to investigate the effects of different:
atmospheric temperature and humidity profiles, air inlet and outlet geometries, air
inlet heights, rain zone drop size distributions, spray zone performance
characteristics, variations in radial water loading and fill depth, and fill
configurations or combinations on cooling tower performance, for optimisation
purposes. Furthermore the effects of damage or removal of fill in annular sections
and boiler flue gas discharge in the centre of the tower can be investigated.
The CFD modelling of NDWCTs presents various options and challenges, which
needed to be understood and evaluated systematically prior to the development of
a CFD model for a complete cooling tower. The main areas that were investigated
are: spray and rain zone performance modelling by means of an Euler-Lagrangian
model; modelling of air flow patterns and flow losses; modelling of fill
performance for oblique air flow; modelling of air pressure and temperature
profiles outside and inside the cooling tower.
The final CFD results for the NDWCT are validated by means of corresponding
one-dimensional computational model data and it is found that the performance of
typical NDWCTs can be enhanced significantly by including protruding platforms
or roundings at the air inlet, reducing the mean drop size in the rain zone, radially
varying the fill depth and reducing the air inlet height. / AFRIKAANSE OPSOMMING: By die ontwerp van ‘n moderne natuurlike trek nat koeltoring (NTNK), moet
strukturele en werkverrigtings eienskappe in ag geneem word. Wanverdeelde
lugvloei en vloeiweerstande moet geminimaliseer word om optimale verkoeling te
bewerkstellig, wat vereis dat die koeltorings twee-dimensioneel en uiteindelik
driedimensioneel gemodelleer moet word om hulle te kan optimeer. Dit is gevind
dat berekeningsvloeidinamika (BVD of “CFD” in engels) modelle in die
literatuur, beperk is tot teenvloei koeltorings gepak met film tipe pakking, wat net
in een vloeirigting poreus is en boonop gewoonlik ook ‘n hoë drukval het, sowel
as suiwer dwarsvloei koeltorings met spatpakking. Hierdie vergemaklik die
analise aansienlik omdat die effekte van vloeiwegbreking by die luginlaat
verklein word en die pakking se werkverrigtingsvermoë bereken kan word
met die analise metode wat oorspronklik gebruik is om die pakkingseienskappe
vanaf toets data te bepaal. Baie teenvloei koeltorings het egter drup- (“trickle”) of
spatpakkings met anisotropiese vloeiweerstand, wat beteken dat die pakking
poreus is in alle vloeirigtings en dat die lug dus skuins deur die pakking kan vloei,
veral naby die koeltoring se lug inlaat. Hierdie verskaf ‘n uitdaging aangesien
beskikbare pakking toetsfasiliteite, en dus ook pakking karakteristieke, beperk is
tot suiwer teenvloei en dwarsvloei konfigurasie.
‘n BVD model word in hierdie tesis ontwikkel wat die werkverrigtingsvermoë van
NTNK’s kan voorspel vir enige sproei, pakking en reënsone konfigurasie deur van
die kommersiële sagteware FLUENT® gebruik te maak. Hierdie model kan
gebruik word om die effekte van verskillende: atmosferiese temperatuur- en
humiditeitsprofiele, lug inlaat en uitlaat geometrië, lug inlaat hoogtes, reënsone
druppelgrootteverdelings, sproeisone werkverrigtingskarakteristieke, variasie in
radiale waterbelading en pakking hoogte, en pakking konfigurasies of
kombinasies op koeltoringvermoë te ondersoek vir optimerings doeleindes.
Verder kan die effekte van beskadiging of verwydering van pakking in annulêre
segmente, en insluiting van ‘n stoomketel skoorsteen in die middel van die toring
ondersoek word.
Die BVD modellering van NTNK bied verskeie moontlikhede en uitdagings, wat
eers verstaan en sistematies ondersoek moes word, voordat ‘n BVD model van ‘n
algehele NTNK ontwikkel kon word. Die hoof areas wat ondersoek is, is: sproeien
reënsone modellering mbv ‘n Euler-Lagrange model; modellering van
lugvloeipatrone en vloeiverliese; modellering van pakking verrigting vir skuins
lugvloeie; modellering van lugdruk- en temperatuurprofiele buite en binne in die
koeltoring.
Die BVD resultate word mbv van data van ‘n ooreenstemmende eendimensionele
berekeningsmodel bevestig en dit is bevind dat die werkverrigting van ‘n tipiese
NTNK beduidend verbeter kan word deur: platforms wat uitstaan of rondings by
die luginlaat te installeer, die duppelgrootte in die reënsone te verklein, die
pakkingshoogte radiaal te verander, en die luginlaathoogte te verlaag.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:sun/oai:scholar.sun.ac.za:10019.1/5440 |
| Date | 12 1900 |
| Creators | Reuter, Hanno Carl Rudolf |
| Contributors | Kroger, D. G., University of Stellenbosch. Faculty of Engineering. Dept. of Mechanical and Mechatronic Engineering. |
| Publisher | Stellenbosch : University of Stellenbosch |
| Source Sets | South African National ETD Portal |
| Language | English |
| Detected Language | Unknown |
| Type | Thesis |
| Format | 1 v. (various paging) : ill. |
| Rights | University of Stellenbosch |
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