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Numerical investigation of fan performance in a forced draft air-cooled steam condenser

Thesis (MScIng)--University of Stellenbosch, 2005. / ENGLISH ABSTRACT: Forced draft air-cooled steam condensers (ACSCs) consisting of multiple fan units are used
in direct cooled power plants to condense steam in a closed steam cycle. Axial flow fans
located below an A-frame configuration of finned tube heat exchanger bundles, force ambient
air through the system. In so doing, heat from the condensing steam is rejected to the
environment via the finned tubes. The performance of an air-cooled system is proportional to
the air mass flow rate and the temperature difference between the finned tubes and the
ambient air. A variation in either will directly affect the efficiency of the steam turbines.
Air flow distortions at the fan inlet caused by structures, wind and other fans may result in a
significant reduction in flow rate as well as fan blade vibration. This phenomenon has an
adverse affect on the cooling capacity of an ACSC, and consequently turbine performance,
due to a decrease in air mass flow rate. In this study the effect of inlet flow distortions on fan
performance (i.e. flow rate and fan shaft power) in an ACSC is numerically investigated by
modelling a section (or sector) of such a system using the commercial computational fluid
dynamics (CFD) code, FLUENT. Fan performance at different platform heights, and
corresponding different degrees of inlet flow distortions, is investigated.
The performance of two types of axial flow fans are also compared. The two fans have the
same diameter, number of blades and rotational speed, but feature different blade designs,
and hub-tip-ratios of respectively 0.153 and 0.4. A fan model based on blade element theory,
better known as an actuator disc model, is used to numerically model the fans.
Previous experimental studies have shown that a solid walkway installed along the edge or
periphery of an ACSC platform can significantly increase the flow rate through the fans
situated along the platform edge. The effects of such a walkway, and other windscreens on
fan performance, are numerically investigated.
Numerical predictions correlate with earlier experimental results: the flow rate and fan shaft
power are decreased by inlet flow distortions. It was found that the fan with a hub-tip-ratio of
0.4 was less affected by these flow distortions. The addition of a walkway increased the flow
rate through the edge fan by up to 48 %. It is furthermore shown that wind effects can only be
accurately modelled if the entire ACSC is considered. / AFRIKAANSE OPSOMMING: Geforseerde-trek lugverkoelde kondensators wat bestaan uit ʼn aantal waaier-eenhede, word
in direk-verkoelde kragstasies gebruik om stoom in ʼn geslote stoomkringloop te kondenseer.
Aksiaalvloei-waaiers wat onder ʼn A-raam-konfigurasie van vinbuisbundels geïnstalleer is,
forseer omgewingslug deur die stelsel. Sodoende word die hitte van die kondenserende stoom
aan die omgewing oorgedra deur middel van die vinbuise. Die warmteoordragkapasiteit van
ʼn lugverkoelde kondensator is eweredig aan die massavloei-tempo van die lug, asook die
temperatuurverskil tussen die vinbuise en die lug. ʼn Verandering in enige van dié faktore sal
die benuttingsgraad van die stoomturbines direk beïnvloed.
Lugvloeiversteurings by die waaier-inlate wat veroorsaak word deur geboue, wind en ander
waaiers kan lei tot aansienlike verlagings in vloeitempo deur die waaiers. Sekondêre effekte
soos waaierlemvibrasie kan ook veroorsaak word. In hierdie studie word die effek van inlaatvloeiversteurings
op waaierwerkverrigting (dws vloeitempo en waaierdrywing) ondersoek
deur ʼn seksie (of sektor) van ʼn lugverkoelde kondensator te modelleer deur gebruik te maak
van die kommersiële numeriese vloeidinamika-pakket, FLUENT. Waaierwerkverrigting
word by verkillende platformhoogtes, en gevolglik verskillende grade van inlaatvloeiversteurings,
ondersoek.
Twee verskillende waaiers word ook vergelyk. Die waaiers het dieselfde diameter, aantal
lemme en rotasiespoed, maar het verkillende lem ontwerpe, en naaf-lempunt-verhoudings van
onderskeidelik 0.153 en 0.4. ʼn Waaiermodel wat gebaseer is op lem-element-teorie, beter
bekend as ʼn aksie-skyf-model, word gebruik om die waaiers numeries te modelleer.
Vorige eksperimentele studies het bewys dat ʼn loopvlak om die rand van lugverkoelde
kondensators die vloeitempo deur waaiers aansienlik kan verhoog. Die effek van so ʼn
loopvlak, en ander windskerms word numeries ondersoek.
Numeriese voorspellings stem ooreen met eksperimentele resultate: die vloeitempo en
waaierdrywing word verlaag deur inlaat-vloeiversteurings. Dit is bevind dat die waaier met ʼn
naaf-lempunt-verhouding van 0.4, minder beïnvloed word deur vloeiversteurings. ʼn Loopvlak
het die vloeitempo deur die randwaaier met tot 48 % verhoog. Dit is ook bewys dat windeffekte
alleenlik gemodelleer kan word deur die hele lugverkoelde kondensator in ag te neem.

Identiferoai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:sun/oai:scholar.sun.ac.za:10019.1/21201
Date12 1900
CreatorsBredell, J. R. (Johann Richard)
ContributorsKroger, D. G., Thiart, G. D., Stellenbosch University. Faculty of Engineering. Dept. of Mechanical and Mechatronic Engineering.
PublisherStellenbosch : Stellenbosch University
Source SetsSouth African National ETD Portal
Languageen_ZA
Detected LanguageUnknown
TypeThesis
Formatxvii, 80 leaves : ill.
RightsStellenbosch University

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