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

Bredell, J. R. (Johann Richard) (2005-12)

Thesis (MScIng)--University of Stellenbosch, 2005.

Thesis

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.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/21201
This item appears in the following collections: