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Natural draft direct dry cooling system performance at various application scales under steady and transient conditions.
Thesis (PhD)--Stellenbosch University, 2024.
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Stellenbosch : Stellenbosch University
2025
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| _version_ | 1867613892887183360 |
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| access_status_str | Open Access |
| author | Strydom, Wian |
| author2 | Pretorius, J. P. |
| author_browse | Pretorius, J. P. Strydom, Wian |
| author_facet | Pretorius, J. P. Strydom, Wian |
| author_sort | Strydom, Wian |
| collection | Thesis |
| dc_rights_str_mv | Stellenbosch University |
| description |
Thesis (PhD)--Stellenbosch University, 2024. |
| format | Thesis |
| id | oai:scholar.sun.ac.za:10019.1/131919 |
| institution | Stellenbosch University (South Africa) |
| last_indexed | 2026-06-10T12:43:21.794Z |
| license_str | Other — see source repository |
| provenance_str_mv | Harvested via OAI-PMH from SUNScholar — Stellenbosch University Repository |
| publishDate | 2025 |
| publishDateRange | 2025 |
| publishDateSort | 2025 |
| publisher | Stellenbosch : Stellenbosch University |
| publisherStr | Stellenbosch : Stellenbosch University |
| record_format | dspace |
| source_str | SUNScholar — Stellenbosch University Repository |
| spelling | oai:scholar.sun.ac.za:10019.1/131919 Natural draft direct dry cooling system performance at various application scales under steady and transient conditions. Strydom, Wian Pretorius, J. P. Stellenbosch University. Faculty of Engineering. Dept. of Mechanical and Mechatronic Engineering. Cooling systems Heat exchangers -- Thermodynamics Coal-fired power plants Unsteady flow (Fluid dynamics) UCTD Thesis (PhD)--Stellenbosch University, 2024. ENGLISH ABSTRACT: Natural draft direct dry cooling systems (NDDDCSs) provide an efficient alternative to traditional forced draft air-cooled condensers and indirect natural draft dry cooling systems. NDDDCSs combine the advantages of these systems, including reduced system complexity, direct condensing, and lower auxiliary power consumption. This study developed 1-Dimensional (1-D) numerical, 3-Dimensional (3-D) computational fluid dynamics (CFD), and co-simulation models to character- ize the steady-state and transient performance of NDDDCSs with vertically arranged heat exchanger bundles. These models were applied to three appli- cations: a large coal-fired power plant (900 MWt), a concentrated solar power plant (100 MWt), and a water desalination plant (1 MWt). The steady-state 1-D and 3-D CFD models were validated against 3-D CFD results from lit- erature, which in turn matched large-scale experimental data under no-wind conditions. The transient models were validated against their steady-state counterparts. Steady-state 1-D simulation results show that increasing the ratio of to- tal tower height to inlet diameter (H₅/d₃) enhances NDDDCS performance. Larger outlet diameter to inlet diameter ratios (d₅/d₃) can also improve perfor- mance, although this reduces outlet air velocities, possibly resulting in cold in- flow effects. Lower inlet diameter to inlet height ratios (d₃/H₄) increase steam velocity through the condenser tubes, whereas higher ratios reduce steam-side pressure drops, leading to higher average saturated steam temperatures. Thus, optimal NDDDCS design parameters vary with scale. Steady-state 3-D CFD simulations reveal that recirculation reduces ND- DDCS performance. Air streams from the upper and lower tower regions converge to create low-velocity vortices in front of most heat exchanger deltas, causing localised reductions in air mass flow rates and increased inlet air tem- peratures, resulting in reduced outlet air temperatures. Under a 6 m/s cross- wind, NDDDCS performance decreases at all scales, with medium- and small- scale systems losing 40-50% in heat transfer rate. Wind mitigation measures are recommended for all scales. Novel performance recovery points are identi- fied as crosswind speeds reach 18 m/s, 15 m/s, and 9 m/s for large-, medium-, and small-scale NDDDCSs, respectively. Transient 1-D simulations indicate that NDDDCS start-up performance does not constrain overall power plant start-up. Step inputs in steam flow admission of 6.25% and 18% from cold start conditions can be managed by large- and medium-scale systems without exceeding statutory pressure limits. Industry-standard thermal load ramps are also effectively managed. Addition- ally, results highlight substantial full and partial load turbine islanding capa- bilities, with load increases of 150% and 135%, respectively, being managed without exceeding typical dry-cooled turbine pressure protection limits. Transient co-simulation results demonstrate substantial reductions in the permissible cold start-up steam flow admission step input for large- (0.8%) and medium-scale (3.25%) NDDDCSs compared to 1-D results. A no-wind, cold start load ramp presents the worst case for NDDDCS start-up performance, exceeding the statutory pressure limit before plant start-up is achieved. How- ever, NDDDCSs achieve start-up effectively under crosswind conditions. Ad- justment of the selected tower-to-heat exchanger arrangement is recommended to improve start-up performance. Finally, co-simulation results confirm the considerable turbine islanding capacity of the system. AFRIKAANSE OPSOMMING: Natuurlike-trek direkte dro¨e verkoelingstelsels (NTDDVs) bied ’n doeltreffende alternatief tot tradisionele geforseerde-trek lugverkoelde kondensors en indirekte natuurlike-trek dro¨e verkoelingstelsels. NTDDVs kombineer die voordele van hierdie stelsels, soos verminderde stelselkompleksiteit, direkte kondensasie, en laer hulpkragverbruik. Hierdie studie het 1-Dimensionele (1-D) numeriese, 3-Dimensionele (3-D) numeriese vloeidinamika (NVD), en medesimulasie modelle ontwikkel om die bestendige en transiente verrigting van NTDDVs met vertikaal gerangskikte warmteuitruilers te karakteriseer. Die modelle is toegepas op drie skale: ’n grootskaal steenkool-aangedrewe kragstasie (900 MWt), ’n mediumskaal gekonsentreerde sonkragstasie (100 MWt), en ’n kleinskaal waterontsoutingsaanleg (1 MWt). Die bestendige 1-D en 3-D NVD modelle is geverifieer teen 3-D NVD resultate vanuit die literatuur, wat op hul beurt ooreenstem met grootskaalse eksperimentele data. Die transiente modelle is geverifieer teenoor hul ooreenstemmende bestendige modelle. Bestendige 1-D simulasies toon dat die verhoging van die totale toringhoogte tot inlaatdiameter verhouding (H5/d3) NTDDV verrigting verbeter. Groter uitlaatdiameter tot inlaatdiameter verhoudings (d5/d3) kan ook die verrigting verbeter, alhoewel dit uitlaat lugsnelhede verminder, wat moontlik koue invloei effekte kan veroorsak. Laer inlaatdiameter tot inlaathoogte verhoudings (d3/H4) verhoog die stoomsnelheid deur die kondensor buise, terwyl ho¨er verhoudings stoomkantdrukvalle verminder, wat lei tot ho¨er gemiddelde versadigde stoomtemperature. Dus varieer die optimale NTDDV ontwerpparameters met skaal. Bestendige 3-D NVD simulasies beklemtoon dat hersirkulasie NTDDV prestasie verminder. Lugstrome afkomstig vanaf die boonste en onderste toring areas bots om lae-snelheid werwels voor meeste van die warmteruilerdeltas te skep. Dit veroorsaak plaaslike verminderings in lugmassavloeitempo en verhoogde inlaatlugtemperature, wat lei tot verlaagde uitlaatlugtemperature. Onder ’n 6 m/s dwarswind verminder NTDDV verrigting by alle skale, met medium- en klein-skaal stelsels wat ’n 40-50% afname in warmteoordragtempo ervaar. Windversagtingsmaatre¨els word aanbeveel vir alle skale. Nuwe prestasieherstelpunte word geïdentifiseer as die dwarswind onderskeidelik 18 m/s, 15 m/s, en 9 m/s bereik vir groot-, medium-, en klein-skaal NTDDVs. Transiente 1-D simulasies wys dat NTDDV aanvangsverrigting nie die aanvangsbedryf van die kragstasie beperk nie. Stapinsette in stoomtoelating van 6.25% en 18% vanaf koue aanvangstoestande kan hanteer word deur groot- en medium-skaal stelsels sonder om die statutˆere kondensorbuis drukgrense te oorskry. Industriestandaard termiese beladingstempos word ook doeltreffend hanteer. Die resultate beklemtoon aansienlike vol- en gedeeltelike-vrag turbineeiland vermo¨ens, met termiese beladingsverhogings van 150% en 135% wat onderskeidelik hanteer word sonder om tipiese dro¨e-verkoelde turbine drukbeskermingsgrense te oorskry. In vergelyking met 1-D resultate dui transiente medesimulasies aansienlike verminderings aan in die toelaatbare stoom-stapinsette vir groot- (0.8%) en medium-skaal (3.25%) NTDDVs onder koue aanvangstoestande. Die slegste operasionele geval vir die NTDDV is tydens die inbedryfstelling van die stelsel onder ’n geleidelike beladingstempo vanaf koue toestande, onder geen wind. Onder hierdie toestande word die statutˆere drukgrens oorskry voordat die kragstasie in bedryf gestel kan word. Die NTDDV ervaar egter geen sulke probleme onder dwarswind toestande nie. ’n Aanpassing aan die gekose toring-tot-warmteruiler uiltleg word aanbeveel om die aanvangsverrigting van die stelsel te verbeter. Laastens bevestig medesimulasie resultate die aansienlike kapasiteit van die NTDDV tydens turbine-eiland situasies. Doctoral 2025-04-09T13:50:14Z 2025-04-09T13:50:14Z 2024-12 Thesis https://scholar.sun.ac.za/handle/10019.1/131919 Stellenbosch University 200 pages application/pdf Stellenbosch : Stellenbosch University |
| spellingShingle | Cooling systems Heat exchangers -- Thermodynamics Coal-fired power plants Unsteady flow (Fluid dynamics) UCTD Strydom, Wian Natural draft direct dry cooling system performance at various application scales under steady and transient conditions. |
| title | Natural draft direct dry cooling system performance at various application scales under steady and transient conditions. |
| title_full | Natural draft direct dry cooling system performance at various application scales under steady and transient conditions. |
| title_fullStr | Natural draft direct dry cooling system performance at various application scales under steady and transient conditions. |
| title_full_unstemmed | Natural draft direct dry cooling system performance at various application scales under steady and transient conditions. |
| title_short | Natural draft direct dry cooling system performance at various application scales under steady and transient conditions. |
| title_sort | natural draft direct dry cooling system performance at various application scales under steady and transient conditions |
| topic | Cooling systems Heat exchangers -- Thermodynamics Coal-fired power plants Unsteady flow (Fluid dynamics) UCTD |
| url | https://scholar.sun.ac.za/handle/10019.1/131919 |
| work_keys_str_mv | AT strydomwian naturaldraftdirectdrycoolingsystemperformanceatvariousapplicationscalesundersteadyandtransientconditions |