This study focuses on the investigation of flow behavior at low Reynolds numbers by the experimental and numerical performance testing of micro-channel heat exchangers. An experimental study of the heat transfers and pressure drop of compact heat exchangers with louvered fins and flat tubes was conducted within a low air-side Reynolds number range of 20 < ReLp < 225. Using an existing low-speed wind tunnel, 26 sample heat exchangers of corrugated louver fin type, were tested. New correlations for Colburn j and Fanning friction f factor have been developed in terms of non-dimensional parameters. Within the investigated parameter ranges, it seems that both the j and f factors are better represented by two correlations in two flow regimes (one for ReLp = 20 – 80 and one for ReLp = 80 – 200) than a single regime correlation in the power-law format. The results support the conclusion that airflow and heat transfer at very low Reynolds numbers behaves differently from that at higher Reynolds numbers. The effect of the geometrical parameters on the heat exchanger performance was investigated. The numerical investigation was conducted for further understanding of the flow behavior at the range of experimentally tested Reynolds number. Ten different heat exchanger geometries with varied geometrical parameters obtained for the experimental studies were considered for the numerical investigation. The variations in the louver angle were the basis of the selection. The heat transfer and pressure drop performance was numerically investigated and the effect of the geometrical parameters was evaluated. Numerical results were compared against the experimental results. From the comparison, it is found that the current numerical viscous laminar models do not reflect experimentally observed transitional two regime flow behavior from fin directed to the louver directed at very low Reynolds number ranging from 20 to 200. The flow distribution through the fin and the louver region was quantified in terms of flow efficiency. The flow regime change was observed at very low Reynolds number similar to the experimental observations. However, the effect of two regime flow change does not reflect on the thermal hydraulic performance of numerical models. New correlations for the flow efficiency � have developed in terms of non-dimensional parameters.

Call number: LD2668 .T4 1984 V46 / Master of Science

Call number: LD2668 .T4 1985 K38 / Master of Science

Thesis (MScIng)--University of Stellenbosch, 2004. / ENGLISH ABSTRACT: An investigation was made into the use of pre-cooling of air with evaporative cooling as a means of improving the performance of air-cooled heat exchangers (finned tube) under conditions of ambient temperatures above maximum design values and during times of increased load. A review of previous research on this subject indicated that the concept is theoretically sound, but that practical application thereof is still limited. It was found that one of the major areas of concern is the wetting of the heat exchanger finned surface and subsequent corrosion. Mathematical models were derived for the behavior of liquid droplets in free air stream conditions and droplets that have penetrated a laminar hydrodynamic boundary layer formed on a flat plate. These two models were combined to determine the behavior of a liquid droplet for its entire lifetime. It was found that evaporation of droplets in a boundary layer resulted in major improvements in heat transfer. In an attempt to prevent droplets from impacting and wetting the finned tube heat exchanger, the use of electrostatically charged water spray was investigated. Experiments were performed to determine the charging performance of a capacitive electrostatic nozzle. It was found that this type of nozzle successfully charged droplets in a spray. Experiments were then performed whereby electrostatic spray was sprayed on to a heat exchanger with a similar electric charge as the droplets. It was found that droplet deposition decreased significantly as the charge on the droplets was increased. However, total prevention of deposition could not be achieved, since the equipment used could not produce high enough voltages. This concept shows some promise, and it is recommended that further research be performed on it. At this stage, no reliable method of evaporative precooling of air has yet been found. The only viable option for cooling capacity shortages at present is the construction of large air-cooled heat exchangers or the addition of wet cooling towers. / AFRIKAANSE OPSOMMING: Voorverkoeling van lug deur middel van verdampingsverkoeling vir die gebruik met droë lug-verkoelde vinbuis warmteruilers is ondersoek as ‘n manier om die verkoelingseffektiwiteit te verhoog gedurende tye van hoë omgewingstemperature en verhoogde las. ‘n Ondersoek van navorsing op die gebied het getoon dat die konsep in teorie moontlik is, maar dat daar nog geen praktiese implementering plaasgevind het nie. Dit blyk dat die benatting en korrosie van die vinbuise een van die hoof probleme is. Wiskundige modelle is afgelei vir die gedrag van ‘n water druppel in ‘n vrye lug stroom en vir ‘n druppel in ‘n laminêre hidrodinamiese grenslaag op ‘n plat plaat. Die twee modelle is gekombineer om die gedrag van ‘n druppel gedurende sy totale leeftyd te bepaal. Die model het getoon dat verdamping van ‘n druppel in die grenslaag ‘n drastiese verhoging in die hitteoordrag koëffisiënt veroorsaak. Die gebruik van elektrostaties gelaaide sproei om te verseker dat die vinbuis warmteruiler droog bly is ondersoek. Eksperimente is uitgevoer om die elektriese laaivermoëns van ‘n kapasitiewe elektrostatiese mondstuk te bepaal. Daar is gevind dat die tipe mondstuk suksesvol is in die laai van druppels. Toetse is toe uitgevoer waartydens gelaaide druppels gespuit is op ‘n warmteruiler met dieselfde lading as die druppels. Daar is gevind dat die duppel neerslag op die warmteruiler merkwaardig afneem namate die lading op die druppels verhoog is. Die warmteruiler kon egter nie totaal droog gehou word nie, aangesien die toerusting gebruik vir die toetse nie ‘n hoë genoeg spanning kon gee nie. Hierdie konsep is belowend, en dit word aanbeveel dat verdere navorsing daarop gedoen word. Op hierdie stadium is daar nog geen betroubare metode gevind om die verkoelings effektiwiteit van lugverkoelde warmteruilers met verdampings verkoeling te bewerkstellig nie. Die enigste sinvolle opsie tans is die kostruksie van groter lugverkoelde warmteruilers of die konstruksie van nat koeltorings.

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.

Thesis (MEng)--University of Stellenbosch, 2000. / ENGLISH ABSTRACT: This report describes the Experimental and Numerical Investigations conducted, during the determination of the structural mechanics of elliptical tubes, viz. the F- and the Atubes. This report is requested in an endeavour to assist Sasol, who is currently busy developing and updating specifications on Air Cooled Heat Exchangers. The objectives of this report therefore are to : (1) determine the strength and the effectiveness of the tube-to-tube-sheet joints. (2) determine the allowable pressure limits on the tubes and (3) investigate the effects of thermal load and vibration on the tube bundle. A series of experiments were conducted to meet these objectives. From a Shear Load experiment it was found that the maximum allowable axial load on the Fand the A-tube is 14.55 kN and 20. 86 kN respectively. Fin Plates were found to have little effect on the bending strength of the tube, w~ilst they have significant effect on the resistance to volumetric expansion of the tube. In fact the more fins per unit length the greater the resistance to volumetric expansion of the tube. These conclusions were drawn from Bending and Pressure Load experiments respectively and supported by FEM analysis of the tube using NASTRAN. When the design pressure limit given by the manufacturer (GEA Air Cooled Systems), were tested using FEM analysis, it was found that they cause no significant deformation and failure of the tubes. Thermal stresses on the tube bundle greatly affect the first tube in the first row (row closest to the flanges) of the tube bundle and it is recommended that provision for thermal expansion be made to reduce these stresses. To reduce vibrations induced by the fan, it is recommended that the natural frequency of the tube bundle must not equal the number of blades (N) times the angular frequency (co) of the fan, or multiples thereof, of each mode of vibration. / AFRIKAANSE OPSOMMING: Hierdie tesis beskrywe die Numeriese en Eksperimentele ondersoeke na die sterkte van elliptiese verkoelingsbuise, naamlik die F- en A- tipes. Hierdie werk sal Sasol, wat huidiglik besig is met die opgradering, van spesifikasies vir lugverkoelde hitteuitruilers van nut wees. Die doelwitte van hierdie tesis is om: (1) die sterkte en effektiwiteit van die buis laste was die buis and die buisplaat verdind, te ondersoek, (2) die toelaatbare druklimiete, sowel as (3) die effek wat hitte en vibrasie op 'n buisbundel het, te ondersoek. 'n Reeks eksperimente is uitgevoer om hierdie doelwitte te bevredig. 'n Skuifbelastingseksperiment het aangedui dat die toelaatbare krag wat op 'n F- en A-buis respektiewelik aangewend kan word 14.55 kN en 20.86 kN is. Die ondersoek het aangetoon dat vinne geen noemenswaardige effek op die buigsterkte van die buise gehad het nie, maar dat dit 'n aansienlike verstywingseffek teen volumetriese verandering as gevolg van interne druk, gehad het. Hierdie gevolgtrekkings is bereik deur die Druk- en Buigtoetse wat numeries bevestig is met die NASTRAN eindige element analise (EEA) pakket. EEA het aangetoon dat die druklimiete voorgeskryf deur die buisvervaardiger (GEA Air Cooled Systems) nie noemenswaardige vervorming van die buise tot gevolg gehad het nie. Termiese belastings het 'n groot invloed op die eerste buise (die rye naaste aan die flense) van 'n buisbundel. Die invoeging van uitsettingslaste word aanbeveel om die spannings hier te verminder. Om vibrasie van die buisbundel te verhoed word aanbeveel dat die resonansfrekwensie van die buisbundel nie gelyk is aan die aantal lemme (N) vermenigvuldig met die rotasie frekwensie (co)van die waaier vir elke vibrasiemode.

Thesis (Ph.D.)--University of Stellenbosch, 2000. / ENGLISH ABSTRACT: The primary purpose of this dissertation is to further the understanding of the influence of plenum chamber aerodynamic behaviour on mechanical draught air-cooled heat exchanger (ACHE) performance. The investigation, which included both forced and induced draught ACHEs, was conducted through the use of a commercially available computational fluid dynamics (CFD) code. A numerical axial flow fan and heat exchanger model that simulates the effect of the axial flow fan and heat exchanger bundle respectively on the flow field within the ACHE was developed and included in the CFD-code through user-programming. Where appropriate the numerical investigation was augmented with experimental data. The plenum chamber aerodynamic behaviour is characterised and included in the draught equations associated with forced and induced draught ACHEs through the introduction of dimensionless plenum chamber recovery and plenum chamber loss coefficient respectively. The influence of changes made to a range of mechanical draught ACHEs on plenum chamber aerodynamic behaviour was investigated. These changes include: • Operating conditions of the axial flow fan. • The height of the plenum chamber in the fan axial direction. • The heat exchanger bundle isothermal flow resistance. • The fan to heat exchanger area ratio. • The axial position of the fan in the fan casing. From the results of the numerical investigation a set of design guidelines are set for both forced and induced draught ACHEs. The design guidelines include recommended values for the dimensionless plenum chamber recovery and plenum chamber loss coefficient to be used in the respective draught equations enabling a more accurate prediction of the operating point of a proposed mechanical draught ACHE. KEYWORDS: air-cooled heat exchanger numerical investigation plenum chamber aerodynamic behaviour / AFRIKAANSE OPSOMMING: Die primere doel van hierdie proefskrif is om die kennis aangaande die invloed van plenum ruimte lugdinamiese gedrag op die werking van meganiese-trek lugverkoelde warmteruilers (LVWRs) te verbreed. Die ondersoek wat geforseerde- asook geinduseerdetrek LVWRs ingesluit het is uitgevoer deur gebruik te maak van In kornmersiele verkrygbare berekeningsvloeimeganika-pakket. In Numeriese aksiaalwaaier- en warmteruilermodel wat die invloed van respektiewelik die aksiaalwaaier en die warmteruiler op die vloeiveld in die LVWR simuleer is ontwikkel en in die berekeningsvloeimeganika-pakket geinkorporeer d.m.v gebruikers-roetines. Waar van toepassing is die numeriese ondersoek aangevul met eksperimentele data. Die plenumruimte lugdinamiese gedrag word gekarakteriseer en ingesluit in die onderskeie trekvergelykings vir geforseerde- en geiduseerde-trek LVWRs deur die daarstelling van In dimensielose plenumruimteherwinnings- en plenumruimteverlieskoeffisient. Die invloed van veranderinge wat aangebring is aan In reeks meganiesetrek LVWRs op die plenumruimte lugdinamiese gedrag is ondersoek. Hierdie veranderinge sluit die volgende in: • Die werking van die aksiaalwaaier. • Die hoogte van die plenumruimte in die aksiale rigting van die waaier. • Die isotermiese lugweerstand van die warmteruiler. • Die waaier-tot-warmteruiler area-verhouding. • Die aksiale posisie van die aksiaalwaaier in die waaierring. In Stel ontwerpsriglyne Vir beide geforseerde- en geinduseerde-trek LVWRs word geformuleer gebaseer op die resultate van die numeriese ondersoek. Die ontwerpsriglyne sluit aanbevole waardes vir die dimensielose plenumruimte-herwinnings- en plenumruimteverlieskoeffisient in wat op hulle beurt aangewend kan word om In meer akkurate aanduiding van die werkspunt van In beplande LVWR te bereken deur gebruik te maak van die toepaslike trekvergelyking. SLEUTEL WQORDE: lugverkoelde warmteruiler numeriese ondersoek plenurnruimte, lugdinamiese gedrag

published_or_final_version / Mechanical Engineering / Doctoral / Doctor of Philosophy

published_or_final_version / abstract / Mechanical Engineering / Master / Master of Philosophy

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