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  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.
1

Performance evaluation of air-cooled heat exchangers

Makhema, Thabo 12 1900 (has links)
Thesis (MEng)--Stellenbosch University, 2000. / ENGLISH ABSTRACT: The main objective of this study to determine the air-side heat transfer and pressure drop performance characteristics of commercially available round and elliptical fined tubes. A . computer code to compare the performance of industrial forced and induced draught aircooled heat exchangers (henceforth referred to as f\CHE's) commonly found in the petrochemical industry is also developed. The comparison is extended to include both round and elliptical finned tubes. From the tests conducted, it is found that there is generally an increase in the heat transfer parameter with a decrease in the fin pitch. The decrease in the fin pitch however also results in an increase in the pressure drop across the tube bundle. The performance of the tubes is compared with round reference tubes having circular or plate fins for which performance correlations are available in the literature. The fan power required by an induced draught aircooled heat exchanger is found to be higher than that required by a forced draught heat exchanger rejecting the same amount of heat. / AFRIKAANSE OPSOMMING: Die hoofdoel van hierdie studie is om die werksverrigting van industriële geforseerde trek -- en geinduseerde trek lugverkoelde warmteruilers (LVWR) soos algemeen aangetref in die petrochemiese nywerheid te vergelyk. Warmteruiier bundels word gewoonlik opgebou uit ronde vinbuise. Die werksverrigting van laasgenoemde word vergelyk met die werksverrigting van warmteruiier bundels wat gebruik maak van elliptiese vinbuise. Die werksverrigting van 'n reeks kommersieël verkrygbare ronde en elliptiese vinbuise word deur middel van toetsing bepaal. In die algemeen word 'n toename in die warmteoordrag-parameter waargeneem met In afname in die vinsteek. 'n Toename in die vinsteek gaan egter gepaard met" 'n toename in die drukverlies oor die bundel. Die werksverrigting van hierdie buise word vergelyk met bestaande korrelasies vir die werksverrigting van ronde verwysingsbuise wat gebruik maak van ronde of plaatvinne. Daar is bevind dat die drywing wat benodig word deur 'n waaier vir 'n geinduseerde trek lugverkoelde warmteruiler, heelwat hoër is as in die geval van 'n geforseerde trek lugverkoelde warmteruiler, vir dieselfde hoeveelheid hitte verwydering
2

Thermal management of hybrid electrical vehicles using heat pipes

Swanepoel, Gerhardus 12 1900 (has links)
Thesis (MScEng) -- Stellenbosch University , 2001. / ENGLISH ABSTRACT: In an effort to improve the fuel efficiency and to reduce emission levels of automobiles, the development of Hybrid Electrical Vehicles (HEVs) has been a major focus area of the automotive industry. The Centre of Automotive Engineering (CAE) at the University of Stellenbosch in conjunction with the Electric and Industrial Engineering Departments are currently developing an HEV. For this thesis, however, the focus is limited to the utilization of Pulsating Heat Pipes (PHPs) for the purpose of the thermal management and control of HEV components. As part of the study of PHPs a theoretical model is developed to simulate the heat transfer rate of PHPs. Several experiments were devised to assist in the understanding of the operating principles of PHPs. An experiment was conducted to determine the average thickness of the liquid film deposited at the trailing end of a liquid plug as it moves down a vertically orientated glass capillary tube under gravity. It was found that the average liquid film thickness varied between 100 and 200 |im for water. The movement of a liquid plug in a vertically orientated U-shaped capillary tube due to gravity and heat transfer was experimentally investigated. It was possible to observe the deposition and the evaporation of a liquid film at the trailing end of the liquid plug with the naked eye. The movement of the liquid plug was then theoretically determined and compared to the experimental results. The theoretical model did not predict the exact movement of the liquid plug but the final steady state values was predicted within 7.39%. The movement of a liquid plug in a horizontally orientated straight capillary tube was experimentally investigated. It was noticed that the plug exhibited a wide variety of movement ranging from irregular oscillations with amplitudes of ~ 50 mm to more steady oscillations with amplitudes of ~ 1 mm. Again it was possible to observe the deposition and evaporation of a liquid film at the trailing end of the liquid plug with the naked eye. A PHP was manufactured using glass and filled with pentane as the working fluid. This made it possible to visually observe the fluid motion inside the PHP. It was found that the liquid plugs moved in an irregular oscillatory manner. It was also observed that two plugs Stellenbosch University http://scholar.sun.ac.za/ sometimes coalesce to form a single plug and that a plug can split up to form two separate plugs. The heat transfer rate was determined for a stainless steel closed end PHP and an aluminium closed loop PHP for different working fluids, power inputs, filling ratios and inclination angles. It was found that the overall heat transfer coefficient varied between 100 and 500 W/m K for the stainless steel closed end PHP using water. The overall heat transfer coefficient varied between 0 and 400 W/m2K for the aluminium closed loop PHP using water. It was found that the stainless steel closed end PHP with ammonia as working fluid was not able to transfer heat in the top heat mode. The inside diameter of the tube (3.34 mm) exceeds the required diameter of 2.96 mm which prevents liquid plugs and vapour bubbles to form causing the PHP to operate similarly to a thermosyphon. The overall heat transfer coefficient varied between 170 and 3000 W/m2K. It is concluded that the experimentally determined heat transfer coefficients can be used to design similar PHPs in the future. The theoretical model was used to predict the heat transfer rate of the stainless steel closed end PHP. The experimental heat transfer rate in the top heat mode was 61 W compared to 60 W predicted by the theoretical model. In the bottom heat mode the experimental heat transfer rate was 63 W compared to the predicted value of 90 W. The theoretical model currently only caters for closed end PHPs. It is recommended that the model be extended to include closed loop PHPs. The internal diameter of the PHPs is too great for ammonia to be used as working fluid. It is recommended that a PHP be constructed with dt < 2.5 mm to allow for ammonia to be used as working fluid. Concepts were generated for the thermal management of selected HEV components. A concept was developed for the thermal management of the HEV batteries. It was found that a Stereo-type heat lane can provide promising solutions for the thermal management of Insulated Gate Bipolar Transistors (IGBTs). / AFRIKAANSE OPSOMMING: In ‘n poging om voertuie se brandstof-benuttingsgraad te verbeter en die vlakke van uitlaatgasse te verminder word daar deur die motorvoertuigindustrie gefokus op die ontwikkeling van ‘n Hibriede Elektriese Voertuig (HEV). Die Centre of Automotive Engineering (CAE) aan die Universiteit van Stellenbosch in samewerking met die Elektriese - en Bedryfsingenieurswese Departemente is tans besig met die ontwikkeling van ‘n HEV. Die fokus van hierdie tesis is egter beperk tot die gebruik van Pulserende Hittepype (PHPe) vir die doel van die termiese beheer van HEV komponente. As deel van die studie van PHPe is ‘n teoretiese model ontwikkel waarmee die warmte oordragstempo van PHPe simuleer kan word. Verskeie eksperimente is prakseer om die beginsels waarvolgens PHPe werk, beter te verstaan. ‘n Eksperiment was uitgevoer om die gemiddelde dikte van die vloeistof-film wat gedeponeer word by die agterkant van ‘n vloeistofprop wat in ‘n vertikaal georienteerde kapillere pypie onderworpe aan gravitasie, beweeg. Dit was gevind gewees dat die gemiddelde dikte van die vloeistof-film het gewissel tussen 100 en 200 (im vir water. Die beweging van ‘n vloeistofprop in ‘n vertikaal georienteerde U-vormige kapillere pyp as gevolg van gravitasie en warmte-oordrag was eksperimenteel ondersoek. Dit was moontlik gewees om die deponering en verdamping van ‘n vloeistof-film by die agterkant van die vloeistofprop met die oog te sien. Die beweging van die vloeistofprop was teoreties bepaal en vergelyk met die eksperimentele resultate. Die teoretiese resultate het nie die presiese beweging van die vloeistofprop voorspel nie maar die finale gestadigde posisie was voorspel binne 7.39%. Die beweging van ‘n vloeistofprop in ‘n horisontaal georienteerde reguit kapillere pyp was eksperimenteel bestudeer. ‘n Wye verskeidenheid van beweging van die vloeistofprop was waargeneem wat wissel van onreelmatige ossilasies met amplitudes van ~ 50 mm tot meer reelmatige ossilasies met amplitudes van ~ 1 mm. Die deponering en verdamping van die vloeistof-film by die agterkant van die vloeistofprop was weereens met die oog waargeneem. ‘n PHP was van glas vervaardig en met pentaan gevul as die vloeier. Dit het dit moontlik gemaak om die vloeistofbeweging binne die PHP visueel waar te neem. Dit was gevind gewees dat die vloeistofproppe in ‘n onreelmatige ossilerende wyse beweeg. Dit was ook waargeneem dat twee vloeistofproppe somtyds saamsmelt om een vloeistofprop te vorm en dat ‘n vloeistrofprop kan opbreek om twee aparte vloeistofproppe te vorm. Die warmte oordragstempo was bepaal van ‘n vlekvrye-staal geslote ent PHP en van ‘n aluminium geslote lus PHP vir verskillende vloeiers, drywing insette, vulverhoudings en inklinasie hoeke. Die algehele warmte oordragskoeffisient was tussen 100 en 500 W/m2K vir die vlekvrye-staal geslote end PHP gevul met water en tussen 0 en 400 W/m2K vir die geslote lus aluminium PHP gevul met water. Dit was gevind gewees dat die vlekvrye-staal geslote ent PHP gevul met ammoniak nie in staat was om warmte oor te dra in die boonste verhittingsmodus nie. Die binnediamter van die pyp (3.34 mm) is groter as die vereiste diameter van 2.96 mm wat verhoed dat vloeistofproppe en gasborrels gevorm word wat veroordaak dat die PHP soortgelyk aan ‘n termoheuwel werk. Die algehele warmte oordragskoeffisient was tussen 170 en 3000 W/m2K. Die eksperimentele bepaalde waardes vir die warmte oordragskoeffisiente kan gebruik word vir ontwerpdoeleindes van soortgelyke PHPe in die toekoms. Die toeretiese model was aangewend om die warmte oordragstempo van die vlekvrye staal geslote end PHP te bepaal. Die eksperimentele warmte oordragstempo in die boonste verhittingsmodus was 61 W in vergeleke met die teoretiese waarde van 60 W. In die onderste verhittingsmodus was die eksperimentele warmte oordragstempo 63 W in vergeleke met die voorspelde waarde van 90 W. Die teoretiese model kan huidiglik slegs geslote ent PHPe simuleer. Dit word aanbeveel dat die model uitgebrei word sodat dit geslote lus PHPe ook kan simuleer. Die binne diameter van die PHPe is te groot om ammoniak te gebruik as vloeier. Dit word aanbeveel dat ‘n PHP vervaardig word met d, < 2.5 mm sodat ammoniak ook gebruik kan word as vloeier. Verskeie konsepte was gegenereer vir die termiese beheer van geselekteerde HEV komponente. ‘n Konsep was ontwikkel vir die termiese beheer van die HEV batterye. Dit was gevind gewees dat ‘n Stereo-type heat lane belowende oplossings kan bied vir die termiese beheer van Insulated Gate Bipolar Transistors (IGBTs).
3

Thermal management of an axial flux permanent magnet machine considering heat pipes

Scowby, Seath 03 1900 (has links)
Thesis (MScEng) -- Stellenbosch University, 2003. / ENGLISH ABSTRACT: Axial Flux Permanent Magnet (AFPM) machines have become attractive because of significant improvements in permanent magnets over the past decade, improvements in power electronic devices, and the ever increasing need for more efficient machines in electric vehicle systems. In comparison with the cylindrical radial flux motor, the AFPM machine is better in a number of aspects: short frame; compact construction; high efficiency; brush less construction; good starting torque and high-power density. The common modes of failure and typical operating conditions of AFPM machines are discussed further. The focus of this research project is a prototype AFPM machine developed by the Electrical Engineering Department of The University of Stellenbosch. The machine considered has a power rating of 300 kW and an operating efficiency of 95 % at a speed of 2300 rpm. This specific machine is used as an example to illustrate the thermal characteristics of geometrically similar AFPM machines. The thermal characterization was achieved with the use of two numerical computer models. Firstly a fluid model was specially developed and experimentally verified. The objective of the fluid model was to calculate the mass flow rate of air through any geometrically similar AFPM machine. The fluid model was further used to investigate the effects of different magnet thickness and axial gaps between the stator and the rotor plates on the mass flow rate of air through the machine. The fluid model was verified with experimental testing that was done on a half-scale Perspex model. During the experimental testing the magnet thickness was varied between 2.5 mm, 5.0 mm, and 7.5 mm along with axial gaps of 6.5 mm, 7.5 mm, 8.5 mm, and 9.5 mm. The fluid model showed a correlation to within 10 % of the experimental mass flow rates. The results of these tests showed that the magnet thickness and axial gap between the stator and the rotor plates had no significant effect on the mass flow rate of air. The fluid model was based on one-dimensional, steady-state, and incompressible flow. The second numerical computer model was a thermal model. This model was used to calculate the transient temperature response of the AFPM machine. The model was based on a twodimensional transient finite difference solution technique. Experimental temperatures taken from the prototype AFPM machine were used to verify the thermal model. Correlations between the experimental and theoretical temperatures were within 5.8 % of each other. The thermal model was used to investigate the effect of geometrical changes on the temperatures in the AFPM machine. It was found that these geometrical changes had no significant effect on the temperatures in the AFPM machine. It was also established that increasing the air mass flow rate over about I kg/s had no further effect on lowering the temperatures. The stator was also identified as being the most critical component as it reached its maximum temperature limit before any other component. Heat pipes were considered as an alternative thermal management technique. The location of the heat pipe was limited to the stator. Further simulations were done to investigate the effect of the heat pipe properties on the amount of heat removed from the stator. Recommendations were made concerning the thermal management of the current and possible future prototype AFPM machines. It was recommended that a further more detailed investigation into the use of heat pipes be considered. This recommendation is substantiated by the fact that in this research project only one type of heat pipe was considered and its location was limited to within the stator. / AFRIKAANSE OPSOMMING: AFPM masjiene het meer aantreklik geword weens betekenisvolle verbeteringe in permanente magnete gedurende die laaste dekade, verbeteringe in elektroniese toestelle en die vraag na meer effektiewe masjiene in elekriese voertuigstelsels. Die AFPM masjien is beter as die Silindriese Radiale Fluksie Motor wat die volgende aspekte betref: die kort raamwerk; kompakte konstruksie; hoe effektiwiteit; borsellose konstruksie; goeie aanvangsdraaimoment; en hoe-krag digtheid. Die algemene vorms van faling en ook die tipiese werkstoestande van die AFPM word verder bespreek. Hierdie navorsingsprojek fokus op die prototipe AFPM masjien wat ontwikkel is deur die Elektriese Ingenieurs Departement van die Universiteit van Stellenbosch. Die masjien onder bespreking wek 300 kW per uur op en is 95% effektief teen 'n spoed van 2300 rpm. Hierdie masjien word gebruik om die termiese kenmerke van geometries-gelyksoortige masjiene te illustreer. Die termiese eienskappe is bepaal deur die gebruik van twee numeriese rekenaarmodelle. Eerstens is 'n vloeistofmodel spesiaal ontwerp en eksperimenteel geverifieer. Die doel van die vloeistofmodel was om die massa vloeitempo van lug deur enige geometries-gelyksoortige AFPM masjien te bereken. Die vloeistofmodel is verder gebruik om die uitwerking van verskillende magneetdiktes en aksiale gapings tussen die stator en die rotorplate op die massa vloeitempo van lug deur die masjien te ondersoek. Die vloeistofmodel is geverifieer deur eksperimentele toetsing wat gedoen is op 'n halfskaal Perspex model. Tydens die toetsing het magneetdiktes gewissel tussen 2.5 mm, 5.0 mm en 7.5 mm en die aksiale gapings tussen 6.5 mm, 7.5 mm en 9.5 mm. Die vloeistof model het 'n korrelasie van binne 10 % van die eksperimentele massa vloeistempo getoon. Die resultate van hierdie toetse het getoon dat die magneetdiktes en die aksiale gapings tussen die stator en die rotorplate geen noemenswaardige uiterking op die massa vloeitempo van lug gehad het nie. Die vloeistofmodel is gebaseer op een-dimensionele, gestadigde, onsamedrukbare vloei. Die tweede numeriese model was 'n termiese model. Hierdie model is gebruik om die transiente temperatuur respons van die AFPM masjien te bereken. Die model is gebaseer op 'n tweedimensionele, transiente eindige-verskil oplossingstegniek. Eksperimentele temperature gemeet op die prototipe AFPM masjien is gebruik om die termiese model te verifeer. Die eksperimentele en teoretiese temperature het binne 5.8% met mekaar gekorrelleer. Die termiese model is gebruik om die uitwerking van geometriese veranderinge op die temperatuur in die AFPM masjien te ondersoek. Daar is gevind dat hierdie geometriese veranderinge geen noemenswaardige uitwerking op die temperature van die AFPM masjien gehad het nie. Daar is ook vasgestel dat 'n vermeerdering in die lug massa vloeitempo yerby I kg/s geen verdere uitwerking het op die verlaging van die temperatuur gehaad het nie. Die stator is ge-identifiseer as die mees kritiese komponent aangesien dit sy maksimum temperatuur limiet bereik het voor enige ander komponent, Hittepype is oorweeg as 'n alternatiewe termiese bestuurstegniek. Die plasing van die pype is tot die stator beperk. Verdere simulasies is uitgevoer om die uitwerking van die hittepyp eienskappe op die hoeveelheid hitte wat verwyder word van die stator te ondersoek. Aanbevelings is gemaak m.b.t die termiese bestuur van die huidige en moontlike toekomstige prototipes van AFPM masjiene. Daar is aanbeveel dat daar in meer besonderhede ondersoek ingestel word na die gebruik van hittepype. Die rede hiervoor is dat daar in hierdie studie net gebruik gemaak is van een tipe hittepyp en dat die plasing daarvan beperk is tot binne die stator.
4

Second order analyses methods for stirling engine design

Snyman, H. 03 1900 (has links)
Thesis (MScIng( Mechanical Engineering)--University of Stellenbosch, 2007. / 121 Leaves printed single pages, preliminary pages a-l and numbered pages 1-81. / ENGLISH ABSTRACT:In the midst of the current non-renewable energy crises specifically with regard to fossil fuel, various research institutions across the world have turned their focus to renewable and sustainable development. Using our available non.renewable resources as efficiently as possible has been a focal point the past decades and will certainly be as long as these resources exist Various means to utilize the world's abundant and freely available renewable energy has been studied and some even introduced and installed as sustainable energy sources, Electricity generation by means of wind powered turbines, photo-voltaic cells, and tidal and wave energy are but a few examples. Modern photo-voltaic cells are known to have a solar to electricity conversion efficiency of 12% (Van Heerden, 2003) while wind turbines have an approximate wind to electricity conversion efficiency of 50% (Twele et aI., 2002). This low solar to electricity conversion efficiency together with the fact that renewable energy research is a relatively modern development, lead to the investigation into methods capable of higher solar to electricity conversion efficiencies. One such method could be to use the relatively old technology of the Stirling cycle developed in the early 1800's (solar to electricity conversion efficiency in the range of 20.24 % according Van Heerden, 2003). The Stirling cycle provides a method for converting thermal energy to mechanical power which can be used to generate electricity, One of the main advantages of Stirling machines is that they are capable of using any form of heat source ranging from solar to biomass and waste heat. This document provides a discussion of some of the available methods for the analysis of Stirling machines. The six (6) different methods considered include: the method of Beale, West, mean-pressurepower- formula (MPPF), Schmidt, idea! adiabatic and the simple analysis methods. The first three (3) are known to be good back-of-the-envelope methods specifically for application as synthesis tools during initialisation of design procedures, while the latter three (3) are analysis tools finding application during Stirling engine design and analysis procedures. These analysis methods are based on the work done by Berchowitz and Urieli (1984) and form the centre of this document. Sections to follow provide a discussion of the mathematical model as well as the MATlAB implementation thereof. Experimental tests were conducted on the Heinrici engine to provide verification of the simulated resutls. Shortcomings of these analyses methods are also discussed in the sections to follow. Recommendations regarding improvements of the simulation program, possible fields of application for Stirling technology, as well as future fields of study are made in the final chapter of this document. A review of relevanl literature regarding modern applications of Stirling technology and listings of companies currently manufacturing and developing Stirling machines and findings of research done at various other institutions are provided. / AFRIKAANSE OPSOMMING:Die tempo van uitputling van die wereld se nie-hernubare energiebronne die afgelope jare het aanleiding gegee daartoe dal daar loenemend fokus toegespits word op die ontwikkeling van hernubare alternatiewe. Meer doeltreffende benutting van die wereld se nie-hernubare energie is reeds 'n fokus punt, vir navorsers reg oor die wereld, vir die afgelope dekades. Die aarde se oorvloedryke hernubare energie bronne word reeds met verskeie metodes ontgin. Die omskakeling van wind-, son- en gety energie na elektrisieteids is net 'n paar voorbeelde. Die effektiwiteid van sonkrag na elektrisietyds omskakeling van moderne fotovo!la'iese selle is in die orde van 12% (Van Heerden, 2003) terwyl die doeltreffendeid van wind energie na elektrisiteit omskakelling in die orde van 50% (Twele et at, 2002) is. Hierdie relatief lae omskelings doeltreffendeid van sonkrag na elektrisietyd, tesame met die feit dat die hernubare industrie nag relatief jonk is, lei lot die soeke na ander meer doellreffende moontlikhede Die Stirling siklus is nie 'n mod erne beginsel nie, maar die toepassing daarvan veral in die hernubare energie industrie is wei 'n relatiewe nuwe beg rip, veral in teme van die omskakeling van sonkrag na elektriese energie (gemiddelde sonkrag na lektriese energie omskakelings doellreffendeid in die orde van 20-24% is gevind deur Van Heerden, 2003). Die omskakeling van lermiese energie na meganiese energie is sekerlik die hoof uitkomsle van die Stirling siklus, alhoewel dit ook toepassing vind in die verkoefingsindustrie. Die feit dat die Stirling siklus van enige vorm van termiese energie (bv. son. biomassa, asook hilte geproduseer as byproduk tydens sekere prosesse) gebruik kan maak. is een van die redes wat die tegnologie 56 aanloklik maak, spesifiek !.o,v. die hernubare energie sektor. Ses (6) metodes vir die analise van die Stirling siklus word in hierdie dokument bespreek. Dit slui! die volgnde in: Beale-, West-, die gemiddelde-druk-krag-metode (GDKM), Schmidt-, adiabatiese- en die eenvoudige analise melodes. Die eerste drie (3) metodes is handige berekenings metodes Iydens die aanvangs en sinlesefase van Stirling enjin ontwerp, lerwyl die laaste drie (3) meer loegespils is op die volledige ontwerps- en analisefases gedurende die Stirling eniin ontwerps proses. Die drie (3) analise melodes is gebaseer op die werk wat deur Berchowitz en Urieli (1984) gedoen is en maak die kern van die dokument uit. Die wiskundige model, implimentering daarvan in MATlAB, sowel as die eksperimentele verifieering van die resultate word bespreek. Tekortkominge van die analise metodes word ook aangespreek in elke hoofsluk. Moontlikke verbeterings len opsigte van die verskeie aannames word in die finale hoofsluk van die dokumenl aangespreek. Verskeie voorgestelde riglings vir toekomslige navorsings projekle word ook in die finale hoofstuk van die dokument genoem. 'n Kort oorsig van die relevanle lileraluur in verband mel huidige loepassings van die Stirling legnologie, asook die name van maatskappye wal tans hierdie tegnologiee ontwikkel en vervaardig, word genoem.
5

The effect of ionization of spray on the wetting characteristics of an adiabatically cooled heat exchanger

Esterhuyse, Bernard de Waal 12 1900 (has links)
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.
6

Thermo-economic analysis of a french fries processing plant at Lambert's Bay

Potgieter, Johan 12 1900 (has links)
Thesis (MScIng)--University of Stellenbosch, 2004. / ENGLISH ABSTRACT: In the literature study energy efficiency is discussed in general, as well as certain critical areas of importance to this study. In addition, measuring and monitoring equipment, and energy inefficiencies in steam and refrigeration systems are reviewed briefly. In the energy analysis, an energy audit strategy is discussed in general. A walkthrough audit of the plant was conducted with specific focus on visible losses in the steam, refrigeration and production line systems. An energy analysis, as discussed in Chapter 3, indicates the main energy consumers, with steam being the biggest consumer of energy. The main consumers of refrigeration energy are the cold stores, flow freezer and blast freezer. Energy consumption in the cold stores can be minimised mechanically, while refrigeration energy of the flow freezer and blast freezer can be minimised through the modification of production activity. The main consumers of steam at the processing plant are the dryers, oil fryer, blanchers and steam peeler. Improved energy savings at the dryers can be obtained through optimisation of moisture and heat transfer mechanisms, while the energy of the blanchers and steam peeler can be combined by means of heat exchangers. The transfer of waste energy by means of a finned-tube heat exchanger from the steam peeler to the blanchers was investigated. The newly installed coal boiler shows capacity for improving the quality of steam, as well as efficiency, by incorporating an economiser and separator for improving steam quality, automatic TDS control and blow-down heat recovery. The product life cycle is discussed considering future automation that could lead to energy and labour savings. Lastly the utilisation of product waste as a future research subject is discussed. A confidentiality agreement was entered into with Oceana. / AFRIKAANSE OPSOMMING: In die literatuurstudie word energie-effektiwiteit oor die algemeen bespreek, asook sekere kritieke areas wat vir die ondersoek van belang is. Hierbenewens word meettoerusting vlugtig bespreek, asook energie-oneffektiwiteite in die verkoeling- en stoomstelsels. In die energie-ontleding word ’n energie-ouditstrategie in die breë bespreek. ’n Deurstapoudit van die aanleg is uitgevoer met spesifieke fokus op sigbare verliese in die verkoelingstelsel, stoomstelsel en produksielyn. Die samestelling van energieverbruik in die aanleg word uiteengesit in Hoofstuk 3, waar dan ook aangedui word dat stoom die grootste energieverbruiker is. Die hoofverbruikers van verkoelingsenergie is die koelkamers, die deurvloei-vrieskamer en die blitsvrieskamer. By die koelkamers kan verliese meganies geminimeer word, terwyl veranderinge aan produksie-aktiwiteite energieverbruik by die deurvloei-vrieskamer en die blitsvrieskamer kan verlaag. Die hoofverbruikers van stoom by die verwerkingsaanleg is die droërs, oliebraaier, blansjeerders en stoomskiller. Energie-effektiwiteit by die droërs kan verhoog word deur vog- en warmte-oordrag optimaal te laat plaasvind deur korrekte instandhoudingsprosedures. Energieverbruik by die stoomskiller en blansjeerders kan deur middel van warmteruilers gekombineer word. ’n Ondersoek na die energie-integrasie van die stoomskiller en die blansjeerders is dan ook uitgevoer. Die pas geïnstalleerde steenkoolketel toon ruimte vir die verhoging van energie-effektiwiteit deur die daarstel van ’n ekonomiseerder – ’n skeier wat die gehalte van die stoom verbeter, outomatiese TDS-beheer en afblaasherwinning. Die produk se lewensiklus word bespreek ten einde toekomstige outomatisering te motiveer in terme van energie- en arbeideffektiwiteit, asook die uitskakeling van onnodige blootstelling van die produk aan omgewingstemperature. Laastens word die herwinning van afvalstowwe as ’n toekomstige navorsingsprojek bespreek. ’n Vertroulikheidsooreenkoms is met Oceana gesluit en word eerbiedig.
7

Technical and economic evaluation of the utilisation of solar energy at South Africa's Sanae IV Base in Antarctica

Olivier, Jurgen Richter 12 1900 (has links)
Thesis (MScEng (Mechanical and Mechatronic Engineering))--University of Stellenbosch, 2005. / There are numerous challenges that have to be overcome in order to generate the electrical and thermal energy required to power Antarctic research stations in a technically, economically and environmentally suitable manner. Consequently the costs associated with generating energy at these latitudes are high, and ways are constantly being sought to improve energy generation methods and protect the pristine environment. These endeavours are strongly encouraged by the Antarctic Treaty. This thesis aims to investigate the technical and economic feasibility of using solar energy at South Africa’s SANAE IV (South African National Antarctic Expedition IV) station in Antarctica. The idea of using solar energy in Antarctica is not novel, and as is shown a number of stations have already capitalised on opportunities to generate savings in this manner. Similarly, at SANAE IV, there exists the opportunity to alleviate an increased summer energy load on the station and reduce diesel consumption through the proper implementation of such a system. There is also ample scope to use wind energy, which would have a marked positive impact on the base’s operation. The data used in this thesis was obtained mainly during the 2004/2005 takeover expedition to South Africa’s SANAE IV station in Antarctica. Included are measurements of total and diffuse radiation that were measured during the months of January and February 2005, and which form an important part of the investigation. Since there are currently no radiation sensors, or any historical record of measured radiation at the station, the only measured data available from SANAE IV was the data recorded during the 2004/2005 takeover expedition. By further collecting archived values of fuel consumption, electricity generation and load profiles, an energy audit of the station was also completed during the 2004/2005 takeover expedition. The expected savings that could be generated by solar systems were calculated by considering the use of both photovoltaic and solar thermal devices at the South African station. The 40 kW photovoltaic system that was investigated was able to significantly reduce the load on the dieselelectric generators, however it was only possible to fully recover the initial costs sunk into commissioning the system after 21 years. The installation of such a system would equate to a Net Present Value of 302 915 Rand at the end of the 25 year system lifetime (assuming a real hurdle rate of 8 % and fuel price escalation rate of 5 %), saving 9 958 litres of diesel annually generating energy at a cost of 3.20 Rand/kWh. It should be noted, however, that under more ideal conditions (i.e. less attractive alternative investment opportunities, higher fuel price escalation rates and a stronger emphasis on environmental concerns) investment into a photovoltaic system could potentially breakeven after approximately 10-15 years, while simultaneously significantly improving base operation. Furthermore, it was found that a flat-plate solar thermal collector utilised with the snow smelter at SANAE IV is better suited to generating savings than photovoltaic devices. The average cost of generating electricity after commissioning such a system with a 143 m2 collector field would be approximately 3.13 Rand/kWh, as opposed to the 3.21 Rand/kWh of the current diesel-only system, and would realise an annual fuel saving of approximately 12 245 litres. The system would arrive at a breakeven point after approximately 6 years, and represent a Net Present Value of 2 148 811 Rand after 25 years. By further considering environmental factors such as the cost of removing soiled snow from Antarctica and diesel fuel emissions the magnitude of the net present savings would increase by approximately 500 000 Rand over the expected 25 year project lifetime. The opportunity to install a solar energy system at SANAE IV therefore warrants action. There is potential not only to generate savings over the operational lifetime but also to preserve the environment in accordance with the desires of the Antarctic Treaty. It is firmly believed that with careful planning and implementation such a project can and should be successfully undertaken. / Centre for Renewable and Sustainable Energy Studies
8

The design of a low-noise rotor-only axial flow fan series

Van der Spuy, Sybrand Johannes 12 1900 (has links)
ENGLISH ABSTRACT: A design routine was derived for designing a series of rotor-only axial flow fans. The routine was applied by designing two different series of axial flow fans. The first design was for a general application rotor-only axial flow fan. This fan series was designed, built and tested in co-operation with Howden Air Industries for both research and commercial purposes. The second design was for a low-noise fan series, which was designed, built and tested by the University of Stellenbosch for research purposes only. The design theory used the principle of blade cropping, meaning that one blade was designed to fit all the different fan sizes. The fan series was designed for diameters ranging from 3 15 mm to 1000 mm. The fan rotors were designed to conform to a velocity profile of minimum exit kinetic flux. The general application fan design was concentrated around the popular fan diameter sizes of 500 rnm, 560 mm and 630 mm and a rotor speed of 1440 rpm, using a commercially available fan series as reference. The low-noise fan design concentrated on one fan size only, namely 630 mm, while also making use of the principle of forward blade sweep. The remaining fan design principles stayed the same as for the general application fan design. The F-series airfoils were used as blade sections for both fan designs. Both fan series were tested for fan noise and performance in accordance with the BS 848 Standards part 1 (1980) and 2 (1985). A selection of fan diameter sizes was tested for the general application fan to verify its perfo!"mance over a range of fan sizes. This indicated a fan series with a wide range of efficient operation, including excellent noise characteristics. A 630 mm diameter fan was used to test the low-noise fan series. It showed both high efficiency and low noise characteristics. The reduction in fan noise achieved with the low-noise fan does not justi1)' the amount of work and costs involved in the designing process, compared to the general application fan. / AFRIKAANSE OPSOMMING: 'n Ontwerpsroetine vir die ontwerp van 'n reeks enkelrotor aksiaalwaaiers is ontwikkel. Die roetine is toegepas deur twee verskillende reekse aksiaalwaaiers te ontwerp. Die eerste ontwerp was vir 'n algemene toepassings enkelrotor aksiaalwaaier. Die waaierreeks is ontwerp, gebou en getoets in samewerking met Howden Air Industries vir beide navorsings - en kommersieIe doeleindes. Die tweede ontwerp was vir 'n lae geraas waaierreeks. Die reeks is ontwerp, gebou en getoets deur die Universiteit van Stellenbosch vir navorsingsdoeleindes. Die onwerpsteorie het gebruik gemaak van die beginsel van lemverkorting, waardeur een lem ontwerp is om op al die groottes waaierdeursnee te pas. Die waaierreekse is ontwerp vir waaierdeursnee tussen 315 mm en 1000 mm. Die rotors is ontwerp om 'n uitlaatsneIheidsprofiel te gee wat 'n minimum verlies in kinetiese energie toelaat. Die algemene toepassings waaierontwerp het gekonsentreer rondom die gewilde waaierdeursnee van 500 mm, 560 en 630 mm. Dit is ontwerp vir 'n rotorspoed van 1440 met 'n kommersieel beskikbare waaierreeks wat as verwysing gebruik is. Die lae geraas waaierreeks het op slegs een waaiergrootte gekonsentreer, naamlik 630 mm. Die lae geraas waaierreeks is ook ontwerp met vorentoe gekurfde lemme. Die res van die ontwerpsbeginsels was dieseIfde as vir die algemene toepassings waaierreeks. Die F-reeks vleuelprofiele is gebruik vir die lemseksies van beide waaierreekse. Beide waaiereekse is getoets vir waaiergeraas en -effektiwiteit deur gebruik te maak van die BS 848 Standaarde deel 1 (] 980) en 2 (] 985). 'n Verskeidenheid van waaierdeursnee van die algemene toepassings waaierreeks is getoets om die waaier se vertoning oor 'n gebied van waaiergroottes te bepaal. Die resuItaat was 'n waaierreeks met 'n wye gebied van effektiewe werking, asook uitstekende geraaseienskappe. 'n 630 mm Deursnee waaier is gebruik om die lae geraas waaier te toets. Die toetse het 'n waaier getoon wat beide hoe effektiwiteit en lae geraaseienskappe het. Die afname in waaiergeraas wat verkry is met die lae geraas waaier, in vergelyking met die algemene toepassings waaier, regverdig egter rue die werk en kostes verbonde aan die ontwerp van die waaierreeks nie.
9

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

Bredell, J. R. (Johann Richard) 12 1900 (has links)
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.
10

Investigation of performance enhancing devices for the rain zones of wet-cooling towers

Terblanche, Riaan 03 1900 (has links)
Thesis (MScEng)--University of Stellenbosch, 2008. / ENGLISH ABSTRACT: The performance of a natural draught wet-cooling tower can be improved by reducing the average drop size in the rain zone. In this thesis, the effect of installing different horizontal grids below the fill on drop size in the rain zone is investigated experimentally and theoretically. A specially designed horizontal grid consisting of evenly spaced slats and a grid made from expanded metal sheeting are tested. Drop size distribution measurements are taken below different cooling tower fills to determine the respective Sauter mean drop sizes and also below different configurations of splash grids to determine the reduction in drop size. Drop break-up through a grid of horizontally placed slats is modelled and compared to measured data to determine the optimum configuration in terms of spacing between the grid and fill, slat width and slat spacing. A cross flow rain zone is modelled under different air and water flow combinations with CFD for two distributions that represent the rain with and without splash grids and the results are compared. The Merkel transfer characteristic for all the flow conditions using both distributions are determined using a Lagrangian, Merkel, Poppe and e- NTU method in order to quantify the increase in rain zone Merkel number. Pressure drop over the cross flow rain zone is also determined and compared for the two distributions under considerations. / AFRIKAANSE OPSOMMING: Die verkoelingsvermoë van ‘n reënsone van ‘n natuurlike trek nat koeltoring kan verbeter word deur die verkleining van die gemiddelde druppelgrootte. In hierdie tesis word die effek wat horisontale roosters op die druppelgrootte het, wanneer dit onder die pakking geïnstalleer is, eksperimenteel en teoreties ondersoek. ‘n Spesiaal ontwerpte rooster bestaande uit horisontaal gepakte latte en ‘n gerolde metaal rooster word onderskeidelik vir hierdie doel gebruik. Druppelgrootte metings word geneem onder verskillende koeltoring pakkingsmateriaal om die Sauter gemiddelde diameter te bepaal, asook onder die verskillende rooster opstellings om die verkleinde druppelgrootte te bepaal wat die rooster veroorsaak. Druppelopbreking deur ‘n laag horisontaal gepakte latte word gemodelleer en vergelyk met gemete data om sodoende die beste kombinasie tussen die afstand onder die pakkingsmateriaal, latwydte en latspasiëring te bepaal. ‘n Kruisvloei reënsone word gemodelleer met CFD onder verkillende lug- en watervoeikombinasies vir twee druppelverdelings wat die reënsone met en sonder roosters verteenwoordig. Die Merkel oordragskoëffisiënt vir die twee verdelings word bereken en vergelyk deur van ‘n Lagrange- , Merkel- , Poppe- en e-NTU metode gebruik te maak om sodoende die verbetering in reënsone Merkelgetal te kwantifiseer. Drukvalle oor die reënsone word ook bereken en vergelyk vir die twee verdelings wat beskou is.

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