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

Esterhuyse, Bernard de Waal (Stellenbosch : University of Stellenbosch, 2004-12)

Thesis (MScIng)--University of Stellenbosch, 2004.

Thesis

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.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/16321
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