The influence of temperature stratification in the lower atmospheric boundary layer on the operating point of a natural draft dry-cooling tower

Hoffmann, Jacob Elisa (1997-11)

Thesis (D.Phil)—Stellenbosch University, 1997.

Thesis

ENGLISH ABSTRACT: Natural draft dry-cooling towers for power stations are designed to reject heat at a prescribed rate under specified atmospheric conditions. In the past, cooling tower designs were based on the annual average ambient air temperature, measured at 1. 5 m above the ground (the reference temperature). Furthermore, the air flow through the tower was calculated according to an adiabatic temperature lapse rate in the atmosphere. However, the air inlet temperature at the heat exchanger may deviate significantly from the reference temperature due to temperature stratification in the atmosphere and the fact that the tower draws in air from considerable heights above the ground. Furthermore, the buoyancy force that drives the air through a natural draft cooling tower is also affected by ambient air temperature stratifications. In this thesis, a logical build-up to a final numerical model is followed. The physics of the formation of atmospheric temperature profiles and their relation to large scale weather patterns are discussed first, followed by a section on the heat and momentum transfer processes encountered in a natural draft dry-cooling tower. This is followed by a discussion of field data, and it all culminates in the application of this theory and data in a numerical example, illustrating the usefulness of the proposed model.

AFRIKAANSE OPSOMMING: Natuurlike trek droë koeltorings vir kragstasies word ontwerp om van 'n gegewe warmtelas ontslae te raak onder heersende atmosferiese toestande. Huidiglik word die jaarlikse gemiddelde omgewingstemperatuur as verwysingspunt gebruik in koeltoringontwerp. Dit is internasionaal die standaard om die temperatuur 1.5 m bokant die grond te meet. Verder word 'n adiabatiese temperatuurverdeling in die atmosfeer aanvaar vir gebruik in die trekvergelyking. Op enige spesifieke oomblik kan die luginlaattemperatuur by die warmte-uitruiler egter beduidend van die gemete omgewingstemperatuur verskil. Die rede hiervoor is dat sterk temperatuurgradiente dikwels in die eerste paar meter van die atmosfeer voorkom. Groot natuurlike trek droë koeltorings suig lug in wat hierdie gebied insluit, vandaar die verskil. Verder beïnvloed die temperatuurgradiënte ook boonop die lugvloei deur die toring, met verdere implikasies vir die warmteoordrag. In hierdie tesis word 'n logiese opbou tot 'n voorgestelde numeriese model gevolg. Eerstens word die fisiese prosesse in die atmosfeer wat aanleiding tot temperatuurgradiënte gee, en hul verhouding met makroskaal weerstoestande, aangespreek. Dit word opgevolg met 'n bespreking van die warmte- en momentumoordragsprossesse in 'n natuurlike trek droë koeltoring. Hierdie teorie word daarna getoets teen eksperimentele data wat by 'n kragstasie gemeet is. Ten slotte word die toepassing van die voorgestelde model aan die hand van 'n numeriese voorbeeld geïllustreer.

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