Doctor of Philosophy / The main contribution of this work is to answer several important questions relating to natural draft wet cooling tower (NDWCT) modelling, design and optimisation. Specifically, the work aims to conduct a detailed analysis of the heat and mass transfer processes in a NDWCT, to determine how significant the radial non-uniformity of heat and mass transfer across a NDWCT is, what the underlying causes of the non-uniformity are and how these influence tower performance. Secondly, the work aims to determine what are the consequences of this non-uniformity for the traditional one dimensional design methods, which neglect any two-dimensional air flow or heat transfer effects. Finally, in the context of radial non-uniformity of heat and mass transfer, this work aims to determine the optimal arrangement of fill depth and water distribution across a NDWCT and to quantify the improvement in tower performance using this non-uniform distribution. To this end, an axisymmetric numerical model of a NDWCT has been developed. A study was conducted testing the influence of key design and operating parameters. The results show that in most cases the air flow is quite uniform across the tower due to the significant flow restriction through the fill and spray zone regions. There can be considerable radial non-uniformity of heat transfer and water outlet temperature in spite of this. This is largely due to the cooling load in the rain zone and the radial air flow there. High radial non-uniformity of heat transfer can be expected when the cooling load in the rain zone is high. Such a situation can arise with small droplet sizes, low fill depths, high water flow rates. The results show that the effect of tower inlet height on radial non-uniformity is surprisingly very small. Of the parameters considered the water mass flow rate and droplet size and droplet distribution in the rain zone have the most influence on radial noniv uniformity of heat transfer. The predictions of the axisymmetric numerical model have been compared with a one dimensional NDWCT model. The difference between the predictions of tower cooling range is very low, generally around 1-2%. This extraordinarily close comparison supports the assumptions of one dimensional flow and bulk averaged heat transfer implicit in these models. Under the range of parameters tested here the difference between the CFD models predictions and those of the one dimensional models remained fairly constant suggesting that there is no particular area where the flow/heat transfer becomes so skewed or non-uniform that the one dimensional model predictions begin to fail. An extended one dimensional model, with semi-two dimensional capability, has been developed for use with an evolutionary optimisation algorithm. The two dimensional characteristics are represented through a radial profile of the air enthalpy at the fill inlet which has been derived from the CFD results. The resulting optimal shape redistributes the fill volume from the tower centre to the outer regions near the tower inlet. The water flow rate is also increased here as expected, to balance the cooling load across the tower, making use of the cooler air near the inlet. The improvement has been shown to be very small however. The work demonstrates that, contrary to common belief, the potential improvement from multi-dimensional optimisation is actually quite small.

Doctor of Philosophy / The main contribution of this work is to answer several important questions relating to natural draft wet cooling tower (NDWCT) modelling, design and optimisation. Specifically, the work aims to conduct a detailed analysis of the heat and mass transfer processes in a NDWCT, to determine how significant the radial non-uniformity of heat and mass transfer across a NDWCT is, what the underlying causes of the non-uniformity are and how these influence tower performance. Secondly, the work aims to determine what are the consequences of this non-uniformity for the traditional one dimensional design methods, which neglect any two-dimensional air flow or heat transfer effects. Finally, in the context of radial non-uniformity of heat and mass transfer, this work aims to determine the optimal arrangement of fill depth and water distribution across a NDWCT and to quantify the improvement in tower performance using this non-uniform distribution. To this end, an axisymmetric numerical model of a NDWCT has been developed. A study was conducted testing the influence of key design and operating parameters. The results show that in most cases the air flow is quite uniform across the tower due to the significant flow restriction through the fill and spray zone regions. There can be considerable radial non-uniformity of heat transfer and water outlet temperature in spite of this. This is largely due to the cooling load in the rain zone and the radial air flow there. High radial non-uniformity of heat transfer can be expected when the cooling load in the rain zone is high. Such a situation can arise with small droplet sizes, low fill depths, high water flow rates. The results show that the effect of tower inlet height on radial non-uniformity is surprisingly very small. Of the parameters considered the water mass flow rate and droplet size and droplet distribution in the rain zone have the most influence on radial noniv uniformity of heat transfer. The predictions of the axisymmetric numerical model have been compared with a one dimensional NDWCT model. The difference between the predictions of tower cooling range is very low, generally around 1-2%. This extraordinarily close comparison supports the assumptions of one dimensional flow and bulk averaged heat transfer implicit in these models. Under the range of parameters tested here the difference between the CFD models predictions and those of the one dimensional models remained fairly constant suggesting that there is no particular area where the flow/heat transfer becomes so skewed or non-uniform that the one dimensional model predictions begin to fail. An extended one dimensional model, with semi-two dimensional capability, has been developed for use with an evolutionary optimisation algorithm. The two dimensional characteristics are represented through a radial profile of the air enthalpy at the fill inlet which has been derived from the CFD results. The resulting optimal shape redistributes the fill volume from the tower centre to the outer regions near the tower inlet. The water flow rate is also increased here as expected, to balance the cooling load across the tower, making use of the cooler air near the inlet. The improvement has been shown to be very small however. The work demonstrates that, contrary to common belief, the potential improvement from multi-dimensional optimisation is actually quite small.

Thesis (Ph.D.)--Rochester Institute of Technology, 2009. / Typescript. Includes bibliographical references (p. 171-175).

Test data of forced draft cooling towers is all too meager, and that available is, in many instances, incomplete. It is the opinion of the authors that putting cooling tower design on a rational basis can be brought about only by two methods. Either complete and intensive study of all existing towers or a thorough study of models, similar to the research conducted on airplanes in wind tunnels and on boat hulls in towing tanks, is necessary. Models used should be so constructed that quantity, condition, and velocity of air; quantity and condition of water; and type and arrangement of filling may be controlled. It is by the second method that the authors believe rational tower design may best be brought about. / Master of Science

Master of Science

Cooling water systems are used to remove excess heat from a chemical process to the atmosphere. The primary components of these systems are the cooling tower and the heat exchanger network. There is a strong interaction between these individual components, thus their performances are interrelated. Most published research in this area has focused mainly on optimization of the individual components i.e. optimization of heat exchanger network or optimization of the cooling towers. This approach does not optimize the cooling water system as a whole. Previous research work in which a holistic approach was used is limited to cooling water systems with single cooling water source. This work presents a technique for integrated optimization of complex cooling water systems. The system under consideration consists of multiple cooling towers each supplying a set of heat exchangers. A superstructural approach is employed to explore all possible combinations between the heat exchangers and the cooling towers. The cooling water reuse opportunities within the heat exchanger networks are also explored. A detailed mathematical model consisting of the cooling towers and the heat exchanger networks model is developed. Two practical scenarios are considered and the mathematical formulations for Case I and II yield nonlinear programing (NLP) and mixed integer nonlinear programming (MINLP) structure respectively. Although the reuse/recycle philosophy offers a good debottlenecking opportunity, the topology of the associated cooling water network is more complex, hence prone to higher pressure drop than the conventional parallel design. This is due to an increased network pressure drop associated with additional reuse/recycle streams. Therefore, it is essential to consider pressure drop during the synthesis of cooling water networks where the reuse/recycle philosophy is employed. The on-going research in this area is only limited to cooling water networks consisting of a single cooling water source. The common technique used is mathematical optimization using either superstructural or non superstructural approach. This work further presents a mathematical technique for pressure drop optimization in cooling water systems consisting of multiple cooling towers. The proposed technique is based on the Critical Path Algorithm and the superstructural approach. The Critical Path Algorithm is used to select the cooling water network with minimum pressure drop whilst the superstructural approach allows for cooling water reuse. The technique which was previously used in a cooling water network with single source is modified and applied in a cooling water network with multiple sources. The mathematical formulation is developed considering two cases. Both cases yield mixed integer nonlinear programming (MINLP) models. The cooling tower model is also used to predict the exit condition of the cooling tower given the inlet conditions from the cooling water network model. The results show up to 29% decrease in total circulating cooling water flowrate when the cooling water system is debottlenecked without considering pressure drop. Consequently, the overall cooling towers effectiveness was improved by up to 5%. When considering pressure drop the results showed up to 26% decrease in total circulating water flowrate. / Thesis (PhD)--University of Pretoria, 2013. / gm2013 / Chemical Engineering / unrestricted

Thesis (MScEng)--Stellenbosch University, 1997. / One copy microfiche. / ENGLISH ABSTRACT: This project concerns the investigation of parameters controlling the behaviour of full-cone spray nozzles of the type used in cooling towers. In the present study large medium pressure hollow and full cone nozzles were investigated. A literature survey provided insight into the relationships between the nozzle dimensions and their spray characteristics, while equations found in the literature were used to correlate the experimental data. It was found that the spray cone angle of hollow cone nozzles could be manipulated by using rounded orifice outlets and this finding lead to the development of a uniquely profiled outlet that actually produces a square spray pattern. More experimental work was done to determine the relationship between the central jet of a full-cone nozzle and the other major nozzle dimensions. These results were then correlated and formulated into a set of guidelines for designing full-cone nozzles. / Digitized at 300 dpi Colour PDF format (OCR), using ,KODAK i 1220 PLUS scanner. Digitised, Ricardo Davids on request from Corinna 01 October 2014 / AFRIKAANSE OPSOMMING: Hierdie projek behels 'n studie van belangrike parameters in volkegel sproeimondstukke soos gebruik in koeltorings. In die huidige studie word groot mediumdruk holkegel en volkegel sproeimondstukke ondersoek. 'n Literatuurstudie het die nodige insig verskaf omtrent die verwantskap tussen mondstuk dimensies en hul spuitkarakteristieke, terwyl vergelykings uit die literatuur gebruik is om die eksperimentele data te korreleer. Dit was gevind dat die sproeir kegelhoek van die holkegelmondstuk verander kon word deur gebruik te maak van geronde uitlate. Afleidings wat gemaak is het gely tot die ontwikkeling van 'n unieke geprofielde uitlaat wat 'n vierkantige sproeipatroon gelewer het. Bykomstige eksperimentele werk is gedoen om die verwantskap tussen die sentralestraal van 'n volkegelmondstuk en die ander hoof mondstukdimensies te bepaal. Die reultate is verwerk om riglyne vir die ontwerp van vierkantige patroon volkegel mondstukke daar te stel.

Thesis (MEng)--Stellenbosch University, 2015. / ENGLISH ABSTRACT: This thesis investigates the flow field and pressure distributions around cylinders at trans-critical Reynolds numbers using the k-ε Realizable turbulence model. A steady state 2-D and 3-D Fluent® model is successfully developed to evaluate the effects of changing various modelling parameters on the static pressure distribution around an infinite and finite cylinder. These parameters include surface roughness, cylinder rotation and air viscosity at the cylinder surface. The subsequent results obtained are compared to each other and to data trends from literature as well as measured experimental results and are found to be in good agreement. In addition a method for calibrating all developed methods based on their shear stress curves over a flat plate model is also successfully developed. The main objective is to find an appropriate single parameter which can be used for the rigorous adjustment of the pressure distribution around a cooling tower, which will allow for improved sensitivity analysis and modelling of cooling tower performance under wind conditions with and without meridional ribs located on the outer shell surface. / AFRIKAANSE OPSOMMING: Hierdie tesis ondersoek die vloeiveld en druk verdelings rondom silinders by trans-kritiese Reynolds getalle deur gebruik te maak van die k-ε Realizable turbulensie model. ‘n Bestendige toestand 2-D en 3-D Fluent® model is suksesvol ontwikkel om die uitwerking van die verandering van verskeie model parameters op die statiese druk verdeling rondom ‘n oneindige en eindige silinder te evalueer. Die laasgenoemde parameters sluit in oppervlak grofheid, silinder rotasie en lug viskositeit by die silinder wand. Die daaropeenvolgende resultate wat verkry word, word met data tendense uit die literatuur asook gemete data vanuit eksperimente vergelyk en goeie ooreenkoms i.t.v die data tendense is gevind. Verder is ‘n metode vir die suksesvolle kalibrasie van die ontwikkelde numeriese tegnieke ontwikkel. Die laasgenoemde kalibrasie metode is gebaseer op die vergelyking van skuifspanning kurwes vir vloei oor ‘n plat plaat model. Die hoofdoel van die navorsing is om ‘n geskikte enkele parameter te vind wat gebruik kan word vir die effektiewe aanpassing van die druk verdeling rondom ‘n koeltoring wat sal lei tot verbeterde sensitiwiteits analise en modellering van koeltoring verrigting onder wind toestande met en sonder meridionale ribbes geleë op die buitenste dop oppervlak.

An investigation has been carried out to determine the effects of the dissolved solids in process cooling water (pcw) and mine water (mw) on concrete corrosion. An experimental set up was designed to simulate the process in the cooling towers of Sasol (Pty) Ltd at Secunda. The investigation was carried out using Ordinary Portland Cement (OPC) and Portland Blastfurnace Cement (PBFC). The corrosion process was monitored as a function of time by determining the concentrations of the ions left in solution. This was done using Inductively Coupled Plasma Optical Emission Spectroscopy (ICP-OES) and Ion Chromatography (lC). The observation, identification and characterization of the secondary phases formed during the corrosion process were analyzed using the Scanning Electron Microscopy(SEM). Energy Dispersive X-ray Microanalysis (EDX) made it possible to identify the various microstructures and quantify their elemental composition. This made it possible to monitor the penetration of sulphate ions in the mortar. Powder X-ray Diffraction (XRD) qualitative analysis was also performed on the test mortar specimens. The organic constituents in process cooling water were determined using Gas Chromatography coupled with a Mass Spectrometer (GC-MS). The corrosion indices which measures the aggressiveness of water solutions towards cement and concrete were calculated for both process cooling water and mine water. The results of the investigation showed that mine water is more corrosive than process cooling water. This observation has been linked to the presence of the organic compounds in process cooling water. The results also indicated that PBFC was mores resistant to chemical attack than ope. / Thesis (M.Sc.)-University of Natal, Pietermaritzburg, 1998.

Please read the "Background" (p v) in the section 00front of this document Copyright 2002, University of Pretoria. All rights reserved. The copyright in this work vests in the University of Pretoria. No part of this work may be reproduced or transmitted in any form or by any means, without the prior written permission of the University of Pretoria. Please cite as follows: Bartie, C 2002, The life and career of the South African dramatric soprano Marita Napier, DPhil thesis, University of Pretoria, Pretoria, viewed yymmdd < http://upetd.up.ac.za/thesis/available/etd-11142007-125718 / > / Thesis (DPhil (Microbiology))--University of Pretoria, 2007. / Microbiology and Plant Pathology / unrestricted