Experimental and numerical evaluation of anisotropic fill performance characteristics in cross- and counterflow

Grobbelaar, Pieter Jacobus (2012-03)

Thesis (MScEng)--Stellenbosch University, 2012.

Thesis

ENGLISH ABSTRACT: The confidence level of modeling cooling towers, where oblique air flow within anisotropic fills takes place, is higher when the change in fill‟s performance cha-racteristics, dependent on the way that air flows through the fill, is better unders-tood. A trickle fill‟s performance characteristics in crossflow are compared to its per-formance characteristics in counterflow by doing crossflow fill tests that are per-formance comparable to counterflow tests with the same fill. In order to do these tests, an existing crossflow fill test facility is critically evaluated and improved. The difference between crossflow and counterflow trickle fill performance charac-teristics is found to depend on air mass velocity (Ga) and water mass velocity (Gw) and to be between 0 and 35% for the Merkel number (Me) and up to almost 200% for the loss coefficient. Additionally, the validity of a recently developed 2-dimensional evaporative cool-ing model is investigated by comparing its predictions to experimental results. The following conclusions are made: - For trickle fill and rain zone tests, the model, with the present assumptions, predicts the average temperature of the outlet air to within approximately 0.4 °C. - Currently, temperature profiles that are experimentally measured at the air and water outlets are subject to significant edge effects, which prevent a fair com-parison to model predictions. - The model predictions can be improved if local variations in Me and the redi-stribution of water by the fill are taken into account.

AFRIKAANSE OPSOMMING: Die betroubaarheid van die modellering van koeltorings, waar lug skuins deur anisitropiese pakking (of “fill” in Engels) vloei, is hoër indien die verandering in die pakking se verrigtingseienskappe, wat afhang van die manier waarop die lug deur die pakking vloei, beter verstaan word. „n Drup pakking (of “trickle fill” in Engels) se verrigtingseienskappe in kruisvloei word vergelyk met dié in teenvloei deur kruisvloei pakking toetse, wat direk vergelykbaar is met teenvloei toetse vir dieselfde pakking, te doen. Ten einde hieredie toetse te doen, word ʼn bestaande kruisvloei toets fasiliteit krities ondersoek en verbeter. Dit word bevind dat die verskil tussen die drup pakking se kruisvloei en teenvloei verrigtingseienskappe afhang van lug massa snelheid (Ga) en water massa snelheid (Gw) en 0 tot 35% is vir die Merkel getal (Me) en so groot as 200% is vir die verlies koëffisiënt. Verder word die geldigheid van ʼn 2-dimensionele nat-verkoelingsmodel wat onlangs ontwikkel is ondersoek deur die model se voorspellings te vergelyk met eksperimentele resultate. Die volgende gevolgtrekkings word gemaak: - Die model, met huidige aannames, voorspel die gemiddelde uitlaat lug temperatuur met ʼn afwyking van ongeveer 0.4°C. - Die temperatuur profiele wat eksperimenteel gemeet word by die lug en water uitlate is onderworpe aan noemenswaardige rand effekte, wat ʼn behoorlike vergelyk met model voorspellings verhoed. - Die model se voorspelling van die profiele kan verbeter word indien die lokale variasies in Me en die herverdeling van die water deur die pakking in ag geneem kan word.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/20282
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