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International Heat Transfer Conference 15

ISSN: 2377-424X (online)
ISSN: 2377-4371 (flashdrive)

The Influence of Surface Roughness on Heat Transfer in the Transitional Flow Regime

Marilize Everts
University of Pretoria, c/o Lynnwood Road and Roper Street, Hatfield, South Africa

Samantha R. Ayres
Department of Mechanical and Aeronautical Engineering, University of Pretoria

Franscois A. Mulock Houwer
Department of Mechanical and Aeronautical Engineering, University of Pretoria

Calvin P. Vanderwagen
Department of Mechanical and Aeronautical Engineering, University of Pretoria

Nicola M. Kotze
Lecturer at the University of Pretoria

Josua Petrus Meyer
Department of Mechanical and Aeronautical Engineering, University of Pretoria, Pretoria, Private Bag X20, Hatfield, 0028, South Africa

DOI: 10.1615/IHTC15.cnv.008338
pages 1626-1637


KEY WORDS: Heat exchanger, Convection, surface roughness, transitional flow, heat transfer

Abstract

Heat exchangers are usually designed such that they do not operate in the transitional flow regime, mainly due to uncertainty and perceived chaotic behaviour in this region. Recent studies, however, show that there might be several advantages operating in the transitional flow regime and the heat transfer and pressure drop characteristics are predictable. However, these studies were limited to smooth tubes and no work has been done on rough tubes. The purpose of this study is therefore to experimentally obtain the heat transfer coefficients in the transitional flow regime as a function of Reynolds number and is work in progress. An experimental set-up was developed, built and validated, and the heat transfer measurements at three different heat fluxes (5.6, 8.4 and 11.4 kW/m2) were taken for water in a smooth and three roughened tubes with a relative roughness of 0.01, 0.02 and 0.04, respectively. All the tubes had an outer diameter and length of 15.88 mm and 1.8 m, respectively. The Reynolds number was varied between 1 000 and 14 000 to ensure that the whole transitional flow regime, as well as a sufficient part of the laminar and turbulent regime were covered. It was found that transition occurred earlier with increasing surface roughness and the heat transfer increased as well, while the secondary flow effects decreased.

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