The effects of surface roughness on fully developed laminar and transitional flow friction factors and heat transfer coefficients in horizontal circular tubes

Although the effect of surface roughness on internal forced convection is considered a mature field, very limited work has been done focussing on the effect of surface roughness in the laminar and transitional flow regimes, especially for fully developed flow. Therefore, the purpose of this study was to experimentally investigate the simultaneous heat transfer and pressure drop characteristics in the laminar and transitional flow regimes for tubes with internally roughened surfaces. Experiments were performed using circular tubes with an internal diameter of 5.14 mm and a length of 5.45 m, with the smooth tube having a relative surface roughness close to 0, and the rough tubes having relative surface roughness values of 0.0 0 03 and 0.0 0 06. Water was used as the test fluid and the experiments were performed for Reynolds numbers between 500 and 9 800, while the Prandtl number varied between 4.32 and 7.01. Three different constant heat fluxes with values of 1, 2 and 3 kW/m(2) were applied to the test section while determining the friction factors from the pressure drop measurements and the heat transfer coefficients, Nusselt numbers and Colburn j-factors, from the measured fluid and surface temperatures. The Colburn j-factors and friction factors behaved similarly throughout the tested Reynolds range and the flow regime boundaries of the heat transfer and pressure drop results corresponded well. An increase in surface roughness favoured an increase in heat transfer in the laminar flow regime, but not in the other flow regimes. When investigating the influence of surface roughness on the critical Reynolds number, three distinct regions were identified and defined, namely, the Dampening region, Enhancing region and Saturating region. For low values of relative surface roughness, the onset of transition was delayed, while transition occurred earlier as the relative surface roughness was increased to moderate and high values. General trends in all three regions were that, an increase in surface roughness decreased the width of the transitional flow regime while increasing the transition gradient. (C) 2022 Elsevier Ltd. All rights reserved.