The effect of large values of relative surface roughness on heat transfer and pressure drop characteristics in the laminar, transitional, quasi-turbulent, and turbulent flow regimes

Abstract

Numerous studies experimentally investigated the heat transfer and pressure drop characteristics of laminar, transitional, quasi-turbulent, and turbulent flow through smooth tubes, however, studies that investigate the effect of surface roughness on the heat transfer and pressure drop characteristics in macrotubes are sparse. This study experimentally investigated the effect of large values of relative surface roughness on the heat transfer and pressure drop characteristics using simultaneously measured heat transfer and pressure drop data. Experiments were conducted using a horizontal circular tube with a base inner diameter of 5 mm, a length of 4 m, and a square-edged inlet. The constricted diameter was used for the rough tubes. One smooth and two rough tubes, with relative roughnesses of 0.04 and 0.11, were tested at different constant heat fluxes between Reynolds numbers of 100 and 8 500. Water was used as the test fluid and the Prandtl number varied between 3 and 7. The smooth tube was used for validation purposes, as well as a reference to compare the rough tube results. The heat transfer and pressure drop results were plotted and discussed using the average Nusselt numbers, friction factors, and Reynolds numbers. Contrary to the trend in the Moody Chart, a significant increase in friction factors with increasing surface roughness was observed in the laminar flow regime. Free convection effects of both Nusselt numbers and friction factors were suppressed by the velocity of the fluid caused by the large roughness elements, even so at low Reynolds numbers. It was found that for a rough tube with a relative roughness of 0.04 at a constant heat flux of 3 kW/m2, the transitional flow regime occurred at a Reynolds number of 560, and the quasi-turbulent flow regime at a Reynolds number of 760. For a tube with relative roughness of 0.11, the critical Reynolds number was below 390 and the quasi-turbulent flow regime occurred as early as at a Reynolds number of 490. In general, for both the friction factors and Nusselt numbers as functions of Reynolds number, there was a clear upward and leftward shift with increasing surface roughness across the different flow regimes in comparison to a smooth tube. The transitional flow regime for friction factors and Nusselt numbers were narrower and had a differing profile in comparison to smooth tubes. The relative roughnesses of both rough tubes were in the saturating region and the influence of heat flux and thus the Grashof number had little effect on the critical Reynolds number. The quasi-turbulent and turbulent flow regimes occurred at lower Reynolds number for increasing roughness. Trends of the friction factors and Colburn j-factors were similar in all the flow regimes for the smooth and rough tubes and the boundaries between the flow regimes were the same for both the pressure drop and heat transfer results. When comparing the relationship between heat transfer and pressure drop, it was found that an increase in surface roughness favoured heat transfer in the quasi-turbulent flow regime. This is useful for rough tubes as the quasi-turbulent flow regime onsets early with regards to the Reynolds number in tubes with large roughnesses.