The effects of surface roughness on the fully developed heat transfer characteristics of laminar and transitional flow through horizontal circular tubes

Abstract

Heat exchangers are essential components in heating and cooling processes that are used in a variety of commercial and industrial applications. The accurate design and development of these heat exchangers rely on correlations determined from experimental work that account for the various conditions that may affect the heat transfer and pressure drop characteristics. This is ever more important as a push towards cleaner and more sustainable processes depend on effective heat exchangers which optimize heat transfer while minimizing pressure drop. Although the effect of surface roughness on friction factors has been widely studied, little work has been done focussing on the effect of surface roughness on heat transfer in macro-tubes, particularly in the laminar and transitional flow regimes. The transitional flow regime is an area of interest because an optimum between high heat transfer and low pressure drop is often found near or in the transitional flow regime, which may be further leveraged by the effects of the surface roughness. This study, therefore, experimentally investigated the effects of surface roughness on simultaneous heat transfer and pressure drop on laminar and transitional flow through horizontal, circular macro-tubes subjected to a uniform heat flux. A smooth (ε/D ≈ 0) 5.45 m long test section with an internal diameter of 5.14 mm was used for validation and subsequently roughened with a sand blasting technique to attain two low relative surface roughnesses of ε/D = 0.0003 and ε/D = 0.0006. Water was used as the test fluid and the experiments were performed at four heat fluxes of 0, 1, 2 and 3 kW/m2, over a Reynolds number range from 500 to 9 800. In the laminar flow regime, the surface roughness disturbed the development of the thermal boundary layers for mixed convection, increasing the thermal entrance lengths, which therefore increased the Nusselt numbers. The low relative surface roughnesses also delayed the critical Reynolds numbers at which transition started. This was initially in contradiction with literature, however, a comparison of the critical Reynolds numbers with literature showed that the relative surface roughnesses attained in this study were at least an order of magnitude or lower than previously investigated. The low relative surface roughnesses dampened the fluctuations in the flow and the disturbance caused by the square-edged inlet geometry, which delayed the onset of the transitional flow regime. Furthermore, it was proposed that three distinct regions can be found when considering the critical Reynolds numbers as a function of the relative surface roughnesses: at low relative surface roughnesses in the Dampening region, the critical Reynolds numbers increase with increasing roughnesses, while at moderate relative surface roughnesses in the Enhancing region and at large relative surface roughnesses in the Saturating region, the critical Reynolds numbers decrease with increasing roughnesses. The key differences are that the effects of secondary flow in the Enhancing region begin to fall off and become insignificant in the Saturating region, and no distinctive transitional flow regime occurs in the Saturating region. Finally, a simultaneous analysis of the heat transfer and pressure drop showed that, when compared to smooth tubes, low relative roughness improved the performance in the laminar flow regime where the heat transfer coefficients increased more than the friction factors.