Effect of Channel Roughness on Heat Transfer and Fluid Flow Characteristics at Low Reynolds Numbers in Small Diameter Tubes

The effect of roughness on pressure drop and heat transfer in circular tubes has been extensively studied in literature. The pioneering work of Nikuradse (1933) established the sand grain roughness as a major parameter in defining the friction factor during laminar and turbulent flows. Recent studies have indicated a transition to turbulent flows at Reynolds number values much below 2300 during single-phase flow in channels with small hydraulic diameters. In the present work, a detailed experimental study is undertaken to investigate the roughness effects in small diameter tubes. The roughness of the inside tube surface is changed by etching it with an acid solution. Two tubes of 1.032 mm and 0.62 mm inner diameter are treated with acid solutions to provide three different roughness values for each tube. The Reynolds number range for the tests is 500-2600 for 1.062mm tube and 900-3000 for 0.62mm tube. NOMENCLATURE B: Bias % Cp: Specific heat (kJ/kg K) Dh: Hydraulic diameter of the tube (mm) e/d: Relative roughness value f: friction factor kf: Thermal conductivity of the fluid (W/m K) Nu: Nusselt Number ×