Experimental Investigation of the Effect of Roughness on Mini-Tubes Friction Factor Under the Isothermal and Heating Boundary Conditions by WU CHOI KENG Thesis Supervisor: Prof. Tam Lap Mou Department of Electromechanical Engineering, University of Macau According to the recent literature survey, it was concluded that the friction factor in mini- and micro-tubes could be predicted accurately by using macro-scale theory. However, certain issues should be further investigated. They are: (a) the effect of inner surface roughness and (b) micro-tube diameter on the start and end of the transition region. Moreover, only a few studies investigated the effect of heating on friction factor in micro-tubes, especially, in the transition region. Therefore, an experimental setup was built in this study to measure the pressure drop for the horizontal mini-tubes under the isothermal and uniform wall heat flux boundary conditions. Distilled water was used as the test fluid. The stainless steel mini-tubes (307µm to 2000µm in inner diameters) with various surface roughness values (1.94 µm to 7.15 µm) were used as the test section. For the pressure drop measurements, different ranging diaphragms were used to measure the pressure drop across the tested tubes. The experimental results clearly showed the effect of heating and roughness on friction factors in the laminar, transition, and turbulent regions. For the friction factors under isothermal condition, the mini-tubes had a narrower transition range due to the roughness when compared to the macro-tubes. For the friction factor under heating condition, the laminar data and the start of transition were different from the isothermal case. Under heating condition, the narrower transition range due to the roughness was also observed in the mini-tubes. However, the effect of heating was not seen on the end of transition and the turbulent region.