The Effect of Inner Surface Roughness and Heating on Friction Factor in Horizontal Mini-tubes

Abstract

According to the recent literature survey, it was concluded that the friction factor in miniand 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) the effect of miniand micro-tube diameters on the start and end of the transition region. Moreover, only a few studies investigated the effect of heating on friction factor in miniand micro-tubes, especially, in the transition region. Therefore, an experimental setup was built in this study to measure the pressure drop for the horizontal miniand micro-tubes under the isothermal and uniform wall heat flux boundary conditions. Distilled water was used as the test fluid. Seven stainless steel minitubes (2000μm to 508μm in inner diameters) with various surface roughness values (1.94μm to 5.30μm) were used as the test section. 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 in the turbulent region.