Flow characteristics of water in straight and serpentine micro-channels with miter bends

Abstract

Flow characteristics of pressure-driven de-ionized water were investigated experimentally in straight and serpentine micro-channels with miter bends. The micro-channels had rectangular cross-sections with hydraulic diameters of 0.209 mm, 0.395 mm and 0.549 mm. To evaluate bend loss coefficient in the serpentine micro-channel and micro-scale size effect on it, the additional pressure drop due to the miter bend must be obtained. This additional pressure drop can be achieved by subtracting the frictional pressure drop in the straight micro-channel from the total pressure drop in the serpentine micro-channel. Since currently there still has a debate on the relationship between the friction factor and Re number in the straight micro-channel, the frictional pressure drop had to be obtained experimentally here. Three groups of micro-channels were fabricated to remove the inlet and outlet losses. The experimental results show that after considering the measurement uncertainties the experimental Poiseuille number can be well predicted by the conventional laminar incompressible flow theory when Re number is less than some value around 1500, the discrepancy observed by the former researchers can be attributed to not accounting for the additional pressure drop in the entrance region. The onset of transition to turbulence might be at 1500–1700. For serpentine micro-channels, the additional pressure drop can be divided into two regions. One is Re < 100. It is very small since no circulation exists. The other one is Re larger than some value in 100–200. At this time the circulation appears and develops at the inner and outer wall of the bend. The additional pressure drop increases sharply with Re number. The bend loss coefficient was observed to decrease and tend to be a constant with decreasing Re number. It is found to be larger than the predicted value for macro-channel turbulent flow and related with the channel size when flow separation appears, namely Re > 100–200.