Effect of surface roughness on liquid friction and transition characteristics in micro- and mini-channels

A general threshold was proposed to subdivide a flow in a smooth or a rough micro- and mini-channel. A database was setup based on the experimental results of liquid flow in rough micro- and mini-channels from the literature, which includes 5569 data and covers wide ranges of the relative roughness and the Reynolds number. The effect of roughness on a flow in micro- and mini-scale channels was investigated by in-depth analyzing the friction factor and the flowing transition characteristics. Existing prediction models were used to predict the friction factors of fully developed flow and critical Reynolds number. Both experimental data and the predictions showed that roughness had little effect on flow characteristics when the relative roughness (epsilon/D-h) is less than 1%, while for the relative roughness larger than 1%, the friction factor and critical Reynolds number gradually deviate from the prediction values of smooth tubes. It was recommended that epsilon/D-h = 1% could be set as a threshold to distinguish smooth and rough micro- and mini-channels. According to this criterion, new correlations to predict friction factor and critical Reynolds number were also fitted. Moreover, it was also concluded that geometries of the channels had little effects on this threshold. (C) 2014 Elsevier Ltd. All rights reserved.