The influence of bacteria-based biofouling on the wall friction and velocity distribution of hydropower pipes

Algae biofouling due to freshwater diatoms has been investigated thoroughly in open channel flows. Their presence has been shown to cause a significant increase in the local skin friction coefficient, the overall drag coefficient and can produce reductions in flow capacity of up to 10%. This study extends previous work to investigate bacteria-based biofouling that forms on the inside walls of pipelines and machinery that are not exposed to direct sunlight. The effect of biofouling on a 1.5 m long internally painted pipe section of diameter 101.6 mm was investigated. The pipe section was installed in a hydropower scheme for an extended period to allow the growth of flow conditioned biofilms at an average flow velocity of U = 1.3 ms–1. The pipe section was tested over a range of Reynolds numbers under fully-developed turbulent conditions. At each flow rate the head loss of the fouled pipe was measured as well as the complete velocity profile at the downstream end of the pipe to ensure the full effect of the biofouling was captured. These results were used to evaluate the pipe friction factor and sand equivalent surface roughness. Trends in the experimentally determined values of pipe friction factor with varying Reynolds number are significantly different from those predicted by empirically-based theory. Experimental velocity profiles show significant deviations from the theoretical prediction of flow through a rough pipe, with a higher maximum velocity observed in the centre of the pipe but a lower velocity in the near wall region.