Abstract
In this paper, experimental results are reported to quantify the effect
of hydrophobic coating LT-8 on frictional drag of water flow in pipes of
450 mm length. Five pipes of 1, 2, 3, 4 and 5 mm inner diameter were
tested. The results from 1, 2 and 3 mm diameter pipes demonstrated an
average frictional drag reduction of 9%, 11.5% and 3%, respectively,
while the results from 4mm and 5mm pipes showed an increase in
frictional drag of 12% and 10%, respectively. The 2mm and 4mm pipes
were also tested with a half application of hydrophobic coating. The
half coated 2mm pipe showed decrease in drag while 4mm pipe showed
increase in drag. The results indicate a relationship between drag
reduction/ increase withnthe percentage of coated surface. The main
conclusions are, the flow changed from laminar state to the liquid-air
wetting surface condition (Cassie-Baxter wetting state) at the pipe
surface and then destabilized by the turbulent boundary layer and
entered the liquid wetting surface (Wenzel wetting state) will be
appeared. This transition lead to a reduction in friction drag for
laminar flow condition and increase in drag for turbulent flow
condition.