Experimental Investigation of Frictional Drag Reduction by Superhydrophobic Coatings in Turbulent Pipe Flow*

Recently superhydrophobic (SH) coatings have been widely investigated for their potential to reduce the frictional drag in turbulent pipe flows and external boundary layers, enabling reduction in energy consumption across a broad range of industrial applications. However, a drag reduction effect may be lost if roughness k+ exceeds ~5, or gas trapped on the surface is lost. We present an experiment investigating the behavior of turbulent flow through circular tubes that have an SH coating on the interior surface. Pressure drop over a tube length of 60 diameters is used to evaluate the effectiveness of SH coatings in reducing frictional drag. The pressure drop is measured for water flow rates across a diameter-based Reynolds number range of 4E3 to 1.1E5. Measurements are taken after the SH surface has been exposed to turbulent flow for a range of time periods with various absolute pressure, temperature and dissolved gas contents. The experiment was designed to enable the use of non-intrusive X-ray computed tomography at the LBNL synchrotron to quantify gas trapped on the SH surface and relate this to the pressure drop.