Experiments on skin friction reduction induced by superhydrophobicity and Leidenfrost phenomena in a Taylor-Couette cell

Abstract

Film boiling and superhydrophobic surfaces may each decrease the skin drag by creating a vapor/air layer encapsulating the body. However, film boiling requires enormous power, and air plastron on superhydrophobic surfaces is not stable. The combined utilization of superhydrophobicity and film boiling is promising as on superhydrophobic surfaces the film boiling (Leidenfrost) temperature is only a few degrees higher than the boiling point. In a recent literature, a Taylor-Couette (T-C) cell was proposed to measure the combined effect of superhydrophobicity and film boiling on skin drag. However, rotation of the inner cylinder of the TC cell creates turbulent Taylor vortices, makes the vapor layer unstable and creates uncertainty in the results. Whereas if the outer cylinder rotated, flow pattern would become azimuthal laminar with weak Ekman vortices. We modified the T-C cell, accordingly; and found that by decreasing the surface wettability (i.e. apparent contact angle), Leidenfrost temperature, and the minimum heat flux to reach the film boiling decrease as well. Furthermore, by increasing the shear Reynolds number from 0.8×104 to 3.2×104, for both heated and unheated superhydrophobic surfaces, skin drag reduces. The heated one results in 67% decrease in skin drag. It is worth mentioning that the heat flux to create this 67% skin drag reduction is less than 2W/cm2, which is 25 times less than the minimum heat flux to create film boiling on a regular aluminum surface.