Surface hydrophobicity change on adiabatic two-phase flow pattern transitions in horizontal tubes

The effect of surface hydrophobicity on the air-water two-phase flow pattern transition was experimentally investigated. Experiments and the two-phase flow visualization were carried out in a hydrophobic polymer tube (4.76 mm ID). The tube material is fluorinated ethylene propylene (FEP) with a hydrophobic surface, and the static contact angle of a water droplet on a smooth surface is 97.28 degrees. The channel surface was roughened to increase the hydrophobicity. The increased surface roughness results in a microstructured surface texture that forms a fine air trapped layer below the liquid droplet on the polymer surface. The channel surface was rotary ground by sandpaper number grit 400, 320, 100, and 80 to develop microstructure textured surface. The measured static contact angle of the roughened surface was 110.7 degrees, 113.26 degrees, 121.03 degrees, and, 126.28 degrees in order of increasing surface roughness. The two-phase flow of the channels with different contact angles was visualized by a high-speed camera and the flow pattern maps were constructed. Even under the same phase superficial velocity conditions, differences in the degree of hydrophobicity lead to transitions to entirely different patterns. The two-phase flow pattern map with the change in hydrophobicity is partitioned by the change in the modified Weber number as a factor indicative of the degree of surface hydrophobicity. The modified Weber number is defined as the ratio of flow inertia to adhesion force, and the new pattern map of the two-phase flow is redefined to take into account the wall hydrophobicity change.