Flow boiling of water in a minichannel: The effects of surface wettability on two-phase pressure drop

Experiments were performed to study the effects of surface wettability on two-phase pressure drop of flow boiling of water at atmospheric pressure. The test channel is a single rectangular channel 0.5 mm high, 5 mm wide and 180 mm long. The mass flux was set at 100 kg/m2 s and 120 kg/m2 s, respectively. The base heat flux varied from 30 to 80 kW/m2. Water enters the test channel under subcooled conditions. The study has been performed at low exit vapour quality (less than 0.1). The samples are either hydrophilic like Polydimethylsiloxane (SiOx), Titanium (Ti), Diamond-Like Carbon (DLC) or hydrophobic like Polydimethylsiloxane (SiOC). These surfaces have static contact angles of 26°, 49°, 63° and 103°, respectively. It was observed that the total two-phase pressure drop significantly increases with the static contact angle. In particular, the average deviation between the highly-wetted and the unwetted surfaces is about 170%. To explain this observation, the “wetting pressure drop” notion caused by the surface tension forces generated at the triple contact lines is introduced. Afterwards, a model is proposed to predict the wetting pressure drop as a function of the static contact angle. This model shows a good agreement with the experimental data with 86% of the data included within the lines of 20% error.