The role of surface wettability on the heat transfer in liquid-liquid two-phase flow in a microtube

Abstract

Liquid-liquid two-phase flow is capable of boosting heat transfer in microdevices compared to the single-phase and gas-liquid flows. A thorough investigation is performed here to characterize the heat transfer in water-oil flow in a microtube. Finite element method along with the level-set model is employed for numerical simulation. A main part of this paper is devoted to studying the effect of wettability on the heat transfer performance. Four contact angles of 0°, 30°, 150°, and 180° are investigated, which revealed that the contact angle of 150° produces the highest Nusselt number (Nu). Triple points form at this contact angle, and the slugs slide on the wall, which results in more significant wall shear and slip velocity on the wall. Based on the observed flow configuration, a novel idea is developed to use the nonuniform distribution of contact angle to augment the local Nu. It is observed that changing the wall from hydrophobic to hydrophilic will locally increase Nu around the transition point. In addition to the contact angle, the slug length, frequency of slug generation, and the film thickness around the slugs affect Nu. Three Weber numbers (We) at four contact angles are examined by varying the flow rate of the oil phase in the next part of the paper. We affects Nu by changing the frequency of slug generation and consequently its length. Finally, the effect of film thickness is scrutinized at various capillary numbers (Ca). The film thickness increases with Ca which reduces the heat removal rate.