Numerical simulation of flow boiling from an artificial cavity in a microchannel Rahim Jafari a,b , Tuba Okutucu-Özyurt a,⇑ a Department of Mechanical Engineering, Middle East Technical University, Dumlupınar Blv. No: 1, 06800 Ankara, Turkey b Mechanical Engineering Department, University of Turkish Aeronautical Association, Bahçekapı Quarter Okul Street No:11, 06790 Etimesgut, Ankara, Turkey article info Article history: Received 15 July 2015 Received in revised form 4 February 2016 Accepted 4 February 2016 Keywords: Bubble nucleation Numerical simulation Evaporation Microchannel Subcooled water Artificial cavity abstract Cahn–Hilliard phase-field method is used to numerically simulate subcooled water boiling in which nucleation occurs from an artificial cavity on the inner surface of a microchannel. The boiling initiates from a cavity and the growth and departure of the nucleated bubbles from the cavity are simulated. The velocity, temperature and pressure distributions inside the microchannel have been analyzed. The bubble generation frequency increases by increasing inlet mass flux. For this case of study, the rising trend continues up to the mass flux of 64 kg/m 2 s. Further increase in the inlet mass flux does not accel- erate the bubble formation. The radius and the shape of the generated bubble compared well with exper- imental data available in the literature. Ó 2016 Elsevier Ltd. All rights reserved. 1. Introduction Flow boiling in microchannels has become an operative cooling method to dissipate the generated high heat fluxes from compact electronic devices. The effect of several design parameters such as mass flux, heat flux, channel dimensions and exit vapor quality have been investigated to clarify flow boiling heat transfer charac- teristics in microchannels. Some review articles have reported these numerical [1] and experimental [2–3] studies and future research directions [4]. In nucleate boiling, vapor bubbles are formed in discrete cavities on heated surfaces and grow up from the active cavities. The expanded bubble along the heated walls fills the entire cross section of the microchannel in milliseconds, and eventually, an elongated bubble or slug flow appears in the microchannel. According to Hsu [5], a cavity gets activated when the liquid temperature at the top of the vapor embryo is at least equal to the saturation temperature corresponding to the pressure in the bubble embryo. Wang and Dhir [6] developed a gas/vapor entrap- ment criterion stating that the cavity will trap gas/vapor if the con- tact angle is greater than the minimum cavity side angle. For spherical and conical cavities, the mouth angle is the minimum side angle. Liu et al. [7] developed an analytical model to predict the incip- ient heat flux as well as the bubble size at the onset of flow boiling based on fluid inlet velocity, subcooling, contact angle, microchan- nel dimensions and fluid exit pressure. Kandlikar [8] presented the relationship between the local bulk subcooling and local wall superheat as a function of nucleation cavity diameters. He obtained the critical cavity radius at the onset of nucleate boiling (ONB) which is the first cavity that will nucleate (if present). Owing to the difficulties in obtaining detailed measurements of two-phase flow and phase-change heat transfer experimentally, numerous investigations have been performed numerically. For this reason, numerical models remain as valuable tools to simulate boiling in microchannels. Zu et al. [9] performed a 3-D numerical simulation of bubble formation using the volume of fluid (VOF) method in the commer- cial CFD software ANSYS FLUENT. In the mentioned study, to avoid the well-known difficulties of modeling bubble generation and growth, the simulations were based on the concept of pseudo- boiling in which the bubble is generated by the injection of vapor from a hole through the heated side wall of the channel. The hole serves as a nucleation site, and the bubble growth is then driven by a constant heat flux. Gong and Cheng [10] simulated the bubble generation in satu- rated water from a microheater wall which has been embedded in a 2-D microchannel. The lattice-Boltzmann method for liquid– vapor phase change heat transfer [11,12] has been adopted to http://dx.doi.org/10.1016/j.ijheatmasstransfer.2016.02.028 0017-9310/Ó 2016 Elsevier Ltd. All rights reserved. ⇑ Corresponding author. E-mail address: okutucu@metu.edu.tr (T. Okutucu-Özyurt). International Journal of Heat and Mass Transfer 97 (2016) 270–278 Contents lists available at ScienceDirect International Journal of Heat and Mass Transfer journal homepage: www.elsevier.com/locate/ijhmt