Abstract—The hydrodynamics and heat transfer characteristics of a vaporized elongated bubble in a rectangular microchannel have been simulated based on Cahn-Hilliard phase-field method. In the simulations, the initially nucleated bubble starts growing as it comes in contact with superheated water. The growing shape of the bubble compared well with the available experimental data in the literature. Keywords—Microchannel, boiling, Cahn-Hilliard method, Two- phase flow, Simulation. I. INTRODUCTION LOW boiling through microchannels has been extensively studied as a cooling alternative for microelectronic devices because of its capability of providing a high heat transfer rate. Vaporized bubble of microscopic size in a microchannel grows rapidly and fills the entire cross section of the microchannel in milliseconds, and eventually, an elongated bubble or slug flow appears in the microchannel. Moreover, at the microscale, the surface tension and evaporation momentum forces are the dominant forces controlling the bubble dynamics [1]. Dong et al. [2] investigated the effect of bubble nucleation, growth and departure on fluid flow and heat transfer in a microchannel via lattice Boltzman 2-D modeling. A single seed bubble, a cavity, two cavities, one seed bubble and a reentrant cavity were simulated in a microchannel with dimensions of 0.2mm×5.3mm. Sun et al. [3] proposed a vapor-liquid phase model in ANSYS FLUENT which considers both superheated and saturated phases. The vapor near the wall gets heated and becomes superheated, which drives the mass transfer at the interface. The vapor stays motionless while the saturated liquid and the interface are driven away from the wall. Magnini et al. [4] implemented ANSYS FLUENT to investigate in detail the bubble dynamics and the wall heat transfer of flow boiling in a circular microchannel of diameter 0.5 mm in 2-D axisymmetrical formulation. Different refrigerants, namely, R113, R134a and R245fa were investigated with two different saturation temperatures of 31°C and 50°C. The bubble nose acceleration to downstream was in good agreement with a theoretical model [5]. R. J. is a PhD. Candidate in the Department of Mechanical Engineering, Middle East Technical University, Dumlupınar Bulvarı, No:1, 06800, Çankaya, Ankara, Turkey (Corresponding author; phone: 534 299 3004; e- mail: e170530@metu.edu.tr) T. O. is an Associate Professor in the Department of Mechanical Engineering, Middle East Technical University, Dumlupınar Bulvarı, No:1, 06800, Çankaya, Ankara, Turkey (phone: 312 210 2575; e-mail: okutucu@metu.edu.tr) Mukherjee et al. [6] studied a vapor bubble growing on a heated wall inside a microchannel with a hydraulic diameter of 229 μm. They solved the continuity, Navier- Stokes and energy equations using the SIMPLER algorithm. Firstly, the water bubble growth rate and the bubble shape were validated by experimental results. Then a parametric numerical study was carried out to analyze the effects of the wall superheat, the inlet liquid flow rate, the surface tension and the contact angle on the bubble growth rate inside the microchannel. The aim of study is to employ the phase-field model to investigate the hydrodynamics and heat transfer characteristics of two-phase flow during nucleate boiling in microchannels. II. PHASE-FIELD METHOD The interface of two immiscible fluids often needs special consideration. One method of handling moving boundaries is to keep track of the motion of material points residing on the interface. Numerically, this may be realized by using grid points moving either with the local fluid velocity or a mesh velocity. This Lagrangian approach is often known as interface tracking. However, interfacial deformation causes some difficulties as remeshing and interpolation increasing the computational cost and error. An alternative to interface tracking is to track the fluid flow of both components on a fixed Eulerian grid, with the interface being determined or reconstructed at each time step by using a scalar indicator function. Examples of this class of methods are the volume of fluid (VOF) method, the level-set method (LS) and the phase- field method [7]. The diffuse interface models for a wide variety of interfacial phenomena such as binary fluids are addressed in literature [8]-[10]. The interface topology is estimated poorly by the volume of fluid approach used to calculate the surface tension force [11]. The phase-field method not only convects the fluid interface as in the level set method, but it also ensures that the total energy of the system diminishes correctly. The phase-field based models replace sharp fluid-material interfaces by thin but nonzero thickness transition regions in which the interfacial forces are smoothly distributed [12]. The phase-field method has been broadly used in physics, material science [13], fracture mechanics [14] and multiphase flow [15], [16]. The basic idea is to introduce an order parameter or phase-field that varies continuously over thin interfacial layers and is mostly uniform in the bulk phases. The order parameter has a physical meaning, and can be applied to different phase change phenomena by a proper modification of the free energy. An extremely thin interface layer is required to properly model the physics of the problem. In addition, CFD Modeling of Boiling in a Microchannel Based On Phase-Field Method Rahim Jafari, Tuba Okutucu-Özyurt F World Academy of Science, Engineering and Technology International Journal of Mechanical and Mechatronics Engineering Vol:9, No:4, 2015 632 International Scholarly and Scientific Research & Innovation 9(4) 2015 ISNI:0000000091950263 Open Science Index, Mechanical and Mechatronics Engineering Vol:9, No:4, 2015 publications.waset.org/10001158/pdf