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ectories of bubbles with various initial diameters that rise in water ( Krishna and Baten, 1999); the coalescence of two gas bubbles (Chen and Li, 1998 ; Annaland et al. 2005 ); the effect of the bubble size/shape and neighboring bubbles on the magnitude and direction of the lift force (Rabha and Buwa, 2009); etc. The simulation results showed good agreements with the experimental data. However, most bubble simulations were limited to cases of adiabatic systems such as air bubble-water flow, where the heat and mass transfer between
each phase were not considered. Those simulation results for bubble behavior are only valid for analyzing bubble behavior in adiabatic systems.

  However, in subcooled boiling flow, bubble condensation is the key parameter to describe heat and mass transfer phenomena. It significantly affects the shape and the area of the varying interface thus the behavior of the condensing bubble becomes different from that of the adiabatic bubble. Therefore, in order to understand bubble behavior in subcooled boiling flow, a numerical study of condensing bubbles considering heat and mass
transfer through the bubble interface is required.

   This study focused on how to simulate bubble condensation with a CFD code. Moreover, using the VOF model, the behavior of condensing bubbles in subcooled boiling flow was investigated. In order to simulate the condensing bubble with the FLUENT code, the bubble condensation was modeled using the user-defined function
(UDF). For the validation of the UDF of bubble condensation, the results of CFD simulation were compared with the SNU experimental results. Through the VOF model coupled with the UDF of bubble condensation, the fundamental behavior of condensing bubble in terms of the bubble velocity, rise distance and moving trajectory was investigated under various conditions. The effects of condensation on bubble behavior were analyzed by comparing the behavior of condensing bubbles with that of adiabatic bubbles.

2. VOF model
  In this study, the volume of fluid (VOF) model in a commercial CFD code FLUENT 6.2.16 (2001) is used to simulate condensing bubbles in subcooled boiling flow. The gas and liquid phases are considered as incompressible fluids and the flow is assumed to be laminar.

References
[1] Annaland, M.S., Deen, N.G., Kuipers, J.A.M., 2005. Numerical simulation of gas bubbles behavior using a three-dimensional volume of fluid method. Chemical Engineering Science 60, 2999–3011.
[2] Bothe, D., Schmidtke, M., Warnecke, H.J., 2006. VOF-simulation of the rise behavior of single air bubbles in linear shear flows. Chemical Engineering and Technol-ogy 29, 1048–1053.
[3] Chen, L., Li, Y., 1998. A numerical method for two-phase flows with an interface. Environmental Modeling & Software 13, 247–255.
[4] Clift, R., Grace, J.R., Weber, M.E., 1978. Bubbles, Drops and Particles. Academic Press. Fluent 6.0 Users Guide Documentation, 2001. Fluent Inc., Lebanon, New Hampshire.
[5] Gopala, V.R., Wachem, B.G.M., 2008. Volume of fluid methods for immiscible-fluid and free-surface flows. Chemical Engineering Journal 141, 204–221.
[6] Hirt, C.W., Nichols, B.D., 1981. Volume of fluid (VOF) method for the dynamics of free boundaries. Journal of Computational Physics 39, 201–225.
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