Heat transfer and flow characteristics of Taylor flow in vertical capillaries with tube diameters ranging from 0.5 mm to 2 mm have been investigated numerically with the Volume of Fluid (VOF) method. Streamlines, bubble shapes, pressure drops, and heat transfer characteristics of Taylor flow were investigated in detail. The results indicate that the dimensionless bubble length increases with increasing Re, while the variation of diameters have slight influence on it. A flat tail and sharper nose bubble with longer bubble length and thicker film thickness are obtained at higher Re for the increasing inertia force. Pressure drops in liquid slug region are higher than single phase flow because of the Laplace pressure drop. The flow pattern dependent model and modified separate model in this work can predict the simulation data well with a MAE of 2.416% and 2.289%, respectively. Bo and Re are adopted in the modified separate model to taking surface tension, gravity, inertia, and viscous force into account. The wall temperature Tw increases along X axis in liquid region, and gets its peak at the tail of Taylor bubble region. Nutp, which is about 1.2∼3 times of fully developed single phase flow with constant wall heat flux, is negatively proportional to the dimensionless liquid slug length (Ls*). Taylor flow can enhance the heat transfer efficiently.

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