In this paper, the rising of a single bubble and coalescence of two co-axial gas bubbles in a viscous liquid are simulated using a transient 2D/axisymmetric model. To predict the shape of the bubble deformation, Navier-Stokes equations in addition to an advection equation for liquid volume fraction are solved. A modified Volume-of-Fluid (VOF) technique based on Youngs’ algorithm is used to track the bubble deformation. To validate the model, the results of simulations for terminal rise velocity and bubble shape are compared with those of the experiments performed by Grace (1973). Next, the effect of different parameters such as initial bubble radius, channel height, and liquid viscosity and surface tension on the shape and rise velocity of the bubble is investigated. Finally, the interaction of two co-axial gas bubbles in a liquid is simulated, and the computed bubble shapes is compared with experimental observations performed by Brereton and Korotney(1991) .

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