The bubble coalescence is discussed in many chemical and metallurgical phenomena. In this paper, the coalescence process of two bubbles moving in a cylindrical tube is studied using both 2D and 3D numerical simulations. The Navier-Stokes equations along with an equation for the interface advection by the Volume-of-Fluid (VOF) method are solved. The results are compared with experimental data reported in the literatures. Simulations are performed for various cases with different configurations of the two bubbles. First, the coaxial bubble motion is studied. The Reynolds number, the density ratio and the viscosity ratio are held constant. The results show that by increasing the Bond number the coalescence time decreases and the leading bubble reveals a more concave interface. Both of these effects make the two bubbles to coalesce faster. Next, the motion of the two bubbles rising side-by-side is studied. On the basis of different Weber numbers based on the approach velocity of the bubbles and the rise velocity, either coalescence or separation will occur.

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