In this paper the three dimensional coalescence of two stationary head-on droplets with equal diameters are numerically investigated. The Navier-Stockes equations are solved in conjunction with the Volume of Fluid (VOF) method to track the free surface of the droplets. Furthermore, surface tension is added to the Navier-Stokes equations as the only body force causing the coalescence phenomenon. Water droplets are considered to have a spherical shape with a specified gap size between their free surfaces. It is found that the coalescence of stationary droplets driven by surface tension is due to the pressure variation in their surrounding fluid. It is also shown that for the coalescence to occur, the gap size has to be less than a certain limit called critical gap size. Finally, the effects of important parameters including surface tension, viscosity and density ratio of two phases on the critical gap size are studied. The results show that increasing the surface tension value and gas-to-water density ratio increases the critical gap size. In contrast, increasing the gas-to-water kinematic and dynamic viscosity ratio decreases the critical gap size.

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