In the present study, two phase bubbly flow phenomenon in a two-dimensional rectangular channel (system) is studied numerically by employing the VOF (Volume of Fluid) method. Hydraulic diameter of the channel is of 0.2 mm and the Bond number, a value usually used to distinguish the regimes of microchannel and minichannel, is of 0.005, far less than 0.05. Air is injected through the inlet into the microchannel which is filled with water to form a bubble. The bubble developed from a hemisphere and reshape with the increase of air from the inlet. During the flow, the volume of the bubble increases and the bubble moves along the microchannel. Slug flow is simulated with single or multi bubbles inside the microchannel. Such two-phase flow at different Reynolds number and of different microchannel length is simulated to get according slug flow image in the microchannel. The dominance of different forces acting on the bubbles, such as the buoyancy force, surface tension, initial force, etc. are analyzed. Small multi-bubbles coalesce with each other after intermittently entering the microchannel if they stay in a short distance, or remain dispersed if they are apart far away. For the large slug bubbles, coalescence is difficult to occur. With a different wettability, the flow pattern displayed in the microchannel is different. For the hydrophilic solid wall, slug bubbles flow along the mainstream liquid, whereas for the hydrophobic wall, plug-type liquid appear in the microchannel with air entrained between them. This result is in accordance with experimental observation.

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