Prediction of Sliding Bubble Velocity and Mechanism of Sliding Bubble Motion along the Surface

Sliding bubbles play an important role in heat transfer enhancement, the mechanism of sliding bubble motion along the surface is required for further comprehension in order to develop reliable heat transfer mechanism models. The momentum equation on a single sliding bubble in the x-direction is built on the analysis of the balance of forces, in which the sliding bubble velocity is obtained by a numerical method, and it is compared with the experimental data of Maity [2000]. The predicted results show that the sliding bubble velocity increases with time. The sliding bubble velocity for horizontal flow boiling is lower than that of local liquid at the sliding bubble center of mass when the bubble just lifts off the surface, whereas the sliding bubble velocity for vertical flow boiling is higher than that of bulk liquid when the bubble just lifts off the surface. The mechanism of sliding bubble motion along the surface can be explained by the analysis of forces in the x-direction, which indicates that the main forces controlling the sliding bubble motion are the buoyancy, quasi-steady drag force, and added-mass force. However, since there is lack of the driving force in the y-direction to lift the bubble from the surface for pool and vertical flow boiling, it is probable that the inertia of the liquid flow beneath the bubble base due to the change in the bubble shape is the driving force to lift the bubble from the surface.