Investigation of bubble structure in a microchannel under microgravity conditions: Effects of discontinuous wettability with dynamic contact angle

Abstract

A thorough understanding of the dynamics of bubble/liquid two-phase flows under microgravity conditions is essential for achieving efficient heat transfer between spacecraft and space stations. This paper examined the influence of the discontinuous wettability of a microchannel's wall on bubble structure movement under microgravity conditions. The volume-of-fluid methodology involving a dynamic contact angle code was employed by considering wall-adhesion equations to predict the form of the bubble/liquid interface at and near the wall. The flow field's governing equations were numerically solved utilizing the finite volume method. Initially, the results were compared to previously reported data for validation. Subsequently, the effects of discontinuous wettability with different configurations on the structure of bubbles were investigated; new insights were obtained into the size and shape of generated bubbles, as well as the slug length as a function of the wall wettability. The results showed that for a uniformly hydrophilic surface, the bubble had a concave shape under microgravity conditions, while for a uniformly superhydrophobic surface, the bubble had a convex shape. A microchannel wall surface with discontinuous wettability led to the generation of an asymmetric bubble, a variation in slug length, changes in the bubble's length and shape, and a continuous phase structure. Furthermore, variations in the wall wettability caused the flow upstream to vary and affected all four stages of bubble generation, namely lag, filling, necking, and the starting point of detachment.