Laminar bubbly flow in an open capillary channel in microgravity

Abstract

The study of a bubbly laminar two-phase flow in an open capillary channel under microgravity conditions was conducted aboard the sounding rocket, Texus-45. The channel consists of two parallel plates of width b = 25 mm and distance a = 10 mm. The flow along the length l = 80 mm is confined by a free surface on one side and a plate on the opposite side. The bubbles are injected at the nozzle of the capillary channel via six capillary tubes of 100 μm in inner diameter. Different liquid and gas flow rates were tested leading to different liquid free surface shape and bubble size. The effect of the gas flow rate on the shape of the free surface is analysed from image processing. The local curvature of the free surface can be related to the local wall friction taking into account the entrance effects in the channel. The wall shear stress is found to be increased in two-phase flow by comparison to single-phase flow. The mean bubble velocities averaged over the channel cross section are also measured by image processing. They are directly proportional to the mixture velocity. The bubble size increases along the channel due to coalescence. The bubble size evolution is well predicted by the coalescence model previously developed by the authors.