A VISUAL STUDY OF SLUG BUBBLES IN MINIATURIZED CHANNELS

This paper deals with the bubble rising behaviors in vertical miniaturized channels closed at the bottom and filled with a stagnant liquid. Our theoretical analysis shows that the drag on a bubble increases significantly as the circular tube diameter decreases and approaches the bubble's diameter. As a result, the up-motion of the gas bubble is slowed down, and ceases completely when the circular tube size is sufficiently small. This bubble rising behaviors is experimentally verified and it is found that the critical tube diameter is 2.9 mm for a given gas nozzle, which equals to the bubble departure diameter in the pool with the same nozzle. More importantly, the results of the experimental investigation show that for the noncircular tubes the elongated bubble always rises upward even though the hydraulic diameter is as small as 0.866 mm. This peculiar phenomenon is of importance for the study of two-phase flow and boiling heat transfer in miniaturized channels.