Experimental study of air-water two-phase flow pattern evolution in a mini tube: Influence of tube orientation with respect to gravity

The evolution of two-phase flow patterns is investigated experimentally in a 1500 mm long capillary tube of 3 mm inner diameter with different orientations with respect to gravity. The flow patterns are recorded with a fast camera (100 0 fps) at two locations along the tube under the same operating conditions. The first is close to the inlet zone at L/D = 10 and the second is far downstream at L/D = 420. Coflowing configurations downward and upward are considered where air is injected through a nozzle of 110 mu m of inner diameter and centered on the tube axis. Unlike many existing studies in the literature, the present work explores ranges of small superficial velocities of the two fluids, from 0.78 x 10(-3) m/s to 79 x 10(-3) m/s for the liquid phase and from 2.3 x 10(-3) m/s to 3.54 m/s for the gas phase, in order to deal with smooth interfaces. The experiments show that the flow patterns are very sensitive to inlet conditions. Although, the superficial velocities considered are very small, several two-phase flow patterns are visualized namely bubbly flow, slug flow, unstable annular and annular flow. In the upward flow, the annular pattern is unstable, while for the downward flow, the bubbly pattern is difficult to obtain. Phenomena like coalescence and breakup yield to the evolution of the flow pattern from the entrance of the tube to the downstream location. The visualizations highlight also that certain flow patterns evolve from one structure to another along the tube from the location L/D = 10 to L/D = 420. Furthermore, the two-phase flow maps are drawn and the effect of gravity on the flow maps as well as on the flow pattern transitions is discussed. (C) 2020 Elsevier Ltd. All rights reserved.