Characterization of Two-Phase Wakes in an Upward Adiabatic Liquid-Gas Flow Around a Cylinder

Two-phase wakes generated from a cylinder in a crossflow were experimentally studied. A water–air mixture traveled through a vertical water channel with a rectangular cross section, in which a cylinder was installed horizontally. Liquid Reynolds numbers, based on a cylinder diameter of 9.5 mm, were varied from Re = 100 to 3,000; the air superficial velocities were varied from j_g = 0.06 m/s to 0.60 m/s; and mean bubble diameters were varied from 0.48 mm to 3.5 mm. Void fraction distribution in the wake of the cylinder was determined from high-speed visualizations, where a correlation was applied to the shadow fraction measurements to account for overlapping bubble images. It divided the wakes into a liquid-phase region with a low void fraction relative to its freestream condition (α/α_∞<1/2) and a bubble-trapping region with a relatively high void fraction (α/α_∞>2). The liquid-phase region occurred in all flow conditions, but its length decreased with increasing Reynolds number. In contrast, the bubble-trapping region occurred only at relatively high Reynolds numbers depending on the bubble size and air superficial velocity. Transitional bubble-trapping behavior was identified at Re = 1,200 for the 3.5 mm bubbles, where bubble trapping only occurred at low air superficial velocities. Once the bubble-trapping region developed sufficiently, the location of the maximum void fraction was consistently located at y/D = 1.3–1.5 downstream from the center of the cylinder.