Development of two-phase flow downstream of a horizontal sudden expansion

Experiments were performed to characterize the development of air–oil two-phase flow downstream of horizontal sudden expansions with area ratios of 0.0625 and 0.25. The tests were performed at oil and air superficial velocities in the range 0.02–0.756 and 0.136–3.75 m/s. In addition to the axial variation of the pressure and area-averaged void fraction, time-averaged local profiles of the void fraction, mean liquid velocity and turbulence intensity were measured downstream of the expansion using hot-film anemometry. The fully developed downstream void fraction can either increase or decrease from its upstream value, and depends on the flow pattern change across the expansion. The turbulence intensity is higher in the immediate vicinity of the sudden expansion. It then decreases with axial distance and asymptotes to an approximately constant value. The phase redistributions immediately downstream of the expansion and the developing length was found to be strongly dependent on the upstream flow pattern and the sudden expansion area ratio.