Effect of forced flow oscillations on churn and annular flow in a long vertical tube

An experimental study has been carried out to investigate the effects of periodic forced flow oscillations on gas-liquid two-phase flows using a 42 m long, 0.05 m ID vertical pipe system. From the knowledge of the authors, there is no experimental work available in the literature on the forced flow oscillations for churn flow regime. Therefore, the main objective of this paper is to experimentally characterize churn and annular flows in a long vertical tube. The time variation of liquid holdup, pressure drop and pressure gradient were analyzed under two superficial liquid velocities (0.017 and 0.3 m/s), and superficial gas velocities oscillations ranging from 4 m/s to 21 m/s. The impact of flow oscillations in two-phase flow regime transitions is also investigated. The observations of flow regimes under oscillatory conditions showed significant differences compared to the ones expected in steady-state flow under the same conditions. Under oscillatory conditions, the local liquid holdup and the amplitude of its waves decreased with the distance from the inlet. For the experimental runs with oscillatory gas flow and low superficial liquid velocities (U-ls = 0.017 m/s), the liquid holdup at the bottom of the tube decreased while the gas flow rate was lowered, whereas in steady-state conditions the liquid holdup should increase while reducing superficial gas velocities. This process is described in this study by the formation of large waves in the bottom of the pipe as a consequence of the flow oscillations. (C) 2016 Elsevier Inc. All rights reserved.