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

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 (Uls = 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.