Similarities and differences in churn and annular flow regimes in steady-state and oscillatory flows in a long vertical tube

This study investigates the effects of induced-low-frequency flow oscillations on gas-liquid two-phase flows using a 42-m long, 0.048-m ID vertical pipe system. This work uses the experimental data set from a previous study on the effect of forced flow oscillations on churn and annular flow. However, in this previous study, the similarities and differences between flows in steady-state and under induced-low-frequency oscillations were not discussed. The time variation of liquid holdup, frequency of large liquid structures, and pressure gradient were analyzed under two superficial liquid velocities (0.017 and 0.3 m/s), and superficial gas velocities ranging from 4 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 flow showed significant differences compared to the ones expected in steady-state flow under similar conditions. For higher pressures and oscillatory flow, churn flow regime was observed, when annular flow regime is expected in steady-state flow. Annular flow regime was not observed for higher superficial liquid velocities and high inlet pressure for oscillatory conditions. Surprisingly, the pressure gradient for oscillatory flow is significantly lower than in steady-state flow. However, this lower pressure gradient effect is only observed for low superficial gas and liquid velocities at the pipe inlet and outlet. This experimental observation is attributed to the formation of large liquid waves at the pipe inlet, which is a consequence of the induced-low-frequency flow oscillations.