Accumulation of liquid, as oil and/or water, at the bottom of an upward inclined pipe is known to be the source of many industrial problems, such as corrosion and terrain slugging. Accurate prediction of the critical gas velocity that can unload the liquid accumulation is of great importance. An experimental study has been conducted in an upwards inclined 0.076m (3″) ID pipe to investigate the effects of the inclination angle on this critical gas velocity for air/water two-phase flow. The parameters of this study cover a range of inclination angle from 2° to 20° and superficial gas velocities up to 32 m/s for a constant liquid superficial velocity of 0.005 m/s. In the transitional region from stratified to slug flow patterns, a different and coherent flow structure has been observed. This flow pattern has been classified as pseudo-slugs consistent with the existing literature. The current study also investigates the distribution of the pseudo-slug frequencies along the pipe as well as their translational velocities as functions of inclination angle and superficial gas velocity.


For an inclined pipeline, liquid accumulation takes place when the amount of momentum transfer from the gas phase to the liquid phase is not sufficient enough to overcome the opposing forces of gravity (acting on the liquid phase) and to some extent friction. The corresponding minimum superficial gas velocity that can continuously remove liquid phase out from the upward inclined pipe is referred to as critical gas velocity. For gas flow rates below this critical gas velocity, the liquid phase starts to accumulate at the bottom of the upward inclined pipe, and partially or completely blocks the cross section of the pipe, forming intermittent flow. Based on the literature, the most susceptible areas for internal corrosion in pipeline correspond to no-flow and water and/or solid accumulation regions. All the methods proposed for internal corrosion management require the use of flow simulators to predict the water accumulation regions (Mogohissi et al. (1), Carimalo et al. (2), Lagad et al. (3), Moghissi et al. (4) and Hauguel et al. (5)). Liquid holdup is observed to increase significantly when gas velocity is reduced below the critical gas velocity, which promotes intermittent flow and steeply increases the pressure loss. Due to the intermittent nature of the two-phase flow, the oscillations of pressure gradient and liquid production could also cause fatigue of the pipeline itself and disturbed the downstream facilities. Therefore, accurate predictions of the critical gas velocity, oil and/or water holdup for the intermittent flow is of great importance for operational purposes. Between the transitions from stratified to slug flows, a different and coherent flow structure has been observed.

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