Effects of swirl on flow patterns and liquid unloading of gas–liquid two-phase vertical flow under stagnant conditions

Liquid loading is a common problem in low-production gas wells faced in many producing regions around the world. Recently, swirl flow has been applied to remove the accumulated liquid. However, the effect of swirl on the flow characteristics in gas–liquid two-phase flow is largely unexplored, which is important for the application of swirl flow in the natural gas industry. Therefore, the effects of swirl on flow patterns and liquid unloading in gas–liquid two-phase vertical flow under stagnant conditions are investigated by visualization experiment in this work. A dimensionless parameter is proposed to quantitatively describe the capability of liquid unloading. The results show that interface morphology varies along the streamwise direction under stagnant liquid situations. In the case of slug flow, a swirler can suppress the fallback of falling liquid film. In the case of churn flow with relatively high gas velocity, the churn flow can be transformed to swirling annular flow once out of the swirler. The swirl is beneficial in unloading liquid when the gas velocity is relatively high. The critical gas velocity, which is defined as the minimum gas velocity to prevent the onset of liquid load up in the gas well, can be reduced in the swirl flow compared with that in non-swirling flow. In addition, the initial position of the liquid level has an influence on liquid unloading, and a swirler located below the initial position of the liquid level performs better in unloading liquid.