Hydrodynamic and statistical parameters of slug flow

In two-phase slug flow pattern, the bulk of the gas is trapped inside large bubbles that are separated by liquid slugs, which may contain small dispersed bubbles. The unsteady nature of slug flow makes the prediction of pressure drop and heat and mass transfer a difficult task. Earlier models that deal with steady slug flow assume constant lengths and shapes of liquid slugs and elongated bubbles, as well as a constant elongated bubble propagation velocity. However, due to the intrinsically irregular character of slug flow, statistical means are required for its proper description. Variation of the flow parameters along the pipes of various diameters and inclinations may strongly affect the resulting flow pattern and should thus be taken into account in modeling the flow. The development of slug flow along the pipe is mainly governed by the interaction between consecutive elongated bubbles. To gain a better insight into the mechanisms that govern slug flow evolution along pipes, experiments with controlled injection of consecutive elongated bubbles were performed in recent years. Due to the complexity of both the continuous slug flow and the liquid flow around injected bubbles, sophisticated experimental methods are required. The latest works regarding the hydrodynamic and statistics of naturally occurring continuous slug flow in pipes, as well as the results of experiments with controlled injection of elongated bubbles are reviewed. It is demonstrated how the information obtained in the controlled experiments can be applied to improve the performance of slug flow and slug tracking models.