Critical review of vertical gas-liquid slug flow: An insight to better understand flow hydrodynamics' effect on heat and mass transfer characteristics

Slug flow is a dynamically complex two-phase flow pattern. A wide variety of industrial gas-liquid systems operate under slug flow conditions for not only flow transportation purposes, but also heat and mass transfer applications. In such systems, heat and mass transfer at the gas-liquid interface are strongly controlled via the hydrodynamic behavior of the flow, which is difficult to accurately predict. Therefore, to understand the interfacial heat and mass transfer phenomena in these systems, one should first comprehend slug flow's hydrodynamic behavior. The present review first explores the literature on the hydrodynamics of two-component, gas-liquid, two-phase slug flow moving upward within vertical macro-scale circular pipes. This exploration includes both stagnant and co-current flow conditions with Newtonian fluids. It then examines the interrelations between flow hydrodynamics and interfacial heat and mass transfer in slug units. The emphasis is placed on explaining and criticizing the experimental approaches, theoretical models, and empirical correlations. Also, the characteristics of slug flow hydrodynamics and how they affect heat and mass transfer at the gas-liquid interface are identified and clarified. Furthermore, the review detects challenges and gaps in literature. Many factors were found to profoundly impact the gas-liquid heat and mass transfer in slug flows. These included the following: the length, rising velocity, and interfacial area concentration of Taylor bubbles; the velocity distribution, turbulence intensity, and stabilization of the liquid phase in the film region; the behavior of the wake region behind Taylor bubbles; the size, rising velocity, velocity fluctuations and distributions, trajectory, passage frequency (i.e., residence time), interfacial area concentration, and shape oscillations of small bubbles in liquid slugs; as well as the void fraction distributions, and the velocity field of the liquid phase and its interactions with such bubbles. Nonetheless, flow development and interfacial heat and mass transfer in slug flow were found to remain not fully understood. Therefore, countless experimental and numerical studies are still required to understand it and develop reliable prediction models and correlations.