An accurate prediction of the liquid holdup and pressure drop using the one-dimensional mechanistic slug flow models requires an accurate estimation of slug void fraction. Considerable theoretical and experimental research has been conducted in the literature to study the slug void fraction. However, almost, all these studies primarily focus on the vertical upward and horizontal slug flows. In this study, we investigated experimentally and theoretically the slug void fraction in vertical downward flow. A series of experiments for measuring the slug void fraction in air–water flow using the conductance probe technique with a pipe diameter of 34 mm were conducted. The relationship between the measured slug void fraction and the mixture velocity revealed the presence of three distinct zones. This observed behavior, which differs from what has been reported in vertical upward flow, was explained through a mechanistic model that considers the varying velocities of liquid slugs, small gas bubbles, and Taylor bubbles. A new empirical predictive model based on the liquid-to-gas superficial velocities ratio and input liquid fraction was proposed. The performance of the proposed model was compared to existing models and demonstrated the lowest error. It showed a good performance with an average relative error of −3.43% and an average absolute relative error of 12.97%. The assessment study of the existing models developed for vertical upward flow showed that they failed to predict correctly the slug void fraction in vertical downward flow.
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