Since it was presented by Wallis1  the unit cell model for slug flow has proved its superiority in predicting the phase fractions and pressure gradient with a reasonable accuracy. However it relies on some crucial assumptions and on the choice of four closure laws. These fundamental issues are addressed in the present paper.

On the one hand, the closure laws guarantees the accuracy of the model. The scientific papers provide abundance of these laws whose physical value is unequal. The effectiveness and weakness of some of these laws are discussed from a comparison with existing data. Their critical role for certain ranges of flow rates is emphasised.

On the other hand, some specific assumptions limit the potentiality of the model. In particular the fully developed flow assumption in long bubble and liquid slug is discussed. A route to improve the model is explored.

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