The mean air-volume fraction model is valid for both internal and external
airline systems, and a simplified version for external airline systems is given by Giot (1982). The
model, although simple in use, has two major drawbacks:
(i) predictions are acceptable for wells up to 11 meters deep, due to the assumption of a single flow regime along the upriser (slug flow), and
(ii) the model is heavily dependent on empirical information (correlations) needed for air-volumefraction and friction drop calculations.