A mechanistic model is formulated to predict the flow behavior of two-phase mixtures in horizontal or slightly inclined fully eccentric annuli. The model is composed of a procedure for flow pattern prediction and a set of independent models for calculating gas fraction and pressure drop in stratified, intermittent, dispersed bubble, and annular flow.

Small-scale experimental data performed in a 50 m long straight 4" (101.6 mm) ID pipe containing a 2" (50.8 mm) OD tube lying at the bottom validate the predictions of the model. Test matrix covered not only the horizontal inclination, but also –4° and +4° relative to the horizontal. A total number of 115 tests were carried out with the following mixtures: (1) air and water were used in 61, and (2) diesel oil and N2 in 54 tests.

The mechanistic model presents also better performance when compared to the results of some empirical models, as the Beggs and Brill1  correlation and a modified version of the Aziz, Govier, and Fogarasi2  method.

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