The thesis presents a general one-dimensional mathematical model to simulate
two-phase, gas-liquid, annular flow in horizontal as well as vertical pipes, and to
mechanistically predict the transition from stratified to annular flow in horizontal
pipes. The method is based on the transient one-dimensional two-fluid model
whereby the two phases are considered as (i) liquid layer and (ii) a mixture of
the gas and liquid droplets in which the droplet concentration in the mixture is
considered as a flow variable. The model entails the introduction of a scalar transport equation for the conservation of mass of liquid droplets accounting for liquid
transfer to and from the film liquid layer. The interface curvature is modelled by
a double circle geometric configuration incorporating a new empirical relation for
the specification of wetted angle. The droplet exchange rate between the liquid
film and gas core is modelled by employing droplet entrainment and deposition
rates derived from modifications of models existing in the literature. Using the
new model the droplet entrained fraction (E), which is defined as the ratio of the
droplet mass flow rate to the total liquid mass flow rate, is computed and validated
against different experimental data for both horizontal and vertical pipes. The
predictions show good agreement with most of the measurements, being within
30% of the data. This is a significant development since, unlike all other exist-
ing models, both horizontal and vertical annular flows can be predicted well with
the same model. Moreover, the transition point from the stratified to the annular
regimes in horizontal flow can also be predicted and the transition points compare
very well with the usual regime boundaries found in existing flow regime maps.