Modelling of annular two-phase flow in horizontal and vertical pipes including the transition from the stratified flow regime

Abstract

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.