A Lagrangian Slug Capturing Scheme for Gas-Liquid Flows in Pipes

Abstract

In this thesis a new Lagrangian numerical scheme for the simulation of gas-liquid flows in pipelines is presented. Based on an approximate two-fluid model, this new scheme, called LASSI (Lagrangian Approximate Scheme for Slug Initiation) is dedicated to the modelling of the transition between stratified and slug flow. It is able to capture directly the slug initiation process and to track the motion of every single slug in the pipe without numerical diffusion. It can thus be qualified as a slug capturing and slug tracking scheme.

The scheme is based on the decoupling between the fast pressure dynamics governing the motion of the slugs and the much slower liquid transport in the bubbles. The liquid motion in the bubbles is then approximately modelled by a modified version of the shallow water equations, in which the influence of the Bernoulli suction force is subtracted from the traditional hydrodynamical term. The fully Lagrangian structure of the scheme makes it possible to accurately capture the transport of the fast interfacial waves whose growth can eventually result in a slug initiation.

The model predictions are compared with some experimental results obtained in the NTNU multiphase flow laboratory or taken from the literature. Several flow conditions are considered, in particular the case of hydrodynamic slugging in near-horizontal flows, slugging in an upwards pipe, severe slugging, and slugging triggered by fast transients. Numerical predictions are also compared with theoretical considerations regarding the stability of stratified flow.