Numerical simulation of stratified-pattern two-phase flow in gas pipelines using a two-fluid model

Two-phase flows in pipelines occur in a variety of processes in the nuclear, petroleum and gas industries. Because of the practical importance of accurately predicting steady and unsteady flows along the line, two-fluid models have been extensively used in numerical simulations. These models are usually written as a system of non-linear partial-differential equations. A reliable prediction of such flows is a difficult task to accomplish due to the numerous sources of uncertainties, such as the basic two-phase flow model, the flow-pattern models, the initial and boundary conditions and the numerical method used to solve the initial-boundary-value problem. Several numerical methods, conservative or not, may be used to discretize the problem. In this paper we analyze the global performance of the numerical model, obtained when we couple a classical one-dimensional single-pressure four-equation two-fluid model to a numerical discretization based on the flux-corrected transport (FCT) method, to solve the stratified-pattern twophase flow that occurs in typical gas-gathering offshore pipelines. The gas phase is considered to be compressible and the liquid phase is incompressible, whereas the flow is assumed to be isothermal. We show that the FCT method is second-order in space and that our numerical kernel produces accurate simulations for steady, as well as unsteady, simulations. We also analyze the hyperbolicity property of the two-fluid model and the adequate prescription of the boundary conditions. The results show that hyperbolicity is lost in part of the area of the stratified-pattern region of a classical flow-pattern diagram. Moreover, the eigenvalues alternate signs at the boundaries, which has implications on how to impose the boundary conditions. Numerical simulations of typical offshore gas pipeline flows show very good agreement when compared to a commercial software. (C) 2016 Elsevier Ltd. All rights reserved.