MODELING TRANSIENT TWO-PHASE SLUG FLOW

A transient two-phase flow simulator has been developed to predict the flow variables of slug structures in pipes as a function of space and time. The transient code is based on a mathematical two-fluid model coupled with a hydrodynamic slug flow model, and solved by a numerical approximation technique. Isothermal flow is assumed, and equations which describe the conservation of mass for the liquid and gas components, and the conservation of linear momentum for the gas and liquid phases constitute the two-fluid model. The concepts of unequal phase velocity and unequal phase pressure are included in the system of equations. A black oil model is used to describe the interphase mass transfer. Averaged slug flow properties are predicted by a mechanistic slug flow model. An explicit finite difference scheme is used to solve the initial value-boundary value problem described by the partial differential equations of the two-fluid model. Transient two-phase slug flow is analyzed by the propagation of an averaged two-fluid flow, and restructuring of this flow to a slug structure at each time level.; Transients which are induced at the flow boundaries of the pipeline by fluctuations in the flow rates of the fluids and flowing pressure can be analyzed with the code. Several example simulation problems were formulated to investigate the boundary conditions associated with the supply-demand situation. The slug flow variables predicted by the simulator during the transient flow period were consistent with physical principles. This led to confidence in the algorithm which is used to couple the two-fluid model with the hydrodynamic slug flow model. Results simulated for upward inclined pipelines indicated that gravitational force can significantly affect the magnitudes of the transient effects. However the general trends in the transient behavior remain the same. It is suggested that the two-fluid model can be coupled with a mechanistic model unique to any of the other flow patterns present in a two-phase flow, to predict the transient flow variables for that particular flow pattern.