Two-phase flow is the simultaneous flow of liquid and gas in the same pipe, duct, or channel. This paper is concerned with transient two-phase flow in pipes, as occurs in gathering, transmission, or distribution systems or in steam pipes. Two-phase flow has generally been of more concern to nuclear or chemical engineers than pipeline engineers, because pipeline practice is to separate the phases where possible, while the presence of two phases may be essential to chemical processes. In analysing nuclear problems, one is commonly faced with forcing emergency cooling water down the same duct from which steam is escaping. However, some pipeline situations necessarily involve two phase flow, namely, sour gas systems, off-shore gathering systems, and distribution systems for evaporated LPG. Sour gas systems commonly use injected oil as a corrosion preventive; for off-shore gathering systems two-phase flow may be an alternative to barge collection or laying a second pipe; and cold weather condensation may be unavoidable for residential LPG distribution systems. It is known that the presence of a liquid phase in a gas pipeline usually increases the resistance to the flow out of proportion to the volume of the pipe filled with liquid. This is basically because of efficient momentum transfer at the gas/liquid interface, which may be enhanced by surface waves and entrainment. Since the liquid generally has a higher viscosity, so that it has more friction with the wall, the net effect is more resistance to the gas. Much of the problem in understanding two-phase flow in detail reduces to that of understanding the interface: its configuration and the momentum transfer. Much of the work done on two-phase flow by various investigators has been directed towards defining the conditions under which various types of interfaces, commonly referred to as "flow regimes", occur. Although there are some differences in nomenclature among the various investigators, it has been established that two-phase flow in pipes can occur as stratified, annular, slug, bubble, and mist. In a previous paper (Modisette. 1983), we have proposed an adaptation of the flow regime concept for modeling steady-state flow in pipelines. Stratified and annular flow are treated in a single model by assuming a transition from stratified to annular flow by a mechanism which consists of the liquid climbing the wall. The transition is smooth, and is driven in the model by the changing value of the Taitel-Dukler (1976) transition parameter which they use to define the boundary between the two regimes. Figure 2 illustrates the geometry of the transition. The transition to slug flow is modeled by increasing the momentum transfer between the liquid and the gas. In the steady-state paper, the model results were compared with the data of Beggs (1972) for air and water in small pipes at low pressure. The results of the steady-state model compared with the Fayed and Otten measurements are shown in figure 3.
The transient model solves the coupled equations of motion for the liquid and gas separately.
