This paper presents velocity measurements performed in hydrodynamic slug flow in a 212 m long horizontal pipe. A post-processing step has been developed to combine Laser Doppler Anemometry (LDA) measurements with liquid height measurements, such that it is possible to build (i) mean axial velocity profiles in the slug, as a function of the distance to the slug front, and (ii) mean axial velocity profiles in the liquid film, as a function of the distance to the slug tail. Based on the velocity profiles, the wake of the gas pocket into the slug can be characterized, and the wall friction can be estimated. Furthermore, it is shown that the velocity profiles in the slug provide hints on the interaction law between successive gas pockets.
Gas-liquid slug flow is a two-phase flow pattern in which the gas and liquid phases flow successively as a large liquid plug containing small dispersed bubbles, also referred to as "slug", and an elongated gas pocket above a liquid film (figure 1). Such a flow regime is common in pipelines transporting oil and gas, and therefore it is important to accurately predict its pressure drop for the sizing of pipelines.
In steady-state conditions, the pressure drop is usually predicted using the Unit Cell Model (Taitel and Barnea, 1990; Fabre and Liné, 1992). The framework of this model is based on the observation that the velocity distribution of the gas pocket front is very narrow, such that it can be well approximated by a mean velocity. As a result, slug flow can be represented as a succession of Unit Cells, consisting of a mean slug and a mean gas pocket, and flowing at the mean velocity of the gas pocket front. Positioned in the frame of reference of the gas pocket, the flow is steady; therefore, the mass and force balances can be greatly simplified. Furthermore, to be able to predict the pressure drop, the Unit Cell Model makes the assumption of fully-developed flow in the slug and gas pocket of the Unit Cell. It means that (i) in the gas pocket region, the flow is modeled akin to fully-developed stratified flow (or fully-developed annular flow in the case of vertical slug flow) along the entire length of the gas pocket, and (ii) in the slug region, the flow is modeled as fully-developed single-phase flow, possibly with a homogeneous distribution of bubbles.