This paper presents experimental data and numerical results on the phase distribution of gas-liquid bubbly flows taking place inside a U-bend pipe curve of square cross-section. The main purpose was to contribute the understanding of analogous processes taking place in gas-handlers. The experimentation was performed with air-water superficial velocities in the range of 0.03 m/s to 0.14 m/s and 0.88 m/s to 2.12 m/s, respectively. The centrifugal force based Froude number (buoyancy/centrifugal force) varied from 0.15 to ~1. To measure the phase distribution, a conductive probe technique was used. The Two Fluid Model was applied to simulate the flow. The numerical algorithm was set-up to account for two turbulence models and other closure relations representing interfacial pressure term and the generalized interfacial force. Results and comparisons are commented.
Phase Segregation in Pipe Singularities
Some pipe singularities are known to act as gas-liquid separators. Pipe Tees, for instance, have been used by the oil industry as phase splitters or partial separators, in order to reduce the size of conventional units and to damp severe flow instabilities. Phase separation in Tees, i.e., the so-called mal-distribution of phases, is credited to the action of various mechanisms, depending on the pattern the mixture flows at the singularity entrance. In annular flow, for example, the flooding limiting condition is a key factor; the pressure rise in the run pipeline also plays a role, as well as the flow diversion induced by fluid extraction, from the main to the branched pipeline. In this regard, Azzopardi and his co-workers (1982, 2002), developed comprehensive approaches to model the phase splitting.
U bend pipe curves may also act as phase separators if properly designed. In this case, the main driving mechanism promoting the phase segregation is the centrifugal field created by the turning flow. The gas, the lighter phase, displaces to the inner curve region and the liquid stays along the outer path. At some point along the curve phase diverters can be inserted to separate the phases. If just one U curve is not enough to provide the necessary time period for the complete phase segregation, a succession of U bend curves, forming a helical path, can be used instead. This is, in fact, the concept behind the helical, or cyclonic, separators, some of which have been described by Shoham and his co-workers (2003), for example, and Rosa et al (2001). According the latter, along a helical path the originally dispersed mixture turns to a stratified pattern due to the action of a centrifugal field. Holes along a channel inner wall then act to divert and extract the gas, separating the phases.
Another example of such a cyclonic separator is the gas-handler used prior to the inlet of electrical submersible pumps - ESP, installed in gassy wells. The gas-handler segregates and separates the gas and liquid streams before the pump eye, displacing the pump surging condition and the more extreme gas blockage to higher liquid flow rates and void fractions. The net result is the improvement of the pump characteristics under two-phase flow conditions.
In cyclonic separators, the phase segregation process depends on the intensity of the centrifugal field, but a number of other factors play a role. The flow pattern at the entrance of the curve, certainly, determines the efficiency of phase separation at the fluids take-off. The mechanisms for flow segregation may be different among the various flow patterns. The viscosity of the liquid, the surface tension, the liquid-to-gas density ratio, the phases flow rates and the turbulence intensity may be some of the phase properties, operational conditions and flow characteristics influencing the process.
Studies on the gas-liquid distribution and phase segregation of bubbly flows in plane U bend curves are, besides the torsion in the flow path that does not exists in this case, a good approach to the analysis ofanalogous flows inside helices. For this reason, in this paper one presents experimental data and numerical results on the phase distribution of gas-liquid turning bubbly flows taking place inside a 180-degree plane curve of square cross-section. The expectation is to contribute to the comprehension of the processes that set the gas-liquid distribution and separation in cyclonic units. More specifically, to the understanding of analogous processes taking place in gas-handlers of square or rectangular flow cross-section.
