Influence of the liquid viscosity on the characteristic parameters in gas-liquid slug flow in vertical pipes

Abstract

In two-phase gas-liquid flows, the slug pattern is characterized by the alternated passage of an elongated bubble and a liquid slug. This pattern can be found in many industrial processes, being particularly common in petroleum production. The literature reports an extensive number of studies on vertical slug flows using water as the liquid phase, and a limited number using high-viscosity oils (µL > 180 cP). Therefore, the current two-phase flow models are based on experimental data with low (10 cP < µL) and high (µL > 180 cP) liquid viscosities. Nevertheless, most petroleum wells in Brazil produce oils with medium viscosity (10 < µL < 180 cP), for which models or empirical data are scarce. In this context, the influence of the liquid viscosity on the evolution of the characteristic parameters of vertical two-phase slug flows was evaluated, focusing on medium viscosities in the range between 1 and 30 cP. This investigation was carried out experimentally in a 50-mm ID, 15-m high vertical section of transparent pipe, with its outlet open at atmospheric pressure. The test fluids were air and water–glycerin mixtures. Four conductance sensors and one capacitive wire-mesh sensor were deployed in the test section to measure the characteristics parameters – namely the Taylor bubble translational velocity, the unit-cell frequency and lengths and gas fractions of both the Taylor bubble and the liquid slug in order to evaluate the role of the liquid viscosity in these parameters. High-speed imaging was also used to visually characterize the effect of the viscosity on the flow pattern structure. The evolution of the slug flow characteristic parameters in a horizontal-to-vertical flow loop was analyzed. Results confirmed the existence of three distinct zones within a unit-cell: the Taylor bubble, its wake, and the liquid slug. As the liquid viscosity increases, the Taylor bubble shape (nose, body and rear) became more distorted, with the liquid slug containing fewer but larger bubbles, which tended to migrate toward the pipe centerline. Higher viscosity also resulted in shorter Taylor bubbles and liquid slugs, along with an increase in the bubble translational velocity. Additionally, the intermittence factor exhibited a bimodal distribution when the unit-cell was defined as a Taylor bubble followed by a liquid slug, whereas a unimodal distribution was observed for a unit-cell beginning with a liquid slug. For water, both configurations exhibited a normal distribution. The experimental data were further compared with predictions from the classical Unit-Cell Model, showing good agreement for global gas fraction and pressure drop; however, the model underestimated the gas fraction in the Taylor bubble region.