Numerical study of the churn-slug transition dynamics in vertical upward air-water flows

Abstract

In gas-liquid two-phase flows through vertical ducts, the spatial distribution of phases may assume several configurations, known as flow patterns. Slug and churn patterns are characterized by an unsteady behaviour and their boundary is a subject of interest for many researchers. Different descriptions of mechanisms occurring at churn/slug transition may be found in the literature. One transition process is described by Taitel et al. (Modelling flow pattern transitions for steady upward gas-liquid flow in vertical tubes. AIChE Journal, 26, pp. 345-354, 1980), in which churn flow is an entrance condition observed upstream of a developed slug flow if there is sufficient pipe length for this to happen. In the present work, vertical upward air-water flows through a 2-inch i.d. tube are simulated with the commercial finite-volume-based code FLUENT, aiming to study this churn/slug transition process through spatial and temporal phase distribution visualizations and statistical analyses of void fraction histories at points along the domain. The incompressible Volume of Fluid (VOF) formulation allied to a Piecewise Linear Interface Construction (PLIC) technique is applied to capture the distribution of phases throughout three-dimensional domains. Turbulence effects are accounted through the Realizable k-epsilon model. Three different tube lengths and two sets of superficial velocities are studied in order to understand their influence in liquid slug collapse and bubble coalescence processes. Void fraction histories at points distributed along the duct centerline are stored during a simulation time long enough to provide statistical data with reasonable errors. Fast Fourier Transform analyses are used to verify the transition point and flow characteristic frequencies. Probability distributions of void fraction at cross sections along the tube are also evaluated. The VOF method was able to qualitatively capture the mechanisms of churn/slug transition processes. Results for entrance length and frequencies are

Optimized orifice geometry to enhance the flow environment and achieve maximum discharge coefficient for effective water resources management

“…For d/D equal to 0.32, the relation is directly proportional, however, for d/D equal to 0.45, the relation is inversely proportional, and for d/D equal to 0.55 and 0.64, nearly there is no variation in discharge coefficient by increasing Reynolds number with a value of discharge coefficient equal to 0.6 for d/D equal to 0.55 and discharge coefficient equal to 0.58 for d/D ratio equal to 0.64. Also, it can be shown from Fig 7 that the discharge coefficient for the circular orifice is in the range of 0.58 to 0.62, and these values are in agreement with ISO standard values (ISO, 2003) presented by Imada et al (2013). In general, the discharge coefficient for a square orifice is higher than that for circular and hexagonal, however, it has less C d values for triangular.…”

“…The computational fluid dynamics technique is used to determine the calibration coefficient of an orifice meter numerically by Oliveira et al (2010). Furthermore, Imada et al (2013) compared the numerical simulation results with the ISO Standard, it achieved by applying the realizable k -ε and kω turbulence models. It was found that the predicted discharge coefficient values agree well with the ISO Standard, with a maximum error of 4.92%, the agreement is slightly higher for the k -ω model.…”

Characteristics of flow through orifice-meter

“…Also, it can be shown from Fig. (5) that the discharge coefficient for a circular orifice is in the range of 0.58 to 0.62, and these values are in good agreement with ISO standard values (ISO, 2003), Imada et al [11]. Fig.…”

“…Oliveira et al [10] compared numerical simulation results to experimental data using Fluent software, when compared to traditional experimental procedures, the numerical methodology used was able to estimate the discharge coefficients well. Furthermore, Imada et al [11] compared their numerical simulation results achieved by applying the realizable k -ε and kω turbulence models to the ISO Standard. It was found that the predicted discharge coefficient values agree well with the ISO Standard, with a maximum error of 4.92%, the agreement is slightly higher for the k -ω model.…”