Core-annular Flow Regime Research Articles

A Three-Dimensional Model of Turbulent Core Annular Flow Regime

In this study, three-dimensional (3D) turbulent core annular flow (CAF) regime is investigated numerically. The proposed model is based on the 3D Reynolds average Navier–Stokes (RANS) equations combined with a pure convective transport equation of the volume of fluid (VOF) to predict the interface between the oil and water phases. The k-ω turbulence model is adopted to better reproduce the oil and water flow characteristics. The two-phase (CAF) regime can be predicted by two inlet configurations: the T-junction (3D-T) and the straight pipe (3D-S). These two configurations are simulated and compared for pipe diameter D = 0.026 m and pipe length L = 4 m . For these two inlet configurations, the computed mixture velocity profile and the water volume fraction at a test section z = 100 D were compared to experimental measurements. The 3D-T configuration gives more appropriate results. The 3D-S slightly overestimates the maximum velocity at the test section and the lower and upper water layer of the (CAF) flow is shifted in the upward direction. For the 3D-T, the relative error in the pressure drop is 3.3%. However, for the 3D-S, this error is 13.0%.

Pore network model for retrograde gas flow in porous media

Gas well deliverability in retrograde gas reservoirs is affected by the appearance of a liquid phase near the wellbore when the bottom-hole pressure is below the dew point. At low enough pressure gradients, the liquid phase may bridge the pore throats, blocking the gas phase flow, leading to what is known as condensate blockage. If the pressure gradient is above a critical value, the capillary forces cannot block the gas flow and the condensate bridges are displaced. Gas production can be estimated using reservoir-scale simulation of the retrograde gas flow near producing wells using relative permeability curves that take into account the different physical phenomena and their dependence on local pressure, temperature and pressure gradient (capillary number). Pore-scale simulators can be used to model retrograde gas flow through porous media and to determine the gas and liquid phases relative permeability curves as a function of different flow parameters. Here, a fully-implicit isothermal compositional pore-scale network model is presented for retrograde gas flow in porous media at high capillary number, at which capillary forces are neglected. The flow pattern in each capillary changes from a single-phase gas flow to a two-phase core-annular flow regime when the pressure and the fluid composition reach a critical condition based on the thermodynamics of the fluid mixture. Newton's method is used to solve the non-linear flow and thermodynamics equations to calculate the pressure distribution and the flow rate of each component through the network. The model was used to predict gas-liquid relative permeability at high capillary number from a depletion process in a 2D network. The results show the evolution of the condensate saturation at different conditions and how the gas permeability falls abruptly at a certain condensate saturation.

Kinetic theory based multiphase flow with experimental verification

Abstract This review is an extension of our 2014 circulating fluidized bed (CFB) plenary lecture. A derivation of multiphase mass, momentum and energy balances is presented, with a review of elementary kinetic theory, to explain the concepts of granular temperature and pressure and the core-annular flow regime commonly observed in CFB. The kinetic theory shows that the particle concentration is given by the reciprocal of a fourth order parabola of dimensional tube radius, in agreement with experiments. Computed flow regimes and heat and mass transfer coefficients in fluidization are also discussed.

Investigation on Hydrodynamics of Triple-Bed Combined Circulating Fluidized Bed Using Electrostatic Sensor and Electrical Capacitance Tomography

To investigate the hydrodynamics, a cold model of triple-bed combined circulating fluidized bed (TBCFB) has been built with an electrostatic sensor and a twin-plane electrical capacitance tomography (ECT) sensor. Experimental results show that with the increase in the superficial air velocity, the flow regime in the riser would transit from a dense plug flow to a core-annular flow, and finally to a dilute suspension flow. In the dense plug flow regime, the passage of the solids plugs in the riser can be monitored and the velocity measured using the twin-plane ECT sensor. In the core-annular flow regime, when the solids holdup is moderate, the measured solids velocities by the electrostatic sensor and the ECT sensor are comparable and complementary. With a dilute suspension flow in the riser, a homogeneous flow is observed with a nearly flat velocity profile of solid particles. On the contrary, in the downer the flow becomes inhomogeneous, and the solids velocities in the center of the downer are higher th...

Experimental Study of the Pipeline Lubrication for Heavy Oil Transport

Heavy oil is accessible in different areas around the world in large amount. Unfortunately its high viscosity makes it difficult to produce and to transport. In this article we focus on transport problems. Pressure drop in the pipes must be lower as possible to limit pump power and to be able to transport in long distance. In the case of heavy oil, the high viscosity leads to huge pressure drop that makes it impossible to simply pump the fluid in single-phase flow, even of large diameter. Different solutions have been developed to transport heavy oil in pipeline like dilution of the crude oil in a lighter one. In this study we experiment a solution that do not modify the viscosity of the oil phase but transform the flow regime of transport: pipeline lubrication. Pipeline lubrication is a technique based on core-annular flow regime and used for pressure drop reduction in the transport of very viscous products. A thin water film adjacent to the internal pipe wall lubricates the internal oil core leading to a longitudinal pressure gradient reduction. An experimental study has been carried out where the influence of the different parameters, which could affect the lubrication process, has been optimised, using water as the annulus and heavy oil as the core of the flow. The tests were conducted in steady laminar flow at moderate flow rates. The results, obtained with annular water injection at the pipe wall, show a pressure drop reduction over than 90% as compared with the same product in flow without lubrication. These results confirm the effectiveness of the lubricating process for heavy oil transport. We also showed that density difference is responsible to particular evolution of the pressure drop with the flow rate.

Hydrodynamics of binary fluidization in a riser: CFD simulation using two granular temperatures

A new computational fluid-dynamic (CFD) model with a separate granular temperature (2/3 random particle kinetic energy per unit of mass) equation for each phase or particle size was developed using constitutive equations derived earlier by Huilin, Gidaspow and Manger. In agreement with the experiment and model of Mathiesen, Solberg and Hjertager the new model computes the observed core-annular flow regime. It predicts the trends of the observed radial and axial particle diameter distributions. For elastic particles the computed particle velocity distributions are parabolic. They are close to the laminar type approximate analytical solution for flow in a pipe, where the mean velocity equals the inlet flux divided by the particle density and volume fraction. The computed turbulent intensity is lower for large particles than for small particles, as measured. This is in agreement with an approximate analytical solution for the granular temperature in the developed flow region of a riser for elastic particles. Computations show that for sufficiently inelastic particles the granular temperature in the center can be lower than near the wall resembling the measured particle fluctuating velocity distribution.

Hydrodynamic Simulations of Gas−Solid Flow in a Riser

Conservation of mass and momentum equations for gas and solid phases were used to compute the hydrodynamics of flow in risers for two core-annular flow regimes. The principal input into the model was a semiempirical viscosity for fluidized catalytic cracking particles. The computed time-averaged radial fluxes and particle concentrations agree with experimental data obtained in a riser by Miller (Ph.D. Thesis, Illinois Institute of Technology, Chicago, IL, 1991) and Knowlton et al. (PSRI Challenge Problem Presented at the Eighth International Fluidization Conference, Tour, France, 1995). The model also computed the dynamics of flow, the dominant frequency of instantaneous particle concentrations, the granular temperature, and the granular pressure in reasonable agreement with measurements.

Riser hydrodynamics: Simulation using kinetic theory

AbstractComputational fluid dynamics (CFD) for fluidization is reaching maturity (Roco, 1998). It has become common practice to compare time‐averaged solid velocities and concentrations to measurements of fluxes and densities. The dynamic behavior of the riser, however, has not been previously compared to experiments. This article shows that the dynamics of solids flow in the riser is in the form of clusters, but the time‐averaged particle concentrations and fluxes give us the core‐annular flow regime in agreement with measurements. The computed clusters, which are essentially compressible gravity waves, produce major frequencies of density oscillations in agreement with measurements. The model and the CFD code compute granular temperature distributions, agreeing qualitatively with data. For volume fraction around 3–4%, which is the average particle concentration in the riser, the computed viscosity agrees with our experimental measurements.

Computation of Circulating Fluidized-Bed Riser Flow for the Fluidization VIII Benchmark Test

Conservation of mass and momentum equations for the solid and for the gas phases was used to compute the hydrodynamics of flow of fluidized catalytic cracking particles in a vertical pipe (riser) for the 1995 benchmark modeling contest. The computer code predicted a new phenomenon: an off-center maximum flux. The computed time-average radial fluxes and particle concentrations agree with PSRI experimental data. The computer code also predicted the observed core-annular flow regime. For the lowest gas velocity and highest solids flux, the code predicted low-frequency, snakelike density oscillations.