A finite volume method-based CFD model has been developed to simulate steady, turbulent, two-dimensional annular gas-liquid flow in a duct. The gas flow is treated as being equivalent to flow through a rough-walled duct. The effect of the liquid film on the gas phase is included in the form of modified wall functions which incorporate the well-known triangular relationship (Annular Two-Phase Flow, Pergamon Press, Oxford, 1970) that exists among wall shear stress, film flow rate and film thickness in annular flow. The presence of droplets is accounted for by solving an additional scalar transport equation for the mass fraction of the droplets. Entrainment and deposition of droplets are included as source term and boundary condition, respectively, in the mass fraction equation. It is shown that the resulting model, while retaining simplicity of formulation, gives good predictions of the literature data of annular flow parameters under equilibrium and non-equilibrium conditions. © 2004 Elsevier Ltd. All rights reserved.

Sreenivas Jayanti

Department Of Chemical Engineering

Computational fluid dynamics

Drop formation

Finite volume method

Shear stress

Two phase flow

ANNULAR GAS-LIQUID FLOW

NON-EQUILIBRIUM CONDITIONS

Flow of fluids

Finite volume technique

GAS-LIQUID TWO-PHASE FLOW

mathematical analysis

Model

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IIT Madras

Chemical Engineering Science

Transient response of two-phase flow is important in many analyses of nuclear reactor accident scenarios. In the present work, we study the response of vertical annular flow to a sudden or gradual pipe contraction. The three component fields of gas-liquid annular flow, namely, the liquid film, the entrained liquid droplets and the gas core, show different behaviour as they pass through an area change section because of their very different inertias. Pressure variation through the contraction is a result of the integrated effect of these responses. In order to throw light on these, pressure profiles along the upstream and downstream sections of the contraction have been measured in air-water flow through a vertical converging pipe section for diameter ratios of 1.5 and 2.0 and for half-cone angles of 8° 15° and 90°. The gas-droplet field interaction has been studied using computational fluid dynamics simulations while the gas-film flow interaction has been studied using an analytical model for film flow. These studies highlight the role of droplet inertia as the principal contributor to the pressure change in the contraction. A semi-empirical model has been developed for the pressure loss coefficient in the contraction. © 2019

Annals of Nuclear Energy

Study of gas-liquid upward annular flow through a contraction

Gas–liquid annular flow draws different responses from its three constituents, namely, the liquid film, the entrained liquid droplets and the gas core, as they flow through a diverging section in a pipe. The resulting change in the pressure profile is a combination of several effects associated with the dynamic interactions among these three fields. Accurate simulation of the response using Eulerian–Eulerian computational fluid dynamics is not feasible because the processes are inherently complex and a framework of relevant and validated constitutive relations describing the physical processes is not yet available. In the present work, a simpler approach is adopted by studying the interactions individually in idealized settings, and bringing together the separate effects into a phenomenological model for pressure loss in upward vertical annular flow. The overall pressure change is expressed as a sum of three contributors: change in area of cross-section available for gas flow, change in the effective interfacial roughness leading to peaking of the velocity profile, and droplet-gas momentum exchange in the immediate downstream of the expansion. Using air–water experimental data from two expansion ratios and three half-angles of the diverging sections, a mechanistic correlation is proposed to evaluate the overall pressure loss coefficient. © 2018 Elsevier Inc.

Applied Mathematical Modelling

A mechanistic model for expansion loss coefficient in upward vertical annular flow

Gas-liquid annular flow through a vertical circular pipe is well-understood and detailed phenomenological models exist in the literature. In the present work, the case of flow through a diverging section is studied experimentally and theoretically. Experiments have been carried out in air-water flow through a vertical diverging pipe section with a diameter ratio of 1.5 and 2.0. Pressure profiles have been recorded upstream, across and downstream of the diverging section for the cases of sudden expansion and gradual expansion with included half-angles of 8° and 15° for the diverging section. These show that the pressure variation is characterized by a strong pressure recovery downstream of the expansion which is in turn influenced by the smoothness of the expansion and the interfacial friction. The non-dimensionalized pressure loss across the expansion, which has been determined by extrapolating the pressure variations upstream and downstream, is found to vary systematically with the diameter ratio, the angle of the expansion and the ratio of superficial liquid-to-gas Reynolds numbers. A method, which uses the equilibrium annular flow model and an empirically determined expansion pressure loss coefficient, has been developed for the prediction of the pressure variation across the diverging section. © 2014 Elsevier Ltd.

International Journal of Multiphase Flow

Experimental and modelling studies of gas-liquid vertical annular flow through a diverging section

In nuclear reactor fuel assemblies, spacer grids are used in the rod bundles to provide mechanical support to the rods and to reduce flow-induced vibration. These are known to play a role in the local heat transfer as well as in the onset of the critical heat flux (CHF) condition. While considerable literature exists on these issues, a detailed study of the flow situation has not yet been reported. The objective of the present study was to investigate the effect of spacer grids on flow and heat transfer in rod bundles. To this end, computational fluid dynamics (CFD) based simulations have been carried out using the Eulerian-Lagrangian framework for calculating single-phase, vapour-only flow as well as the droplet-laden flow through a typical bundle geometry. The results show that the single-phase convective heat transfer coefficient increases locally by 30-40%, leading to a consequent decrease in the wall temperature, in the spacer grid region. Droplet trajectory calculations show that the droplet deposition rate is increased considerably in the spacer grid region and that about 15% of the droplet flux is deposited in the spacer grid. Estimates of the size of the re-entrained droplets have shown that these are much smaller than those of the undeposited droplets present in pre-CHF flow. It is hypothesized here that these smaller re-entrained droplets may cause enhancement of the evaporative heat transfer coefficient between the droplets and may help in bringing down the surface temperature further downstream of the spacer, which has been observed experimentally. © 2013 Elsevier B.V. All rights reserved.

Nuclear Engineering and Design

Effect of spacer grids on CHF in nuclear rod bundles

Air-water upward annular flow experiments were conducted in a 10.8 m long tube of 31.8 mm internal diameter. Local values of the pressure gradient, film thickness, wall shear stress, film flow rate, disturbance wave velocity and frequency were measured over a range of gas and liquid flow rates. The results show that rapid changes in the film flow parameters occur within the first 50 tube diameters or so from the inlet. Entrained fraction and the film thickness appear to be the slowest to respond and these may take 100 to 300 tube diameters to develop fully. © 2001 Elsevier Science Ltd. All rights reserved.

Flow development in vertical annular flow

Numerical Heat Transfer and Fluid Flow

Heat Conduction

Fluid Dynamics

On the modelling of multiphase turbulent flows for environmental and hydrodynamic applications

Statistical Fluid Mechanics: The Mechanics of Turbulence

A multidimensional model for annular gas-liquid flow

Journal	Chemical Engineering Science
ISSN	00092509
Open Access	No