14,533 research outputs found
One-dimensional modelling of mixing, dispersion and segregation of multiphase fluids flowing in pipelines
The flow of immiscible liquids in pipelines has been studied in this work in order to formulate
a one-dimensional model for the computer analysis of two-phase liquid-liquid flow in horizontal
pipes. The model simplifies the number of flow patterns commonly encountered in liquid-liquid
flow to stratified flow, fully dispersed flow and partial dispersion with the formation of one or
two different emulsions. The model is based on the solution of continuity equations for dispersed
and continuous phase; correlations available in the literature are used for the calculation of the
maximum and mean dispersed phase drop diameter, the emulsion viscosity, the phase inversion
point, the liquid-wall friction factors, liquid-liquid friction factors at interface and the slip
velocity between the phases. In absence of validated models for entrainment and deposition
in liquid-liquid flow, two entrainment rate correlations and two deposition models originally
developed for gas-liquid flow have been adapted to liquid-liquid flow. The model was applied
to the flow of oil and water; the predicted flow regimes have been presented as a function
of the input water fraction and mixture velocity and compared with experimental results,
showing an overall good agreement between calculation and experiments. Calculated values
of oil-in-water and water-in-oil dispersed fractions were compared against experimental data
for different oil and water superficial velocities, input water fractions and mixture velocities.
Pressure losses calculated in the full developed flow region of the pipe, a crucial quantity in
industrial applications, are reasonably close to measured values. Discrepancies and possible
improvements of the model are also discussed.
The model for two-phase flow was extended to three-phase liquid-liquid-gas flow within
the framework of the two-fluid model. The two liquid phases were treated as a unique liquid
phase with properly averaged properties. The model for three-phase flow thus developed was
implemented in an existing research code for the simulation of three-phase slug flow with the
formation of emulsions in the liquid phase and phase inversion phenomena. Comparisons with
experimental data are presented
The Richardson's Law in Large-Eddy Simulations of Boundary Layer flows
Relative dispersion in a neutrally stratified planetary boundary layer (PBL)
is investigated by means of Large-Eddy Simulations (LES). Despite the small
extension of the inertial range of scales in the simulated PBL, our Lagrangian
statistics turns out to be compatible with the Richardson law for the
average of square particle separation. This emerges from the application of
nonstandard methods of analysis through which a precise measure of the
Richardson constant was also possible. Its values is estimated as
in close agreement with recent experiments and three-dimensional direct
numerical simulations.Comment: 15 LaTex pages, 4 PS figure
MRI-driven angular momentum transport in protoplanetary disks
Angular momentum transport in accretion disk has been the focus of intense
research in theoretical astrophysics for many decades. In the past twenty
years, MHD turbulence driven by the magnetorotational instability has emerged
as an efficient mechanism to achieve that goal. Yet, many questions and
uncertainties remain, among which the saturation level of the turbulence. The
consequences of the magnetorotational instability for planet formation models
are still being investigated. This lecture, given in September 2012 at the
school "Role and mechanisms of angular momentum transport in the formation and
early evolution of stars" in Aussois (France), aims at introducing the
historical developments, current status and outstanding questions related to
the magnetorotational instability that are currently at the forefront of
academic research.Comment: 51 pages, 16 figures, to appear in the proceedings of the Evry
Schatzman School 2012 of PNPS and CNRS/INSU on the "Role and mechanisms of
angular momentum transport during the formation and early evolution of
stars", Eds. P.Hennebelle & C.Charbonne
Construction and execution of experiments at the multi-purpose thermal hydraulic test facility TOPFLOW for generic investigations of two-phase flows and the development and validation of CFD codes - Final report
The works aimed at the further development and validation of models for CFD codes. For this reason, the new thermal-hydraulic test facility TOPFLOW was erected and equipped with wire-mesh sensors with high spatial and time resolution. Vertical test sections with nominal diameters of DN50 and DN200 operating with air-water as well as steam-water two-phase flows provided results on the evaluation of flow patterns, on the be¬haviour of the interfacial area as well as on interfacial momentum and heat transfer. The validation of the CFD-code for complex geometries was carried out using 3D void fraction and velocity distributions obtained in an experiment with an asymmetric obstacle in the large DN200 test section. With respect to free surface flows, stratified co- and counter-current flows as well as slug flows were studied in two horizontal test channels made from acrylic glass. Post-test calculations of these experiments succeeded in predicting the slug formation process. Corresponding to the main goal of the project, the experimental data was used for the model development. For vertical flows, the emphasis was put on lateral bubble forces (e.g. lift force). Different constitutive laws were tested using a Multi Bubble Size Class Test Solver that has been developed for this purpose. Basing on the results a generalized inhomogeneous Multiple Size Group (MUSIG) Model has been proposed and implemented into the CFD code CFX (ANSYS). Validation calculations with the new code resulted in the conclusion that particularly the models for bubble coalescence and fragmentation need further optimisation. Studies of single effects, like the assessment of turbulent dissipation in a bubbly flow and the analysis of trajectories of single bubbles near the wall, supplied other important results of the project
- …