196 research outputs found
Two Phase Bubble Columns: the Determinants of the Flow Regime Transitions
The fluid dynamics in large-diameter bubble columns can be described by an analytical relation between two global flow parameters, the drift flux and the gas holdup. This relation, named bubble column operating curve, builds on five flow regime transitions. In order to determine the variables influencing the flow regime transitions, a statistical approach was derived by coupling: (1) the ordinary least squares method (OLS) to determine the relationship between the variables, (2) the variance inflation factor (VIF) to check for multicollinearity issues, and (3) the least absolute shrinkage and selection operator (LASSO), to select suitable variables. It was found that the geometrical characteristics of the sparger strongly influence the flow regime transitions, and uniform aeration is essential for all the regimes to exist. Increasing the superficial liquid velocity in the counter-current mode destabilises the mono-dispersed and poly-dispersed homogeneous flow regimes. As for the aspect ratio, an increase in the column aspect ratio slightly destabilises the existing flow regimes. The statistical method identifies viscosity as the only significative variable concerning the liquid phase properties
Acid gas removal from natural gas by water washing
Projections in the future energy scenario outline an important role played by fossil fuels to meet the increasing
global energy demand. A “golden age” has been recently outlined for natural gas, in particular, as the fastest
growing and the cleanest of all fossil fuels. Although natural gas is mostly considered to be a “clean” fuel with
respect to the emission of pollutants from its combustion, the raw natural gas found in reservoir deposits is not
free of contaminants. Among the others, hydrogen sulphide and carbon dioxide are two undesired
compounds, which are responsible for the sour or acidic nature of natural gas and must be removed for
operational and safety reasons. Acid gas treating is typically performed in facilities built at surface locations,
mainly by means of chemical absorption into aqueous amine solutions. However, subsurface technologies
may allow to possibly separate the gas undesired compounds directly downhole. The high pressure
encountered in this environment makes the use of water as liquid absorbent worth considering. This work
investigates the possibility of acid gas removal from natural gas by downhole water washing and presents a
preliminary evaluation of the performances of the process, which is assumed to be carried out in the gas
production casing that can be represented as a bubble column. A previously proposed correlation for the gas
holdup in this type of contacting device operated counter-currently has been used to determine the volumetric
mass transfer coefficient for design purposes, considering different raw gas flow rates and inlet acid gas
concentrations. By solving a simplified model of a bubble column and by using water flow rates compatible
with reinjection into the reservoir, it has been found that it is possible to reduce the H2S content from the inlet
concentration to the commonly accepted value to meet pipeline specifications and, depending on the inlet CO2
concentration, to perform a bulk removal of it
CFD study of an air–water flow inside helically coiled pipes
CFD is used to study an air–water mixture flowing inside helically coiled pipes, being at the moment considered for the Steam Generators (SGs) of different nuclear reactor projects of Generation III+ and Generation IV. The two-phase mixture is described through the Eulerian–Eulerian model and the adiabatic flow is simulated through the ANSYS FLUENT code. A twofold objective is pursued. On the one hand, obtaining an accurate estimation of physical quantities such as the frictional pressure drop and the void fraction. In this regard, CFD simulations can provide accurate predictions without being limited to a particular range of system parameters, which often constricts the application of empirical correlations. On the other hand, a better understanding of the role of the centrifugal force field and its effect on the two-phase flow field and the phase distributions is pursued.
The effect of the centrifugal force field introduced by the geometry is characterized. Water is pushed by the centrifugal force towards the outer pipe wall, whereas air accumulates in the inner region of the pipe. The maximum of the mixture velocity is therefore shifted towards the inner pipe wall, as the air flows much faster than the water, having a considerably lower density. The flow field, as for the single-phase flow, is characterized by flow recirculation and vortices. Quantitatively, the simulation results are validated against the experimental data of Akagawa et al. (1971) for the void fraction and the frictional pressure drop. The relatively simple model of momentum interfacial transfer allows obtaining a very good agreement for the average void fraction and a satisfactory estimation of the frictional pressure drop and, at the same time, limits the computational cost of the simulations. Effects of changes in the diameter of the dispersed phase are described, as its value strongly affects the degree of interaction between the phases. In addition, a more precise treatment of the near wall region other than wall function results in a better definition of the liquid film at the wall, although an overestimation of the frictional pressure drop is obtained
Novel gas holdup and regime transition correlation for two-phase bubble columns
The gas holdup is dimensionless parameter of fundamental and practical importance in the operation, design and scale-up of bubble columns. Unfortunately, the many relationships between the bubble column fluid dynamic parameters and the various variables characterizing the system make it difficult to find general correlations for the precise estimation of the gas holdup. Wilkinson et al. (1992), in their pioneering paper, proposed a correlation to predict the gas holdup in industrial-scale bubble columns, based on the physical properties of the phases and the operating conditions. However, this correlation lacks in generality, as it does not take in account the bubble column design. In this paper, we propose a generalization of the Wilkinson et al. (1992) gas holdup correlation to take into also the bubble column design parameters. Starting from considerations concerning the flow regime transition, corrective parameters are included to account for the effects introduced by the gas sparger openings, the bubble column aspect ratio and the bubble column diameter. The proposed correlation has been found to predict fairly well previously published gas holdup and flow regime transition data
Capability of non liner eddy viscosity model in predicting complex flows
Paper presented at the 6th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, South Africa, 30 June - 2 July, 2008.The research field of the study is related with turbulence
modeling. The general objective is the implementation in a
commercial code, of two equations Non Linear Eddy Viscosity
Model (NLEVM) which removes Boussinesq linear
approximation for the Reynolds stress tensor. The work
described in the paper implements a second order k-ε model
based over Shih, Zhu and Lumley (1993) [1] and Craft,
Launder and Suga (1996) [2] in the finite volume commercial
code ANSYS-FLUENT v. 6.3.26, by writing additional
subroutines. The model has been validated through
experimental and DNS data available in the literature. The
benchmarks shown in this paper are the straight Square Duct
[8] and the Backward-Facing Step [9, 10]. After the validation,
the model has been used for predicting the flow behavior for
complex industrial applications. The geometry used is similar
to the bowl-shape downcomer of nuclear reactor. This is an
application field of interest still under study by the same
research group and an international consortium.vk201
Thermo fluid dynamic Euler-Lagrange CFD analysis applied to wet flue gas desulphurisation technology
Paper presented at the 6th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, South Africa, 30 June - 2 July, 2008.Wet Flue Gas Desulphurisation (FGD) technology is the
most frequently used scrubbing process for sulphur dioxide
(SO2) reduction from coal-fired utility boilers. Wet limestone
FGD-plants using Open Spray Tower (OST) technology are the
most commonly used.
CFD has been used to investigate the gas-liquid fluid
dynamics inside a counter-current OST and the heat transfer
between the phases. The continuous phase (gas) is modelled in
the Eulerian framework while the discrete phase (liquid
droplets) in the Lagrangian frame of reference. Simulation
results show good agreement with measurements on a pilot
plant flue gas cleaning unit. The commercial code Fluent
6.3.26, completed with the necessary subroutines for liquid
phase properties and slurry wall interaction, has been used for
the calculations.vk201
The Borexino Thermal Monitoring & Management System and simulations of the fluid-dynamics of the Borexino detector under asymmetrical, changing boundary conditions
A comprehensive monitoring system for the thermal environment inside the
Borexino neutrino detector was developed and installed in order to reduce
uncertainties in determining temperatures throughout the detector. A
complementary thermal management system limits undesirable thermal couplings
between the environment and Borexino's active sections. This strategy is
bringing improved radioactive background conditions to the region of interest
for the physics signal thanks to reduced fluid mixing induced in the liquid
scintillator. Although fluid-dynamical equilibrium has not yet been fully
reached, and thermal fine-tuning is possible, the system has proven extremely
effective at stabilizing the detector's thermal conditions while offering
precise insights into its mechanisms of internal thermal transport.
Furthermore, a Computational Fluid-Dynamics analysis has been performed, based
on the empirical measurements provided by the thermal monitoring system, and
providing information into present and future thermal trends. A two-dimensional
modeling approach was implemented in order to achieve a proper understanding of
the thermal and fluid-dynamics in Borexino. It was optimized for different
regions and periods of interest, focusing on the most critical effects that
were identified as influencing background concentrations. Literature
experimental case studies were reproduced to benchmark the method and settings,
and a Borexino-specific benchmark was implemented in order to validate the
modeling approach for thermal transport. Finally, fully-convective models were
applied to understand general and specific fluid motions impacting the
detector's Active Volume.Comment: arXiv admin note: substantial text overlap with arXiv:1705.09078,
arXiv:1705.0965
Bridge pier scour measurement by means of Bragg grating arrays
Abstract. This paper deals with a new method to measure scour level at bridge piers. The proposed technique is based on an array of Bragg grating temperature sensors, heated by an electrical circuit. The Bragg gratings in water sense a lower temperature than those buried in the river bed, because of the different heat scattering principles in the two situations. Furthermore the response of each sensor is slower if it is buried in the bed, with respect to the case it is in water. The paper presents laboratory tests, showing the method effectiveness and reliability, and it explains the advantages with respect to other more traditional methodologies to measure scour level
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