Post on 06-Apr-2018
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Commercial CFD codes
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Structure of a CFD code
PRE-PROCESSING
Domain definition Mesh generation Definition of teh
physical model (e.g.
governing equations,
boundaryand inlet
conditions).
SOLVING POST-PROCESSING
Pre-processing: mesh generation
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Pre-processing:
definition of the physical model
FLOW TYPE
fluid/solid
steady/transient;laminar/turbulent
isotherm/non isotherm
single/multiphase
reactive/non reactive
TURBULENCE MODEL
Zero equation
k-
RNG k-
k-
SSG Reynolds Stress
QI Reynolds Stress
Reynolds Stress
Ecc.
COMBUSTION MODEL
Eddy dissipation
FinitE rate chemistry
Finite rate chemistry/eddydissipation
Laminar flamelet with PDF
RADIATION MODEL
Rosseland
P1
MontecarloDiscrete Transform
BOUNDARY CONDITIONS
APPROACH
DNS
RANS
LES
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Solving
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Post-processing
Some examples O&G
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Vane type eliminator
Objectives
Prediction of removal efficiency of a vane-type separator with acommercial CFD code (Ansys CFX): comparison between CFD andexperimental data
CFD helps designing and investigating new configurations
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Vane type eliminator: computational
domain and grid Computational domain:
2D
Computational grid:
Structured (34,000 cells)
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Vane type eliminator: physical model
Euler-Lagrangian approach (Lagrangian tracking)
One-way coupling
GAS PHASE:
turbulence model standard k- (STD k-)
shear Stress Transport (SST)
DROPLETS
drag is considered
turbulent dispersion is cosidered through original Eddy InteractionModel (EIM; availablein the code);
modifiedEIM (implemented with a subroutine in Fortran language)
dropletdroplet interaction is negligible,
dropletfilm interaction at the walls is negligible,
droplets behave as hard spheres, unsteady forces (virtual mass and Basset history), pressure gradient and lift forces are negligible.
once the droplets collide with the walls, they do not rebound but are removed immediately from thewalls.
re-entrainment not taken into account.
Injections from 1000 locations
Vane Type Eliminator: Eddy Interaction
Model for turbulent dispersion Droplet equation of motion
The instantaneous gas velocity
The continuous phase simulation provides the mean velocity, turbulence levels andeddy dissipation rates.
Such characteristics have to be used to reconstruct a fictitious turbulent flowfield seenby the droplets and responsible for the turbulent dispersion.
Eddy lengthscale
Eddy timescale
The droplet sees a gas velocity
Nris a random number taken from a Gaussian (normal) distribution with zero meanand standard deviation equal to 1,
Ug is usually updated when- ever the droplet crosses a grid element, whereasug andNrare computed at the end of the eddy interaction.
Vane Type Eliminator: Eddy Interaction
Model for turbulent dispersion
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Vane type eliminator:
flow field
Turbulence model: STD k- Turbulence model: SST
SST model describes in more detail the recirculation regions near drainage channels,
whereas the STD k- describes such regions as just low velocity regions.
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Vane type eliminator:
particle trajectories
Dp = 3 m Dp = 6 m
no turbulent
dispersion
varied EIM
original EIM
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Vane type eliminator:
removal efficiency
Turbulence
model: STD k-
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Vane type eliminator:
removal efficiency
Turbulence
model: SST
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Vane type eliminator:
removal efficiency
Size
distribution
after vane type
eliminator
bends
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Cyclones
Many studies have demonstrated that CFDcannot produce a very accurate description ofthe flow field because of difficulties in modelingthe phenomena occurring in swirling flow.
Calculated results for pressure drop agree onlymoderately well with the experimental data.The experimental pressure drop was larger thanthe calculated pressure drop by 60%, 15%, and16% for standard k-e , RNG k-e , and Reynoldsstress model, respectively.
Recently, large eddy simulation LES was used topredict the unsteady, spiral shape, and vortexcore characteristics of a cyclone separator.
Results are encouarging but LES iscomputationally expensive.
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Cyclones
Hydrocyclone flow field
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Slug catchers
Problem:
Slug catchers are designed to stratify slug flow The gas/liquid ratio will change over the lifespan of a well, and flow rates will vary during different operation regimes (e.g. cleaning)
Questions:
At high flow rates, does the liquid overflow into the gas pipe, causing problems downstream? Is the slug catcher long enough to promote stratification and deal with the largest slug volumes? Will excessive gas quantities enter slug catcher, requiring flaring? What will happen if I double the flowrate? Whats the force loading on the structure?Solution:
CFD can be used to simulate the multiphase, transient characteristics of the slug catcher at different flow rates and gas/liquid ratios
Detailed understanding of slug catcher performance and operational limits
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Slug catchers:
boundary conditions
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Slug catchers:
computational domain and grid
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Slug catchers:
physical/solver model
Master in Progettazione di Impianti Oil & Gas 24
Slug catchers:
animation of oil
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Master in Progettazione di Impianti Oil & Gas 25
continuity eq.
momentum[ ] ( ) ( )
gggsgggggggg
UUgPUUUt
+++=+
rrrrrr
[ ] ( ) ( ) ( ) ssgsggsssssssss GUUgUUUt
++=+
rrrrrr
g,skt
U
t
skkkk ==
+
0
r
Multiphase flows (G/S)
Dense G/S flows (fluidised beds), no reactions
CFD code: CFX 5.7 by Ansys Inc.
transient simulations
Eulerian-Eulerian model
IMPORTANT: CFX neglects the solid stress
tensor s
(simplified Gidaspow model)Master in Progettazione di Impianti Oil & Gas 26
Multiphase flows (G/S): computational
domain and grid 2D domain
structured grid 14,220 cells
3D domain unstructured grid
600,000 cells
too CPU time
3D simulations on a simplified
(shorter) geometry
Master in Progettazione di Impianti Oil & Gas 27
Multiphase flows (G/S): solid volume
fraction
ds = 200 m
ug = 1.5 m/s
BubblingFluidised Bed
sand volumetric fraction at different t
Master in Progettazione di Impianti Oil & Gas 28
Multiphase flows (G/S): solid volume
fraction
ds = 200 m
ug = 2 m/s
CirculatingFluidised Bed
core annulusregime
sand volumetric fraction at different t
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Multiphase flows (G/S): advanced Gidaspowmodel for particle-particle treatment
[ ] ( ) ( ) ( )ssgsggssssssssg Gt ++=+
UUgUUU
IUU ssssS
sss =3
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solid stress tensor
[ ]Tsss
SUUU +=
2
1
+
+
=z
z
U
y
y
U
x
x
U
comp
x,s
s
x,s
sx,s
s
sx
+
+
=
z
z
U
y
y
U
x
x
U
comp
y,s
s
y,s
s
y,s
s
sy
+
+
=z
z
U
y
y
U
x
x
U
comp
zs
s
zs
s
zs
s
sz
,,,
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Multiphase flows (G/S): comparison of simplifiedand advanced models for particle-particle treatment
simplified model (available in CFX) implemented model
Advanced Gidaspow model:
larger bed expansion
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Flares
Motivation:
Conventional approaches for blowouts and flares:
Fluid dynamics used near the release section (jet behaviour); Advection and diffusion equations used far away (plume
behaviour);
Dedicated radiation models for blowout/flare scenarios.
Can be CFD used as single tool? CFD capability anddrawbacks?
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Flares: physical model
Subsonic flow fictitious release section
Stationary flow RANS
Euler-Lagrangian approach with two-way coupling
Gas-liquid mixtures: Hydrocarbons are represented with C1-CX mixtures
The liquid phase (oil) is represented with one hydrocarbon of equal molecular
weight
The oil is assumed to be 100% volatile
Evaporation is modelled through Antoine equation
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Flares: physical model
BuoyancyTurbulence model
k- model with C1 = 1.6 (Morse, 1977) Sensitivity analysis: RNG and standard k- models
Combustion model and kinetic scheme
CH4, C2H6, C3H8, C4H10 oxidations: Eddy Dissipation Model, 1-step global mechanism
prompt and thermal NO formation : Arrhenius integrated with PDF of T soot formation/oxidation: Eddy Dissipation Concept, 2-step mechanism by Tesner et al.
(1971)
Radiation model
P1 (spherical harmonics)
Sensitivity analysis: Discrete Ordinate and Discrete TransferSpectral model
WSGG (Smith et al, 1982) Soot radiation properties from Mie-Scatter theory
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Flares
200 m
80 m
Vertical single-phase Horizontal two-phase
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Flares