Introduction to CFD - Examples

8/2/2019 Introduction to CFD - Examples

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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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particle trajectories

Dp = 3 m Dp = 6 m

no turbulent

dispersion

varied EIM

original EIM

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removal efficiency

Turbulence

model: STD k-

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removal efficiency

Turbulence

model: SST


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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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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


Multiphase flows (G/S): solid volume

fraction

ds = 200 m

ug = 1.5 m/s

BubblingFluidised Bed

sand volumetric fraction at different t


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

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