2008 Esssugm Aguirre A

Guidelines on numerical modeling of cyclones and hydrocyclones

João Aguirre, M.Sc. ([email protected])

Ricardo Damian, M.Sc. ([email protected])

Discussions

• Cyclone / Hydrocyclone flows;

• Geometry and mesh generation;

• Simulation time behavior;

• Physical modeling:

Turbulence modeling;

Multiphase modeling.

• Boundary conditions and initialization;

• Numerical issues;

• Monitoring and post-processing.



Cyclone / Hydrocyclone flows

• Strong swirl component;

• Phases separation due to density difference;

Or classification due to particle size.

• Formation of a vortex core:

Low pressure region;

Formation of a gas-core (hydrocyclones);

If discharge surfaces are openedto gas regions.

Vortex-core precessing.

Intrinsically transient phenomena.

feed

overflow

underflow

heavier particles (droplets) dragged outwards

inner upward and outer downward spirals

cyclone/hydrocyclone flow structure from Cullivan et al. (2004) - modified

gac

rr >>


Cyclone / Hydrocyclone flows

• Strong velocity gradients in the core region:

Very sharp for tangential velocity.

• Anisotropic turbulence:

Effect of the strong streamline curvature;

Challenge for turbulence models.

radius

velo

city

forced vortex

free vortex

conceptual hydrocyclone tangential velocity profile

experimental visualization of

the flow inside of a hydrocyclone – detail of the

air-core and the vortex core precessing

Geometry and mesh generation

• Usually cyclones and hydrocyclones present simple geometries:

Cono-cylindrical geometries with tangential injection;

Different models may present special features.


conceptual hydrocyclone

geometry

conceptual cyclone

geometry


• Some issues about geometry design for CFD simulations:

Inlet and outlet piping extension:

Adequate boundary conditionmodeling;

Inlet condition improved withusage of developed boundary profiles;

As usual it is a good practice toextend boundary regions to reachdeveloped flow regions:

If possible extend overflowand underflow regions untilno reverse flow occurs.


hydrocyclone inlet and overflow extension



Inlet piping tangential to body or displaced:

Actually a mesh issue (mainly to hexa meshes).


tangential inlet piping case

displaced inlet piping case



Use of blends (fillets) on inlet edges:

Actually a mesh issue (mainly to tetra meshes).


tangential inlet(no blends)

tangential inlet(blended)


• Mesh generation guidelines:

Cyclone cases can be simulated both with tetra/prism or hexa meshes;

Hydrocyclone cases are much more mesh sensitive, it is strongly recommended to use hexa meshes:

The use of hexa meshes also reduces the number of nodesusing the same grid size and, due to higher quality, helpsin stability and simulation speed.

Hexa meshes can be created using ANSYS Meshing Tools or ANSYS ICEM CFD.

Meshes should have refinements near walls and near geometry transitions (such as cylinder to cone transitions) and in in the vortex-core region:

Vortex core region usually well-defined using the overflow diameter as a reference;

Verify the y+ values during simulation run!





refinements near the walls (to capture boundary-layer effects) refinement in the vortex-core region (to

capture correct velocity peaks and gradients)

refinements near geometry “transitions” (to stabilize the

solution and capture flow changes)

smooth transition between the refined vortex-core mesh and the outer mesh



Examples of mixed and tetra/prism meshes:


tangential inlet(blended)

cyclone tetra/prism mesh –details on core refinement

and inlet blend mesh



Meshing tips:

Using ANSYS ICEM CFD to generate tetra/prism meshes, delete all pointsand curves that do not define important features and where you want to mesh approximately the surfaces (e. g. tangential inlet) using the Repair Geometry Tool. Similar to that, in ANSYS Meshing Tools, use the Virtual Topology feature to “clean”and “smooth” the geometry;

Also in ANSYS ICEM CFD, when working on hexa meshes, use the “Smooth orthogonal” tool to betterdistribute hexa elements, after this ortho smooth always perform some smooth by quality iterations.

In ANSYS Meshing Tools, cut a small “square tube”in the hydro/cyclone axis to reproduce the “o-grid”feature of ICEM CFD.


Simulation time behavior

• Intrinsically transient phenomena:

Core precessing;

Flow feature (not just a turbulent feature);

Not “absorbed” by the turbulence modeling.

Complex turbulence.

• Transient simulation!

Physical time scale for time step selection:

Montavon et al. (2000) - modified;

Time step also limited by Courant number!

Decrease in mesh size reduces the maximum time step.

Adaptive time step simulations must have a large interval between updates.


( )inlet

underflowoverflowmax v

ddt

⋅=∆

10

,min

Maximum time step:

x

tuCFLCourant

∆∆⋅==

Courant number:

1 < Courant RMS < 10, 50 < Courant MAX < 100

Simulation time behavior

• Transient simulation!

Time scale reference for total simulation time:

Characteristic flow time:

Useful to determinate continuous phase flow development;

Initial estimate for multiphase flows.

Flow development is case-dependent!

Some multiphase flows need a longer simulation time to develop.

Development defined by flow data monitors.


inlet

nehydrocyclocyclonesticcharacteri V

Vt

&

/=∆

Flow characteristic time:

Total flow time (single-phase flow):

sticcharacteritotal tt ∆⋅= 3

1·∆tcharacteristic for flow development

+

2·∆tcharacteristic for time averaging

Physical modeling

• Turbulence modeling:

Single-phase flows:

Due to intrinsic flow characteristics common turbulence models not apply to this case:

Unrealistic results achieved with usual k-ε and k-ω models.

Prediction of exaggerated diffusion anddissipation leading to a “rigid body”rotation profile.

Models that can account for anisotropy or strongstreamline curvature must be applied:

Some two-equation models with additional termsfor high streamline curvature effects available butstill needing “fine tuning”;


conceptual hydrocyclone tangential velocity profile -

solved using a two-equation model (blue) and a RSM model (red)


Fluent Viscous Model panel

CFX Fluid Models panel

Physical modeling


Definition of the turbulence model:

Based on what information the engineer needs.

The simpler turbulence model that gives coherent results is the RSM model withsecond order pressure-strain correlation(SSG in CFX).

More detailed approaches like SAS(Scale Adaptive Simulation), LES orDES can be used but increasing computational cost.

Physical modeling


Multiphase turbulence:

Same observations for the single-phase flow apply to primary phase on multiphase flows:

Also in cases where the homogeneous turbulence is applied.

Choice of homogeneous/inhomogeneous turbulence modeling should be based on case conditions:

For inhomogeneous turbulence, choose the appropriate dispersed phase turbulence model;

Turbulence on phases interaction:

Also, for the phases interaction check the turbulence dispersion modeling (in CFX)!


Physical modeling


Multiphase turbulence set-up:


Fluent Viscous Model panel for multiphase flows

CFX Fluid Models panel for homogeneous turbulence

CFX Fluid Models panel for inhomogeneous turbulence

– detail of the dispersed phase turbulence models

Physical modeling

• Multiphase modeling:


10-5 10-210-3 0.2 0.6

Secondary Phase Volume Fraction

... ...... ......

* : Possible coupling with Population Balance;Attention to Non-Drag Forces;Strong case dependency! – Contact ESSS engineers for help.

Boundary conditions and initialization

• Boundary conditions:

Basic rules to impose boundary conditions on cyclone or hydrocyclone simulations:

Whenever possible, include inlet, underflow and overflow piping:

As discussed in “Geometry and mesh generation” section.

Be careful with boundary conditions types:

A common combination is to prescribe mass flow rates (or flow rate) at the inlet and one outlet (overflow or underflow) and pressure at the other outlet;

Setting all boundaries with prescribed pressure is very unstable, if pressure is all the information you have you can use user functions (UDF’s in Fluent and User FORTRAN in CFX) to control flow rates to match the head loss data;

Be very careful to define the pressure values imposed to the outlet boundaries, due to rotation the radial pressure gradient is very intense – using prescribed values as pressure averages can lead to inconsistent results;

Do not specify velocity direction (or velocity components) on outlet boundaries unless you are sure that the flow is developed at the given position.





overflow pressure contours – pressure difference on axis and near walls

overflow velocity vectors and contours –non-normal velocity at boundary



Multiphase boundary modeling:

Secondary phase boundary conditions:

Pay attention to the definition of velocity and spatial distribution of the secondary phase in the inlet region, prescribing correct slip velocities and inhomogeneous spatial distribution (e.g. when there is a curve right behind the inlet section) lead to more realistic results.

Observe transient boundary behaviors:

In many cases there are effects of variation (in time) of flow rates, gushes, variation of secondary phase concentration, etc. These events have strong influence on the equipment efficiency.



• Initialization:

A suitable flow initialization can save a lot of computer time (reducing the time to develop the velocity and pressure fields), mainly for the primary phase.

Use of initial conditions from simulations using coarser meshes and/or cheaper turbulence models;

Interpolate results from previous studies;

Use of analytical solutions to velocity and pressure fields (approximations):

The “Burgers vortex” solution is an analytical solution of the Navier-Stokes equations that provides a velocity field close to a cyclone or hydrocyclone field (for the tangential component) based on two-parameters.


zvvrvv zr ˆˆˆ ⋅+⋅+⋅= θθr

Burgers vortex solution

zvz α=

−−=

υα

πφ

θ 4exp1

2

2r

rv

rvr α2

1−=

Numerical issues

• Numerical set-up:

Discretization/advection schemes:

Use second order schemes to moment and volume fraction equations;

Use second-order turbulence schemes verify consistency after first results;

Usually not necessary.

Convergence criteria and precision:

On CFX decrease the convergencecriteria default value to assure volume fraction equations convergence;

The run on double precision can improvestability and results:

Mainly for hydrocyclone cases.


Simulation Control panel on CFX

Solution Controls panel on Fluent

Numerical issues

• Numerical set-up:

Attention to pressure discretization!

On Fluent use the PRESTO! scheme on single-phase simulations:

Pressure and velocity fields solved in a staggered fashion.

On CFX force the trilinear scheme for pressure element interpolation:

Pressure interpolation inside the element using all element nodes (more precision calculating the pressure gradients).

Automatically set using multiphase flows with buoyancy.

On Fluent use the Coupled solver for single-phase analysis.


Advanced Options on Solver Control Panel in

CFX – detail of the Trilinear interpolation scheme selection for

pressure

Solution Controls for single-phase flow in Fluent – detail of the Coupled solver and the PRESTO!

Scheme for pressure discretization

Monitoring and post-processing

• Simulation monitoring:

Flow development must be assured by monitoring main flow variables!

Common monitor used in cyclones and hydrocyclones cases:

Pressures at boundaries (or head losses);

Head loss ration (inlet-overflow / inlet-underflow);

Disperse phase mass flow rates / balance:

In some cases these values will take much longer than pressure and velocity fields to develop.

Verify transient averages:

Define time-averaging to variables of interest:

Velocity components, pressure, turbulence intensity, etc.

After flow development and correct averaging the averaged values should stabilize.




Fluent monitors set-up:

Again, be aware of area-averaged pressurevalues!


Fluent Surface Monitors and Define Surface Monitor panels

Fluent Monitor window, simulation in progress, development when monitors reach a constant value (or time-averaged constant)



CFX monitors set-up.


CFX Monitor Points definition and User Point monitors tab in

Solver Manager

CFX Transient Statistics panel – definition of variables for time-averaging on the run


• Simulation post-processing:

Common cyclone/hydrocyclone post-processing:

Head losses;

Velocity profiles (axial, tangential, radial);

Separation efficiency;

Contour plots (pressure, volume fraction, velocity, etc.)

Turbulence post processing;

…





Tangential velocity profile

-5.00

-3.00

-1.00

1.00

3.00

5.00

-0.020 -0.015 -0.010 -0.005 0.000 0.005 0.010 0.015 0.020

radius [m]

tang

entia

l vel

ocity

[m

s-̂1

] .

CFX

experiment 1

experiment 2

tangential velocity profile – experimental

validation

overflow and underflow

streamlines –inner (in red) and outer (in blue) spirals




efficiency by particle size – experimental validation

(from Fluent News)

particle streamlines colored by

residence time

gas volume fraction in a

hydrocyclone simulation

with gas-core




turbulent structures colored by vorticity

turbulent structures colored by Q-criteria

turbulent structures colored by vorticity

Date post:	13-Dec-2015
Category:	Documents
Upload:	ruben-ramirez
View:	15 times
Download:	2 times

2008 Esssugm Aguirre A

Documents