Presentation

eResearch Intern Showcase 2011

Generation of Nonwoven Filter Geometry for CFD Simulation of

Oil Mist Filters

Dr Andrew King, Fluid Dynamics Research Group, Curtin University

Dr Ben Mullins, Fluid Dynamics Research Group, Curtin University; Atmospheric Environment Research Centre, Griffith University

Robert Howie, Curtin University


The Need for CFD Simulation of Oil Mist Filters

Oil mist filters are used to remove aerosolized oil droplets from gas streams.

Currently new designs are developed by trial and error.

Computational Fluid Dynamics (CFD) simulation would allow faster development bringing improved performance and lower design costs.

(Edwards High Vacuum International 2004) (LEADWELL 2009)


The Need for CFD Simulation of Oil Mist Filters

(Donaldson Torit 2010)(Donaldson Torit 2010)

Diagram of an Oil Mist Filter Oil Mist Filter Media Cartridge


The Problem

Simulating oil mists using the standard CFD solvers is infeasible due to the micrometer cell size required.

This was the impetus for the development of a hybrid particle and volume of fluid solver [1]

Up to this point the hybrid solver had only been tested on simple geometry.

More realistic filter geometry was required to analyse the behaviour and compare it to theoretical and experimental results.


Project Goals

The objective of the project was to develop a method of creating more realistic nonwoven filter geometries that could be used for developing and testing the hybrid solver.

The filter geometries had to be more realistic but not perfect. The long term goal for the hybrid solver is testing on 3D scans from real filter media.

(Mullins, B 2004)


Solution Features

Required features:

• Output in a format that snappyHexMesh can read (.stl)• Provide a way of controlling the solidity (alpha)• Provide control of the fibre diameter

Beneficial features:

• Fibre diameter distributions• Control over the fibre orientations

Extra features:• Curved fibres


Approach – Platform Selection

We chose to extend Blender 2.5 to take advantage its:

• Interface, 3D view, rendering capabilities• Import and export capabilities (including .stl)• Mesh manipulation tools• Python scripting API [2]


Trial of the Soft Body Approach

We decided to build up a web of fibres using the physics engine built into Blender in an attempt to replicate the manufacturing process of nonwoven media.

This method was too computationally expensive


First Approach – Rigid Body Simulation

We moved to a rigid body approach because the fibres are quite straight at the micrometer scale, and the curvature would not have had a significant impact on the behaviour of the oil mist.

3D models of fibres were created in a box and then let drop to the floor using the Blender Game Engine (BGE).

The script had to be modified to make the face lengths acceptable for CFD meshing and then, later, to increase performance.


First Approach – Rigid Body Simulation

Drop Initialised Drop in Progress Final Product


Shortcomings of the Rigid Body Approach

The final script could produce geometries of a few thousand fibres, but wasn't very reliable.

The fibres didn't seem to be behaving realistically during the physics simulation.

These problems are likely caused by the large load on the BGE which is designed to run in real time.

The script didn't provide enough control over the solidity. We could change it but not specify it.

We decided we needed a more reliable solution that also gave us more control over the final product.


Second Approach - Cyclic In Place

We decided that the next iteration needed to produce a filter geometry with a specific alpha value.

We decided to create the fibres in their final positions with controls over their orientation.

The user inputs the fibre parameters and the dimensions of the region. And the script calculates the number of fibres to create.

Each fibre is created 8 times (one per octant in the 3D Cartesian grid), and the region in the centre is exported to ensure that the total volume of fibres expected is present within the region.


Second Approach - Cyclic In Place

Creation of Fibres(Octant One Highlighted)

Final Product Orthographic Top View(Highlighting Wrap Around)


The Need for a More Flexible Approach

The user has to ensure that the longest diagonal of the fibres is less than the smallest dimension of the region or else fibres may not "wrap around" correctly. This would create a model with lower solidity than expected.

Creating large regions with many fibres requires more resources than it should because each fibre has to be created eight times.

We needed the ability to create these larger regions without this overhead when a cyclic simulation is not required.


Final Approach – In Place Non-Cyclic

The final approach was to generate a large region and export a smaller section of it.

This is required because the solidity would not be accurate around the edges where parts of the fibres centred there lie outside the region.

The inner solidity should be accurate but it may vary because the fibre placement is random. This small scale variation is realistic.


Final Approach - In Place Non-Cyclic

Creation of Fibres(Export Region in Orange)

Final Product


Final Approach - In Place Non-Cyclic

Screenshot of the Add-On Running in Blender


Results

This script provided the most flexible way of producing the fibres and was the only solution capable of producing large geometries with minimal overhead.

To minimise the overhead created by the unusable region the fibre length should not be much larger than the export region.


Outcome

Using the filter models generated in the course of this project we were able to move up from simulations using 4 fibres to realistic sections of filter media.

Currently there is a simulation of a tangible size with around 6 million cells running.


Outcome

(King, A 2010) (King, A 2011)

Initial Geometry New Geometry


Conclusions

The Blender add-on created is useful for testing CFD solvers for oil mist filters and dust filters.

The software can produce simulated filter geometries at a tangible scale.

The software is going to allow further development of the solver.

Future work in this area (geometry generation) with a longer term commitment should probably focus around an new open source geometry generation project where the developers and users have more control. It would be possible to use Blender to examine the models produced by a non GUI tool.

The next immediate step is to further develop and test the solver.


Acknowledgements

iVEC [ivec.org]

Curtin University [curtin.edu.au]

Centre for Comparative Genomics, Murdoch University [ccg.murdoch.edu.au]

Dr Andrew King, Curtin University

Dr Ben Mullins, Curtin University

Ms Valerie Maxville, iVEC

Mr Paul Newman, iVEC

Mr David Schibeci, Murdoch University


References

[1] A. J. C. King et al., "Hybrid volume-of-fluid and discrete particle solver for oil-mist filter simulations", presented at the 17th Australasian Fluid Mechanics Conference, Auckland, 2010.

[2] The Blender Foundation. (2011, February 22). Blender v2.56.1 - UNSTABLE API documentation [Online]. Available: http://www.blender.org/documentation/250PythonDoc/


Thank You for Listening

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License: Creative Commons Attribution-ShareAlike 3.0 - http://creativecommons.org/licenses/by-sa/3.0/(Excluding referenced images)

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