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A Short Guide To Fan CFD Simulations

A Short Guide To Fan CFD Simulations

A couple of years ago I had the chance to work together on a project with CFD specialists of one of the top fan manufacturing companies in the world. Based on what I had learnt from running a benchmark project I made a short summary of the most important guidelines how the professionals run CFD simulations of fans for HVAC and automotive applications.

1. Simulation of rotating motion. It is not usual to simulate the real rotating motion, later you will see why. For most of the cases where flow field is axisymmetric to the rotation axis the so called frozen rotor technique is used, where the position of rotor and stator is fixed. When doing so we only need to describe type of motion (rotation as type and angular speed that is coming from the electric motor RPM) and we do not care about sliding or as we call it in SC/Tetra discontinuous mesh parameters and contact surfaces.
What do we win with this approach? A whole bunch of time, and time is what we have the least in real industrial environment. It is so because simulations are steady-state, run fast and this is important since during a ventilator development a CFD guy has to run a lot of simulations.
What do we loose with this approach? Sure you are right: a little bit of accuracy. It is so because the results could depend on the actual position of rotor and stator and also because transient features like pressure variations, secondary flows can not be seen.

2. Fixed volume flow rate as boundary condition. Application of a fixed volumetric flow has a straightforward reason and it comes from the fan-curve. The pictures below show the fan-curve of an axial ventilator. If we applied the pressure difference between inlet and outlet and measured the flow rate, we would analyse our brand new ventilator along the red horizontal line. That is not so good because – depending on the fan-curve – our red line may intersect with the fan-curve in more than one points and you would be in trouble to tell which volume flow is you are looking at for the specific pressure drop [1].

Pressure difference as boundary condition means simulations along the red line
Pressure difference as boundary condition means simulations along the red line
Volume flow rate as boundary condition means simulations along the green line
Volume flow rate as boundary condition means simulations along the green line

On the contrary when applying fixed volume flow between inlet and outlet there will only be one specific pressure drop (green line on the figure above) and this way volume flow and pressure drop pairs can obviously be matched. For the outlet usually p=0 Pa static pressure is applied.

So, ventilator developers run CFD simulations across the whole fan-curve: for a given geometry (and mesh) they calculate pressure drop for a set volume flow. And here this is why simulation time becomes important: you may have many volume flows to analyse. Luckily in SC/Tetra this procedure can almost fully be automated.

3. Turbulence model. There is no single, universal solution for many great questions of life, and so it is with turbulence.
Choosing the right turbulence model is affected by for example Reynolds-number and flow behaviour (are there any separations from blade surfaces).
Depending on the actual task a good choice could be for high Re-numbers [2]:

  • realizable k-ε,
  • SST k-ω,
  • according to my experience rarely but LES (Large Eddy Simulation) is also used.

For low Re-numbers (in general low Re turbulence models are written to work well for high Reynolds numbers as well):

  • AKN (Agabe-Nagano-Kondoh) k-ε,
  • MPAKN k-ε.

4. Number of prismatic boundary layers. Besides turbulence modeling meshing is the subject that is still good for many books and papers to write.

If we just concentrate on boundary layer mesh made of prismatic elements, then the two most important parameters – thickness and number of layers – highly depend on the actual task and turbulence model. Our RnD colleagues at the fan manufacturing company for example use 6-8 layers in general, and one of the references [2] mention maximum 10 layers for high Reynolds numbers and for low Re turbulence models even 40 layers may be necessary.

5. Torque on the axis of rotor. One of the results professionals look at is the torque measured on the axis of rotor. Both from pressure and from viscous forces are calculated, because torques from viscous forces could reach 6-8% of the overall amount which can not be neglected if you want accuracy.

6. Shear stress on blade surfaces. Shear stress can be highlighted both as scalar (the magnitude only) and as vector on blade surfaces. The reason for calculating and analysing it is that it pretty obviously shows what is happening on the blades: if the magnitude is small, the flow is separated from the surface. This can be used as a sign to show where blade geometry should be modified.

Magnitude of shear stress on fan blades
Magnitude of shear stress on fan blades
Shear stress vectors on blade surfaces
Shear stress vectors on blade surfaces

Dr. Robert Dul

References:

[1] Source of pictures: www.electronics-cooling.com
[2] http://www.cfd-online.com/Wiki/Best_practice_guidelines_for_turbomachinery_CFD

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