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[ICEM] Questions about turbomachinery meshing

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Old   January 30, 2018, 14:08
Default Questions about turbomachinery meshing
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I have a single Igs file of a steam turbine last stage consists of two blades (one for stator and one for rotor) and i want to use ICEM for meshing but i am beginner? so i dont know how to capture the stator rotor interactions? do i have to use sliding mesh for that? and is it okay to model single passage and use periodicity if the stator and rotor rows have different number of blades (60 for stator, 65 for rotors)? also i have the two blades (stator and rotor) together in one file, can i mesh them together?
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Old   February 1, 2018, 01:24
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Hi,
All turbomachinery geometries are axi-symmetric and they are periodic in nature. So you can do the grid for one set of stator-rotor combination.

If you are having only one combination of stator-rotor than you can generate one-to-one connected grid for it. But if you are doing an optimisation study with multiple geometric variations with varing number of stator and rotor blades, than generating sliding mesh grids are preferable.

About the igs, it donesn't matter whether stator/rotor are there in the same igs or not. You can group the required surfaces as you want and start with the gridding.

I don't know much about structured gridding in ICEMCFD, but use GridPro for turbomachinery cases.

Hope these inputs helps you.
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Old   February 1, 2018, 08:19
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Thank you so much for this useful answer. but i have a difficulty understanding how to capture stator-rotor interactions there are many techniques like mixing plane,frozen rotor and sliding mesh
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Old   February 1, 2018, 23:21
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Hi,
What type of technique you use depends on the type of simulation to be performed.

I am ignorant of the mixing plane,frozen rotor approach. But sliding mesh approach is generally used when both stator and rotor are rotating.
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Old   February 3, 2018, 07:30
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If stator is also rotating then it must be a rotor.

Mixing plane is used to mixed out the downstream and upstream flow along the tangential direction. Downstream component (it is rotor in case of compressor and rotor in case of turbine and vice versa) prescribe the average static pressure condition for the upstream component through mixing plane model. Only averaging is done at circumferential direction, but radial profile is real one.

Problem or disadvantage of this method is that you cannot see the effects of wake on downstream component. But the advantage is that most of the turbomachinery components are designed for some fixed performance parameters like efficient, pressure ratio and mass flow rate which are global quantities and does not take effect from local unsteadiness. So if you are interested to find out the performance of turbomachinery at or near design condition then mixing plane is the highly recommend method. .

Another advantage of this method is that you dont need to use full wheel geometry. Only one passage (one blade) from each blade row is enough to model the effects completely.

But but you cannot use this method on the non asymmetric geometries,
e.g. pump impeller and volute around it.

Frozen Rotor Method
As the name suggests that geometry is frozen at particular location in space and time. Unlike the mixing plane, you need to match the pitch (section of circle) of upstream and downstream component. This is problematic but the turbomachinery component blades are in even and prime number to avoid some issue to natural frequency occurring. So we cannot get any multiple of blade in rotor and stator which can make pitch equal. So we end up modelling the whole stator and rotor. But it is not a necessarily happening in all the cases.

Second problem with this method is that, it does not average the flow quantities and scale it according to circumference, so to get the required flow rate (for example) you have to model equal pitch on both sides. Due to this major issue arises which is fact that the position of stator and rotor are fixed in space and time. So if stator is at front of rotor, it will give the different solution when the stator is mid way between the two downstream rotor blades. This second limitation is not the case when you are modelling the impeller with van less volute. Moreover you cannot use the mixing plane model in this situation. Another situation where we can use frozen rotor successfully is the simulation of wind turbines.


Sliding Mesh
Above two methods are steady state and sliding mesh is transient method. It involves time, so it is costly in terms of resources and time required to get the solution.

This is the method of choice when you are really interested in the stator and rotor interaction. For example you want to see the effect of changing distance between rotor and stator. This can improve or decrease the performance by few percent. It is known fact that reducing the distance between rotor and stator in compressor increases the performance and in turbine opposite is true due to recovery factor.
Second situation where you want to use this approach, is to check out the clocking effects of upstream and downstream stator (i.e in multistage compressor, one stator is upstream of rotor and one downstream of rotor). Here you want to check out the effect of changing the position of downstream stator w.r.t to upstream stator in circumferential direction by few degrees in +ve or -ve direction.
Another fact is that sliding mesh is the general approach for the frozen rotor model, becuase in frozen rotor, position is fixed for rotor and stator, but in sliding mesh it is changing with each time step. So it means frozen rotor is subset of sliding mesh method at some particular fixed position of rotor and stator.
Sliding mesh is not a recommend method for every turbo-machinery simulation.

Last edited by Far; February 4, 2018 at 17:16.
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Old   February 4, 2018, 17:03
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Thank you so much far for this detailed useful answer, it made everthing clear for me
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