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Old   July 18, 2007, 13:21
Default Fundamental Problem
  #1
Naghman Khan
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Hello,

I'm finding it difficult to picture the domain in which flow occurs. I have a geometry, 3D centrifugal fan without any casing i.e. just the impeller. I have made the blades and the two ends which hold the blades, all are volumes and meshed and successfully exported.

But what about the hollow space in the impeller? Is this space hollow and therefor not subject to any calculations by fluent? Do I need to make a separate volume in this space, mesh it and then define this as what (Boundary types)? And also would I define my blades as walls, for the air to flow around them and then through the impeller and out past the opposing blades?

And where would the flow out from, do I need to explicitly define outflows as the spaces between the blades?

Thanks people, I'm new to CFD, still can't picture it.

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Old   July 18, 2007, 16:34
Default Re: Fundamental Problem
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red lemon
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Dont think you have a clear idea of what you are doing. Look at some other turbo style meshes for pumps/turbochargers etc or turbo examples available from Fluent.
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Old   July 18, 2007, 17:33
Default Re: Fundamental Problem
  #3
Naghman Khan
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Red Lemon,

Hi you are right I'm not completely sure of what I am doing. Unfortunately my department does not have the Turbo add on. Anything else you can suggest? Thanks
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Old   July 19, 2007, 18:02
Default Re: Fundamental Problem
  #4
red lemon
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You dont need GTurbo to access the turbo tutorials. Just go to fluentusers.com or learningcfd.com and advanced tutorials. All setups for pumps & fans (MRF or sliding) are there.
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Old   July 24, 2007, 01:14
Default Re: Fundamental Problem
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Raj
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Hi, To start with, what is the significance of that hollow space? I meant rhetorically. If there is no heat transfer calculation going on, then you dont have to worry abt the conduction, if at all, in the impeller blade. So you dont have to create that volume and hence no meshing involved there. Yes, just define the faces of that hollow region as walls and zero heat flux.

Now, lest consider just one blade...ok... Here, if the space around that blade has been meshed, then you would have six faces(if the meshed region is cuboid in shape). One would be inlet, one would be outlet. Two would be walls and two more would be given periodic, ie peridic interfaces between the previous blade and the next blade. That way you can do the analysis for only one blade and then replicate the others in post processing.

I hope this helps. Later. Raj.
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Old   July 24, 2007, 07:09
Default Re: Fundamental Problem
  #6
Naghman Khan
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Raj,

Hi there, thanks so much for your reply. I am beginning to picture things now.

So what you are saying is that I only need to physically make and mesh one blade and adjacent open space and define them as periodic. Or do i need to make two blades with the space in between defined as inlet, outflow, walls etc?

What if I wanted to use a multiple reference frame i.e. to make the blades rotate, should I still just model one blade or in this case would I ahve to model all of them?

Thanks Raj, hope my questions make sense. Best wishes

Khan.
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Old   July 24, 2007, 08:02
Default Re: Fundamental Problem
  #7
Raj
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Hi, If you are working in Gambit for modelling and meshing for turbo m/c, then I am a lil rusty. In my experience, the meshing software which is best for turbo blade meshing is Turbogrid. Anyway....

lets say, your fluid domain has three blades in it. Then the whole fluid region can be divided in 3 equal parts 60 deg apart. but where would you start that division in your domain? Generally at the mid section between any two blades. lets name these three blades as A, B and C. Then the fluid domain can be divided in to three parts with three sections, between A and B, between B and C and finally between C and A. We will name these faces or sections as 1, 2 and 3 respectively. So when you take the first blade A, it is bounded by faces 1 and 3 on either sides, hub and casing curves on top and bottom and finally two more faces, which are your inlet and outlet. Got the picture?!! Now, 1 and 3 are given periodic and the other two with the baldes in them are repeated and it will automatically take care of the other blades` position and all, since in the beginning you`d define the num of blades as three (in this example). So just model the one blade and model and mesh the fluid domain around it. You dont have to model the blade as a volume and mesh it. Rather think that your fluid domain has a hollow space in it and the shape of the hollow space is the shape of the blade!!!

Sometimes it becomes difficult to picturise things in the scientific world when we put it only in words.

MRF-stage or frozen interfaces can all be worked out in the solver.

I dont know if I am able to make any sense to you or not!!! Later. Raj.
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Old   July 24, 2007, 11:21
Default Re: Fundamental Problem
  #8
Naghman Khan
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Raj,

Thanks bro, for all your help. Your advice does make a lot of sense, I'll give it a go.

By the way, I am using Gambit but I don't have the Turbo function as this is an extra which you have to pay extra for.

On the question of MRF, when I define a fluid as having a rotational velocity of say 20 rpm, in the rotation axis origin and rotation axis direction what input need I give?

For the rotation axis origin, is this asking me which coordinate does the actual axis about which rotation takes place originate, e.g. 0,0,0 (x, y, z)? And for rotation axis direction under x, y, z values do i need to put basically the length of the rotating body? For example if my axis started at 0,0,0 and was 10 units in positive z direction, would I put 0,0,0 as the origin and 0,0,10 as the rotation axis direction values?

Thanks again bro,
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Old   August 1, 2007, 11:01
Default Re: Fundamental Problem
  #9
Remsy Tharakan
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I WOULD LIKE TO SEE A ROTATING MOTION OF THE PROPELLER INSIDE A PIPE. What is the way to introduce this function in the Fluent. There is already a liquid flowing through the pipe and through the propeller an another liquid flows out. How can I simulate this in Fluent. Which way to proceed to get the end result.
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