# MRF, 3d Centrufugal Fan Analysis in FLUENT

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 December 16, 2014, 13:30 MRF, 3d Centrufugal Fan Analysis in FLUENT #1 New Member   alireza esmaily Join Date: Dec 2014 Posts: 17 Rep Power: 4 Hi Friends Now i'm working on a centrifugal fan.(same as photos) I've a lot of questions. i need help. I know i should use MRF method (and Steady-State), but it doesn't work right (= I can't reach to right solution). my questions: 1- meshing? (in detail pls.) 2- cell zones? (in detail pls.) 3- boundary area? 4- solution methods? 5-... however, i tried to solve a 2d,mrf,fan problem, but i couldn't.

December 16, 2014, 16:14
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 Originally Posted by kaktus10 Hi Friends Now i'm working on a centrifugal fan.(same as photos) I've a lot of questions. i need help. I know i should use MRF method (and Steady-State), but it doesn't work right (= I can't reach to right solution). my questions: 1- meshing? (in detail pls.) 2- cell zones? (in detail pls.) 3- boundary area? 4- solution methods? 5-... however, i tried to solve a 2d,mrf,fan problem, but i couldn't.
First off, MRF is known to be the least accurate method for a fan. It is accepted for general engineering problems and is typically used as the quicked way to get a solution. If you have time, I suggest you use a sliding mesh approach. It is more accurate but you will have to sacrifice more computational time.

2D models are ill-advised unless you have symmetric flow. Even then, they don't always solve correctly.

If you are only looking at the flow, then use a coarse enough mesh to obtain a y+ close to 1 on your fan surface for enhanced wall functions.

 December 16, 2014, 16:31 #3 New Member   alireza esmaily Join Date: Dec 2014 Posts: 17 Rep Power: 4 Now i'm using mrf method. There is a problem. The interfaces between two zone dosen't work right? What operation should I do? Thanks

December 16, 2014, 16:57
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 Originally Posted by kaktus10 Now i'm using mrf method. There is a problem. The interfaces between two zone dosen't work right? What operation should I do? Thanks
The interface between the zones should be cylindrical. You have the make sure your mesh follows the cylinder surface very closely. Also If the two regions contrain different size cells, there is a maximum ratio of 5:1 for a fluent interface. When you create the interface you also have to select the smaller size surface as your first interface.

Since you are looking for steady state values, you can initialize your sliding mesh with mrf results if you go that route.

December 17, 2014, 00:55
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alireza esmaily
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Quote:
 Originally Posted by jpando The interface between the zones should be cylindrical. You have the make sure your mesh follows the cylinder surface very closely. Also If the two regions contrain different size cells, there is a maximum ratio of 5:1 for a fluent interface. When you create the interface you also have to select the smaller size surface as your first interface. Since you are looking for steady state values, you can initialize your sliding mesh with mrf results if you go that route.
Thanks jpando

I'll solve a very simple problem at first. please see my result.
1- what's your mean about ratio? (mesh ratio at interfaces or size of cell zones?)
2- OK -> smaller size is interface1 and bigger is interface2.
3- How can i use the sliding mesh with mrf? (what it means?)
Thanks

 December 17, 2014, 00:57 #6 New Member   alireza esmaily Join Date: Dec 2014 Posts: 17 Rep Power: 4 In this result ,the dissconection between two zone is very clear. There is no inlet & outlet. Inner zone has Frame Rotation. Outer zone is stationary area. fluid is Air. Rotational Velocity is -2500rpm.

 December 17, 2014, 01:03 #7 New Member   alireza esmaily Join Date: Dec 2014 Posts: 17 Rep Power: 4 In this result ,the dissconection between two zone is very clear.

 December 17, 2014, 05:57 #8 New Member   alireza esmaily Join Date: Dec 2014 Posts: 17 Rep Power: 4

 December 17, 2014, 05:57 #9 New Member   alireza esmaily Join Date: Dec 2014 Posts: 17 Rep Power: 4 Please Help

 December 17, 2014, 09:39 #10 New Member   Join Date: May 2012 Posts: 26 Rep Power: 7 Unfortunately I am unable to see you pictures. I think it has someting to do with the network I am on. The ratio I am talking about is the cell size ratio on the surface of the interface. See section 5.4.2. Non-Conformal Interface Algorithm in the fluent user's guide. They give very good details on what is required for an interface. So the sliding mesh is setup when you have two separate zones. The rotating cell zone is set with frame motion for MRF and mesh motion for sliding. Mesh motion requires a transient calculation to move the mesh. Start off with very small time steps and increase until each time step turns the cell zone about 1 degree (0.017453293 radians). You can increase the step size up to about 2 degrees if the model is behaving well at 1 degree. From what you are saying, "In this result ,the dissconection between two zone is very clear. There is no inlet & outlet. Inner zone has Frame Rotation." It sounds like the interface is not set as an "interface" in the boundary conditions. I can only guess since I don't see the picture. kaktus10 likes this.

 December 18, 2014, 03:02 #11 New Member   alireza esmaily Join Date: Dec 2014 Posts: 17 Rep Power: 4

 December 18, 2014, 09:37 #12 New Member   Join Date: May 2012 Posts: 26 Rep Power: 7 From what I see, the biggest issue is that you are trying to do this problem in 2-dimensions. There is no symmetry in this problem where you can ignore the third dimension. I recommend rebuilding your mesh in 3D and considering all the geometry.

 December 22, 2014, 03:11 #13 New Member   alireza esmaily Join Date: Dec 2014 Posts: 17 Rep Power: 4 Hi friends, I solve the problem in 2d and now i'have some problems: 1-At the output in center of the output line there is reverse flow. 2-Flow Rate at the output(outlet) is very low. 3-Flow Rate at the input(inlet) is Zero!!! photo of solution: https://www.dropbox.com/s/jfuchg3mqz...sults.PNG?dl=0

 December 22, 2014, 03:27 #14 New Member   alireza esmaily Join Date: Dec 2014 Posts: 17 Rep Power: 4 ok my friends, i found the problem. the input velocity magnitude in boundary conditions was zero. Amazing mistake!

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