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November 15, 2012, 02:36 
inlet has higher pressure than outlet in axial flow fan

#1 
Senior Member

Hi ,
I am simulating axial flow fan. the results looks wrong. the simulation results shows inlet has higher pressure than outlet pressure. I have given volume flow inlet(2 m3/s), pressure outlet (100000), rpm750. I am not able to figure out the mistake in my case, Can you people help me to sort out this issue. I am giving some more details about my case. I have attached the initial condition files, please have a look and give me your suggestions and idea for my problem. Thanks, Sivakumar 

November 15, 2012, 07:07 

#2 
Member

Hi,
Calculate the corresponding inlet static pressure and set the inlet boundary as total and static pressure. In such devices the inlet boundary is fixed as pressure and the outlet boundary is a fixed pressure or a quadratic law with respect to the outlet mass flow rate. Regards. 

November 15, 2012, 09:52 

#3 
Senior Member

Hi Vicarious,
Thank you for your reply, I am giving here the experimental condition, flowRate = 16 m3/s at inlet rpm = 750 pressure outlet = 101325 Pa I need to reflect the same condition in CFD. I have attached the 0/U and 0/P please have look and give me you correction and suggestion. Thanks, Sivakumar 

November 15, 2012, 11:22 

#4 
Member

By setting the inlet as mass flow rate, there is always a chance that the inlet pressure decreases.
Is there any dimension of the experimental setup? If there is, you calculate the inlet velocity and the inlet dynamic head; from total pressure you will be able to calculate the inlet static pressure corresponding to the 16 m3/s. Try to set the inlet boundary as another p.txt. As I mentioned, Numerical simulations for such devices need constraints as pressure in both inlet and outlet so the solver can handle the solution. I'm afraid I am not familiar with files you've attached but the procedure I've explained works with the SIMPLE algorithm for axial flow fans and compressors. 

November 15, 2012, 13:40 

#5 
Member
Aathavan
Join Date: Nov 2012
Posts: 70
Rep Power: 4 
Hi Vicarious,
Thank you for your kind information, I bit struggling to implement your information. I have calculated the dynamic pressure = 64.50 pa can you please tell how to calculate static pressure. I know P0 = Pd +Ps, as well I am not getting the question Is there any dimension of the experimental setup? you mean for inlet section? if you give me some detailed information for the inlet pressure condition, it will be grate help for me. if you need any information please ask me. thanks, Aadhavan 

November 15, 2012, 15:31 

#6 
Member

Hi Aathavan,
Yes I meant the inlet section area. Since you've already calculated the inlet dynamic pressure, do the math from the same correlation (Ps=P0Pd), and set it as your inlet total and static pressure. For the outlet boundary, if the flow discharges into atmosphere the pressure you have set will handle the solution. For other devices such compressors, the outlet static pressure must be set based on the pressure rise. 

November 15, 2012, 15:54 

#7 
Member
Aathavan
Join Date: Nov 2012
Posts: 70
Rep Power: 4 
Hi Vicarious,
thank you for your message, what value I need to use for P0. do i need to use 101325 Pa. or how can I calculate. Thanks, Aadhavan 

November 15, 2012, 16:27 

#8 
Member

Yes the atmospheric pressure or the free stream pressure is the total pressure, which is 101325 Pa in your case.
Regards. 

November 15, 2012, 16:39 

#9 
Member
Aathavan
Join Date: Nov 2012
Posts: 70
Rep Power: 4 
Hi Vicarious,
Thank you, I will try to implement it tomorrow. since I am using openFOAM, I dont know what is the usage, I will have a look. set it as your inlet total and static pressure do i need to give both (inlet and outlet) as pressure initial condition. which condition i need to use for inlet and outlet Pressure inlet value  (static pressure or total pressure) outlet as well pressure value (101325) is it right if both are pressure? I am sorry for not understanding, thanks, Aadhavan, 

November 15, 2012, 16:56 

#10 
Member

Yes you need to give them both so the solver will be able the calculate the inlet velocity itself. The total pressure is fixed in the solution. The static pressure is used to guess the initial values.
Yes both are set as pressure. inlet: total and static; outlet: static. 

November 15, 2012, 17:11 

#11 
Member
Aathavan
Join Date: Nov 2012
Posts: 70
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Good, As I said before my dynamic pressure is 64.5 Pa
P0=Pd+Ps P0=101325 Pa So my Ps = 101260.5 Pa Now my Inlet : total+static outlet: 101260.5Pa is it correct? thanks, Aadhavan 

November 15, 2012, 17:31 

#12 
Member

Yes your inlet Ps is correct.
But as I said, the outlet pressure condition is calculated based on the pressure rise that fan produces. I suppose axial fans do not increase the pressure too much, am I correct? In that case the rise could be negligible and you can set the same static pressure. 

November 15, 2012, 17:47 

#13 
Member
Aathavan
Join Date: Nov 2012
Posts: 70
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Yes, fan works with moderate pressure.
actually i need to get the pressure difference approximately 200 Pa, ok Vicarious, I will implement tomorrow, I am not able to digest the inlet pressure, if I add P0+Ps I am getting 202585.5 Pa, is it right value? thanks, Aadhavan 

November 15, 2012, 18:10 

#14 
Member

If you are able to take the pressure difference into account, then use it to set the outlet pressure as 200 Pa less than the inlet for example.
No, you just can't sum these two values. I meant to enter these values as two different inlet boundaries. you need to enter a total pressure as a constant in your whole domain. From the static pressure, the solver will guess the velocity and calculate it for the whole domain and marches in time. If you're using a software to do this, it should be available for both inlet total and static pressure (for example FLUENT). 

November 16, 2012, 09:09 

#15 
Member
Aathavan
Join Date: Nov 2012
Posts: 70
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Hi Vicarious,
I am back, I am trying to implement the idea in to openFoam, I am bit confused, I have attached my boundary condition as pdf for your consideration. please have a look and tell me where do I need a change. you asked me too me define total pressure and static pressure at inlet thanks, Aadhavan 

November 16, 2012, 15:31 

#16 
Member

Pardon me but something is wrong with the file you attached, showing something else.


November 16, 2012, 15:48 

#17 
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JPConway
Join Date: Nov 2011
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November 16, 2012, 16:21 

#18 
Member
Aathavan
Join Date: Nov 2012
Posts: 70
Rep Power: 4 
Hi Vicarious,
Yes I am wrong, I have attached the wrong file, I am rally sorry for that. I will sent it to you tomorrow morning thanks, Aadhavan 

November 17, 2012, 05:54 

#19 
Member
Aathavan
Join Date: Nov 2012
Posts: 70
Rep Power: 4 
Hi Vicarious,
I have attached the pdf, please have look, I got some other reply as well in that "if I keep constant total pressure for my whole domain, there will be no dynamics" that is also makes valid point. please have look and guide me what to do. Thanks, Aadhavan 

November 18, 2012, 00:01 

#20  
Member

Quote:
The constant total pressure I mentioned was not for the whole domain as a fixed value (It's only valid for irrotational flows), I meant for the calculation the inlet total pressure will be a constant to solve the entire domain. Since the static pressure changes through the flow, therefore the dynamic pressure also changes with it. Pardon me if I explained it in a bad way. Your boundary setup looks appropriate. Do the simulation and let me know what happened. If anything went wrong, consider the fan's pressure rise and set it as outlet boundary. 

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