Learning Forum

[lzhang]

Hi,

I'm modeling an axial fan with some air gaps in Fluent, as shown in the picture below. The blades are set to be rotating, and there are some air gaps surrounding the fan. The model has two cylinder inlet and outlet ducts, and the inlet and outlet are set on the two sides of the ducts. I'm trying to simulate the reverse flow in the air gap. Both the inlet and outlet should be atmospheric pressure, and the fan zone will create a pressure rise across it, which pushes some air exiting the fan zone to enter the air gaps and flow back to the inlet side. However, I'm having trouble simulating this scenario in Fluent. Since the pressure inlet boundary condition only sets the total pressure at the inlet, the inlet static pressure can't be set to 0. I've also tried using velocity inlet or mass flow inlet. The result shows large pressure at the fan inlet side, and the flow in the air gaps is in the same direction as the flow in the fan zone. How should I set the inlet and outlet boundary conditions so that there's atmosphere pressure on both sides? Any help is appreciated. Thanks!

[C N]

Hello Izhang,

I  recommend you to give the inlet boundary condition of type inlet vent because it accounts for the pressure loss coefficient . I am attaching the user guide link of the boundary condition. Kindly follow it and implement this in your simulation. also uncheck the prevent reverse flow option. Hopefully this should solve your problem.If not please try enabling the 3d fan zone region in cell zone conditions of the rotor which is the rotating part. Also try using this option of intake fan as inlet type it accounts for the pressure jump and leaves the prevent reverse flow unchecked. I hope this helps you.

7.4. Boundary Conditions (ansys.com)- 7.4.7.11 , 7.4.8.11(refer)

Thanks,

Chaitanya Natraj

[lzhang]

Hi Chaitanya,

Thanks for your reply. I tried setting the inlet as an inlet vent. The static pressure at the inlet is around -60 Pa. However, when I look at the total pressure on the x-plane, there is a large pressure inside the air gap. I've attached a screenshot below. When I look at the velocity vector inside the gap, there is a large tangential velocity. Since the air gap is not rotating, I find this a little confusing. Is this because of the boundary conditions I set or some other reason? Also, a question on the 3D fan zone: is there any resources on how to get the inflection point ratio? I know how to model the blades separately, but how do I use the results in Fluent to calculate this value? 

Thanks again!

[lzhang]

 

Hi Chaitanya,

Thanks for your reply. I tried setting the inlet as an inlet vent. The static pressure at the inlet is around -60 Pa. However, when I look at the total pressure on the x-plane, there is a large pressure inside the air gap. I’ve attached a screenshot below. When I look at the velocity vector inside the gap, there is a large tangential velocity. Since the air gap is not rotating, I find this a little confusing. Is this because of the boundary conditions I set or some other reason? Thanks again.