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 D-man May 25, 2020 05:31

Axial fan simulation setup

Hello, i'm new to cfd and flow works.
I'm trying to simulate an axial fan to make fan curve.
i have a fan blade design, rpm, and results of experiment.
Experiment is very simple - setting 10, 20, 30, Pa extra pressure in backwards direction of a fan and measuring pressure to calculate airflow.

As for now i made enclosure, set up fan inside of it and made rotational region that is covering blades completely but not touching enclosing tube.

I'm getting airflow at surface 1.25D from fan without any goals or boundary conditions (except for stator surface).

How to apply extra pressure in backwards direction?
How to make results closer to experimental ones?
Is there any guide or tutorial with explanation? Or anything ro read about blade design flow works test?
Idea or direction for future learning needed.
Thanks in advance. (yeah, quite tough task i chose for newbie, i think)
Sorry for my poor english.

I think i should better illustrade what i'm doing.
Velocity

 Boris_M June 30, 2020 08:00

Hi D-man,

First, how did you set up your fan experiment in reality?
The reason is, that with your setup in the simulation, how do you want to apply any pressure difference if inlet and outlet are connected around the fan?

1st Rule in CFD:
You can only be as accurate as you model the reality. Any simplification or deviation from the reality (either in geometry or boundary conditions) will impact the results (some more, some less).

The simplest way is to create a long tube with lids at the inlet and outlet. You can then apply a flow rate at the inlet and environment pressure of 1 atm at the outlet.
I know, you will wonder that you want to find out the flow rate and not dictate it. But simply take any flow rate and you will see that the pressure difference you can measure with goals at the inlet and outlet will be a point on the curve.
Why?
Because the fan has its RPM and imagine you have barely any flow dictated, the fan is rotating like crazy trying to get more flow going through but it is fixed by your BC (boundary condition). So what will happen?
The pressure will change and adjust and the pressure difference is what he is working at in this flow rate condition.

Now the next thing for a fan curve is to run through several flow rates and this can be done with a parametric study where you simply change the flow rate from A to B in X steps to get multiple points for your curve.

There are two tutorials on how to set up fans and pumps in general but nothing specific about blade design as this is a user task, not a how to set up a simulation for rotation.
There are gazillions of user-specific tasks they want to do, not a chance to have a tutorial for all the cases that need to be considered. The tutorials are there to show you how to set up a simulation task. When the reason on how to set up such a task and the limitations of the tools or physics is known, the user can develop his experiments himself.
Simply imagine how you would do it in an experiment and then reproduce it in CFD. After all, it is a virtual testbench.
Sometimes you just have to turn your way of thinking upside down, like with the flow rate. Eg. a plane flies through the air, but you can also hold it still and let the air fly over the plane, same result.

Hope that helps,
Boris

 D-man July 6, 2020 01:28

Quote:
 Originally Posted by Boris_M (Post 776433) The simplest way is to create a long tube with lids at the inlet and outlet. You can then apply a flow rate at the inlet and environment pressure of 1 atm at the outlet. Hope that helps, Boris
Wow, i've tried a lot of ways, but that is really unexpected.
Thanks a lot. I will try that way.

I managed to learn rules of basic blade design. Now i need to make cfd tests correspond to reality.
Thank you for hint how to do it.

 D-man September 10, 2021 09:49

Quote:
 Originally Posted by Boris_M (Post 776433) The pressure will change and adjust and the pressure difference is what he is working at in this flow rate condition.
I must thank you one more time.
That was best explanation ive seen. Returning to this problem after a long time, searching a lot of video, articles and tutorials brought me back here and... your advise helped me most.
Thank you.

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