Rotate airflow 90 degrees and distribute evenly in a long box
 

Good afternoon everyone,

I have a question about distributing an airflow over a long length.
The situation is as follows:
We want to blow out a large amount of air (+30.000m3/h) over an area of 300x1800mm. The airflow comes from the side so it must also be turned 90 degrees. The inlet opening is 650x650.
The discharge surface is very long and we want the air distribution to be as even as possible.
Above the discharge opening is a varying resistance of goods.

We are familiar with angle vane design rules and are thinking of applying them in our 'box'.

We are afraid that we will lose too much pressure in the front of the duct, so that the airflow is not proportional.

We are looking for help to properly design this box so that we get the most equal possible outflow of air. Are there design rules for this? Do you have any advice for this? See also attached illustrations

Hello all,

The above post did not give much response, but we are now a bit further with performing tests.
As mentioned, we want to rotate an airflow 90 degrees and distribute it evenly. We have placed angle vanes in a box for this. We have done several tests with different boxes.

Our finding is that the air is fairly well distributed over the discharge sections. However, we have the problem that the air is very concentrated along the angle vanes. A kind of vortex is created in the inner bend.

The question we now have is: How can we ensure that we prevent this vortex and that the air is better distributed over the outlet openings.

Okay, I'm describing a design which might reduce losses, and improve airflow ... imagine the design to be something like a water hose, and you drilled holes after every 1 meter, so water starts leaking out of the holes, and using one pipe, you're able to irrigate multiple plants.

1. put metal sheets after every hole, so multiple chambers of air are formed, and each chamber 's air will only be able to escape through one way, i.e the top.

2. Use a thin channel to distribute the air, not a thick channel. This will increase pressure and speed in the channel (but maybe a little bit lossy).

3. Cut holes into your thin channel, so the air can go to the chambers described in step 1.

4. Use quarter cut PVC pipes in each chamber described in step 1, so the air can smoothly flow up while following the curve of the pipe.

Simple design, no vanes required, it will reduce loss, but initial pressure needs to be high otherwise the air won't enter the chambers at a high enough velocity.

An engineering drawing to explain the solution.

Thanks for your reply.

For us, an equal air distribution is much more important than fuel/energy consumption. So if we find a way that causes more energy consumption but ensures good air distribution, then we will do this.

What we see in testing is indeed that we need more back pressure to better distribute the air.
We have done tests with a mesh/grid on the outlet opening. This grille has a passage opening of 75%. We already see a much better result, but we still see that there is a lower flow speed in the inner bend of the bends.

This phenomenon is worst in openings C and D. It also applies to the other openings, but slightly less.


If you have any ideas please let us know.

If you would like to distribute equally to each chamber, then ...

How about guiding the air with channels, instead of only deflecting with vanes?

This will ensure air is only going into the channels, and turbulence won't be formed since the other vanes don't obstruct the airflow.

I hope I understood correctly what you mean by "equal air distribution" . Because currently I'm thinking of how the air will be distributed to the different channels, and not about how the air will be distributed in the room once it comes out of the long box.

Yes this is what we were thinking of as well.
We have done tests yesterday with those horizontal plates for each vane.

But unfortunately this makes no difference at all in air distribution.
So we are still with the issues as mentioned before.

Sorry, Can't say what's causing the issue by only guessing. If it was me, I would next reduce the sudden turning angle of the channels, and see what happens. That is, try to bend the air over of a gradual and smooth bend, not a sudden bend. Instead of a 90 degree bend, I would try out a 60 degree bend and see if it improves the air distribution.

Since the turbulence arises in the channels, how about using layers of mesh to make the flow less turbulent? This technique is used in wind tunnels.