# Fan blower with less airflow in the center

I'm building a ducted fan and I would like it to:

• have a uniform airflow at the output
• have good static pressure
• have a square section output (see figure: air enters the circular edge and exits from the square edge)

In my application the output section will be covered by a breathable membrane.

While doing some tests I noticed that the airflow in the center was much weaker than at the sides, with this blade design (counter-clockwise rotation):

How can I get more uniform airflow?

Here are some thoughts on how to improve the design, but since I don't know virtually anything about fan design and fluid dynamics I'm asking more competent folks here to judge whether these ideas could work:

• make the output section smaller than the input section

Do you know if any of these would work? Do you have better suggestions?

• Have you considered a matrix of smaller fans? Each fan would have its own center/slower throughput, but the total assembly would have the aggregate of the fans and would also "mix" more thoroughly. Jul 14, 2020 at 15:29
• @fred_dot_u Thank you for your comment. Unfortunately I cannot go for the matrix of smaller fans, so I can only try to improve this design... Jul 14, 2020 at 15:36
• Did you check it with the duct in place, or was it in the open flow condition? The duct will massively change the downsteam jet characteristics. Also, check the area ratios of the round and square sections. Changes in area will cause changes to the velocity profile across the duct. You won't know what you've got until you have the whole thing built, but the breathable membrane should be tailored to achieve the uniform flow downstream of the membrane. Jul 14, 2020 at 16:57
• @PhilSweet With the duct in place. I added some sand on the breathable membrane and the sand would "bubble" only close to the edge of the duct. Talking about the ratio of the areas (say $\displaystyle \frac{\text{output}}{\text{input}}$): would you suggest to make it bigger or smaller than $1$? Jul 14, 2020 at 17:19
• Look at how this is achieved in wind tunnels Jul 14, 2020 at 18:11