Fan blower with less airflow in the center

Question

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:

adding more blades
make the blades flatter
make the blades wider
reducing the radius of the mounting cyclinder of the blades
make the output section smaller than the input section

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

Solar Mike · Answer

There are several possibilities:
A longer outlet tube to allow the velocity profile to develop but there will always be a profile
The output may have a rotational component, which you could reduce by using a flow straightener - think of a "block" of drinking straws end on.
To check what your profile is you could use a pitot tube and move it across the outlet section taking measurements to see the pressure profile. Then consider some baffles earlier in the output tube to reduce the maximums etc.

Fan blower with less airflow in the center

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