Because of the hub there is always a dead air spot. Why can't fan makers make a fan that's shaped so air angles more towards the center. I'd think that would be easy to do.
The above is zalman fan bracket which I had from one of their flower coolers. You can buy the brackets seperately. I wanted to have it straighter but my video card is in the way. That's a 92mm ys-tech fan. I have a 120mm on the way to try with the bracket.

