TMD fans have tiny hubs because there's no motor there. This produces greater airflow. The manufacturer claims about 30% more airflow compared to ordinary fans of the same diameter. In addition, there's more airflow close to the center of the fan.

It's a very clever idea. It seems remarkable to me that nobody thought of it sooner.

http://www.tmdfan.com/

Phanes, I hope you'll keep posting. You obviously have some useful things to say.