A New Way of Testing Fan Airflow

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We began our quest for better measurements by starting with the most obvious error: The Arctic Fan 12. We measured less than half the airflow of conventional fans at similar RPM, and we were fairly certain we knew why: The direction of rotation was wrong for our anemometer.

The rotation of a fan's blades causes air to exit the fan in a swirling vortex, not a straight line. Like a miniature tornado, the vortex spins in a particular direction. Because 99% of fans turn in the same direction, this effect can usually be ignored. However, a the Arctic Fan 12 produces a vortex that spins in the opposite direction from "normal", which has a serious affect on our anemometer since it spins in the "normal" direction.

Our anemometer in use.

The solution was fairly obvious: Find a way to straighten the vortex airflow. The photo below shows our first attempt. A bunch of tightly packed straws mounted on a foam base. The straws are set about two inches in front of the fan so that the airflow is more evenly distributed across the straws.

Unfortunately, our hopes for this contraption were dashed when we started measuring airflow. The measured airflow of most fans that we tried in the device dropped by almost half (and even more at low rotation speeds) compared to our directly-in-front measurements. The one exception was the Arctic Fan 12L, whose measured airflow barely dropped at all and thus brought its result in line with the rest of the fans we tested. This suggested that we had fixed the problem we set out to solve — the airflow reaching the anemometer blades was no longer swirling, so the spin direction of the fan did not affect the measured airflow. But the device introduced far too much impedance, especially at low speeds.

The fan mounts in the square cut-away.

Looking straight down the airflow path.

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