3 ms·

The airflow into a fan is in response to the low pressure created by the blades, an so will approach over a wide angle. The outflow, however, is initially stron

The airflow into a fan is in response to the low pressure created by the blades, an so will approach over a wide angle. The outflow, however, is initially strongly biased to a direction perpendicular to the plane of the blades (together with some swirl imparted by the blades.) Because of this, the anemometer shown is likely to intercept a larger fraction of outflow than the inflow. Maybe the swirl will also have an effect, depending on whether the fan blades and anemometer blades rotate in the same or opposite directions.

kqr 4y ago
Hm, but does this explain why some fans show higher inflow than outflow?
- mannykannot 4y ago
  Try this with a desk fan: turn it on and stand a meter away on the outflow side - you will feel the flow. Stand a meter away on the inflow side, and the flow is much weaker and probably undetectable. That's because it is flowing in over a whole hemisphere upstream of the fan, but flowing out in essentially one direction. Here, as with the anemometer, you are not measuring the total mass flow; what's being measured is the velocity of part of that flow, and you are intercepting more of that flow on the outflow side. If you were to measure and integrate the velocity over two hemispheres centered on the fan and divided by the plane of the blades, with a barrier in that plane to prevent recirculation, and calculate the mass flow from that (you can assume constant density as the pressure change is tiny), you will find that the total outflow downstream equals the total inflow upstream.† † To actually do this experiment, you will need a sensitive anemometer which measures over a small area and does not unduly interfere with the flow, such as a hot-wire probe type. There may be additional difficulties in averaging turbulence. https://electricalworkbook.com/hot-wire-anemometer/ https://electricalworkbook.com/hot-wire-anemometer/

kqr 4y ago

Hm, but does this explain why some fans show higher inflow than outflow?

mannykannot 4y ago
Try this with a desk fan: turn it on and stand a meter away on the outflow side - you will feel the flow. Stand a meter away on the inflow side, and the flow is much weaker and probably undetectable. That's because it is flowing in over a whole hemisphere upstream of the fan, but flowing out in essentially one direction. Here, as with the anemometer, you are not measuring the total mass flow; what's being measured is the velocity of part of that flow, and you are intercepting more of that flow on the outflow side. If you were to measure and integrate the velocity over two hemispheres centered on the fan and divided by the plane of the blades, with a barrier in that plane to prevent recirculation, and calculate the mass flow from that (you can assume constant density as the pressure change is tiny), you will find that the total outflow downstream equals the total inflow upstream.† † To actually do this experiment, you will need a sensitive anemometer which measures over a small area and does not unduly interfere with the flow, such as a hot-wire probe type. There may be additional difficulties in averaging turbulence. https://electricalworkbook.com/hot-wire-anemometer/ https://electricalworkbook.com/hot-wire-anemometer/