Some fans have three blades, and some fans have four blades. If I add a fourth blade to the three-blade fan, does the air flow increase proportionally? Assume everything else is unchanged (i.e: rpms, blade pitch, blade surface area and overall dimensions)
I’d say you’d see a 133% (or 4/3) increase.
Jinx: Some fans have three blades, and some fans have four blades. If I add a fourth blade to the three-blade fan, does the air flow increase proportionally? Assume everything else is unchanged (i.e: rpms, blade pitch, blade surface area and overall dimensions)
I’d say you’d see a 133% (or 4/3) increase.
Sorry, but wrong.
The total blade area for a given circumference fan is constant. If you stuck in a fourth blade, all the blades would have to shrink accordingly. (One factor of airflow calculation is total blade area.)
(You could overlap blades, but the part of one blade in front would block air from one in back, and no significant airflow would result.)
The reason for three is that it’s the minimum blades needed to create a stable spin for the hub. One or two blades would be too unstable. More blades would just up the cost for no significant airflow gain.
There are fans with more (many more) blades, notably jet engine intake fans and mining ventilation fans. For these, more is better to prevent cavitation, or at least excess cavitation. Any fan blade creates high pressure on one side and low pressure on the other (cavitation). The wider the blade is, the greater this pressure difference is. Too big a difference will overstress the blade, as well as create drag which will hamper airflow.
With small household fans, desired airflow speed is only a few feet per second (100-150 ft[sup]3[/sup]/min) to provide a cooling effect and perhaps remove odors. So the rpm of the fan itself isn’t that fast.
But for mining fans, transfer of gases is a matter of life or death. As much fresh air as possible needs to be delivered, with as much “bad” air being removed. So the big circumference of these fans needs to be compensated with higher RPM and skinnier blades.
Judges 14:9 - So [Samson] scraped the honey into his hands and went on, eating as he went. When he came to his father and mother, he gave some to them and they ate it; but he did not tell them that he had scraped the honey out of the body of the lion.
Hmm…I thought cavitation only applied to pumping liquids with enough suction to “boil” the liquid within the pump. How can a gas cavitate? …create full vacuum? Does this have the same damaging effect as liquid cavitation on pump impellors?
Don’t be so sure that three blades are required for stability. Two blades would balance nicely. It would not be appropriate for a house fan but single bladed propellors have been used for model airplanes in speed contests. A single blade propellor requires an articifial counter weight but cna be more efficitnt in extreme high rpm application.
On a airplane, where the fan is REAL important, a fan with more blades is a bit less efficient, quieter with less vibration (lower tip speed for a given rpm), more expensive, and suffers less erosion from rain/water.