The discussion revolves around the efficiency of fan blades, specifically whether a fan with half the blade length would be more efficient. Participants explore various aspects of fan design, including rotational inertia, torque, air movement, and the relationship between blade length and efficiency. The conversation includes theoretical considerations, practical examples, and comparisons to other aerodynamic structures.
Participants express multiple competing views on the efficiency of smaller versus larger fan blades, with no consensus reached. Some argue in favor of smaller blades being more efficient, while others contend that larger blades are typically more effective in transferring momentum to the air.
The discussion highlights ambiguities in defining "efficiency" and the assumptions underlying various claims. Participants acknowledge that different conditions and parameters may lead to different conclusions about fan efficiency.
This discussion may be of interest to those studying fluid dynamics, mechanical engineering, or anyone involved in the design and application of fans and similar aerodynamic devices.
That question is unclear. The obvious answer is no. If you scale down the wing in all dimensions, the "tip" retains the same percentage of the total area.CWatters said:Fan blades are essentially wings. The tip of a wing is usually less efficient than a bit in the middle of the wing. If you reduce the length of the wing does the tip represent more or less of the wing as a percentage?
That's only if you ignore the hub / bearing / shaft.jbriggs444 said:That question is unclear. The obvious answer is no. If you scale down the wing in all dimensions, the "tip" retains the same percentage of the total area.
Agreed. The vision I had was that with a larger diameter blade, one would scale down the rotation rate, thus having the same tip speed, flow speed and the same angle of attack, but it seems clear that other folks are making different assumptions.sophiecentaur said:Perhaps the conditions implied in the OP not are not precise enough for a proper answer.
Unfortunately, this (Original Post) is that it contains a misapprehension. "Torque" is not particularly relevant because the fan could be driven with a suitable motor to produce that torque. What counts is Power, which is torque X rotational speed. Both the speed of the fan and the torque are relevant and a small fan would (optimally) have different shaped blades and rotate at a different speed.kolleamm said:Would a fan with half the blade length be more efficient? My reasoning is that there would be less mass to spin, also a shorter blade length would make the lever arm smaller thus reducing more of the torque the air would create on the blade.
jbriggs444 said:That question is unclear. The obvious answer is no. If you scale down the wing in all dimensions, the "tip" retains the same percentage of the total area.
No, it would be less efficient in terms of transferring momentum to the air for a given power. That's why glider planes have long wings, and human powered airplanes have long propeller blades. See also most wind turbine blades.kolleamm said:Would a fan with half the blade length be more efficient?
That would be only relevant if you are switching the fan on and off all the time.kolleamm said:My reasoning is that there would be less mass to spin,
But you also move less air with the shorter blade.kolleamm said:also a shorter blade length would make the lever arm smaller thus reducing more of the torque the air would create on the blade.
And short blades need to move faster, creating more noise. The best fans for shifting air in a room are the large overhead ceiling fans. Their only disadvantage is that they are obtrusive and can injure your head if you stand on a chair.A.T. said:But you also move less air with the shorter blade.
That could be said of a vast number of PF threads.CWatters said:Old thread.
Shame. I used to be one of your greatest fans, Dave.DaveC426913 said:Maaaaan!
Nuthin' like poring over 14 posts with interest, forming ideas and responses in one's head - only to find out at the last moment that it's years old...Who knows what's changed... CWatters and sophiecentaur might be so old and decrepit by now that they have to type using a stick in their mouths...
Pinkyverma said:Yes, a smaller fan blade would be more efficient.
@gneill has a good point; it is highly dependent on the application. In HVAC, you tend to have an airflow and pressure requirement and efficiency is efficiency: power/power. Small fans are better at generating high pressure at low airflow.cjl said:No, by the usual measures of efficiency (air flow per power or thrust per power), a smaller fan blade is less efficient. High efficiency fans usually have as large a diameter as possible.
Absolutely. It's only when a fan is operating in free space that the efficiency can be worked out in a straightforward way. Fans are often used in confined spaces where cooling requires an optimum volume of air to flow through an equipment cabinet. Same applies to automotive cooling fans. As @russ_watters says, the relevant figure is Power / Power and a fan may have to be fitted into a non optimal place. There's also the issue of acoustic noise which is often a factor.russ_watters said:it is highly dependent on the application.