fan velocity

[scap]

Does anyone know how to find this or cpf of air the fan pushes. this is for a physics project i am workin on.
thanks

[Paul -The Mad Hatter]

I think hoot did something like this. He used a garbage bag and then timed how fast it took to fill the bag w/ air. then he divided the time by the size of the garbage bag (i think it was 5 gallon bag). Hoot mite be able to help you w/ this.

[KeyboardCowboy]

that is actually a really good idea paul, i was gonna go on a rant about the size of the blade, angle, physics, the rpm of the fan ect.

[Crazy Jayhawk]

Originally posted by Paul -The Mad Hatter
I think hoot did something like this. He used a garbage bag and then timed how fast it took to fill the bag w/ air. then he divided the time by the size of the garbage bag (i think it was 5 gallon bag). Hoot mite be able to help you w/ this. Weird - I posted almost the exact same thing in this thread and my post disappeared. :confused:

[cjtune]

EDIT: Double post: see the one below.

[cjtune]

You might do better by finding the manufacturer's performance chart of the fan you have. It's a plot of pressure/head vs. flowrate -but you need to find out how much 'back' pressure the system is throwing at your fan (ie. resistance to flow). Calculating it the easy way is to get a anemometer (wind speed measurement device) and try and get an averaged speed for your fan and multiply it with the cross-sectional area spanned by the outlet of the fan. If all else fail, then try the garbage bag method but it'll be a very rough estimate as the 'back' pressure will vary as the bag is filled more and consequently the fan airflow throughout the process drops continually (till the bag is full).

EDIT: One more idea: Just measure the thickness of your fan, the cross-sectional area of the openings ("big circle minus small circle"). Multiply the rpm of the fan with the number of blades its has and then multiply again with both the thickness and cross-sectional area. This will give you a volume per minute figure. Again: Thickness*CArea*RPM*Nblades = volume per minute. The unit for volume depends on what you use (cm^3, in^3) so be consistent. This aprroximate measurement is only valid for free-flow conditions, where there is nothing immediately in front or at the back of the fan to provide obstruction. If you fan is in a duct or if it fixed on a HSF, then this approx. is no good....