PDA

View Full Version : Velocity of a fan

scap
08-23-01, 07:23 PM
Does anyone have any idea how to do this. It is for a project i am workin on for physics. also does anyone know how to find the CFM of air that the fan pushes.

joey_rjm5
08-23-01, 10:22 PM
Be more specific. Do you want to just convert rev./min. to m/s or are you suppose to figure it out some other way. I don't see how you could measure it on your own because what can the everyday person use to measure rev./min. on something that goes so fast.

edit- I forgot we can look in the bios to see the rev./min of our fans.

Crazy Jayhawk
08-23-01, 10:52 PM
If you're looking for flow rate, find a plastic bag where you know how many cubic feet it can hold. Seal it to the fan's outlet and see how long it takes the fan to inflate.

Cubic feet of bag divided by number of minutes to fill it = CFM of the fan.

scap
08-24-01, 05:09 PM
cool idea with the bag
and yes i want to messure the velocity in m/s meter/sec if possible the reason i need to doit is because i was also going to do it at 5 volts to see how everything is effected.

parkan
08-24-01, 05:46 PM
I remember measuring the velocity of a lego pulley with a LEGO mindstorms light sensor (yes, I still build robots out of that stuff and programm them in C). The basic idea is that you have to find how fast are the blades 'replaced' by empty space between them, measure the diameter of the fan and multiply it by pi, divide it by number of blades plus number of empty spaces, provided that spaces=blades in size then divide it by your timing and you got the velocity, I think (I'm out of caffeine so the above post may be absolutely crazy and wrong).

Wiseass
08-24-01, 06:39 PM

Kingslayer
08-24-01, 10:31 PM
I do it the easy way.

Place the fan on a hard flat surface. Blowing down on the surface. Hit the power. If it doesn't levitate IT"S CRAP!

Crazy Jayhawk
08-26-01, 12:20 AM
Originally posted by Kingslayer
I do it the easy way.

Place the fan on a hard flat surface. Blowing down on the surface. Hit the power. If it doesn't levitate IT"S CRAP! I think my 120mm would pass that test. :cool:

joey_rjm5
08-26-01, 02:28 PM
If you know revolutions per minute of the fan you can just convert it to the velocity of the tip of a fan blade. First find the circumference of the circle in cm that the fan blades make. Then multiply that by the # of revolutions in a minute. That will give yo distance over time which is velocity but it will be cm/min, not m/sec. To fix this... divide the amount of centimeters by 100 (because there are 100cm in 1 m), then divide that number by 3600 (there are 3600 sec. in 1 min.) This is how to convert rev./min to m/s. The m/s is the distance the tip of the fan blade travels in 1 sec. (velocity)

To find the rev./min at 5v you would need to run it at 5v and look at some sort of fan monitoring program like mbm or look in your bios to find out what the rev./min. would be at 5v. Then use the procedure above to get the m/s.

I hope this helps.