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I have mounted a standard PC case fan blade onto a big motor. I currently have it spinning at about 11,000 revs.
If the outer diameter of the blade is 120mm and the diameter where the blades meet the hub is 60mm this should give me a surface area of...
(Pi * (60 * 60)) - (Pi * (30 * 30))
= 8482mm^2
I reckon the pitch of the blades is about 28 degrees.
Is there a way to calculate roughly the volume of air that passes through the fan?
PS Currently the fan easily lifts its own weight!
Get some cardboard or other material.
Build box the size of fan.
Cut across one side of the box.
Make a door to go in cut to stop flow of air (until the fan is at full RPM).
Tape a bag to one side of the box.
Tape other side to fan or...
Fill bag will a quantifiable material. Possibly water. Or pellets of some sort.
Either way, time how long it takes for the bag to fill with air from fan.
And viola.
Lol, that project in itself would be larger than what I'm trying to achieve!
Well... if you want to know....Quote:
Originally posted by wossname
Lol, that project in itself would be larger than what I'm trying to achieve!
I was thinking more along the lines of some kind of algebraic formula.
Without making some huge modelling assumptions that's pretty much impossible. Try asking a fluid mechanics professor...
I had a dream in which I solved this. But I cannot recall the math.
It was probably bogus anyway. :wave:
Google.com?
Resolved!
558 cubic feet per minute +/- 70cfm
(about 15 cubic metres per min)
Dayummm!
lol.
he said revolved.
:p
I wondered if anyone would notice. :D
Very good that U got the solution. Now can U please let us know the method of solving this problem. We all will appreciate.
My work colleague has a graphing calculator with a program on it that worked it out for me. You'd have to ask him! :)
I'll see if I can get it off him.
But I doubt the calculations take account for the curve of the fan blades. The curve makes a big difference. How well it can 'catch' the air then redirect it.
The only accurate way to measure would be a scientific method, as I described above. ;)
He gave me a list of all the things he reckons it compensates for...
RPM
Blade cross section (curvature I suspect)
Fan diameter
Fan hub diameter
Duct length and diam
Dist from fan to duct entrance
Dist from fan to duct exit
Local ambient pressure (pascals)
Local humidity (%)
Blade angle (wrt. axis of rotation)
Number of blades
Temperature (???)
And he reckons it takes about 2 minutes to run this program on his TI-82 graph calc.
I'm not entirely convinced his results are accurate since my own observations have been generally incongruous with his results.
Hmmm.
I'd think it'd have to be pushing more than 70 CFM for it to 'float'.
How big is the motor?
Yeah I know. The 70cfm was just the margin of error I'd allowed for.
Motor spec:Quote:
Originally posted by wossname
558 cubic feet per minute +/- 70cfm
(nondescript type fom Maplin's)
7.2v, 6 amp, 42watts. 19,000 rpm unloaded.
Running from a 7.2v 1.8ah nicad race battery.