~ Converting CFM to MPH??

Craig 930 RS · 03-10-2005, 11:56 AM

Is there a method to convert Cubic Feet per minute (fan air flow) into an equivalent MPH rate?

jpnovak · 03-10-2005, 11:59 AM

Only if you have an area that the mph is flowing through. This will give you a cubic dimension for the conversion. Then just convert mi > ft then ft x area (ft^2) = ft^3 and convert your time scale.

randywebb · 03-10-2005, 12:07 PM

and that will be the average - the flow will vary thru the cross-section, with the highest speed in the middle, the lowest near the 'wall'

Bill Verburg · Bill Verburg's Garage

Quote:

Is there a method to convert Cubic Feet per minute (fan air flow) into an equivalent MPH rate?

They are not dimensionally equivalent, so the answer is NO

MysticLlama · 03-10-2005, 01:48 PM

You need a couple of things for this.

1. Sunny Day
2. Calibrated speedometer
3. Small protractor and ruler
4. Dog with long floppy ears

You go for a drive on a sunny afternoon and observe the angle and stretch of the dogs' ears at different speeds.

Then you set the dog on the floor in the living room and spin the fan at x rpm and observe the angle and stretch again.

Then comparing numbers from the two prior experiments should yield a reasonable estimate mph of fan flow.

You didn't say it had to be a scientific way.

jpnovak · 03-10-2005, 01:52 PM

OK Craig, Are you trying to use a leafblower to replace the cooling fan? 'fess up.

Craig 930 RS · 03-10-2005, 02:10 PM

That was pretty humorous stuff, Mystic - that was good!

So.......here we go:

17"X4" oil cooler = 72 sq in
4" dia SPAL fan; capacity is 240CFM

'Ram air flow' vs fan air flow..........how do the two compare at speed(s)

(And you thought that trig class etc was worthless......)

TIA!

MysticLlama · 03-10-2005, 02:21 PM

Well, I'm poking around looking for stuff.

Here is something interesting...

http://www.airflow.co.uk/terms/

With the formulas and conversion tables in here, surely we can come up with some sort of comparison?

Bill Verburg · Bill Verburg's Garage

Quote:

'Ram air flow' vs fan air flow..........how do the two compare at speed(s)

you don't, just know that at speed an efficient front mount is light years ahead of any fender mount.

when stopped the fan cooled will be clearly superior.

one exception will be dual ducted fender mounts which have a nice large intake and efficient ducts

Similar to what the 993 used.

masraum · 03-10-2005, 02:38 PM

The area of the fan assuming it's round would be ~12.6sqin --> .0873 sqft.

240/.0873 = 2749 fpm to convert that to mph would give you an average of 31mph. Not exact, but I think that'd be a basic, and close enough for what you are asking.

Assuming you want to know the cfm of the radiator at speed (again, not really 100% accurate, but I bet it's good enough for comparison's sake)...

17"x4" --> 68sqin = .47 sqft

.47sqft x 2749fpm = 1292cfm

So with the fan when you are sitting still assuming an average air through the fan speed of 31mph(2749fpm) you would be getting 240 cfm through the fan. If you were driving at a speed where the air going through the fan was averaging 31mph you would have 1292cfm going through the fan.

Even with 3 fans you are going to be just over half because the fans are round and so you would be missing all of that area.

Bill is right, the exact conversion isn't possible, but I think this is a decent comparison showing the sort of info that you are interested in.

MysticLlama · 03-10-2005, 02:43 PM

Air volume flow rate (qv)
The volume flow rate is obtained by multiplying the average velocity (v) by the cross sectional area (A) where the velocity is measured. It is expressed in cubic metres/second (m3/s).

qv=v * A

I'm going to make average velocity 100mph to make the math a little easier.

Since that has to be expressed in cubic meters, we'd have to assume to attain the velocity measurment, it has to be a square meter since I used the index of M/sec to MPH.

A 4" circle is about 12 square inches. This is about .0075 square meters.

Index to convert mph to m/sec is .447, so this would make the velocity @ 100mph in (m3/s) 44.7 given a 1sq meter area.

qv=44.7 * .0075

.33525 = 224 * .0075

.33525 cubic meters = 11.84 cubic feet/sec

So it looks like a 4" hole should produce about 710CFM at 100mph?

This might be 150% wrong, I haven't done math like this in a LONG time. I just kinda made it up as I went and looked up some formulas / conversions.

Craig 930 RS · 03-10-2005, 02:45 PM

Good stuff!
This does help with some ideas I have.........fans vs. more efficient cooler ducting, etc

THX!

randywebb · 03-10-2005, 02:52 PM

Craig - where are you headed with this? That might be the best way to get the "right" answer.

If for a street car, it might depend on how much time you spend at a slow speed (i.e. near gridlock) on how hot a day. See Bill's response above.

Hills in the heat can be tough too. That's the only time I ever had my street cars get too hot.