Building a Low Speed Wind Tunnel: Fan/Motor Size Considerations

[good_ken]

I am trying to build a low speed wind tunnel, and have a question. I understand that the contraction cone speeds up the air (proportional to the area ratio).

What I am confused on is how does this "mechanical advantage" factor into sizing my fan / motor?

I am planning on a tunnel with a velocity of 225 ft/s. That yields 33,000 cfm (for a 1.5' x 1.5' area). But if I build a 12:1 contraction cone, doesn't that help me downsize the fan/motor combo I need? Or does the 33,000 cfm number already represent this advantage of the contraction cone?

 

[FredGarvin]

CFM is CFM is CFM. The only thing the test area buys you is an increase in velocity. The volumetric flow remains the same for a given fan speed.

 

[good_ken]

After posting, I had that realization.

Thanks

 

[Cyrus]

I have not heard the term 'contraction cone' used before, are you in the UK or something?


Its just called a nozzle.


Thats 150mph wind, that's pretty quick.

 

[good_ken]

No, California. Might as well be a foreign country (relative to the other states).

I picked up that term from http://observe.arc.nasa.gov/nasa/aero/tunnel/tunnel_parts.html"


Yes, we are trying to test model rocket. Specifically to get the Cd. To get the Reynolds number right, we need to be at about 225 ft/s.

 

[Cyrus]

33,000 cfm is a lot of air, I would look into a leaf blower.

 

[russ_watters]

33,000 cfm is a lot of air and 225 sq ft is a big test section. You're probably not being realistic about how big you can make your wind tunnel and what you can do with it. But if you are really serious about this and can do it, this fan is about what you need: http://www.grainger.com/Grainger/items/7F883

 

[good_ken]

33,000 cfm is a lot of air and 225 sq ft is a big test section. You're probably not being realistic about how big you can make your wind tunnel and what you can do with it. But if you are really serious about this and can do it, this fan is about what you need: http://www.grainger.com/Grainger/items/7F883

I am shooting for a 2.25 ft^2 test section (1.5 feet on a side).

I found a 10 bladed tube axial fan. I am going to run those calcs to see how many horses that'll require.

 

[FredGarvin]

Leright is asking pretty much the same thing over in the Engineering forums. Are you two working together?

You need to look at a BAF. I just like the company for their name. They're not applicable here.

http://www.bigassfans.com/

 

[russ_watters]

Sorry, misread that test section size... still, 33,000cfm is an awful lot. You can't even use a normal house receptacle to power it. It's about 40 amps at 240V - probably the same as an electric dryer or medium-sized water heater.

 

[trose3310]

good_ken

I am currently designing a wind tunnel that will need to 50m/s (about 164 ft/s) and was wondering if you would be willing to share some design details?

Specifically I am looking for contraction cone area ratio and length diffuser length guide lines. Also I was wondering what you used for your flow analysis?

I would like to have a 2' test section and keep it below 15' long. I have access to 240W outlet for motor powering.

Any help would be much appreciated!