How can I achieve 30mph in my homemade wind tunnel?

[apollo8910]

Hey everybody,

I have built a 7' wind tunnel. The test section is 7''x7'' and 24" long. The diffuser opens to 22''. I would like to achieve around 30mph through the test section. I am having trouble finding a fan that can meet that need.

I have looked into using an electronic radiator fan found on a car, however, the problem is then finding a DC source. It seems like converting to AC is more trouble than finding a fan that is just in AC.

I have also looked into using an HVHC fan? Has anyone gone that route?

I know I will need around 2000cfm because I am also using a 7''x7'' square of drinking straws 4.5'' long to create my uniform stream. Kinda restrictive.

I have read some other threads on wind tunnels and haven't really seen any suggestions on a power source. If any of you could share some knowledge on what works I would greatly appreciate it!

P.S. I'm a student... so the industrial grade fans that are around $300 are way to expensive

Thanks

 

[jim hardy]

I have looked into using an electronic radiator fan found on a car, however, the problem is then finding a DC source.

Look for a 10 amp battery charger. They're only about forty bucks new. I prefer the old fashioned ones with a transformer not the new ones with a computer and inverter inside. You can tell them apart by by the weight, about a pound per amp for transformer type.

My local metal salvage yard sells junked ones for thirty cents a pound, about three bucks.
If the transformer doesn't smell burnt usually all that's wrong is a broken wire or the thermal overload is corroded from being left out in the rain. At that price you could parallel two of them. If you luck out you might find a twenty amp.

Good luck.

Nice workmanship, by the way.

 

[apollo8910]

Hi jim,

Thank you for replying and your suggestion! I was wondering if you would also have a suggestion on making it a variable speed setup? I believe that most of the battery chargers I have used have only had two amp settings? Do you know of anything I could incorporate into this to give me more control of the amps?

Also, thank you for the compliment! It has been a fun project

 

[etudiant]

Air duct booster fans for HVAC systems are pretty cheap and run off AC, plus they come in relatively compact 8" diameter size, so they are also easy to install and operate.

However, your 2000cfm spec is about what a whole house blower will provide, duct boosters usually are maybe a tenth of that. Apart from cost, house blowers will take serious duct work to fit.
I don't think a car fan will work, it seems underpowered for your needs, especially after factoring in the back pressure created by the air straightener.
You need about 1000cfm of clean air flow through your tunnel, so you specify 2000cfm to allow for the pressure losses, which in that envelope means a pretty impressive air pusher. Perhaps a diesel air intake turbo from a junk yard, but that would a project on its own.
Maybe putting a larger exhaust fan on the back end of the tunnel would help.

Good luck on your work. It looks well done, as Jim Hardy has already said, so I'm confident you will find a satisfactory power source.

 

[jim hardy]

Hi jim,

... I was wondering if you would also have a suggestion on making it a variable speed setup?

The easiest thing is to power your battery charger through an adjustable transformer. They're usually called "Variac" though that's a trademarked name of General Radio.
Variacs are a genuine adjustable transformer that doesn't chop the waveform like lamp dimmers do, so you can use them with a transformer load. I used one with a battery charger and windshield wiper motor to make an adjustable low speed speed drive for another hobby..

Variacs go for 50 to 100 dollars on ebay - here's one cheap right now, take a look and ask the fellow its rating and if it works. If you don't buy it I might.
http://www.ebay.com/itm/140935651161?ssPageName=STRK:MEWAX:IT&_trksid=p3984.m1423.l2649

I believe that most of the battery chargers I have used have only had two amp settings? Do you know of anything I could incorporate into this to give me more control of the amps?

Fan motor needs probably around 10 amps .
A hobby motor controller that robust tends to be pricey.
http://www.hobbyengineering.com/H2977.html

What I would try first is get the speed control switch and nichrome resistors from an old Ford car or truck airconditioner and see if it'll work with your radiator fan. I say Ford because I am familiar with them - if you're a Chevy or Toyota guy use what you know.

Is a radiator fan going to move enough air for you? Ten amps at 12 volts is only $\frac{1}{6}$ hp.
If not there's another source for el-cheapo variable speed drive.

Most thrift stores have old exercise treadmills.
Many of those use a 2hp 90 volt DC permanent magnet motor with a nice PWM speed controller.

I got a treadmill for free because it wouldn't run.
I just wanted the motor for a drill press.
So rather than take trouble to fix the computerized speed controller I used a full wave bridge rectifier and an ordinary in-wall type lamp dimmer mounted in an outlet box on a plank.
The motor is capable of 6,000 rpm and runs "smooth as silk" on the dimmer/bridge combination.
A 6 amp dimmer is good for up to about half the motor's rating, or ~ 1hp.
If you find you need more power there are heavy duty lamp dimers at home supply stores.

Here's a 35 amp 1kv bridge :
http://www.digikey.com/product-detail/en/GBPC3510A/GBPC3510A-ND/357383

You should mount the bridge on a piece of aluminum for heatsink and place it in the airflow.
Put some silicone grease between the bridge and aluminum heatsink and through-bolt it snug for good heat transfer..

Getting late and I'm fading.. if this needs clarification please advise. You an PM me if you wish.

Hope this gives you some ideas.
Neat project - keep us posted.

old jim