I want to measure the air flow from some fans, which means I need an air flow straightener to smooth out the wind enough to make the numbers less error-prone. You can, of course, buy cute little straighteners that bolt onto the outlet side of the fan, but what’s the fun in that?
The general idea is to pass the air through a set of thinwall tubes to damp out the turbulence. A downstream gap between the fan outlet and the passages eliminates / reduces the dead spot caused by the fan rotor. About 1 diameter downstream of the tubes, the air flow becomes reasonably uniform and a few more diameters produces the familiar parabolic velocity profile found in HVAC ducts.
A few minutes with a bandsaw extracted a 2-diameter-long tube from a 4-inch diameter heavy cardboard mailing tube. A pull saw and a miter box converted some surplus cigar tubes (which I got a long time ago for just such an occasion; I’m not a cigar smoker!) into 3-diameter lengths. Lay as many cigar tubes into the mailing tube as will fit, jam in one more, and they’ll remain in place with sufficient tenacity for my purposes. I suppose, if you were fussy, you could dribble in some adhesive.
I pushed the cigar tubes to the middle of the mailing tube, mostly because that seemed sensible. As nearly as I can tell, this is one of those things where it’s easy to get a reasonable result (as witness the variety of straighteners used by overclockers) and nearly impossible to get a truly trustworthy quantitative setup (as witness the bizarre vanes used in real wind tunnels by actual engineers). An overclocker discussion lives there.
A quartet of board spacers screwed into 90-mm (92-mm, whatever) fan fit neatly around the mailing tube’s OD, where I simply hot-melt-glued them into place.
A cardboard gasket seals off the gaps between the fan and the tube.
The gasket looks like this; the next time I will print this picture and cut it out, rather than repeating some fussy layout and getting it wrong twice. Scissors around the outside, a hollow punch for the four screw holes, and a razor knife for the interior. I considered a CNC project, but …
And then it Just Worked.
The “before” flow, measured about 1 diameter downstream of the bare fan standing in mid-air, ranged from 0.8 to 1.4 m/s, with the expected completely dead zone in the center. The “after” flow, 1 diameter downstream of the tube, was 0.9 to 1.1 m/s across the entire width, with no decrease in the middle.
The cross-section area is 12.5 in2 and the flow is maybe 40 in/sec, so the fan is pushing 17.5 ft3/min. More or less, kinda-sorta; it’s a quiet CPU case fan from an ancient Dell PC. I have a box of 60 cfm fans arriving shortly, so we’ll see how they stack up.
The anemometer is a La Crosse EA-3010U, which may be the wrong hammer for the job, but it doesn’t require me to dope out a hot-wire anemometer just to get a few numbers…
The Smell of Molten Projects in the Morning 
I once built a small wind tunnel demo for kids and as part of the demo showed why you want to have the blower downstream rather than upstream. But for the upstream part, I used cheap (non-bendy) drinking straws packed into a tube to do much the same thing as your cigar tubes, and it likewise did a great job.
A year or so ago we looked high & low for non-bendy straws, only to discover that they simply can’t be found; my shop assistant was sore disappointed.
I’m sure there’s a complex empirical formula giving the ideal diameter / length / location for flow straightener tubes, but ya gotta use what ya got… and it’ll be close enough, for sure.
I’m curious how much resistance smaller tubes would produce. If I can find some plain straws, maybe I’ll send ’em to you so you can put them in your existing rig and measure the new flow. Or I could do it myself (my anemometer [originally bought for Skywarn use] is a Kestrel, but they’re pretty similar).
Just what I need… another project! [grin]
If you happen to see ’em in a Standard Retail Outlet, let me know and I can pick them up locally. Better than shipping a few hundred of the things, methinks.
BJ’s has boxes of them – you’ll have to task your shop assistant with removing all of the wrappers however.
My first thought was a restaurant supply or the like. I’ll look around and see if I can find bulk boxes sans wrapping, because now I’m curious.
because now I’m curious.
Now, that is gonna get you in trouble…
Should be ^3 not ^2, I think?
Exactly!
Thanks…