Simple Air Flow Straightener for Simple Fans

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?

Air flow straightener - overview
Air flow straightener – overview

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.

Air straightener - cigar tubes
Air straightener – cigar tubes

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.

Air flow straightener - fan mount
Air flow straightener – fan mount

A cardboard gasket seals off the gaps between the fan and the tube.

Fan gasket in place
Fan gasket in place

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 …

Air flow straightener gasket
Air flow straightener gasket

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…

12 thoughts on “Simple Air Flow Straightener for Simple Fans

  1. 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.

    1. 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.

  2. 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).

    1. 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.

  3. BJ’s has boxes of them – you’ll have to task your shop assistant with removing all of the wrappers however.

    1. 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.

  4. the fan is pushing 17.5 ft2/min.

    Should be ^3 not ^2, I think?

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