Wind Tunnel Construction

[ProHandyman]

I'll check my lighting catalogue. I'm aware of older 2x4 and 4x4 commercial ceiling trouffers having 1,2 and 4" square aluminum. Not sure if still available though.

[ProHandyman]

googling finds many sources... here's one: http://www.americanlouver.com/eggcrate.html

[BillA]

Thanks PH

ok Ben, where can we scrounge some ?

[bigben2k]

I'll keep an eye out. Thinking of office surplus outfits (many around Houston). If I find any, I'll pick'em up for you; we ought to be due for a beer by then.

Putitng nozzle calcs on Excel isn't obvious; still at it.

[bigben2k]

About to go home. Came across a site that provides a BASIC program:
http://www.delta-tcompany.com/nozzles/program.php

The theory:
http://www.delta-tcompany.com/nozzles/paper.php

Will try to tackle the spreadsheet again tmo.

[Albigger]

May also try honeycomb core for flow straightener, if can source cheap enough.
first google link (http://www.plascore.com/honeycomb/default.asp) - comes in many sizes/materials.

At least two wind tunnels at my Univ. use some type of honeycomb material, if I had to guess about 2-3" thick.

I would be worried that light-fixture gratings wouldn't be thick enough, but not really sure of your goals / tolerances with this project.

[bigben2k]

I used the basic program to run a few numbers.

First:

The flow range to be covered is determined by the nozzle diameter. In order to have better resolution, the maximum flow should be near the 3.0+ inches w.g. range in differential pressure. ... If the differential pressure is below 1 inch w.g., select a smaller nozzle. If the differential pressure exceeds 4.0 inches w.g., select a larger nozzle or combination of nozzles.

(from fantester)

so 1" is min, and 3" is max.

Using the program, I calculated the following flow rates:

DIAMETER    Flow at 1" wc    Flow at 3" wc
0.75"        11.8cfm        20.5cfm
1"        21.5cfm        37.5cfm
1.25"        33cfm        57cfm
1.5"        48cfm        83cfm
1.6"        54cfm        94cfm
2"        85cfm        147cfm
2.5"        133cfm        231cfm
3"        192cfm        333cfm

Nozzles listed are from the standard size list from Helander.

Back Monday.

[BillA]

what is that ?
what equations ?
std air ?

did you read post #2 ? it is from Fantester
recommended minimum and maximum flows:

NOZZLE DIA ./ CFM @ 0.1 INCH W.G./ CFM @ 4.0 INCH W.G.
0.7 “ . . . . . . . 3.5 . . . . . . . . . . . . . . . 21.0
1.0“ . . . . . . . 6.7 . . . . . . . . . . . . . . . 42.0
1.6“ . . . . . . . 17.0 . . . . . . . . . . . . . . 110.0
2.0“ . . . . . . . 28.0 . . . . . . . . . . . . . . 172.0

max flow best @75% (3"WG)

(running at lower pressure is going to be easier on the ears)

EDIT
Ben, have you looked for a fan pumping ~200CFM @ 4+"H2O ?
now I understand why they are in enclosures

[BillA]

Thoughts on the supply fan rating – 250 CFM being the nominal target

At max flow the dP across the nozzles will be 4�WC. This can be reduced by adding nozzle capacity, not sure w/o nozzle pricing how this relates to the cost effectiveness.
The internal straightening vanes drop I assume to be negligible.
As I intend to control the air temp, a heat exchanger will be used which has a pressure drop of 0.35�WC at 225 CFM, assuming ~0.45 at 250 CFM.
The 8� flexible air supply hose pressure drop is insignificant if the hose is kept straight.

From http://www.connel.com/freeware/airduct.shtml
This is what you input
Flow Rate 250 CFM
Velocity 714 FPM (based on 8� dia)
Pressure 14.7 PSI
Temperature 60 F
Duct length 10 FT
Duct Roughness 0.01 FT (canvas, wire wound)

These are the results
The duct diameter is: 8.01 inches
The air density is: 0.0764 pounds per cubic feet
Reynolds Number = 51056.22
Friction Factor fa = 0.04308519
Friction Factor f = 0.04458125
loss PSI = 0.00077848
loss inches of H2O = 0.02157016
loss inches of H2O per 100 ft = 0.21570163

To the above must be added the expected max drop across the DUT; difficult to imagine more than 0.4 as such pressure requires a very noisy fan. (most apps will see a dP <0.2�WC using an axial fan)

All of this suggests ~5�WC, w/o the rad and a lower DUT pressure perhaps 4�WC could do.
This is not a typical blower as most top out at 1.5 or 2�WC.
…………………………………………†¦â€¦â€¦â€¦â€¦

Another question is about the air control mechanism.
Blast gates seem recommended, I guess because they are cheap. Is there any reason why the motor cannot be speed controlled ? (a LOT less noise)

[bigben2k]

I haven't looked much into the blower, but I did note a concern about it being to push air with some decent pressure, as possibly a more important/relevant spec than the cfm: 4"wc is not obvious, for any fan.

An axial fan would fail miserably, without a doubt, at any diameter and in any configuration. A blower ought to do well, but I suspect that it would have to be of a fair diameter, in order to achieve a pressure of 4"wc. I'm guesstimating 8" diameter, possibly more, hence the large box (also a noise suppressor). I'm all for DC control, but these are limited, so I believe that the blast plate is still needed. Agreed on the duct; an elbow is going to be the equivalent of ~24 feet of the same duct!

As for the calculation, I used the Basic program, with these:
Duct Diameter: 32 (could be wrong, but impact is negligeable)
Throat diameter: [insert nozzle size]
Upstream static pressure: 14.7
Upstream static temperature: 70
name of gas: air
The program then returns gas viscosity, upstream gas density, beta, gamma, and the critical pressure ratio.

then I put in the dP (0.1, 1, 3 or 4, "wc) and it returns:
downstream static pressure, pressure ratio, exit velocity, Mach#, downstream static density, Reynolds, Mass flow rate, CD, the flow rate (converted in various units), and the "pressure recovery".

I see that 0.1" wc as a minimal pressure drop for the nozzle, in the sense that a dP that is any lower, will not produce results that are meaningful. I'm going by the fantester instructions, which state that 1.0"wc is the minimum dP, before one should switch to a smaller nozzles. I'm interpreting the instructions as:
min dP: 1.0"wc and max dP: 3.0"wc (across any nozzle). This seems to fall inline with a "desired measuring range".

Today I've got a lot of catching up to do, having been offline since Thursday, but I'll look into pressure drops of larger blowers, and see how to get to 4"wc+. Sidenote; might be able to use a combination of two (or more) 5" blowers, with DC control.

[edit: removed off topic ramblings]

[bigben2k]

Confirmed (went over ebm-papst product line); can't do 4" with a single blower, regardless of diameter. It'll have to be multiple blowers. This makes DC control easier (turn some on/off).

[BillA]

no
google radial blower
I bought this to see http://cgi.ebay.com/ws/eBayISAPI.dll...tem=7544689100

[bigben2k]

Dandy, but 3 phase isn't practical for me.
http://www.allproducts.com/tami/chuanfan/05-pf.html
http://www.kooltronic.com/b-blower/hprad/hprad.htm

cute animation:
http://www.rtpumps.com/rtpumps/web/h...256EE5007976A9

I'm going to pursue the multiple blower configuration:
1-easier DC control (turn some on/off)
2-variable voltage of an EBM-Papst (spec'd at 24 volts) can be from 9 volts to 32 (much "play" room)
3-should be cheaper than a specialty unit (unless I can get lucky on eBay, as you did).

I'm assuming, for now, that running a pair side-by-side isn't going to be an issue.

[BillA]

get a phase converter

[BillA]

well I got a blower lesson (link in #37)
super construction, but . . .
made in Italy by Nassetti USMCA, an Italian conglomerate gone bust in the '90s
they did not actually make the fan but it will be difficult to find who did to get the specs (I speak Italian but surf poorly, don't know the right keywords)

I had assumed (wonderful word) to change the motor to a TENV for the torque with a speed control, BUT it does not have a std NEMA frame

back to eBay it goes

[bigben2k]

Bummer. No rpm indication either?

[edit: removed off topic ramblings]

Otherwise I completed my New Year's resolution to "clean house"; got the (drug addicted) brother-in-law gone, the live-in boyfriend kicked out, and the step-daughter opted to follow the boyfriend. Am strapped for cash, but otherwise at peace .

[BillA]

bleh, good cleansing

[bigben2k]

[edit: removed off topic ramblings]

[bigben2k]

[edit: removed off topic ramblings] My boss quit; I am boss-less but applying for it.


I've been scouring eBay for a blower. Nothing useful yet, but the Grainger website (www.grainger.com) comes in handy for specs. I'm still keeping in mind finding a 32" tube; I think this is going to come down to a lucky find, while driving around town.

More links that may come in handy, for those interested:
http://www.ceesi.com/pubs_gasflow.aspx
http://ts.nist.gov/ts/htdocs/230/233.../NCSL_4e03.pdf

[bigben2k]

Today I thought I'd spend a bit of time on the nozzles.

First I'd like to clarify wether we are indeed going to keep the dP during testing between 1.0 and 3.0 "wc, as per the fantester guidelines, or if we're doing something different.

I'm also thinking about nozzle selection; what's the word on using multiple one at the same time? Ok for 2? Ok for 3, 4? (I understand how it would work; this is an accuracy concern only).

Knowing that, I can figure out the minimum number of nozzles, their sizes, and the right combination to get to 250 cfm. Do we have a minimum cfm requirement?

I expect to be mobile again Wednesday; I'll start to scour the area for tube and egg crates.


Speaking of which...

Originally Posted by from fan tester

...The chamber has flow straightening screens installed upstream and downstream of the nozzle array. The screens break up turbulence in the airstream and provide a uniform flow approaching the nozzle array. Each set consists of three screens with open areas of 60%, 50% and 45%. ...

Any thoughts if we're going to be ok with the egg crate, or if we're way off here?

[BillA]

review the spec in the first post

the screens have 2 purposes, the increasing density is to make the pressure more uniform
note the max velocity differences permitted (need a thermal anemometer over 400CFM as I recall)

[bigben2k]

According to the paper from the Portland State University... using up to 3 nozzles is still within the standard. That answers one question I had.

The nozzles used are: 0.75", 1" and 1.6". That gives this tester the following cfm ranges:
(as stated by the man)

0.75" only: 9 to 19 cfm
1.0" only : 15 to 35 cfm
1.6" only : 39 to 96 cfm

0.75" and 1.0" : 24 to 40 cfm
0.75" and 1.6" : 48 to 71 cfm
0.75" and 1.0" and 1.6" : 63 to 150 cfm

A simple addition, as expected, but max for {0.75" and 1.0"} ought to read 54 cfm (instead of 40), and max for {0.75" and 1.6"} would read 115 cfm (not 71). Not sure why there's a difference.

The cfms calculated are consistent with a dP range of 1.0"wc to 3.0" wc (both our calculations appear to be off, but mine are closer ). I don't understand why the graphs in that paper have a pressure range between ~0 and ~45 Pa: 1.0" wc is 249 Pa.

Am I missing something here? I'd like to master the calculations. That'll be my goal this week. I'll retry the calcs at 60F and 32F, to see if I can sort out the differences.


I read you (clear) about the screens: I just wanted your opinion on wether the egg crate would suffice for us, or if we should consider looking at twpinc.com (I've ordered samples from them in the past, for water filtration). Going over the site quickly, it is possible to replicate the 60, 50 and 45% open area in mesh, but it might be pricey (i.e. in aluminium, ~4.00$ per square foot, off of a 36" wide roll).


I don't understand you on "max velocity differences permitted". We won't be anywhere near 400 cfm, right?

[BillA]

ck, preserver
no idea on the calcs, have not seen them

[BillA]

bought a blower
http://cgi.ebay.com/Dayton-2C820-Rad...QQcmdZViewItem

[bigben2k]

Yep, am tracking same model. Darn, 220v...