Flow Conditioning In Tube 2

[carockwell]

We are trying to measure the air velocity and pressure in aircraft air conditioning hoses. We have been using "laminar flow tubes", basically just long straight tubes about 15 times the diameter of the hose. Now we are flowing more air, which means we have increased our hose sizes. The laminar flow tubes are now too long to fit in our space. What simple techniques can we use to "straighten" the air in our laminar flow tubes so that the tubes do not need to be so long?

 

[reidh]

Honeycomb material, such as Alcore.

Reidh

 

[rb1957]

are you using "laminar flow" ducts to increase the flow volume (presumably given a certain pump) ?

maybe your pump (compressor, whatever) isn't big enough to provide the required demand and overcome the losses ?

 

[carockwell]

1. Where can we get small quantities of honeycomb?

2. The laminar flow tube is used to measure the static and total pressures of the airflow in a ventilation hose. The flow tube is installed for test purposes only.We connect a vaccum fan to the laminar flow tube and draw air through a large cabinet. We are interested in measuring the pressure drop across the cabinet.

 

[vortexman]

I'm assuming that the issue is that your measurements are more accurate if the flow in the "laminar" tube is fully developed. Depending on the Reynolds number, the flow in the tube may not be laminar at all. A short length of some kind of flow straightener could help the flow to reach its fully developed laminar or turbulent state more quickly. If the flow coming out of the cabinet is fairly clean, this may not even be necessary. I don't know that any kind of flow conditioning can take the place of allowing the flow to fully develop. If flow conditioning is appropriate, a bundle of small-diameter straws, about 10 times longer than their own inside diameter, should work. Whether you condition the flow or not, 15 diameters may not be enough for the flow to fully develop. Look in your Fluid Dynamics book, it should tell you how many diameters of length are required for the flow to fully develop, as a function of Reynolds number.

Good luck

 

[reidh]

Your best bet may be to ask some of the large distributors of it for some samples.

Reidh

 

[Surestick]

Mythbusters used full drinking-straw boxes with the ends cut off to straighten airflow in a recent show.
If the flow through the tube isn't enough to blow the straws out of the boxes and it's not for a permanent test stand that idea might be worth a look, at least from a cost point of view!

 

[GregLocock]

Making hex is easy.

Crease the paper into strips, using appropriate fold directions.

flatten them back out. Apply glue to one strip in three. Stick another piece of the paper to it. Repeat.



Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[magicme]

if you use long straws to straighten the flow, you will get a large pressure drop across them and you may have a tough time getting the velocity you need downstream of the straws.

i did exactly that years ago, making a small wind tunnel in the basement ..... flow was VERY straight and VERY slow.

also.... tie them down ..... do you know what 2000 straws look like scattered across the room? (i do !)

regards

magicme


------------------------------------
"not all that glitters is gold"

 

[carockwell]

I was recently told that using the foam from off road racing aircleaners was the best way to get the desired result. I will try this in the next few weeks and compare it to the straw method.

 

[davidjh]

Why not set up the airplane ducts in a lab in the physical layout they exist in the airplane with your measuring sensors; attach some "long straight" tubes with sensors to provide "truth" data and just obtain a calibration of the "as installed" ducting/sensor config. Since this is likely a steady state scenario, could even use the same sensors keeping costs down and improve relative accuracy of the data.