Anyone work with laminar air-flow or in aerospace?

[absolom7691]

I have finally lost my mind and am going to build another fog screen. The previous one I built has a few problems that will (hopefully) be ironed out in the new and improved design. In my fog screen here: http://www.photonlexicon.com/forums/...screen-Project I used drinking straws that are only .23" OD. The problem I was noticing was the accretion of water droplets in the straws that carried the fog. The buildup would eventually form a meniscus of water and block the straw completely. After running for only about 10 minutes, the blockage would get to the point where very little fog would flow, ruining the effect. I figure I can combat this by using a larger diameter straw.

My question is, how big a diameter of straw can I get away with before I lose the ability to create laminar air-flow? If I increase the length of the straw along with the diameter, will that help keep the air-flow laminar? Any info would be fantastic.

[planters]

Your question is too difficult without knowing the velocity of the air and the micro-roughness of the straw's plastic. Even the fog will change the Reynolds number of the air...gasp. Before trying to rebuild this box what about a short reset with either a high pressure blast or a reverse to suction every so often? Maybe even a brief shock with an acoustic shaker like the ones we mounted under our gaming chairs.

[absolom7691]

Well, I need to rebuild the box anyway. I am going to make the laminar column of air wider for more stability as well as making everything internal even down to the fog generation. I am also going to rebuild out of aluminum instead of wood. I'll run some tests tonight with different diameters to see if the water buildup continues. Since I'll be rebuilding, I could implement something to combat the water buildup, should it occur. Thanks for the response. Once I get this started, I'll get another build thread going.

[zerowaitstate]

is it possible heating the area / straws to a specific temp might aid in the reduction of condensation ?

[absolom7691]

is it possible heating the area / straws to a specific temp might aid in the reduction of condensation ?

It would be too difficult to set that up. I am pretty sure the larger diameter straws will take care of the issue I am having.

[LaNeK779]

maybe cut the ends at an angle? could help to drive condensation away and increases output surface (replicating a larger diameter)

just my 2p worth

[danielbriggs]

If you don't fancy checking the maths, with the Reynold's Number < 2000...
Air velocity is the biggest factor(!)

What about heating the pipes a little? It should stop the fluid condensing out (not easily done with plastic drinking straws!)
Maybe if you've a cheap source of copper tubes, electrically isolate + use the array as a big resistor, and connect to a constant current source?

[absolom7691]

Thanks for the suggestions, guys. I have some larger diameter straws on order, they should get here soon. I am going to run a few tests and see how they do. As car as copper pipes, that would bump they weight up well beyond where I would like it.

As far as cutting angles, I think that would affect the flow but the water blockage isn't always forming at the ends of the straws. Some of the accretion is in the middle of the straws too. Hopefully the larger diameter should combat it a little. I am also going to put some high CFM fans in the the system that will periodically kick on and "blast" any water build up out. I can't wait to break ground on this and get another build thread going.