Air Turbulence and DIY laminar flow hoods

Question

So for years on the mycology, plant tissue culture, and DIY laboratory websites there has been this ongoing debate on how to achieve laminar flow in a home built laminar flow hood.

Flow hood link!

It is generally accepted that a squirrel blower is effective at creating this laminar flow using the set up in the image posted above. Many people are interested in using inline centrifugal fans because they are cheaper than squirrel blowers, more quiet, more energy efficient, and potentially more compact. The build for a laminar flow hood using these inline fans look, generally, like the two samples at the bottom of this link (http://www.fungifun.org/English/Flowhood#construction).

The main question is whether these inline fans will produce laminar flow. The argument is that the air pushed by these fans is turbulent and so will exit the HEPA filter in a turbulent fashion. It seems to me that so long as the plenum is air tight besides the entry point(fan location) and exit point(HEPA filter), the plenum will achieve a static pressure (based upon the cfm of air driven by the fan and the resistance of the HEPA filter) and flow smoothly out of the HEPA.

Would this be totally wrong? If so, is there a way to build the DIY flow hood to achieve a laminar flow using one of these inline centrifugal fans?

Answer 1

I think that such a deep filter will eliminate the need for a screen.

My guess/gut feeling is that (due to high pressure loss and flow through a porous medium) the air leaves the filter in a laminar fashion (even if it was turbulent before), so what kind of fan you use shouldn't matter if e.g. the volume flux (i.e. cfm) achieved is the same.

Everything hinges on the pressure loss across the filter (i guess pretty high) and the resulting volume flux of the whole system (depending on the characteristic/power of your fan). With this you get the flow velocity, and you can build a Reynolds number, and if this is higher than a critical number, you'll get turbulent flow.

The choice of length scale is important here. Do you pick the width at the end of the small channels in your filter? (but they are triangular/have a non-constant diameter, and flow through the walls to boot). The width of the fume hood? (But this is only ~one width long, so hardly counts as a "channel"...)

The problem is to find the critical Reynolds number that applies to your situation, which is not exactly standard. Critical Re for e.g. (round) pipe flow is ~2300 (but you have rectangular channels, could maybe pick the hydraulic diameter), and for a free jet I found a number of 1300, but take that with a grain of salt.

All in all, you're back to gut feelings. You could just build it though, and make experiments: Hold an incense stick or cigarette near the filter (downstream) and observe the smoke - is it smooth for the first couple centimeters (->laminar), or already jagged (->turbulent)?