Equal flow distribution in manifold

Mechanical Engineering general discussion
#1

QUESTION
I need to design a manifold of 100 needles arranged in a 10x10 grid all connected to the bottom of a liquid container.The container itself has 4 inlets on top in which liquid is pumped using a peristaltic pump. The requirement is that each needle will provide with a flow of 10 microliters per second +/- 5 microliters. The needles themselves have an ID of 1 mm. The problem so far is that the flow distribution is not equal among the needles. Some have a flow of 10 microliters, some have no flow at all.
My thought was that by drastically increasing the pressure drop in each inlet of a needle (maybe by introducing a porous sheet with a very small porosity) I could perhaps compensate for the large ID of the needles in comparison to the very low flow rate which seems to create unequal distribution to the 100 outlets. My questions:

  • Has anyone faced such a problem before?
  • Does the porous sheet solution seem viable? if so, does anyone know of a material that could work in this case?

Dispenser
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REPLIES

gruntguru
At the very low flow rates, it is unlikely that velocity distribution in the plenum is a problem (unless the plenum has a ridiculously small cross section). The peristaltic pump is a good choice - positive displacement, accurate metering. Seems your lack of pressure in the plenum (lack of pressure drop across each metering orifice (needle)) is allowing non-uniform factors (gravitational head, capillary action/surface tension etc to dominate)

Yes you need 100 metering points with equal cross section and discharge coefficient, small enough to generate a significant pressure drop - say 50% of the pump rating at the specified flow. The simplest way may be a screen as you suggest. Make sure the 100 holes the screen material rests against are equal. Something like a reverse osmosis membrane might work, the important thing is uniform permeability across its surface. Make sure there are no solids in the working fluid - they will be filtered and block the membrane.

SOURCE
https://www.eng-tips.com/viewthread.cfm?qid=379068
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#2

@gruntguru, Solid, practical advice - as always.

#3

This is an old problem. …much older than the old discussion on the other site.
Unfortunately, the usual flow equations sort of fall apart because the Re is very very low, and the liquid in the plenum is not obliged to do what you think it will do.

The least awful solution is a 100 channel peristaltic pump, which can probably achieve the specification given. Peri pumps are nowhere near ideal, and nowhere near as stable, precise, durable or reliable as innocents might assume.

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