Equal flow distribution in manifold
Equal flow distribution in manifold
(OP)
Hi all,
This is probably a concept design problem so I apologize in advance if its trivial.
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:
1. Has anyone faced such a problem before?
2. Does the porous sheet solution seem viable? if so, does anyone know of a material that could work in this case?
Thank you for reading,
George
This is probably a concept design problem so I apologize in advance if its trivial.
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:
1. Has anyone faced such a problem before?
2. Does the porous sheet solution seem viable? if so, does anyone know of a material that could work in this case?
Thank you for reading,
George





RE: Equal flow distribution in manifold
The very first thing (if possible) is to have the largest possible plenum to feed the needles. You want to minimize any localized stream flow from plenum intake to exit (needles).
Diffusing the pump discharge into the plenum will also help.
Peristaltic pump seems like a poor choice. You want consistent pressure in the plenum. Some sort of positive displacement pump (a pair of small gear pumps?) with good pressure regulation and maybe even some recirculation to help regulate.
Quality of your needles is of paramount importance. Even microscopic variations in ID and length can cause significant variation.
Temperature regulation counts for a lot if your fluid is viscous. Warmer needles will have more flow than cooler ones.
http://www.EsoxRepublic.com-SolidWorks API VB programming help
RE: Equal flow distribution in manifold
Hello and thank you for the prompt reply. The liquid is not viscous, it is close to 2 cP , so lets say water.
Diffusing the flow was my first thought, but I was afraid it will not be enough. The very low flow rate and the comparably huge size of the needles result in an extremely small pressure drop along the flow paths. This is why I though of introducing a high pressure drop component before the needles, to render the manufacturing deviations among the needles unimportant. Another reason is that I have noticed that even tiny errors in the orientation of the system (not entirely normal to the gravity vector) has a tremendous effect on the flow difference among the needles. The thing is, the setup is pretty much set by the client, so I am trying to make this work with a minimally invasive solution to his device.
RE: Equal flow distribution in manifold
RE: Equal flow distribution in manifold
RE: Equal flow distribution in manifold
Maybe a 10x10 array of inserted carriers, each bearing a sapphire orifice with a small hole. You may still have to flow-match them because orifices are so sensitive to aperture diameter and entry and exit radius.
If you don't need continuous flow, maybe a 10x10 array of small plunger pumps, all driven by a translating plate. Then all you need is a 10x10 selector valve to charge the pumps. Easy-peasy; hah!
Mike Halloran
Pembroke Pines, FL, USA
RE: Equal flow distribution in manifold
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.
je suis charlie
RE: Equal flow distribution in manifold
The problem with an upstream restriction is that an orifice with a diameter of 0.2 mm needs a head of only 15 mm or so to give the flow you want. Using an orifice with a diameter smaller than that is risking endless blockages, and even with 0.2 mm you will need to ensure that your fluid is well filtered.
Also, with such a low flow the liquid coming out of each hole is not going to be continuous. It will grow to a droplet size before over coming the surface tension and falling to the needle. To get the drops to fall in exactly the right spot will require some sort of pin or pipe at each orifice that will allow the drop to grow at a specific point and fall where you want it. Rather than trying to get the drops to fall exactly into the needles I would build a grid of "pools" around the needles to catch the drops and allow the liquid to flow into the single needle in each "pool".
Katmar Software - AioFlo Pipe Hydraulics
http://katmarsoftware.com
"An undefined problem has an infinite number of solutions"
RE: Equal flow distribution in manifold
je suis charlie
RE: Equal flow distribution in manifold
RE: Equal flow distribution in manifold
with the largest plenum, maybe add baffle ribs to break up the inlet flows and direct flow into the corners ?
another day in paradise, or is paradise one day closer ?
RE: Equal flow distribution in manifold
gkan - you could try a sheet of filter paper as a first guess - add layers if the pressure drop is still insufficient.
je suis charlie
RE: Equal flow distribution in manifold
...and thank you for a plethora of ideas! I lean towards the initial idea, which is to use a sort of filter sheet to the inlet of the needles. I think an equal distribution in the plenum would only be useful if dynamic pressure differences were actually the reason for an unevenly distributed flow. In this case even orifices directly below the entry points of the plenum will display low to zero flow if you tilt the plenum for a couple of degrees. The idea of individual choking points is way over the budget of the project plus I really don't want to risk blockage in individual needles. Gruntguru, when you say filter paper, you mean like american coffee filter paper or is there another product?
Thanks again for your time!
Best Regards,
George
RE: Equal flow distribution in manifold
je suis charlie
RE: Equal flow distribution in manifold
RE: Equal flow distribution in manifold
This got me thinking along a different tack - the pressure drops through the needles can be increased by making the neeedles longer. If the needles could be made 100 or 150 mm long the allowable variation could be doubled or tripled. Several posters have pointed out the need to have the levels equal across the whole plenum. If the extra needle length was achieved by having them extended up through the base of the container (with still 50 mm or so projecting below) the 4 entry points could be made below the liquid surface and this would have a stilling effect on the surface level compared with allowing the entering liquid to fall and splash into the liquid.
Katmar Software - AioFlo Pipe Hydraulics
http://katmarsoftware.com
"An undefined problem has an infinite number of solutions"
RE: Equal flow distribution in manifold
je suis charlie