Hole where water cannot escape at certain pressure?

[j7on]

Hi eveyrone!

I am completely new to this forum but didnt know where else to turn for a "problem" of mine..

I am trying to design a new kind of waterpump and still in the teory part of the design, i would be interested to know how i can calculate the "thickness" of water at a certain pressure?

How small must a hole be that water cannot escape at certain pressure?

Does it matter if it is a direct hole or a gap that is exactly as wide as the hole(do many same sized holes require the same amount of pressure as only one)?

How much does water temperature affect outcome?

Hard to explain, but then again, i am no physicist...;)

 

[Dr Lots-o'watts]

If I understand correctly, you are looking for a relation between water pressure and rate of escape through a fixed-diameter hole. Then, by decreasing the pressure, you want to reach a rate of escape that you can consider to be negligible. The pressure causing this threshold rate is your unknown.

By decreasing hole diameter, you will lower the rate, but not "prevent" it completely from escaping. Ultimately, water will evaporate a molecule at a time for as long as its dryer outside.

As for temperature, staying clear of the freezing level is probably your main concern for now.

 

[j7on]

Yes, i guess, i mean the problem is quite simple, i just don't know how to calculate it.

Imagine a syringe with a large needle hole, there is quite little pressure to apply to push water throught it, then, decrease the size of the hole until you reach your desired pressure but water is still not coming out(apply even more pressure and it will ofcourse).

I would just like to know how i can count how thick water is at some pressure.

Example: How small must a hole be that water cannot go throught it at a 10bar pressure(but will at more)?

Now, the second question, if the hole would be like 1/50th mm, will i have the same result if there is a 1/50th mm gap instead of hole or will water sip out before i reach 10bar?

Damn, the hardest thing is to explain(and English is not my first(or even second) language so.. bare with me)

 

[russ_watters]

Yes, i guess, i mean the problem is quite simple, i just don't know how to calculate it.

Imagine a syringe with a large needle hole, there is quite little pressure to apply to push water throught it, then, decrease the size of the hole until you reach your desired pressure but water is still not coming out(apply even more pressure and it will ofcourse).

I would just like to know how i can count how thick water is at some pressure.

Example: How small must a hole be that water cannot go throught it at a 10bar pressure(but will at more)?

Dr Lots o Watts (cool name) is correct: there is no such relationship. Water will come out of a syringe at any pressure, at a velocity determined by Bernoulli's equation.

Now, the second question, if the hole would be like 1/50th mm, will i have the same result if there is a 1/50th mm gap instead of hole or will water sip out before i reach 10bar?

Velocity is pressure dependent only. The area of the hole times the velocity gives you flow rate.

 

[russ_watters]

Sorry, guys - I got a spyware/malware attack that coincided with my posting in this thread and deleted it temporarily as a precaution...it looks ok, so I restored it.

 

[rcgldr]

At some point of diminishing hole diameter, it would seem that friction, viscosity and skin effect of the water would effective prevent low pressure flow (except for the evaporation effect), or reduced the flow to insignificant, depending on the definition of insigificant.

 

[LURCH]

Is there a reason why a simple pressure valve would be undisirable?

 

[russ_watters]

At some point of diminishing hole diameter, it would seem that friction, viscosity and skin effect of the water would effective prevent low pressure flow (except for the evaporation effect), or reduced the flow to insignificant, depending on the definition of insigificant.

Surface tension will do it at very low pressure, sure... not sure if that's what the OP was going for, though.

 

[LURCH]

As a compramise between the pricisely sized hole and the pressure valve, you could try a surface made fo rubber or some other very elastic material, riddled with holes that are hld closed by compression. At sufficient pressures, the water would force its way through the holes.

 

[Loren Booda]

What limiting properties does an osmotic membrane and solution have that would cause water to travel one way?

 

[Emreth]

The pressure can get sufficiently large for very small holes.
you can calculate it using the capillary distance as in the 1st link. You have to calculate the pressure corresponding to certain height of water, as in a manometer.Eqn is in the second link. You need to go to nanoscale to get substantial pressures.
http://en.wikipedia.org/wiki/Capillary_action
http://en.wikipedia.org/wiki/Pressure_measurement

 

[QuantumPion]

This is actually how some pumps are designed, they use what is called a Labyrinth seal. I know that the shafts of nuclear reactor coolant pumps are designed this way (very large 2250 PSI >8000 HP pumps).

 

[j7on]

Well i was thinking of using a ceramic ball inside a ceramic tube(ceramic because it does not wear out and leaves no taste trace because there will be water inside).

Caeramic balls & tubes can be made with very small precision tolerances, a hydraulic shaft underneath would push the ball creating pressure on top where the water is, now is it even possible to make a clearance that is so small that water can reach temporary pressures(1 min. max.) of 10-20bars and still be able to move freely(return down with just gravity)?

?

 

[QuantumPion]

It sounds like you are trying to make a positive displacement water pump, using a piston like a car engine. You might want to google some manufacturer web pages for similar pumps and see what seal designs they use.

 

[j7on]

Its just that, there isn't any, i am thinking if the gap between the ball & the tube be so small thus creating a natural seal for some time without the need for an actual seal.