pipe full of water: opening to the air/opening to water - different results
pipe full of water: opening to the air/opening to water - different results
(OP)
Hi to all,
We are involved in a project where we are making a first prototype and we found the following:
We have a pipe under pressure full of water and small circular openings. the diameter of the pipe is 50mm and the holes 5mm. we have 5 holes, pointing downwards. The water goes out to the air.
The problem of calculating flowrate per opening and the pressure loss in the pipe, is easy to solve by bernouilli equations tanking into account the adequate discharge and contraction coefficients. we have checked it measuring the pressure drop and everythin is ok.
But now we move the pipe to a tank. And we put the pipe so that the openings are in contact with the water (we insert them a few mm so that there is no signigficant pressure increase "outside" the opeinings). In this case the pressure drop goes down (not much about 8%) and the flowrate increases.
Is there any change of the velocity or in any coefficent that should be taken into account in this case?- when we put it in contact with water instead of coming out to the air?
thanks a lot
gorostidi
phd mechanical engineer
We are involved in a project where we are making a first prototype and we found the following:
We have a pipe under pressure full of water and small circular openings. the diameter of the pipe is 50mm and the holes 5mm. we have 5 holes, pointing downwards. The water goes out to the air.
The problem of calculating flowrate per opening and the pressure loss in the pipe, is easy to solve by bernouilli equations tanking into account the adequate discharge and contraction coefficients. we have checked it measuring the pressure drop and everythin is ok.
But now we move the pipe to a tank. And we put the pipe so that the openings are in contact with the water (we insert them a few mm so that there is no signigficant pressure increase "outside" the opeinings). In this case the pressure drop goes down (not much about 8%) and the flowrate increases.
Is there any change of the velocity or in any coefficent that should be taken into account in this case?- when we put it in contact with water instead of coming out to the air?
thanks a lot
gorostidi
phd mechanical engineer





RE: pipe full of water: opening to the air/opening to water - different results
There's probably some other coefficient to use for this but try that one and see if it works.
My motto: Learn something new every day
Also: There's usually a good reason why everyone does it that way
RE: pipe full of water: opening to the air/opening to water - different results
David Simpson, PE
MuleShoe Engineering
"Belief" is the acceptance of an hypotheses in the absence of data.
"Prejudice" is having an opinion not supported by the preponderance of the data.
"Knowledge" is only found through the accumulation and analysis of data.
The plural of anecdote is not "data"
RE: pipe full of water: opening to the air/opening to water - different results
Independent events are seldomly independent.
RE: pipe full of water: opening to the air/opening to water - different results
Ted
RE: pipe full of water: opening to the air/opening to water - different results
Your resistance to flow - as noted above correctly - is changing based on the difference between effect of air and water, even if there is very little back pressure involved. It's a spray nozzle type effect: entraining just a little bit of air into a flowing stream through a different shape hole a different place in the nozzle will greatly change the speed and shape of the output spray, even if all of the nozzles on your garden hose have nearly the same area for flow.
You will also have a effect of the flow down the pipe: the final hole - if the same size as the first flow, will have notably less pressure behind it. Each hole will not get 20.0% of the flow.
RE: pipe full of water: opening to the air/opening to water - different results
All comments are very wellcome.
Our idea of what is really happening is very close to what zdas04 and racookpe1978, suggest, that the Cv is for some reason in this case changing. Cc should be the same, as i understand the vena contraction is because of the internal flow inside the pipe. But what BigInch and hydtools say is also reasonable. The fact is that when inserting into water the flow increases instead of decreasing (which is what we expected). In any case it seems complex to prevent.
From the mathematical point of view we will also try LittleInch suggestion. Seems to be the right one.
Our REAL AND COMPLETE experiment is the following:
1) experimentally obtain the cc cv and discharge coefficients. Water inside, air outside. this is very close to any reference found in books.
2) Then put the pipe inside the water (as i described). the results are similar but as i said they change a bit, and we did not undersand the physicall reason.
3) now go the other way round. air inside the pipe water outside, so we inject air into a water tank. (in fact the pipe will be in a tank of a filtering system, where sometimes we insert air, sometimes water). If we take the results taken from point 1) the results are quite approximate to the experimental ones.
We have done it this way because it is very easy to get a very precise data of the flowrate with water, and all the coefficients as described in point 1). On the other hand, with air, we need (and we use) a flowmeter, which is ok, but has some small error. In any case if we use this data in points 2) and 3) we obtain acceptable results but not completely precise.
An error of 5% is acceptable in our development, its ok, but we think that it will be worth getting data from 2) and 3) experiments in real conditions, this is with water outside.
We will go for it and let you know. in any case, new ideas will be wellcome and reference books or articles that have been involved in something similar, i guess we are not the first ones.
in any case from the comments we will make the experiments and the the coefficients experimentally. but it is not easy sometimes to find the real reason of why they change depending on the experiment parameters
thanks
gorostidi
mech eng phd
RE: pipe full of water: opening to the air/opening to water - different results
thanks
gorostidi
RE: pipe full of water: opening to the air/opening to water - different results
http://webwormcpt.blogspot.com/2007/10/how-to-appl... as Figure 10-20. The curves on this graph cross each other several times.
I suspect that it is mostly related to the formation of the vena contracta, but I agree with David that surface tension is probably a factor too.
Katmar Software - AioFlo Pipe Hydraulics
http://katmarsoftware.com
"An undefined problem has an infinite number of solutions"
RE: pipe full of water: opening to the air/opening to water - different results
The experiment looks simple but the coefficents behind are not that easy to get (or find!) in books or articles . We will use this graph as a reference to compare with.
gorostidi
RE: pipe full of water: opening to the air/opening to water - different results
we have made a ver simple experiment:
we take one pipe of 25mm and install put a manometer to measure the pressure just before it. after the pipe we put an elbow open to air. the flow is big enough (the manometer measures about 2 bars) so that the pipe is completelly full
now we just insert the system (just the elbow) a few mm in a tank. the pressure shown in the manometer decreases, from 2 bars to about 1.6 bars. it is the same effect as we described bellow but in a much simpler system.
but the flowrate at the two cases remains the same.
does anyone have a clue of what we are misssing?
thanks a lot to everyone
gorostidi
RE: pipe full of water: opening to the air/opening to water - different results
Independent events are seldomly independent.
RE: pipe full of water: opening to the air/opening to water - different results
Will do again the tests...completelly lost...
RE: pipe full of water: opening to the air/opening to water - different results
Anyway, try calculating the velocities, then including velocity head. At least you might not be totally lost.
Independent events are seldomly independent.
RE: pipe full of water: opening to the air/opening to water - different results
Katmar Software - AioFlo Pipe Hydraulics
http://katmarsoftware.com
"An undefined problem has an infinite number of solutions"
RE: pipe full of water: opening to the air/opening to water - different results
finally solved the mystery. everything was ok, except that in we had some air in some parts of the circuit. this caused a wrong measure in the pressure value. when the holes are in contact to the water, the pipe gets rid of the air and the pressure value is ok. as soon as we have made a "closed circuit" then everything goes fine.
really thanks a lot for all your help.