• Some users have recently had their accounts hijacked. It seems that the now defunct EVGA forums might have compromised your password there and seems many are using the same PW here. We would suggest you UPDATE YOUR PASSWORD and TURN ON 2FA for your account here to further secure it. None of the compromised accounts had 2FA turned on.
    Once you have enabled 2FA, your account will be updated soon to show a badge, letting other members know that you use 2FA to protect your account. This should be beneficial for everyone that uses FSFT.

Fluid Dynamics

H

HiTech-Hate

Gawd
Joined
Oct 10, 2002
Messages
986
Can any of you physicists out there let me know how to convert pressure (in PSI) to flowrate? Any equations you can provide would be helpful, and if you ever need chemistry help in return i'm your man :p (physics has always been a plague and burden to me, thermodynamics, kinetics, and organic chemistry are my favourites :)
 
Thanks ... but that wont help :(

I'm not looking to convert units here, I need to find the eqs necessary to calculate flowrate given a pressure (in other words im not going from pressure to pressure or flowrate to flowrate in terms of units, im going from pressure to flowrate :) )
 
http://www.efunda.com/formulae/fluids/calc_pipe_friction.cfm
http://www.efunda.com/formulae/fluids/calc_orifice_flowmeter.cfm
http://hyperphysics.phy-astr.gsu.edu/hbase/pber.html
http://www.cr.org/publications/MSM2002/pdf/195.pdf
http://www.ing.unipi.it/~d6600/papers/vencann.pdf

Hopefully one of the links above have what you're looking for. I just skimmed over the first bit of each page, to make sure they were relevant. The last link started talking about an equation between pressure and flow, so that might be your best bet. Happy reading!
 
Although this may have occurred to you, you realize there is not necessarily a relationship between pressure and flow. A tank or pipe of a fluid can have a given pressure and have a flow rate of 0.

Is this for a wc setup? Do you have information on the pump?
 
Chemists are not chemical engineers (plumbers). They don't need to know silly bits of physics.

Flowrate and pressure can only be related once all the conductances of the system have been calculated, which is way the hell too much trouble. Watercooling is no exact science.
 
ok perhaps I should clarify myself a bit better. First off mwarps i'm not looking for 'silly bits of physics' i am genuinly concerned that there is a problem with flowrate in my setup. I have this ...

http://www.dtekcustoms.com/index.asp?PageAction=VIEWPROD&ProdID=49

in my loop (from Pump --> Gauge --> heatercore ...)

and I thought that there may have been an easy relationship between the pressure it is reading and the flowrate of my loop .... apparently I was wrong, thanks ehZn for pointing that out. I was actually totally unaware that there was no relationship between pressure and flowrate, I just assumed there was (woops), like I said, im a chemist, and HATE physics, I really know NOTHING about fluid dynamics heh.

As for my setup, I have a danner mag 3 pump, i'm using 1/2" tubing, only a cpu block (white water), a heatercore I purchased from dangerden, and I went with a t-line due to spacial restrictions. I have two problems that are leading me to question the flowrate, firstly it has been nearly a week and not nearly all of the air has bled out of the t-line, it is at the highest point of my setup, and not plugged. I'm also not getting very good performance, in fact i'm getting pretty bad performance, my idle temps havnt changed one degree since migrating to wc from an SLK947U.

So if any of you have any suggestions on either calculating the flowrate, or even a simple way of testing the flowrate to put my mind at ease, i'd be very appreciative.
 
HiTech-Hate said:
PS: Thanks RickyJ for the links, I guess I better start reading :p
Click to expand...
There are equations for max flow in a certain sized pipe. My dad has such equations in a book, but I am over 1000km from home this summer, so I can't get them to you.
 
HiTech-Hate said:
ok perhaps I should clarify myself a bit better. First off mwarps i'm not looking for 'silly bits of physics' i am genuinly concerned that there is a problem with flowrate in my setup. I have this ...

http://www.dtekcustoms.com/index.asp?PageAction=VIEWPROD&ProdID=49

in my loop (from Pump --> Gauge --> heatercore ...)

So if any of you have any suggestions on either calculating the flowrate, or even a simple way of testing the flowrate to put my mind at ease, i'd be very appreciative.
Click to expand...

Alrighty we have a little to work with here. While a pressure gauge will give you some general idea of how the flow is going, it won't help at all in calculating the flow rate.

This is how I would do it. Your pump should have a given pumping rate, given in liters per min or something like that. This means that when fully supplied with adequate fluid reservoir it would pump that much liquid in that amount of time. I found this about the mag3
It pushes 350GPH max and 10.5FT max.
Click to expand...

So ideally in any tubing (total length < 10.5 ft) you would have 350 GPH (gallons per hour) passing through it. Unfortunately, there aren't many ways to check this empirically short of hooking a flow meter into the system.

What you might be able to do is find a review of the danner mag3 pump where they use it in a wc setup, and they tell you what pressure it runs at with 1/2" tubing. This would allow you to use the gauge to compare your flow to theirs. If your psi is higher, it means your flow is not what they got, but if it is lower, it means you are getting better flow than they did.

Hope that helps...
 
I appreciate the speedy responses, I will do some research. I did know what the 'theoretical' flowrate of the pump was, im just not sure if the pump itself is damaged or whatnot, im not very satisfied with the flow i'm seeing through my system, that and it leaked like a mofo until I sealed the inlet housing up with marine goop, having to do that made me a little aprehensive abotu putting the loop in my pc to start with, but seeing as i'd already bought everything I figured I had no choice heh.
 
I found this very informative graph for converting Head to Pressure (PSI)...

http://www.engineeringtoolbox.com/34_406.html

My pressure gauge was reading 4 PSI ... so from the graph

2.3106 * 4 = 9.24 ft.

and from this PQ graph ...

http://phaestus.procooling.com/pqcurves.jpg

with a head of 9.24 ft. it would seem that I am getting extremely low flowrates, less then 50gph. So unless i've done something wrong, i've got a new problem on my hands .. to solve this apparently restrictive flowrate in my loop, I wonder wtf could be going wrong?
:confused: :confused:
 
When I have to check my flowrates, I take the loop out, and set it up in a bathtub or sink with a fresh supply of water and a graduated measuring device. Keeping the loop as close to as it was in the case, I disconnect the pump inlet put it in the gallon jug or whatever you have. Then you should turn on the pump and time how long it takes to fil the gallon (or 5 gallons or anything that you can use to accuratly measure that kind thing) and calculate your flowrate from that. Aside from buying a flowmeter (an accurate industral one and not one from a watercooling 'shop'), thats the easiest way I've found to get a very accurate flowrate. Less than 50gph does seem a little strange, especially because most low restriction loops with a decent pump can push over 1 gallon per minute if not a lot more.
 
yeah well that brings up anothe prob I forgot to mention (i'm forgetting a lot today lol sorry guys, im not really with it, super tired ... and super busy at work). I went to drain my loop the other day (I want to try lapping the waterblock), so I figured the logical way to do it is to remove the tube from the pump inlet, turn on the pump, and it should pump all the water out and into a container. Well, turning on the pump did NOTHING!!!! No water was pumped out into the container, the only water that did was the water already in the tubes that drained out cuz of gravity!! I couldn't figure it out which is why I came here. If ANYONE has any clue on why this may be :( I cant really return the pump or anything if it is pooched (since i sealed it with marine goop). bah this sucks lol
 
ehZn said:
Although this may have occurred to you, you realize there is not necessarily a relationship between pressure and flow. A tank or pipe of a fluid can have a given pressure and have a flow rate of 0.

Is this for a wc setup? Do you have information on the pump?
Click to expand...

This man speaks the truth..
 
HiTech-Hate said:
yeah well that brings up anothe prob I forgot to mention (i'm forgetting a lot today lol sorry guys, im not really with it, super tired ... and super busy at work). I went to drain my loop the other day (I want to try lapping the waterblock), so I figured the logical way to do it is to remove the tube from the pump inlet, turn on the pump, and it should pump all the water out and into a container. Well, turning on the pump did NOTHING!!!! No water was pumped out into the container, the only water that did was the water already in the tubes that drained out cuz of gravity!! I couldn't figure it out which is why I came here. If ANYONE has any clue on why this may be :( I cant really return the pump or anything if it is pooched (since i sealed it with marine goop). bah this sucks lol
Click to expand...
That's an ass-backwards method for draining a system. You want to remove the outlet, and not the inlet of the pump. Then turn it on. A pump needs something to push through, and without the inlet and a steady supply of water, there is nothing to push. Gravity will do most of the work too.

Keep in mind that pumps can not be run dry for any reason...so taking off the inlet and running it for a while like that could have b0rked it perhaps...In any case its not good for the pump at all so don't do that again.
 
ah thanks OMP. Like I said, I was shooting in the dark as to how to drain the loop, couldn't find anywhere that explained how it should be done, and considered it too stupid a question to ask on the forums lol. I didn't run it for long, maybe 15 seconds. That WAS my logic though as to why it didnt drain, that the pump may not operate pushing water correctly when there is no water in it ... thats why I didnt explain this issue in my original post :).
 
a simple way to test flowrate? fill something that you can measure and time it how long it takes to fill. Not sure if you can do this or not, just in case it helps. you can get your velocity from this by dividing by doing your (flowrate/area of tube). Roughly ft/s or m/s.
 
There are relationships between pressure DIFFERENTIALS and flow rates (bernoulli's principle). I'll get on later and explain.
 
what's the point of using bernoulli (don't leave the ground without him)? How are you going to find/calc the q?

gz = easy
p = static probe (head port)
q = ? unless you plan on using a pitot tube
 
Use a venturi (with a pressure tap at the throat) or an orifice (with a pressure tap at the vena contract). The difference in pressure between that and the established flow pattern upstream is proportional to the square root of the flow rate.

...my texts are at work. I'll try to bring one home and give the equation. It's something to the tune of

Code: 
Q = Cd * ( (P1 - P2)/((Ao/At) - 1) )^.5

Cd = discharge coefficient
P1 = upstream pressure
P2 = downstream pressure
Ao = area of orifice
At = area of tube
 
K2Rage101 said:
Use a venturi (with a pressure tap at the throat) or an orifice (with a pressure tap at the vena contract). The difference in pressure between that and the established flow pattern upstream is proportional to the square root of the flow rate.

...my texts are at work. I'll try to bring one home and give the equation. It's something to the tune of

Code: 
Q = Cd * ( (P1 - P2)/((Ao/At) - 1) )^.5

Cd = discharge coefficient
P1 = upstream pressure
P2 = downstream pressure
Ao = area of orifice
At = area of tube
Click to expand...
looking at a Nobel if you can pull this off, pressure <-> flowrate
don't lose sight of the original question
 
q = 1/2 rho v^2

though, you get your velocity and can find flowrate from that using

pi*r^2*V (volume flow rate) m^3/s


K2Rage101 > I'd be interested in the equations for this, I probably learned them in fluids but i'm aero so pipes aren't really my thing. I like flows over bodies. :D

Also how are you going to get a venturi for 3/8" tubing? won't the boundary effects screw it all up anyway? I don't have a good enough feel to if the flow will be developed at these velocities and with water.
 
I'll get the equations soon, forgot my book today.

I wouldn't use a venturi, I would use an orifice plate. Simpler and gives a higher dP for greater accuracy. Just a plate sandwiched in the line with a hole in it roughly 1/2 the diameter of the normal pipe.

The other thing you could do is ask your water pump manufacturer for a pump curve. The discharge pressure - intake pressure = pump head. This number plus pump RPMs = flow rate off of a pump curve.
 
BillA said:
here is an article that every poster in this thread can benefit from
BASIC PRINCIPLES INVOLVED IN HEAT TRANSFER AND COOLING RELATED TO PROCESSORS

a bit long, but well worth the effort
Click to expand...


howdy BillA ;)
stumbled across this while looking for something else
as always great info ;)


this seemily pointless post, made so I can relocate this thread in need :p
and to say hey to an old friend

while not fluid dynamics reference per se....
A Heat Transfer Textbook Lienhard & Lienhard (MIT & U of Houston) PDF
might be usefull
 
You must log in or register to reply here.
Back
Top