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  #61 (permalink)  
Old 26th December, 2004, 11:31 PM
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What Danrok said. We are likely to be arguing this back and forth until NEXT Christmas, LOL. In any case, the inventor's spirit lies in experimentation when there is no clear answer.
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Old 26th December, 2004, 11:32 PM
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Quote:
Originally Posted by gizmo
But one of the rads has twice the surface area of the other one. Assuming that the large rad is twice the size of the small one, you want 1/3 of the water going through the small rad, and the remainder going through the large rad.

All else being equal, if they are both regular tubing type rads, then the larger rad will have roughly twice the flow resistance of the smaller rad. However, if they are the flat-tube rads like what I use, it is more likely that the large rad will actually have roughly HALF the resistance of the small rad. (Flat-tube construction is generally a two-pass affair, so a larger rad will have more tubes in parallel on each pass, reducing flow resistance).
Also, not forgeting that he wants to use a TEC as well!
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  #63 (permalink)  
Old 27th December, 2004, 07:10 AM
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Quote:
Originally Posted by gizmo
Yes and no. Depends on the radiator design. I use regular car heater cores, which are a flat tube arrangement. The pressure drop across this kind of rad is very low. I have two of these in my system in series right now, and they work very well. My main concern with running two rads in parallel is that he'll end up in a situation where he isn't getting maximum cooling because the water isn't staying in the rad long enough. Of course, valves could be used to help this situation, but we will get to that in a moment.
This is the problem with concept design. You get very fast heat transfer if the water flows quickly through the radiator. You also get as effective a cooling job if the water flows very slowly through the radiator but spends long enough in there that it still cools down despite the lower efficiency. The former is a more elegant solution, but the latter is rather easier to build.

I have a tendency to err on the side of caution where radiator pressure drops are concerned - the design for most "for overclocking" radiators is a single-pass, many-leg radiator with somewhat tight bends, and a not particularly large pipe diameter. They're small and neat, but promise to provide quite some impediment to flow.

The type of radiator gizmo has specified is already two-pass, in effect, two radiators in parallel in the same package. Being of a flat profile, its wetted contact area is rather greater than that of round-tubed variations (though it will have a higher pressure drop per metre pipe than a round tube). In addition its liquid capacity will be rather high. I rather suspect that on entering the radiator and separating into each of the two passes, the water slows to a snail's crawl and spends a very long time being cooled at low efficiency - remember, car heater cores are designed to remove many kilowatts of heat at high efficiency. In effect it's a reservoir with cooling abillity I have occasionally wondered about using a central heating radiator for the same reason...

Quote:
Originally Posted by gizmo
Aye, there's the rub. Each valve is a flow restriction, and it is significant (more so than a turn or two of tubing). I would think the reduced flow from all those valves would be even worse than the pressure drop from the rads in series.
Possibly if you had an awful lot of them, but I rather doubt it. I posted this up back in 2002 in answer to another question, when I had the relevant book to hand. It uses equivalent pipe diameters, so is rather easier to calculate than by trying to work out the acceleration on the fluid at each corner and the coefficient of discharge of every valve. It's empirical, so just gives a rough answer...
Quote:
Originally Posted by Kaitain in 2002
Pressure drop in pipes due to friction is given by:

Pf =8 f (L / di ) .(pu^2)/2

Where Pf = pressure drop in N/m^2
f = friction factor
L = total pipe length in m
di = pipe inside diameter in m
p = fluid density in kg/m^3
u = fluid velocity in m/s

The bits you'll need:

Friction factors: found from a nasty looking graph. A typical value is 0.0035

p for water is approx 1000kg/m^3

Method of equivalent diameters: For each of the following fittings, multiply the number of equivalent pipe diameters by the inside diameter of the pipe and add this to the length of your tube. Use the total sum in the above equation:

45deg std elbow = 15
45deg long radius elbow = 10
90deg std radius elbow = 30 - 40
90deg long radius elbow = 23
90deg square elbow = 75
Tee - entry from leg = 60
Tee - entry into leg = 90
Union and coupling = 2
Sharp reduction (tank outlet) = 25
Sharp expansion (tank inlet) = 50
Gate valve (fully open) = 7.5
Globe valve (fully open) = 300
Plug valve (fully open) = 18

(Numbers after the = are the number of equivalent pipe diameters). This method assumes that the pressure drop due to each of the above fittings is equal to that caused by an equivalent length of the straight tubing.

Whatever value you get for the pressure drop is the pressure head your pump needs to provide to make the system flow.

Shout if you need more info.

(Source: Coulson and Richardson's Chemical Engineering volume 6, ed 3, R K Sinnot, 1999, Butterworth Heinemann)
As you can see, a 180 degree std radius U bend (most common in made-for-watercooling radiators) gives a pressure drop of up to 80 pipe diameters per bend. A well chosen valve results in a pressure drop of a tenth that.

I'm not clear on whether this is meant for turbulent or laminar flow - I suspect it's for laminar, but that the modifications for turbulent flow would be in scale. I don't have the relevant book handy though.
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  #64 (permalink)  
Old 27th December, 2004, 04:39 PM
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Quote:
Originally Posted by Kaitain
This is the problem with concept design. You get very fast heat transfer if the water flows quickly through the radiator.
Really? Why? Is it due to the increased turbulence at the boundary reducing the boundary layer?

Quote:
Originally Posted by Kaitain
In addition its liquid capacity will be rather high. I rather suspect that on entering the radiator and separating into each of the two passes, the water slows to a snail's crawl and spends a very long time being cooled at low efficiency.
Yup, that's about the size of it.

Quote:
Originally Posted by Kaitain
It's empirical, so just gives a rough answer...
As you can see, a 180 degree std radius U bend (most common in made-for-watercooling radiators) gives a pressure drop of up to 80 pipe diameters per bend. A well chosen valve results in a pressure drop of a tenth that.
Empirical answers are still useful for getting a feel for the nature of the problem, though. Thanks for posting this. This is good info.
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  #65 (permalink)  
Old 27th December, 2004, 06:43 PM
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Quote:
Originally Posted by gizmo
Really? Why? Is it due to the increased turbulence at the boundary reducing the boundary layer?
I don't have the book handy to quote the equation, but when you work through the math for the overall heat transfer coefficient for a system, it is directly proportional to the bulk velocity of the fluid. Most of these equations are, again, empirical combinations of various dimensionless numbers (Nusselt and Prandtl and viscosity correction usually).

In terms of boundary layer conditions, at higher flow rates, you get either a transitional or turbulent boundary layer at a shorter distance from the last obstacle (at which point the boundary layer disappears and you have little bits of laminar bulk flow). Obviously the best mixing, hence best heat transfer, occurs when you have a fully turbulent bulk flow and fully turbulent boundary layer. This is rather more detail than we need.

Quote:
Originally Posted by gizmo
Empirical answers are still useful for getting a feel for the nature of the problem, though. Thanks for posting this. This is good info.
My entire profession is a collection of one empirical result after another
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  #66 (permalink)  
Old 27th December, 2004, 07:59 PM
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Quote:
Originally Posted by Kaitain
My entire profession is a collection of one empirical result after another
I wish software engineering was that disciplined. LOL. Most of the time, we are guessing.
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  #67 (permalink)  
Old 27th December, 2004, 10:56 PM
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This is why I'm no good at programming
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Old 28th December, 2004, 03:30 PM
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its because they keep us guessing from early on I'd guess.
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