[myv65]

Originally posted by gmat
the quite low temperature differential accors the circuit (1°C or less). Is 1°C enough to produce a noticeable static pressure ?

I don't have my texts with me, they're at work. Water reaches its peak density at 4°C. It is sort of parabolic going away from 4°C. 1°C won't change the density much, probably on the order of 0.01-0.1%. To know the pressure rise, I'd have to verify this number, know the tubing rigidity, and look up the *real* bulk modulus. You may be surprised how much static pressure such a small temperature change can generate if the system is truly rigid.

i've been wondering what was the *real* pressure distribution around a closed loop of water. Maybe CFD would answer that..
You seem to say that in every case the pressure is positive around the whole circuit, which i doubt by experience. (say, i only "doubt", ie i may be quite wrong on this).
What i'm considering here is the dynamic effects.
In a turning flow; the radial kinetic forces tend to create a pressure on the outer side, and an underpressure in the underside (Bernouilli, if my memories serve me right). This effect is mostly noticeable in rads ans waterblocks, under CFD or other flow analysis tool scrutiny. The question is, whats the difference when "ambient" or "static" pressure of the flow at this point is "high" or "low" ?

No, I don't say that pressure is positive around the whole loop. It may or may not be, depending on the system. I will guarantee you that it is positive in an open system provided the reservoir is at the high point. In an open system, it may be under a slight vacuum in the last leg if the reservoir is at a lower elevation.

In a closed system, "most" often it is at a slight vacuum leading to the pump suction. You can easily change this with a standpipe filled with water, though you may need a higher ceiling. If you place a T ahead of the pump suction and run a vertical (open at the top) line and fill it with water, the water height in that run will define the pressure at the suction (and eliminate any chance of having a vacuum). This is obviously not a practical way to run a system, but does serve to illustrate some points about pump systems.

I'm not quite sure what you're getting at with your comments about radial flows. Yes, there are secondary flows caused by bends and temperature changes among other things. Yes, they create localized variations in velocity. This manifests itself as localized variations in convection coefficient. Sure, you could model this with CFD, but your answers are only as good as your assumptions placed on the model.

More practically, you can run pressure taps at the inlet and outlet of each individual component if you want to know the overall restriction imposed by each part. Knowing the delicacies of secondary flows may be useful to a block designer, but I've seen little evidence that the major manufacturers really understand what they're doing from an engineering standpoint. Fact is they don't have to understand it well. I say this for two reasons. First, marketing drives sales because the majority of buyers buy stuff that "looks cool" or they heard about from a buddy. Second, there really isn't a dramatic difference in performance among the main blocks. So long as the block is sufficiently restrictive as to create a high velocity via a pressure drop, it'll do just fine. Where blocks will disappoint is if something else in the system constricts flow so much that the block velocity is too low.

To answer your final question about ambient or static pressure, it's irrelevent. Static pressure by its very definition is the same in all directions. This means that it can't affect flow rate (other than increasing flow area in soft tubing), so has no, repeat no effect on heat transfer. Relative pressures determine flow rate and are all that really matters.

Notes:
* I doubt that with rigid tubing the pressure "suddenly" drops at pump inlet. Correct me here if i'm wrong.
* i've got a rather powerful pump (rated at 27W), but i doubt it dumps all its 27W in water. Actually it's quite hot to the touch, and since it has fins i suspect it dumps a good part of its heat in the air. Besides the engine (hot part) is totally insulated from the water chamber, since it's a mag drive. The percentage of heat dumped by the pump into water has been a hot topic but until now i've seen no convincing answer.

Item 1, you are correct. Sudden drops only occur when there are sudden changes in the flow path. Fittings, bends, Ts, valve bodies, etc., cause "step" changes in pressure. Tubing causes a linear "loss per length" based upon flow and internal surface roughness along with Reynold's number.
Item 2, tough to answer. Most motors run on the order of 80% efficiency. Most centrifugal pumps that we use peak out around 60% efficiency. This peak occurs when discharge resistance is moderate, neither a minimum (peak flow) or a maximum (dead headed with zero flow). Because of our systems, we tend to run higher head pressure on the pump than the "peak efficiency" head pressure. So figure your pump is ~40% efficient overall. It also doesn't run at 100% rated motor current, say maybe 80%.

Take all this together and you probably dump ~27*.8 (load)*.2 (motor inefficiency) as heat from the motor fins. The remainder of energy input (27 * .8 * .8) gets put into the water. Of this, flow rate * delta-P (outlet - suction pressure) is "useful work" and the remainder is pure heat. Even the useful work is still energy, though, so is energy that enters the system and must be removed by the radiator.

Tell ya what, I'm part way through a series for AMDMB discussing all facets of water cooling. There's also a couple of articles discussing heat transfer and fans. For the water cooling parts, all that is currently posted is the introduction and pump section. The fluids section should be up within a week or so. The real fun starts with the next one after that covering radiators and blocks. There's sure to be material there that throws some people into a tizzy fit. If you haven't already seen it, feel free to stop on by for a look. The pump article is "stickied" in the liquid cooling forum and the others are "stickied" in the cases&cooling forum.