You are correct Ewan, using equivalent lengths typically yields a rough solution, though for what I was taught to do it is typically good enough. Keep in mind that I am not an expert on fluids by any means (I am a student civil engineer specializing in concrete materials) though I have had some formal training in the subject. In any case, there are more exact methods than using equivalent lengths, but for what we are doing I would question whether or not the more complex solutions are warranted.

If you don't mind, I would like to see an example for multiple devices in parallel. Particularly, dissimilar devices in parallel.

Thanks.

How many legs would you like? Two or three (or more?)?

Three legs of with three different length equivalents would be great.

Now I truely understand the crudeness and errors in my calculations ... always good to learn something new eh?!?
<--Says here that I'm a noob :D

Another question. Would it be accurate in saying the most amount of pressure in the system is right out of the outlet of the pump?

If so, would it not give you a small advantage with center inlet blocks to go from the pump directly to the block? Instead of having the pressure drop from the rad first then the block?

:shrug:

to your first question. Yes, the highest pressure in the circuit is at pump outlet.

to your second and third questions. No, it is the flow rate that determines the performance of the components. The higher the flow rate the better the performance.

It is the total pressure drop in your circiut that determines the flowrate (for a given pump).

Yeah I am totaly lost still I see....

If the inlet of the block is getting the highest amount of pressure then doesn't that mean the water will "press" harder against the block and lower the boundry layer creating better heat transfer?

I'm afraid it doesn't work like that. You can reduce the boundary layer by increasing velocity and turbulance which are linked by what the fluid is flowing around and the reynolds number.

The pressure drop across the block is an indication of the turbulance created hence the heat transfer coefficient acheived. This pressure drop will be the same no matter where the wb is positioned in the circuit for a given flow rate


sorry if I'm not very good at explaining it

I am just not recieving it well. :D I understand now though. Thanks! :)

It's quite a hard thing to visualise, but I think you're getting it.

Consider it this way.

The pressure drop across a block at any point in a loop of identical components will be the same, since the flow rate will be the same, yes.

So while there may be a greater pressure at the block inlet by situating the block right next to the inlet, but the pressure at the outlet of the block will also be considerably higher than if the block were at the tail end of the loop. The relative difference between these two values should not change regardless of positioning, and it is the pressure difference, essentially a pressure GRADIENT which causes flow, much like a temperature gradient causes flow of thermal energy.

Hope this helps

8-ball

Just for accuracy one may wish to remember that the head of water adds to the absolute pressure at any given point. Therefore in addition to the pump's pressure there is pressure at the bottom of the loop which is equal to the height of the loop. So if the waterblock is mounted 20 cm above the pump, then the absolute pressure will be 20cm of water less than the pump outlet pressure. This information is completely useless though because the absolute pressure of any particular point in the loop is irrelevant. If you wished to raise the pressure of your system you could hook up a T connector to your water cooling loop and plug that into your water mains which will be anywhere between 2- 6 bar. However flowrate will remain the same since the pressure drop through the loop will remain unchanged. Thge high pressure will stress all your components though, so don't do this.

One could assume that having a higher pressure would mean a greater contact force against heat transfer surfaces which would be beneficial in much in the same way that heat will transfer better between a CPU and water block the harder you clamp them together. But it doesn't work this way.
When you increase force between two solids you are in effect increasing the contact surface area, since irregularities between the surfaces get squashed out. While the materials seem hard, on a molecular scale they will get better squashed together the harder you squeeze them. This squashing together means that more mocules from one surface will come into contact with molecules from the other, thereby increasing the heat transfer surface area.
This doesn't happen to the same extent with liquids since liquids find their way into gaps anyway. Increasing the pressure may help them into very tight gaps, but it's not an effect which I think would be noticable unless you had extremely high pressures. If you had pressures of 50 bar or thereabouts then the effect may be noticable but that's a far stretch from the 1.1 bar that one would find in a watercooling situation (1 bar being atmospheric pressure and the 0.1 bar provided by the pump).

Uh, yeah, what he said... ;)


Pressure is tricky, and I have to really concentrate to explain it, because it was explained to me wrong the first time.

First, there's relative pressure (relative to atmosphere). Imagine that you have a tube, standing up, capped at the bottom, and full of water. There is some pressure inside the tube, from the water pressing down (thanks to gravity). That pressure is slightly higher than atmospheric. Now... at the water level itself (top), the difference between the pressure inside, and the pressure outside is zero: it's the same.


When you throw in a pump, everything goes awry: you create pressure where there might not be any, normally. If you can imagine a PC loop without a pump, you still have the relative pressures (relative to atmosphere), and the whole thing is predictable.

Add pump.

Now you've got an additional set of variables. If we stick to relative pressure (relative to atmosphere), the pump outlet will register the highest pressure point, and the pump inlet will register the lowest (in fact, the pump inlet pressure can be below atmospheric).

But none of that is a concern, for water cooling, except to note that if there's a leak in the loop, it'll either spill water, or suck in air, depending on where the leak is.

Obviously, the relative pressure drops, from the pump outlet, to the pump inlet, as the water goes through various components. It really doesn't matter in what order the components are, they'll all drop the same pressure.

The only thing that's different, is the relative pressure. The only effect it has, is on the joints. Example:

A heatercore drops 1 psi at a given flow rate. If the core is right after the pump, then those joints will have the highest pressure points, relative to atmosphere, but the core will still drop 1 psi.

If the core is at the pump inlet, then those joint will have the lowest pressure. If there's a leak there, it's even possible for the joint to suck air inside the core. (It's often the "invisible" leak that some people can't figure out).

The different relative pressure will have no measurable effect on performance: water is still a mostly incompressible liquid.

I'm sorry guys about not having those examples up... I am swamped with marking this evening, and will be lucky to get through it. I'll get them up this weekend though.