Yeah the second one, pump>Rad>CPU>any other blocks>res>
This is my 2C of what's going on:
- Water flow rate is constant throughout the loop, it's independant of pump location.
- Water velocity changes depending on the restriction but it's idependant on pump location. Velocity increases at smaller openings becasue water has to move faster to keep up with the systems flow rate. It's the water block's purpose to increase velocity and create turbulent water so that the maximum amount of cold water transfers heat from chip. This is the idea behind impingment/nozzles/etc for water blocks. High velocity/turbid water mixes and so a higher percentage of the water flowing by comes into contact with the hot metal in the block. This is where the challenge comes in for water block design, more restrictive means better fluid heat transfer, but at the same time you're reducing flow rate as pressure drop increases. In addition blocks are trying to create more copper surface area to contact the water, so it becomes a careful balance of increasing velocity(pressure drop) and maintaining a good flow rate for the pumps and the rest of the system.
- Water pressure changes and gradually lowers throughout the loop but it doesn't really matter regarding to placement in a loop. It drops the most blocks and fittings, but this is simply a matter of energy loss...the pump adds the energy, and the friction eats it up. Dependant on pump location, but the only important thing about pump location is that it's easy to bleed the air out of and has a res/t-line to feed it. The cumulative losses around a coolant loop are what dictates how much flow rate your pump can produce. The important point to note is that it's an accumulative loss, just because you have a 1/8" impingement block doesn't mean you're wasting your time with making other improvements to the system. The pump is working agains the friction loss of the CPU block + any other block + radiator + fittings + tubing. If you reduce the pressure drop at one point, you'll reduce the overall system pressure drop which is what the pump is working against. It's the P/Q curve where each pump type has one, and the flow rate or Q depends on the system pressure P..nothing more.
- Water temperature is the lowest after a radiator and increases across any water blocks or pumps. The radiator exchanges the water heat into the ambient air temperature, so it is at it's coldest immediately after the radiator.
So yes, the highest priority part you want to cool should be located immediate after the radiator.
Now for reality of it and why it's probably argued to some degree, the difference is small
. We're only talking a couple of C depending on how much heat you're dumping and getting rid of in the radiator. I only see about 1C drop across my radiator, this could be more if you were moving more heat and you had a higher performance radiator with higher cfm fans, but it's still only a few degrees. I would still rather go this route if you can, but if it requires a whole bunch of extra tubing, I would just do what works best for your setup, the only must have is to have a reservoir or T-line right before the pump so you can easily prime and bleed the setup. Folks that are running pelts probably see some pretty big drops across the radiator, but that's because they are also dumping several times the amount of heat in the coolant and the difference between water temperature and ambient temperature increases so much.