[Cathar]

Originally Posted by Jabo

This is quite interesting discussion going on here indeed
my 5 pennies:
Lokking at pictures and analyzing decriptions of aformentioned deustch testbed system I came to following conclusions (based on my limited knowledge of fluid mechanics and thermodynamics):

1. Heatload simulating element used (it's size and configuration) produces massive thermal energy density per mm^2. Such setup preffers blocks with thicker base plate (Fourier's law, isotropic heat diffusion etc.), which may in turn explain better performance of Murks block (VERY thick base plate)

Actually cooling performance (in terms of W/m²K) is not affected by the thermal density of the heat-load. The coolant flowing through the block is applying a fixed rate of cooling effect, which is proportional to the rate of thermal convection that's going on inside the block, and since the cooling area is fixed, then the amount that the heat source will warm up by is directly proportional to the heat that it emits.

i.e. a block that cools better at 50W of heat load, will still cool better at 100W or 200W, and in fact will provide increasingly better temperatures.

What a thick base-plate does is determine the amount of thermal spread of the heat by the time that the heat reaches the convectional zones within the block. Stick on a thick base-plate, and all of a sudden you have effectively more surface area for the water convection to work on (presuming that the heat source is initially far smaller than the waterblock).

What we strike here is a balancing act though. The thicker base-plate increases the thermal resistance as well. As the rate of thermal convection is improved (typically through higher flow rates) the thickness of the copper increasingly becomes a barrier to further improvements in cooling performance, as it becomes the predominant source of thermal resistance.

For thin base-plate blocks, these blocks rely on the rate of thermal convection being high enough to overcome the lack of thermal spread. Less convective surface area is available for the coolant to operate on, but the rate of thermal convection is high enough to overcome this drawback. There is the added benefit here that as the thickness of the copper is reduced, this too offers less thermal resistance.

So we have a balancing act. If designing a block for ultra-low flow rates, one would naturally choose a thicker base-plate to offset the reduction in thermal convection by increasing the effective area available to the coolant. If designing a high-flow block, then assuming it's done properly, going down to quite amazingly thin base-plates is exactly what you want.

The amount of heat load of the heat-die has nothing to do with it.

A thin based block will suffer at flow rates below its design balance point in comparison to a thicker based block. A thicker based block will not see any significant gains above its designed flow-rate balance point in comparison to a thinner based block.

Originally Posted by Jabo

2. Taking into account the above and low flow (jet impingement design is based on localized increase of coolant's density/pressure which entails increased thermal capacity maintaining the highest possible dT) it's totally expected that Murks performance is better then Cascade's (sudden 4C drop is simply impossible, unless Tom discovered how to dump excesive heat into another dimenssion or used sth like foamed graphite insert combined with laser beam micro channeling of copper )

Speaking of jet impingement, another important aspect here is the jet power. As flow rate is dropped, so too does the jet power and its ability to impinge on the surface. Higher jet velocities demand a wider cup-jet width ratio as the size of the primary impingement zone increases with velocity. Higher jet velocities also demand that the jet stands off from the base of the cup further, as the increased velocity allows for greater mixing/turbulation of the incoming jet stream without it losing significant power. With lower jet velocities (the result of lower flow rates and pumping pressures) the jets need to be placed closer to the base-plate to offset the loss in impingement "power" that would be experienced if one kept the same parameters as a higher-velocity setup.

i.e. for low flow setups, one would both decrease the jet/cup width ratio, and bring the jets closer to the base of the cups (within 2.5-3.5d).

However, doing this will then impact higher flow performance. One won't see quite the gains that is possible at higher flow rates.

Co-incidentally, the Murks 3.1, from what I can see of it, does just what I'm highlighting as issues for low-flow/pressure setups.

Originally Posted by Jabo

3. Results are just numbers and like with all statistics one reading it has to know very well how to interpret results or not be suprissed arriving at incorrect conclusions.

To summarize, as Cathar stated above, there are no universal designs and every piece of h'ware shows it's potential only if used within it's design perameters.

Exactly. Waterblock design is a game of trade-offs. For myself, I focus on the highest possible design performance, accepting that in doing so I am sacrificing low flow performance.

I do still believe though that the results with the Cascade at WCP are at the very least between 1-2C worse than I would have expected, but given that the block had been modified by the user with no guarantees of anything after that event, then I guess anything is possible. The Cascade as it ships is still a very powerful low-flow performer, but I happily accept that had I been focussing on lower-flow performance then it could be made to perform better in that scenario.

Changing the base-plate to 2.5-3mm thickness, dropping the jet/cup ratio to 2d (presently above that), dropping the jet standoff distance to 3d (presently significantly above that), and increasing inter-cup area ratio slightly, would yield a design that at 100W would probably pick up 1-2C or so at 1-2LPM flow rates, but we'd see much flatter performance curve beyond 4LPM at which point the present Cascade design would overtake it.

Horses for courses.