Quote:
Originally posted by gone_fishin
I have a question. Once the boundary layer is gone at X velocity, is there any point in going faster? It will expand the no boundary layer region but here it is being limited by a cup anyway.
|
Yes, it does benefit to go faster as more of the water comes into contact with the metal directly as the increased downwards velocity will increase the chance the water molecules that would not have made it to the metal will now actually reach it.
There's a limit of course, which happens when every single water molecule comes into contact with the metal and each molecule absorbs the same amount of heat each. This would be "perfect" convection within the limits of the coolant itself, which is what you're talking about. We're not close to that limit though (but are getting closer).
Perfect convection will occur when the heat source rises by exactly the amount caused by the thermal paste layer, the metal conduction interface, and the coolant rise in temperature as a result of the coolant's flow rate.
If you graph that on a flow vs C/W chart you get what would be the absolute limits to waterblock efficiency. Given that we know the TIM resistance and can work out the metal conduction resistance, we can then plot a real data graph of our waterblock much like BillA has done in his tests.
Doing that for the White Water shows it to be still quite a long way enough, despite it being an impingement design, but it grows closer to the "perfect" line the more the flow rate is increased, showing us that increasing flow rates still helps, even though the boundary layer may have gone.
The theory papers talk about the stagnation region convection efficiency in terms of the Prandtl and Nusselt numbers, where higher is better, and higher is achieved through increasing the flow rate.