Quote:
Originally posted by satanicoo
And the impingement is the best kind of turbulance to break the boundary layer. It simply breaks it and "new water" (fresher) comes in contact with copper.
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Agreed. Although I dont really view impingement as turbulence. Impingement doesn't necessarily require turbulent flow for it to be effective.
Impingement is costly though in terms of pressure drop, but there is no free lunch here.
A further part of the Cascade's impingement effect is the creation of turbulence in the incoming jet, at the cost of some of the jet's power. When the jet rushes down into the cup, it shears against the water coming back up out of the cup. In this fashion the Cascade uses the cups to both create a dual impingement effect at the middle base of the cup, and the cup walls, as well as churn the water up to more effectively scavenge any heat that makes it up the cup walls.
Again, I am struggling to find the exact link to the paper that I read, but basically it was showing that the effect of extra turbulence in the jet stream by standing it off from the base and having it shear against the water around it was actually more effective at stripping the boundary layers despite the force of the jet's effect being slightly reduced. I believe that I've been able to mimic and observe the paper's statements through experimentation myself. The paper was talking about simple submerged jet behavior and not really talking about jet-in-a-cup behavior which is something else that I had to explore.
So basically if the jet is too close it doesn't gather any turbulence as it descends. The further away the jet is, the more turbulent the jet becomes but the more power the jet loses. There is a cross-over point between the two effects where as the jet is moved yet further away, the added turbulence is no longer enough to offset the loss in the jet power. The actual ideal distance is related to the jet diameter and the jet velocity, with lower velocities achieving better performance with closer jets, and higher velocities with more distant jets. Picking a design point amongst all that mess of variables is the challenge.
Anyway, the end summary is that it's all about impingement and turbulence and attempting to find the right balance in the design to make the most of the "typical" pumping pressure that gets applied. It's about stripping away the boundary layer most effectively. No matter what you do, the boundary layer is always there, and the best we can hope for is to reduce it as much as possible across as much of the surface area that's closest to the CPU heat as possible.
There is danger in going too extreme though in that in much the same way that water shapes mountains and valleys out of granite for the Earth, metal erosion can become a factor.