Quote:
Originally posted by Long Haired Git
I read Cathar saying that the actual measurement is between 0.6mm and 0.8mm per fin. Or was that channel? Cathar?
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Huh?
1:0.75 ranging to 1:1.5 came from a research paper I read, and seemed to match up with my anecdotal experience.
It specifically applies to (mini)-channelled designs - being channels less than 2mm in size. With channels larger than that you can basically forget chasing leading edge performance.
It does not apply to the impingement cup design.
0.1mm channels/fins would result in a waterblock who's pressure drop is WAY too high. The block would perform quite poorly unless you had a gear pump applying ~50PSI to it to get sufficient flow. If in a White Water style design, you'll also lose the breadth of the jet impingement region. Part of the reason for the width of the White Water's jet is because it dictates the resultant width of the impingement region. Making the block shorter would also alter the width of the impingement effect that is responsible for a fair portion of the block's extra "push" up the performance slope.
The area of effect over which the sprayers operate is determined by working out how far the heat will spread for the largest CPU die you intend to cool. Currently that's the P4 with an IHS. With a thin base-plate, you need to focus on about a 16x16mm area. For the longer (~14mm) off-centre barton dies, the area also needs to be about 18mm wide, so a 16x18mm area is the focus here which works well for the CPU's current and forwards looking.
The next step will have over 50 sprayers in a slightly smaller area. Hitting the limits of conventional machining here. The goal is to pack as many into the target area as possible.
The total pressure drop for the sprayers is worked out on the total orifice area open for the pump to push through. Through experimentation I've determined what I believe to be the best trade-off of flow vs pressure-drop vs jet velocity, and the total orifice area which gives me that best trade-off point. The target total orifice area divided by the number os sprayers dictates the orifice area that each sprayer should use, and simple geometry then tells me the diameter of each sprayer hole to meet that requirement.
The pump parameters are important, but I don't tend to work in those terms directly. I work more by guaging what seems to be working for real actual pumps that people actually use, and finding the best balance point for them.
Best base thickness can determined by trial and error fairly easily. It is directed tied to the convectional efficiency of your design. Lower convectional efficiencies require thicker bases. Using the results of established research, it's possible to estimate your convectional efficiency, and establish a good starting point to try with the base-plate thickness to suit.
Turbulent water helps to disrupt the boundary layer that forms along the metal surface. The boundary layer is a thin slow-moving insulative layer of water that hugs the wall and reduces thermal transfer efficiency.
Jet impingement is extremely effective at stripping away the boundary layer by blasting a narrow focussed jet of water at a surface. Where the jet impingement in most effective there is effectively no boundary layer occuring. This is the region of extremely high thermal transfer efficiency, far higher than simply trying to brute force the water into turbulence by using a bigger pump to make the water go faster.
Well, that's my take on some of the answers to the questions.