In my reading of various papers, those that achieve k~=1 factors are doing so at flow rates so low that the thermal capacity of the water becomes a major limitation. I've pretty much always considered pursuit of "k=1" as a bit of a misdirected quest for the holy grail as in "Great! k=1 but now the water's boiling!"
Quote:
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A maximum surface heat flux of 310 W/cm2 was achieved using water jets of 173.6 μm diameter and 3 mm spacing, impinging at 12.5 m /s on a circular 19.3 mm diameter copper surface. The impinging water temperature was 23.1°C and the surface temperature was 73.9°C.
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So if we calculate that, the average convectional rate is ~61000 W/m²K. That's nice but that's also against a totally flat surface so h(conv) == h(eff). Compare that to even my conservative (by Les's statements above) estimate of h(eff) as being ~105000 W/m²K and 61000 W/m²K isn't that spectacular. Sure, k is going to be a heck of a lot closer to 1 in the micro-jet implementation, but that's rather pointless if it can't deliver a better h(conv) than ~61000 W/m²K.
I'm all about lowering temperatures as far as possible, and doing so without the theoretical constraints that some of these guys place over themselves of achieving close to k=1 efficiencies. Once I see research papers start to discuss k efficiencies I take that as a pretty good sign that they're on the wrong track for purposes of maximising cooling performance. It's a false grail IMO.