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Originally Posted by Les
Here we have the h(conv.coeff) relation with h(effective) problem |
Yes, I couldn't agree more. It's something that I constantly find myself beating my head against. Whilst a very high conv. ceoff. may be very attractive, sometimes it doesn't produce the highest effective h, and I believe that the Cascade in comparison to a "classic" impingement block is a very good example of this.
The Cascade offers more "useful" surface area, and despite, to my best estimates, offering a convectional h of around 80% of that of a "classic" impingement effect, achieves more effective cooling.
Something I struggled long and hard with was the inter-cup geometry. Decreasing inter-cup spacing puts more cups into the same area, but now there is reduced vertical (and to some degree lateral) heat spread, and performance goes down. Put too much inter-cup spacing, and you manage to engage more of the cup walls as part of the cooling effect, but performance goes down because the spreading resistance goes up.
I tried hard to computationally model this effect based on emperical evidence, but was unable to achieve anything that I had any confidence in. Basically I found myself falling back to established measured good ratios. I found myself in that mode that Bill disdainfully describes as "measuring the data, and then assembling the theory to fit".
In the end I gave up, and just satisfied myself with approximated guesses as to h