Surely your first though is the non dimensional numbers as they give you all the important parameters. Alright you mihgt need an intial "good" block to work from but it should help.
the modified reynolds analogue/chilton colburn analogies state that the averge boundary layer thickness is
St*Pr^2/3 for hydrodynamic condesiderations
and
St.mean*Sc^2/3 for themral boundary layer
[sc = schmidt number, ratio of momentum and masss difusivities. Cc=kinematic viscosity/D)
From that you can guess a good width of channel. Fine equations are known so that can be approximated well to start off with as well. Fancy stuff like jet impingement cant be done first off, but jet impingement numbers are alwasy given dimensionless and you should be able to scale results using the St number.
That tell yous you how big to make the fins on a block so that a good amount of fluid is going to get through. For a specific gemometry it allows you to scale sizes up and down.
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