Heat sink design help needed

[par]

I am designing two heat sinks one with fins and the other with Pin-fins that extrude from a rectangular base. The base is approx 20"x4"x0.5" lxwxthick respectively. The heat sink is A356 cast Aluminum and can be ussumed to have uniform temperature. I am attempting an analysis on the channels between fin/pin fins for each design. The pin fins have 2d air flow through the pin-fins while the fins have only 1d natural flow through the fins. There will be no forced convection just free. My question is should I perform a free convection analysis on rectangular air channels between fin/pin-fins as internal flow or treat it as external flow. Goal is to dissapate a given heat loss without increasing the surface temperature of microwave wave-guide the heat sink is fixed to.

Thank you for any help.
Regards
Phil Revell

 

[prodeng]

I think you have to look at the distance between the two fins. If they are close enough, you'll have to use internal and external characteristics (depending on edge area). I would base that on the boundary layer formed on each fin. I'm assuming you are familiar with these types of calculations. Good luck.