Thermal resistance model for two heat sinks in series 1

[Tunalover]

Folks-
I've been searching the web and Eng-Tips for some hints on how to handle this seemingly simple problem. I have two identical plate-fin heat sinks mounted in line on an insulating surface. There is a substantial distance between them so that the air volume between them needs to be considered. On each heat sink are two power-dissipating devices mounted to the flat side opposite the fins. The airflow is contained in a duct slightly larger than the envelopes of the heat sinks so that the airflow is directed through the fins without loss.

I know the junction temperatures of the upstream components (knowing the flowrate, resistances, power dissipation, and temperature of the incoming air). But how do I quantify the temperature of the air flowing OUT of the upstream heat sink and into the downstream heat sink?

Thanks in advance for your help. This is NOT an academic problem.

Tunalover



Bruce aka Tunalover

 

[IRstuff]

?? I guess I'm not understanding your question very well. The downstream air temperature must essentially be the result of the Joule heating of the air from the heat being removed from the first sink.

TTFN
faq731-376

 

[Drexl]

Yeah i don't follow. Are you asking what the temperature of the air is after you have heated it with a known amount of energy?? Maybe a picture would help..

 

[IRstuff]

perhaps that's all you're asking:

volumetric flow * density * specific heat --> watts/kelvin

divide the power dissipation of the first sink by the above gives you the ideal-case temperature of the downstream air. However, since it's unlikely that the air is perfectly mixed by the time it hits the downstream sink, the apparent air temperature might be considerable higher. Note that this also requires some determination of the amount of heat lost in the ducting between the two sinks.

TTFN
faq731-376

 

[Tunalover]

Drexl and IRStuff-
The first heat sink heats the air up to an average temperature. What is this average temperature? I could use Q=mdot*Cp*delta T on the air but what temperature terms make up deltaT? Is it simply Tout-Tin? Somehow the average temperature of the heat sink comes into play doesn't it?
Tunalover

Bruce aka Tunalover

 

[IRstuff]

Again, it's all in the convective heat transfer. Q=area*h*deltaT. The end result should still lead you to the equation I posted before.

TTFN
faq731-376

 

[Tunalover]

IRstuff-
Thanks for the inputs. You hit the nail right on the head.
Regards,
Tunalover


Bruce aka Tunalover