Air flow for a heat sink

[sn0wman]

Hi,

I am working on a project where I incorporated a parallel finned heat sink. The design right now has air flow, from a fan, parallel to the fin with regards to length. I now need to place the fan above the heat sink and have the air flow directly down on to the surface. A team member is telling me that the dissipation will increase with this design. I disagree with him and just wanted someone to back my assumption before I pull out my heat transfer book and crank out some numbers. Any help would be appreciated.

snowman

 

[pablo02]

I tend to agree with your colleague that the dissipation will increase in that you should have more surface area exposed to the air flow...

 

[riceboy]

I agree that the dissipation would increase unless your heat sink is undersized, can your heat exchanger be better designed to better utilize the air flow to it from above?

 

[hacksaw]

in "parallel" alignment the fins see an LMTD air temp while in the down flow more of your finned surface sees cold (unheated) air, but at ~ half the flow (half in each direction for example).

generally more efficient heat transfer possible in down flow, but you may need to increase the fan capacity to achieve dramatic increase in heat removal(dissapation).

 

[sn0wman]

Hi,

Thank you very much for the input. I cranked out a few numbers and the air flow from above was the better choice. thanks again.

 

[julrx]

hi, may i ask you what are the formulas you used to make the calculations to prove that?

 

[cbiber]

julrx,
I've written a paper and article on the subject. It's at

http://www.coolingzone.com/Guest/News/NL_NOV_2002/Biber/Biber_Nov_2002.html

with an example at

http://www.coolingzone.com/Guest/News/NL_DEC_2002/Cathy/Biber_Dec_2002.html.

One of the big reasons air flow from above is better could be that this configuration eliminates fan air flow bypassing the heat sink fin array.

Cathy Biber


Biber Thermal Design

http://www.biberthermal.com

 

[hacksaw]

Good suggestion Cathy, but the original questioner has been asked about his new found knowledge. Knowledge that would have obviated his question in first place.

 

[julrx]

Hi, just another question about heat sinks... if i would like to optimise a heat sink design, is even 1% of improvement justifiable?

 

[chicopee]

Heat transfer formulae for finned surfaces are pretty well established but my question is what values for convective heat transfer coefficients did you use for air flow parallel to the length of the fins and perpendicular to the fins when you positioned the fan above the fins and were these values, bulk values?.