Heatsink Cooled In Liquid Nitrogen 1

[dscbob]

Hello,

Can anyone offer advice on the design of a finned heatsink that will be submerged in liquid nitrogen to cool an analytical instrument? I am a mechanical engineer with minimal thermal experience and I am designing a coldfinger or heatsink that is attached to a heat source. The coldfinger must bring the heat source (block) temperature from ambient to minus 170 degrees Celsius and then 60 watts will be applied to the heat source in order to bring the temperature up to 200 degrees Celsius.
Can you recommend equations, values for heat transfer coefficient of liquid nitrogen or a design approach? Do I simply use the convection heat transfer equation Q=hA(delta-T)? Where A is the surface area of the coldfinger exposed to liquid nitrogen?

Thanks,
Bob

 

[ko99]

Q = hA(deltaT) is the correct formula but it's not so simple to determine h.

Here's an excellent article on how to do that, at least for air cooling. Maybe you can apply it to liquid nitrogen. It also has a good list of references:

http://www.electronics-cooling.com/html/2000_sep_a4.html

Or, if you have access to CFD software you can model it fairly easily.

ko (

http://www.ecooling.biz)

 

[EdStainless]

You are going to need a lot of surface. You have to keep the nitrogen from boiling, because if it goes to a gas you loose most of your heat transfer.
A large number of very fine Cu fins is the most effective approach. Soem of the ones available for aftermarket computer use use skived fins, they are literally shaved from a block of Cu.

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http://www.trent-tube.com/contact/Tech_Assist.cfm

 

[dscbob]

Thanks for the information gentlemen.

Should I be calling it a coldfingeras opposed to a heatsink?
Anyways.....Is there any advantage to maximizing the mass of the heatsink so it will store/maintain a cool temperature to help increase cooling rates when energy is applied?

~Bob

 

[ko99]

Never heard the term coldfinger, then again "heatsink" is a poor name for a hunk of metal that doesn't behave like a "sink" in heat transfer terminology...

Greater mass will slow the temperature change, if that's what you're after. Once at steady state, the mass doesn't matter.



ko (

http://www.ecooling.biz)

 

[IRstuff]

Coldfinger is usually applied to the point at which a Stirling cooler expander makes contact with the object being cooled, which is usually inside a vacuum Dewar.

TTFN