Heatsink color

[zappedagain]

Does a black aluminum heatsink have a lower thermal resistivity than a machined aluminum heatsink? A colleague put a conformal coat on a heatsink and saw the performance improve (65C rise instead of 90C). What explains this?

Radiated heat?
Surface finish (area)?

The heatsink is on a bench in lab in an ambient temperature of 25C dissipating 7.7W.

Is that why OTS heatsinks are typically (always) black anodized?

Thanks,

John D

 

[KENAT]

There's a lot on the internet about this.

As far as I could tell the black anodized gives a good compromise for convective & radiation cooling relatively cheaply etc. for most applications.

That doesn't mean it's the best, or even most cost effective, for every application.

Posting guidelines faq731-376

http://eng-tips.com/market.cfm?

(probably not aimed specifically at you)
What is Engineering anyway: faq1088-1484​

 

[corus]

Must be radiation as convection has nothing to do with the colour or surface finish. Polished aluminium has an emissivity of about 0.1 whereas blackened plate will have a value close to 1.

Tata

 

[IRstuff]

Black anodize is preferred, because:
> Anodize is thin enough to not impede convection
> Higher emissivity results in higher heat loss; there is no downside on this

TTFN

FAQ731-376

 

[ione]

When heat transfer takes place throughout both natural convention and radiation, these two processes may contribute each one with approx the same order of magnitude. So radiation definitely plays a not trivial role when coupled to natural convection and in this case both the surface finishing and the colour improve the radiative heat loss. On the other hand when forced convection comes into play, adopting black anodized solution won’t produce such that improvement.

 

[ione]

In my previous post I have forgotten to say that black heatsink are probably not the best option also when there are around other emitting sources, in the vicinity of the heatsink itself, as a black heatsink not only emits more radiation but can also absorb more radiation.

 

[HDS]

The conformal coating could have improved the heat transfer between the part and the heat sink (or the 2nd installation was better than the first).

 

[zappedagain]

HDS - the part and the heatsink and the temperature sensor were all conformal coated at the same time, so there was no assembly variation.

Thanks for the responses, all!