Re: ATM 3-Vane or 4-Vane Spiders

["Arjan te Marvelde (ELN)" <Arjan.te.Marvelde@eln.ericsson.se>]

Nils Olof Carlin wrote:

> I believe (until somebody offers a better explanation) that the temperature
> of the vane is an equilibrium between heat lost by radiation and heat gain
> from conduction through an essentially stationary boundary layer of air.

Very much so. 
Surface temperature (and for practical vane temperatures also the radiative heatflow) is solely determined by the surface absorption coefficient.
Insulation only affects the time domain; it takes longer for the interior to cool down and the surface itself cools quicker. The ultimate vane temperature is identical to the non-insulated case (with equal absorption coefficient) and so is the air boundary layer.

So the only advantage of using insulation is that the air boundary layer establishes more quickly.
As Nils Olof points out, the only (passive) way of obtaining a higher vane temp is keeping the radiative flow down, i.e. by using a high reflection surface.

However: is the air boundary layer really static? I would say that, depending on the vane geometry, you can expect more or less convective effects. 
If you have a classical blade-shaped vane, there is relatively much surface to cool much air to several degrees below ambient, which will start flowing. Thinking back of last years' S&T article about mirror cooling (where 1 deg of temp difference in the air was very well visible in schlieren images) this does seem a legitimate reason for concern. 

With the choice of materials we have, i'd say that surface reduction probably has more effect than reducing the radiation.


/Arjan te Marvelde