[Stoves] the 150 gasifier in operation in Vietnam

[ajheggie at gmail.com]

On Tue, 6 Mar 2012 09:26:39 +0100, Crispin Pemberton-Pigott wrote:

>
>>...as it is in the flame we can assume the conduction and convection have
>little effect. 
>
> 
>
>Wow. Well, I need to think about that one. From the things I have calculated
>I estimate the convection to be a large fraction even for things that are
>not very hot so I might need to go back to the spreadsheet to look at
>something this hot.

OK I was assuming a case where the wire was not extracting a
significant proportion of the flame's energy. To get hot by conduction
from the flame to the wire, and neglecting any boundary effects, there
has to be a temperature difference. i.e. the flame has to be hotter
than the wire and hence the gases after the wire are cooler than
before. both gas streams before and after the wire will remain hotter
than the wire so the wire will not contribute heat to them.



> 14% comes to mind for some reason, for radiation. I
>guess you could also say that the colour is a measure of the gas temperature
>if the material is radiating poorly. The more efficiently it radiates, the
>larger will be the difference between the gas temp and the material doing
>the radiating, no? It is effectively a heat sink.

I'm not sure I understand you but see above, the better the wire
radiates the bigger the difference in temperature between the gas from
the flame and the gas after the wire.
>
> 
>
>My point (sort of) is that yes it is radiating but not very efficiently in
>the IR band, which makes it so loud in the visible.

I take it by the IR band you mean you can feel heat from the steel but
not discern any glow? If so then this suggests the steel is below
450C.


> As the emissivity does
>not change much with temperature (in spite of many opinions to the contrary)
>a shiny silver part will not emit well in the infrared so the temp keeps
>going up.

Yes, the objects temperature will continue rising in the hot gas until
it radiates the same amount of heat as it is receiving, so a poor
emitter will rise to a higher temperature before it reaches
equilibrium.
>
>>The only conclusion I can see from this is that at equilibrium a poor
>emitter will become hotter than a good emitter.
>
> 
>
>Exactly. And the worse, the closer it will get to the gas temp. Yes?

Yes, I am unsure how close it will get though my guess is a few
hundred degrees lower.

AJH