[Stoves] Heat transfer and in-line water heater

[andrew]

On Sunday 09 December 2007 23:21, Steve Taylor wrote:
> >> Temperature DIFFERENCE.
> >>    
> >
> > I don't think so Steve, that's true of conduction (where
> > adjacent atoms jostle each other to exchange energy) but
> > radiation is absolute and is related to the 4th power of the
> > absolute temperature.
> >  
>
> So do two objects at identical temperature radiate to each other ?

Yes but you also have to consider the size and shape of the objects 
and their albedo and absorption/emission characteristics.  

Each point on the surface of a body will emit radiation through all 
the angles it can see. In the case of the stove I cited I assumed a 
disc, so each point would radiate through Pi steradians.

Now the pot is likely to be larger than the hot bits of the 
combustion chamber but I think we can ignore the re radiation from 
the pot to the fire because it is very small, as the pot is cool and 
as you see it's a 4th power effect not directly proportional to 
difference in temperature.

The interesting bit is how the radiation changes from the incident 
radiation to the re radiated radiation, the hot body has send 
electromagnetic waves to the cool body which takes them up as energy 
in discrete amounts but the electromagnetic waves re radiated as 
discrete amounts leave at longer wavelengths, the greenhouse effect 
is caused because the longer wavelengths can no longer pass through 
the medium that was transparent to the initial incident radiation.

Consider heat transfer from pot skin to water, this is driven by the 
difference in temperature between the two materials and this will be 
the same if the pot is at 200C with the water at 80C ans when the 
pot is at 220C and the water at 100C.

> I thought thats why a black body radiator works, and why you can't
> see the inside of a fire very well

Yes The best black body is one that absorbs all the incident 
radiation, so a hole which doesn't let any incident light out 
becomes a good emitter when it is hot.

AJH