[Stoves] Heat transfer and in-line water heater

[andrew]

To quickly explain the different look of this post from the last, 
I've been moved back to my own computer by another family member.

On Thursday 06 December 2007 21:17, Crispin Pemberton-Pigott wrote:
> >...it looked like the maximum radiation
> >from a stoichiometric wood burn at 1600C in a combustion chamber
> > 150mm diameter was 13kW(t), use wet wood and increase excess air
> > such that the combustion chamber dropped to 800C and you reduce
> > this tenfold.
>
> Very impressive. 

It is a theoretical figure, I'd hoped someone would come up with a 
likely figure.

> On a shiny pot it might be greatly reduced 
> (another 50%?)

Who knows? The little infra red thermometer I use assumes E=0.95 and 
in practise a paintend radiator doesn't read much different from a 
copper pipe.

>
> I was trying to point out that if you absorb all the heat
> available in the first 5 cm of pot radius, there isn't much left
> to collect. 

OK but it looks like we are looking at 20-30% heat transfer into the 
pot, that looks like quite a lot of scope for improvement, if we're 
wasting it it must be measurable as temperature of flue gas above 
the pot.

> Increasing the heat transfer rate in the region above the flame
> will only make things worse and leave less heat to be absorbed at
> the edges.

I'm not a cook but intuitively I'd disagree with this.
>
> Increasing the gap between the flame and the pot will reduce the
> rate of heat transfer in the centre of the pot and all more even
> heating of the whole bottom be allowing the edges to pick up more
> of it.

Or by allowing more flue gas re circulation?

> Something one must think of when doing this is that the added
> height affects the excess air volume and thus lower the
> efficiency, not because of a lower gas velocity, but because of
> more heat lost to more unneeded air flow.

I cannot see why this would result in more airflow, I grant it will 
increase the volume of the stove between flame and pot, and hence 
the propensity to lose heat through the sides.

> Increasing the gap and 
> reducing the EA can result in lower gas velocity, a lower heat
> transfer rate immediately above the flame, and a higher thermal
> efficiency, all at the same time.

Yes cutting massflow by controlling excess air will up the 
temperature and decrease flue gas velocity.

I cannot answer your questions about the boundary layer and what 
attaches it to the pot, it may be weak forces but equally it could 
just be drag, what we do know is that it can be modelled as a poor 
conductor in series with the metal of the pot.

AJH