[Stoves] Heat transfer and in-line water heater

[Crispin Pemberton-Pigott]

Dear Andrew

The important bits:

>I don't see pulse combustors being viable as stoves, 

Agreed!

>I've had a few exciting moments with combustion experiments but not
>with wood, the most startling was with a series of small acetylene
>explosions, my ears did ring!

The fake machines guns on WWI biplanes work that way with acetylene and air
fed into a tube with a spark plug at the top end.  Sounds like a 303!

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

>>Increasing the velocity will increase the heat transfer RATE in J/sq cm /
>>second. 

>Which is the same as saying the power into the pot increases, for
>efficiency to remain the same the flue gases after the pot must be the
>same as the previous case.

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. For example
putting a black aluminum pot on a concentrated natural gas flame means the
food gets burned to the bottom centre of the pot and the edges are
relatively cold.

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.

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.  In such a case,
some of the time, the heat transfer efficiency will not drop as the exit
temperature could be the same.  In others, it might have a higher exit
temperature.

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.
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.

It is not as simple as changing only one and predicting a better or worse
performance.

I have seen very little discussion of the 'boundary layer' (however defined)
that included a thickness.  In normal engineering any part of the gas flow
that is influenced by a nearby body is considered to be in the 'boundary
layer'.  Dean frequently speaks of a 'stationary layer of air'.  There may
well be ionic attachment of air molecules to the surface of the pot.  This
is sometimes (not always) modelled as a layer 0.1mm thick which is
considered to be conductive only and all heat must pas through it.  French
engineers are trained that way.  Bejan in his recent convective heating book
does not insist on it for modelling though he agrees that it is becoming
more common to think that way.

If a pulse combustor is 'removing' or getting through such a layer, what it
the expected final thickness?  0.01mm?  Nothing going on in a natural draft
stove is going to accomplish something akin to that so the effect must have
to do with having a small enough gap to minimize excess air and not decrease
the dwell time (gas against pot) too much so the exit temperature does not
rise.

Thanks
Crispin