[Stoves] Heat transfer and in-line water heater

[Jeff Davis]

Dear Tom,


A true treasure chest of an e-mail, indeed rare!

You wrote, "weaning us from cheap oil", I would add that the cost is more than 
a dollar amount that we pay for oil. Biomass could enable the many to be free 
from the control of the few.

You also wrote, "But it is directly proportional to the flame velocity.", this 
explains why my noisy forced secondary air producer gas burner can heat metal 
to a red color.



Many thanks,


Jeff

  

On Sunday 09 December 2007 10:18 am, Thomas Reed wrote:
> Dear Crispin and All:
>
> You can quote me that "Biomass Thermal Conversion is much more complex
> than Nuclear Energy".  We managed to understand the principles and
> practice of nuclear energy from fusion to fission in one 20th century.
> In 1000 millenia Humans discovered fire and thermal conversion, but more
> is still being discovered than all past.
>
> We need both "Top Down" and "Bottom Up" thinking to crack this important
> part of weaning us from cheap oil.  You have made an excellent attack on
> the complexities of heat transfer and a plea to become quantitative by
> Crispin here.
>
> Working at the Linde Air company on flames and arcs in my youth, I was
> fascinated by the intricacies of heat transfer in flames and arcs ...
> and now in electron beams, lasers, pulse jets, ....
>
> We need first to know about all of these mechanisms, then to apply them
> to BIOMASS THERMAL CONVERSION.  We can study the theory of heat
> transfer, but we can also learn a lot by observation.
> ------------------------------------------------------------------------
> THEORY (Top Down Thinking)
> In most general terms, heat transfer from convection and conduction
> increases proportional to the temperature difference between the source
> and the sink.     Heat transfer from directed flames is proportional  to
> between the 1/2 power and first power.  But it is directly proportional
> to the flame velocity.
>
> Qc(T, V) ~ (T2-T1)*V
>
> Well mixed air flames all have a temperature ~ 2000C +/- 200 C.
>
> But heat delivery is proportional to both temperature difference AND gas
> velocity.  The velocity of flames is proportional to the "laminar
> burning velocity" which is ~ 40-50 cm/s for all hydrocarbons, but 300
> cm/s for hydrogen and acetylene, so both acetylene and hydrogen air and
> oxygen flames can deliver much more heat than methane, propane or other
> flames.
>
> Oxygen flames have a temperature of ~3000C +/- 200 C and based on
> temperature alone, the heat transfer from an oxygen flame should only be
> 50% higher than from an air flame. The flame velocity of oxygen flames
> is typically 5-10 times that of air flames and so heat transfer can be
> 20 times that of air flames, hence the use for brazing and welding.
>
> But radiation heat transfer increases as the FOURTH power of temperature
> and can travel significant distances.
>
> Qr(T) ~ T2^4
>
>
> ------------------------------------------------------------------------
> OBSERVATION (Bottom up thinking)
> Since hot gases have very little mass, they have to be 100 times as
> large as a solid to radiate like a solid.  Put your finger next to a
> candle flame and you can barely feel the radiant heat.  Now pass your
> finger quickly through the flame :-P and you will see that the direct
> radiation from small gas sources is negligible by comparison to direct
> convection.  Compare the radiation from a blazing fireplace fire and the
> coals that are left when the blaze dies down.  You will feel MUCH more
> heat at distance from the coals than the bigger flame.
>
> Now if you substitute a blow torch for the candle flame (no finger test)
> you will increase heat transfer 10 - 100 times, since heat transfer is
> directly proportional to flame velocity.
>
> There are many refinements on the above observations, but the
> combination of "top down" and "bottom up" thinking will move you toward
> the solution much faster than either alone.
>
> Yours truly,
>
> TOM REED                The Biomass Energy Foundation