[Stoves] Heat transfer and in-line water heater

Sun Dec 9 10:18:12 EST 2007

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

Crispin Pemberton-Pigott wrote:
> Dear Calculating, Heat Transferring Friends
>
> I think it was Andrew who wrote
>   
>> I'm told that pulse combustors are actually able to disrupt this
>> boundary layer but in general the boundary layer is the limiting
>> factor in convection.
>>     
>
> It recall having a conversation about these a few years ago on this list and
> someone mentioned being accused of having a machine gun hidden in the woods.
>
>
>   
>> Of course despite Crispin not considering radiant heating being
>> significant it is not limited by this boundary layer.
>>     
>
> Let's not over-react....
>
> Let's see some numbers attached to all  these claims.
>
> I think Dean wrote
>   
>>> Higher velocity flue gases get more heat into the pot.
>>>       
>
> We have to separate the very different concepts of thermal EFFICIENCY and
> the RATE of heat transfer.  They are very different and are confused with
> abandon in these conversations.
>
> Increasing the velocity will increase the heat transfer RATE in J/sq cm /
> second.  This is like the wind speed reducing the 'effective' temperature to
> give a wind chill effect on a winter's night. A wind chill of -50 does not
> mean the temperature is -50 and it does not mean to tip of your nose will
> drop to -50 deg. It only means that the rate of cooling will be AS IF it was
> -50 deg without wind.
>
> Increasing the gas speed and therefore rate of heat transfer does not
> increase the amount of heat available and it does not increase, on its own,
> the % of heat transferred from a fire to a pot.  Further, the increase in
> speed, while changing the rate of heat transfer, does not do so by
> increasing the effectiveness of heat radiated from somewhere to the pot.
> That part of the pot that 'sees' the fire can receive radiation from it.
> There is some heat radiated from hot stove components to the pot. Changing
> the gas velocity will only help increase the rate of heat transfer by
> convection and by a tiny amount related to conduction through the 0.1mm
> layer of gases against the pot often used in modelling that is assumed to be
> conductive only.
>
> Increasing the gas speed for any given set of dimensions will a) increase
> the RATE of heat transfer per second to any given area of surface and in
> nearly every case, reduce the overall heat transfer EFFICIENCY.  Like
> putting you foot down on a car's accelerator, you can go faster, but at a
> reduction in efficiency.
>
> It is quote true to way you can transfer more heat by changing only the gas
> velocity, but not unless you add more fuel to the fire, or unless you change
> the gap instead, you can't win.  However velocity is not always a winner.  I
> just returned from Malawi to find an interesting example of decreasing the
> velocity of the gases by changing only the volume of gases, and increasing
> both the heat transfer efficiency and the rate of heat transfer.  The reason
> is that the length of the gas path was sufficient to take advantage of the
> change in gas volume.  Increasing the velocity by increasing the excess air,
> not only increased the gas velocity, it reduced the combustion temperature,
> reduced the amount of heat transferred, and it also increases the
> temperature of the exhaust, wasting more heat x more air.  The change in
> overall performance was a difference of 4-fold! With the high velocity and
> high excess air it was 17% efficient. With the low excess air and low
> velocity it was 70% efficient.
>
> In that case more velocity = less heat transferred.  Simple as that.
>
> If you fix the speed of the gases and change the DIMENSIONS of the stove,
> then you can get quite different results.
>
> If you fix the dimensions and change the VOLUME of gases (by reducing or
> increasing the excess air ratio) you also get different results.
>
> There are ways to optimise the whole thing of course which is what heat
> exchange designers do.  A stove is nothing more than a fuel-burning heat
> exchanger and follows the same rules.
>
> Regards
> Crispin in a snowstorm Waterloo
>
>
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