[Stoves] Heat transfer and in-line water heater

[Crispin Pemberton-Pigott]

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