[cdt-l] Black pot = increased heat

[James McCreight]

was there a follow up to this? I havent been paying the closest attention...


anyway, yes, conduction, convection, and ratiation are acting on the pot to
heat it (or cool it). However, if it's windy, advection will be important.
to distinguish advection and convection: advection is something you fight
with a windscreen, convection you cant do much about:

convection: if the air is still, the air around the flame and pot heats. It
becomes less dense in warming and rises. It is replaced by cooler, more
dense air, which heats. This is why bedouins wear black in the desert, they
get a convective air flow through their clothes.
advection: Transfer of heat by wind not caused by heating the pot.

of course, this is being technical. often times you will find them lumped
into one (eg a decent link on the topic:
http://www.me.tulane.edu/Faculty/Zhang/Courses/ME371/Chapter12A.ppt )

what's important is what fraction of the change in temperature (per unit
time) is determined by each process; the total change in temperature is the
sum of the 4 terms of conduction, advection, convection and radiation.

What should be happening here is that the conduction term forms the largest
part of the sum - that is assuming we are heating from below with a flame.
The heat of the flame is conducted by the pot. The expression for heating
the pot, the rate of change of temperature, is positive; the pot warms up.
(what we are really interested in here is the temperature of the inside of
the pot, we want to raise this quickly.)

Note that advection, convection and radiation are probably all negative,
heat is being transfered from the pot to the environment; the pot is loosing
temperature via these processes. (The total heat of the flame in contact
with the pot will also be diminished by these, but let's just talk about the
pot, not the pot-flame system ;) The pot will always be loosing heat to it's
environment after cooking (unless you're on a really hot planet - but then
you'd probably be dead). This is because energy flows from high
(temperature) to low (temperature), the system strives for equilibrium. This
is also why heat flows from the flame to the inside of the pot.

Now advection and convection depend on conduction in a sense; flow of air
around the pot is likely replacing hot air with colder air near the surface
of the pot. This causes a greater temperature difference and causes more
heat to be conducted out of the pot. (Remember that the pot is not heated
uniformly, it may actually be loosing heat somewhere... )

Now note that if we heat the pot with a flame, the temperature difference
between the pot and environment will increase. So, in other words, as we
heat the pot the amount of loss from convection, advection, and radiation
will both increase!! (It's a nonlinear partial differential equation!) The
conduction will also increase as a function of temperature! (eg
http://galileo.phys.virginia.edu/classes/152.mf1i.spring02/HeatTransport.htm
)

What will be true at the temperatures we are operating at is that the
advection and convection terms will dominate, if it's not windy then there
will be no advection, just convection. We've all tried to cook in wind,
soups not hot! In the absence of wind we really have no problem heating up
the stove: convection is relatively unimportant as is radiation.

Now for conduction: we apply flame, the pot heats up accordingly. What
matters most in heating up the pot, or the inside of the pot, are the
properties of the pot 1) thickness 2) thermal conductivity. Considering we
keep the pot shape the same and only change the material, or the thermal
conductivity. Then you can look here, for example, to compare types of
materials and their conductivities:
http://www.engineersedge.com/properties_of_metals.htm The double edged sword
here is that material which transfers heat best will also transfer heat away
best! So your pot will cool down faster as well as heat up faster. (Think
about hooking in a ceramic dish vs a tin pan.)

So, assuming that our pots are equal size and shape but different matierial,
the difference in heating is govered by the thermal conductivity. The chart
shows that Al will conduct about 10x better than Ti, but those are for pure
metals. Who knows what alloys we are working with here....

Introducing a black surface will only change the radiation term by some
percentage. But this was already shown to be a small term. So it really wont
matter. Putting the pot in your jacket before you cook would probably do
more. Of course we could put numbers to this... but that would take a
while...

james

J




On 12/28/06, Mark Dixon <mkdixon1 at excite.com> wrote:
>
>
> Radiant heat is what you feel standing in front of a campfire. Conduction
> is heat transfer by direct contact, like if you reach in and grab a hot coal
> with your hands. Convection is the movement of heat with air currents, for
> example. I think cooking on a campstove involves radiation and convection,
> but not conduction. The other Marks tests with different materials and
> colors (not in direct sunlight) makes sense from what I remember from way
> back: that there should be no difference. I'll dig up my old text book and
> see if I can find something.
>
> Mark
>
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