Re: Constant area

["Crispin" <crispin@newdawn.sz>]

Dear Ken (and Dean)

My late brother Andrew, a historian, would say that it it the historian's
role to say, "It is really not quite as simple as that,"  so I now quote
him.

>My mentor, Larry Winiarski, discovered a long time ago that tapering the
gap
>under a pot, decreasing towards the rim, assists better heat transfer and
>therefore more even temperatures.

I think the emphasis there should be on the words 'more even'.

Because of the nature of your application I suggest the follow for
consideration (not a total solution):

First, make the steel plate as thick as possible, especially as it is a
comercial application.  The heat loss getting it hot in the morning is
amortized over a long cooking day and the cost will be well worth it if it
helps cook over a larger area.  The thinner the plate the worse your temp
variations will be no matter what else you do.

Second, one of the simple ways to greatly reduce the temperature right over
the fire is to put another steel plate under that spot.  This is not a
matter of thickening it, but of 'hanging' another plate with an air get
between them.  The heat protection so provided varies with the distance it
stands off.  Try a 200mm diameter 4mm plate 15mm down. The little plate will
get very hot (I would use some sort of stainless like an old pot bottom or
lid) but it will only pass a certain amount of heat through to the point on
the top directly over the fire.

This plate conserves heat in the flue gases so that as they get farther away
they are hotter than they used to be.

Third, as the gases get farther away they cool down and conduct less heat to
the plate.  This has to be factored into your calculation making if FAR more
complicated.  For example, if you follow the method you described, the gases
would drop from, say, 750C to 250C as it passes under the plate from center
to chimney.  Clearly even though you are letting the gas touch the plate for
the same amount of time at each point by tinkering with the gap, you should
also have to slow it down so that it has more time to transfer remaining
heat to the plate as the temperature is lower and the heat transfer rate is
lower.  Nifty, huh?  So in fact the gap should be larger (to slow it down)
than the simple calculation of 'equal areas' (as I call that type of
solution).

Fourth, as the transfer of heat is greatly enhanced by turbulence, you
really should not have a smooth taper between two surfaces but rather have a
series of 'annular rings' each of which is progressively higher than the
previous (inner) one thus reducing the gap at that point only.  This would
look like a stadium seating arrangement.  One might call it a Stadium Flue
Control.

Now combine that idea with the change in depth to speed up the flue gases,
and you will have a series of donut-shaped chambers that get shallower
either towards the outside (or the inside - it is not yet clear).  They will
create chambers where the heat can 'rest a while' to mix and then rush
through the gap to transfer heat to the plate.  If the first ones are small
and shallow with a large clearance to the plate's underside, the hot gases
will speed by, and then as they cool down, they will linger longer
languishing around to dump their final heat upwards.

You can make these rings by planting a sheet metal strip into a clay
undersurface with the plate supported above.

Because the calculation is so complex, I suggest you simply start and
experiment a bit to see what you get.  My suspicion is that you should be
able to use an inexpensive combination of a small reflector plate directly
above the fire and annular strips around it to move the majority of heat out
towards the periphery getting a large percentage of the plate to a
surprisingly even heat, given a steady fire.

Regards
Crispin


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