[Stoves] Are we being bluffed....or no?

[Paul Olivier]

See comments.

On Mon, Mar 5, 2012 at 12:57 AM, Richard Stanley
<rstanley at legacyfound.org>wrote:

> Andrew Crispin,
>
> And this (bluff body) in itself: Is it  is enough to explain Paul's noted
> reduction of boiling time by half ?
>
> Richard
>
> On Mar 4, 2012, at 8:23 AM, Alex English wrote:
>
> Hi Crispin and Paul,
> Bluff bodies (BB) are refered to in fluid dynamics generally   for which
> combustion of gases is a subset. The definition of a BB
> "a solid obstruction in a fluid stream, having a broad flattened front and
> providing a shelter for small scale turbulence and zones of low velocity; a
> stability assister."
> (from the  North American Combustion Handbook (Vol 2))
>
> So does a screen qualifies as a bluff body? It can be a flame holder as
> can a single wire in the path. We have  a screen burner on our  propane
> refrigerator but the flame is down sream of the screen. I don't think the
> screen glowed.
>
> Paul's burner appears to have good flame holding at the holes in the
> burner head. He mentions  the screen protects these flames from the wind
> and that combustion is below the screen.  So in this case it is not clear
> that there is any combustion happening in or on the outer surface of the
> screen although it could be invisible to our eyes.


If the fan is turned too high, gas will exit the wire mesh dome and burn
above it.
This is a situation I try to avoid.

The burner housing supplies hot secondary air to the burner.
At the same time the wire mesh restricts the flow of air and gas through
the burner housing.
A dome of a fine aperture restricts more than a dome of a coarse aperture.
It all behaves as a single flow.
Getting the draft created by the housing to match up with the aperture of
the dome is not easy.
I do not think that I have optimized this in any way.

It seems logical that the dome reflects thermal radiation downwardly as
much as it reflects upwardly.
All that goes down obviously does not hit the pot.
I want to place a piece of aluminum foil about 3 mm from the bottom of the
burner.
Hopefully this will reflect back up a lot of the thermal radiation that has
been reflected downwardly by the dome.

A dome is 3-dimensional, whereas a circle is 2-dimensional.
A dome has a greater surface area than a circle of the same periphery.
Would it be correct to say that a wire mesh dome reflects more thermal
radiation than a flat wire mesh circle of the same periphery?

Note that it is the diffusion tail that burns within the dome.
If there were no diffusion tail, then the wire mesh would not have to have
the shape of a dome.
The diffusion tail requires a 3-D dome to burn correctly,  and, instead of
being a liability, as it is so often perceived to be,
it now becomes an asset insofar since it necessitates the construction of a
3-D electromagnetic emitter.

Does this make sense?

Thanks.
Paul



> So you could say that there are two screens, the holes in the burner head
> being the first screen, the BB perhaps, and the second is simply a
> radiator/wind screen. The small pressure drop across the screen also helps
> distribute the flow evenly.
>
> All in all,  great result= great design.
>
> Alex
>
>
>
> On 04/03/2012 8:26 AM, Crispin Pemberton-Pigott wrote:
> > Paul, what you are describing are the effects of a 'bluff body'. It is
> why they are sometimes used above a flame.
> > Crispin
> >
>
>
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-- 
Paul A. Olivier PhD
27C Pham Hong Thai Street
Dalat
Vietnam

Louisiana telephone: 1-337-447-4124 (rings Vietnam)
Mobile: 090-694-1573 (in Vietnam)
Skype address: Xpolivier
http://www.esrla.com/
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