[Stoves] the 150 gasifier in operation in Vietnam

[Crispin Pemberton-Pigott]

Dear Alex

I base my answer on the experiences I had trying different flame holders on
the FSP stove, the same one you saw close up. 

The fact that Paul's wider spaced wire screen did not work is relevant.
Until the flow is interrupted enough to 'retain' the flame, the effect is
not pronounced. Above a critical threshhold, the effect of a BB becomes
evident, that is, when the flow is impeded. I found that a thin plate with
lots of small holes punched through with a plasma torch also worked but that
the CO was still not zero. The cause I suspected was the cooling of the
flame (flamelets) as they passed through the small holes. The wire gets hot
enough to actually assist the ignition. 

The visibility is an issue - a CO flame is not necessarily visible. Remember
the FLOX discussion we had a while back. 

There is CO at the screen that is not there 50mm higher up even when there
is no flame visible. 

The thin wire grate used on the downdraft coal stove at SeTAR employs the
same idea of assisting the ignition of smoke and CO with a hot wire, kept
hot by radiant heat from the flame directly underneath. It not however the
BB effect. Just the ignition. 

The propane heaters with a ceramic element do pretty much the same thing
(turning hot gas into radiant energy) without a bluff body by having
hundreds of very short flames coming through the ceramic plate which gets
red hot. 

Regards
Crispin in Johannesburg - summer!
------Original Message------
From: Alex English

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.  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