[Stoves] Nozzles for TLUDs Re: venturi system -ratios of air and gas?

[Julien Winter]

Dear Crispin;

Thanks for adding you points of clarification (below).

You are right that the Australian work applies only to a TLUD with
concentrator burners.  I have tried the same thing, and I get both lower
gasification rates, and an increase in  soot as concentrator ring diameter
is reduced.  If the mechanisms of gas combustion change down stream, then
we should not assume these finding apply, because they probably don't.

I have put a cap with a smaller hole on the top of risers, but I haven't
played with the hole size, nor measured its effects on gasification rates
or emissions.  I usually use out of my toolkit a 'Crispin pot' which is the
plate with a multitude of 2.5 cm holes in it.  This is especially important
if the diameter of the riser is wider than the TLUD or initial combustor.

Alex made an interesting point about how increasing the height of the riser
can increase the turbulent mixing  of gas with secondary air.  I have a
video forth-coming on a counter-current burner showing the increase in
turbulent mixing of gas and air, and a modest lowering of flame height as
riser height was increased from 8, 14, 20, 26, and 32 cm.   It also seems
to me that the burner worked better at low gasification rates with the
taller risers, because with a small flame, it was important to get as much
buoyant force as possible to stabilize the flow of both primary and
secondary air.  With taller risers, I got a more reliable response to
restricting or releasing primary air, because the gasification rate
responded more quickly.  Critically, however, I still don't know how riser
height and other burner dimensions affects flame excess air (above
stoichiometric needs).  I provisionally suggest a riser height that is
sufficiently tall to contain the largest desired flame.

Cheers,
Julien

PS, here is Crispin's posting, because I couldn't read it on "Stoves", and
other may experience the same problem.  I had to open the html attachment,
paste it into a text file, and save it on my computer as a *.html file,
then read it.  As a Canadian, a hate to say it, but I think the problem
has something to do with using a Blackberry phone.

> Crispin Pemberton-Pigott crispinpigott at outlook.com
> Tue Jan 12 05:49:38 MST 2016
>Dear Julien

>I want to address something that could become a generalisation for
designers:

>"Research in Australia showed that when the aperture of a concentrator
ring was reduced below 70% of the reactor area, the gasification rate
started to decrease."

>There are many factors contributing to the total draft and to the burn
rate. I feel it would be best if a 70% figure was not treated as a
generalisation. A reduction of 30% in area is an extremely small change in
flow resistance at those velocities. The position of the flame (expanding
gases) vertically relative to constriction could easily have more effect
than that reduction. Plus the shape as you note, is an influence. As the
shape of a hole can change the flow rate by a factor of three (assuming
constant pressure) before considering the influence of the flame (gas
expansion) position let's tag the Aussie experiments as "related to that
device under those circumstances" and not a general rule.

>Paul A's point about preventing air from entering the top and descending
into the chamber above the fuel is relevant. An open system with such air
entry, closed a bitby a destructor plate to prevent it (because it has a
flame destabilising effect) would change the primary flow rate as well. Up
or down? Who knows. It depends on the fuel packing and air holes and the
position of the plate relative to the fuel.

>The SeTAR Centre's BLDD burners are natural draft. The exit hole is a
quarter of the area of the point at which the gas exits the fuel. The
reactor area is nine times larger than that exit hole area. All the
secondary air is directed into the gas stream between the gas exit and the
final exit hole, the smallest point. ‎The gas velocity in the exit pipe
doesn't reach the speed at which friction is a really significant issue,
which happens at gas speeds above three metres per second.

>Alex's corn burner reactor diameter is about 1 foot. The exit is what,
about 2 inches? I am not saying the restriction isn't having an effect on
the primary air flow rate, just that a compensation in height or shape can
overcome it, perhaps without even noticing.

>The reacting fuel volume in the GTZ-7 stove series (diagrammes on my
website) is about 2.9 litres and measures 330 x 150mm with an exit choked
to 100 x 80mm. That leads to a conical (sort of) combustion chamber that
creates the expansion space for the flame as it rises and completes the gas
burn. Rather than backing up the gas flow, it adds draft.

>The secondary air is added in the middle of the 100 x 80 throat because
that is where the pressure difference between ambient and the gas stream is
greatest. One usually expects to see additions to the gas flow to be made
at the point of constriction in a venturi, not above or below it. ‎All
this discussion has been interesting from that point of view.

>Alex's later 'contraptions' with multiple valves and entry points mostly
don't have a restriction, though he does have a sudden diameter increase
part-way through the flame length that worried me. When I tried it I got an
increase in CO and he didn't. I am still thinking about that. Blasting a
flame into a large space is usually an invitation to CO disaster, but not
always.

>Those who have experimented with a Vesto will have observed that the
secondary air turns down into the fuel bed when the primary is virtually
closed, and upwards when it is wide open. The 'directing force' is neither
vanes nor hole angle as all are horizontal. It is all done by draft
circulation within the combustion zone.

>I must say I have appreciated these detailed sets of descriptions from
everyone with their views on why things do and don't work well. Most
stimulating.

>Best regards
>Crispin


-- 
Julien Winter
Cobourg, ON, CANADA
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