<div dir="ltr"><div style="width:100%;font-size:initial;font-family:Calibri,"Slate Pro",sans-serif,sans-serif;color:rgb(31,73,125);text-align:initial;background-color:rgb(255,255,255)">Dear Crispin;<br><br></div><div style="width:100%;font-size:initial;font-family:Calibri,"Slate Pro",sans-serif,sans-serif;color:rgb(31,73,125);text-align:initial;background-color:rgb(255,255,255)">Thanks for adding you points of clarification (below).<br><br></div><div style="width:100%;font-size:initial;font-family:Calibri,"Slate Pro",sans-serif,sans-serif;color:rgb(31,73,125);text-align:initial;background-color:rgb(255,255,255)">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.<br><br></div><div style="width:100%;font-size:initial;font-family:Calibri,"Slate Pro",sans-serif,sans-serif;color:rgb(31,73,125);text-align:initial;background-color:rgb(255,255,255)">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.<br><br></div><div style="width:100%;font-size:initial;font-family:Calibri,"Slate Pro",sans-serif,sans-serif;color:rgb(31,73,125);text-align:initial;background-color:rgb(255,255,255)">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. <br><br></div><div style="width:100%;font-size:initial;font-family:Calibri,"Slate Pro",sans-serif,sans-serif;color:rgb(31,73,125);text-align:initial;background-color:rgb(255,255,255)">Cheers,<br></div><div style="width:100%;font-size:initial;font-family:Calibri,"Slate Pro",sans-serif,sans-serif;color:rgb(31,73,125);text-align:initial;background-color:rgb(255,255,255)">Julien<br><br></div><div style="width:100%;font-size:initial;font-family:Calibri,"Slate Pro",sans-serif,sans-serif;color:rgb(31,73,125);text-align:initial;background-color:rgb(255,255,255)">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.<br></div><div style="width:100%;font-size:initial;font-family:Calibri,"Slate Pro",sans-serif,sans-serif;color:rgb(31,73,125);text-align:initial;background-color:rgb(255,255,255)"><br>> Crispin Pemberton-Pigott crispinpigott at <a href="http://outlook.com">outlook.com</a><br>> Tue Jan 12 05:49:38 MST 2016<br>>Dear Julien</div><div style="width:100%;font-size:initial;font-family:Calibri,"Slate Pro",sans-serif,sans-serif;color:rgb(31,73,125);text-align:initial;background-color:rgb(255,255,255)"><br></div><div style="width:100%;font-size:initial;font-family:Calibri,"Slate Pro",sans-serif,sans-serif;color:rgb(31,73,125);text-align:initial;background-color:rgb(255,255,255)">>I want to address something that could become a generalisation for designers: </div><div style="width:100%;font-size:initial;font-family:Calibri,"Slate Pro",sans-serif,sans-serif;color:rgb(31,73,125);text-align:initial;background-color:rgb(255,255,255)"><br></div><div style="width:100%;font-size:initial;font-family:Calibri,"Slate Pro",sans-serif,sans-serif;color:rgb(31,73,125);text-align:initial;background-color:rgb(255,255,255)">>"<span style="line-height:initial">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."</span></div><div style="width:100%;font-size:initial;font-family:Calibri,"Slate Pro",sans-serif,sans-serif;color:rgb(31,73,125);text-align:initial;background-color:rgb(255,255,255)"><span style="line-height:initial"><br></span></div><div style="width:100%;font-size:initial;font-family:Calibri,"Slate Pro",sans-serif,sans-serif;color:rgb(31,73,125);text-align:initial;background-color:rgb(255,255,255)"><span style="line-height:initial">>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. </span></div><div style="width:100%;font-size:initial;font-family:Calibri,"Slate Pro",sans-serif,sans-serif;color:rgb(31,73,125);text-align:initial;background-color:rgb(255,255,255)"><span style="line-height:initial"><br></span></div><div style="width:100%;font-size:initial;font-family:Calibri,"Slate Pro",sans-serif,sans-serif;color:rgb(31,73,125);text-align:initial;background-color:rgb(255,255,255)"><span style="line-height:initial">>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. </span></div><div style="width:100%;font-size:initial;font-family:Calibri,"Slate Pro",sans-serif,sans-serif;color:rgb(31,73,125);text-align:initial;background-color:rgb(255,255,255)"><span style="line-height:initial"><br></span></div><div style="width:100%;font-size:initial;font-family:Calibri,"Slate Pro",sans-serif,sans-serif;color:rgb(31,73,125);text-align:initial;background-color:rgb(255,255,255)"><span style="line-height:initial">>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. </span></div><div style="width:100%;font-size:initial;font-family:Calibri,"Slate Pro",sans-serif,sans-serif;color:rgb(31,73,125);text-align:initial;background-color:rgb(255,255,255)"><span style="line-height:initial"><br></span></div><div style="width:100%;font-size:initial;font-family:Calibri,"Slate Pro",sans-serif,sans-serif;color:rgb(31,73,125);text-align:initial;background-color:rgb(255,255,255)"><span style="line-height:initial">>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. </span></div><div style="width:100%;font-size:initial;font-family:Calibri,"Slate Pro",sans-serif,sans-serif;color:rgb(31,73,125);text-align:initial;background-color:rgb(255,255,255)"><span style="line-height:initial"><br></span></div><div style="width:100%;font-size:initial;font-family:Calibri,"Slate Pro",sans-serif,sans-serif;color:rgb(31,73,125);text-align:initial;background-color:rgb(255,255,255)"><span style="line-height:initial">>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. </span></div><div style="width:100%;font-size:initial;font-family:Calibri,"Slate Pro",sans-serif,sans-serif;color:rgb(31,73,125);text-align:initial;background-color:rgb(255,255,255)"><span style="line-height:initial"><br></span></div><div style="width:100%;font-size:initial;font-family:Calibri,"Slate Pro",sans-serif,sans-serif;color:rgb(31,73,125);text-align:initial;background-color:rgb(255,255,255)"><span style="line-height:initial">>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. </span></div><div style="width:100%;font-size:initial;font-family:Calibri,"Slate Pro",sans-serif,sans-serif;color:rgb(31,73,125);text-align:initial;background-color:rgb(255,255,255)"><span style="line-height:initial"><br></span></div><div style="width:100%;font-size:initial;font-family:Calibri,"Slate Pro",sans-serif,sans-serif;color:rgb(31,73,125);text-align:initial;background-color:rgb(255,255,255)"><span style="line-height:initial">>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. </span></div><div style="width:100%;font-size:initial;font-family:Calibri,"Slate Pro",sans-serif,sans-serif;color:rgb(31,73,125);text-align:initial;background-color:rgb(255,255,255)"><span style="line-height:initial"><br></span></div><div style="width:100%;font-size:initial;font-family:Calibri,"Slate Pro",sans-serif,sans-serif;color:rgb(31,73,125);text-align:initial;background-color:rgb(255,255,255)"><span style="line-height:initial">>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. </span></div><div style="width:100%;font-size:initial;font-family:Calibri,"Slate Pro",sans-serif,sans-serif;color:rgb(31,73,125);text-align:initial;background-color:rgb(255,255,255)"><span style="line-height:initial"><br></span></div><div style="width:100%;font-size:initial;font-family:Calibri,"Slate Pro",sans-serif,sans-serif;color:rgb(31,73,125);text-align:initial;background-color:rgb(255,255,255)"><span style="line-height:initial">>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. </span></div><div style="width:100%;font-size:initial;font-family:Calibri,"Slate Pro",sans-serif,sans-serif;color:rgb(31,73,125);text-align:initial;background-color:rgb(255,255,255)"><span style="line-height:initial"><br></span></div><div style="width:100%;font-size:initial;font-family:Calibri,"Slate Pro",sans-serif,sans-serif;color:rgb(31,73,125);text-align:initial;background-color:rgb(255,255,255)"><span style="line-height:initial">>Best regards </span></div><div style="width:100%;font-size:initial;font-family:Calibri,"Slate Pro",sans-serif,sans-serif;color:rgb(31,73,125);text-align:initial;background-color:rgb(255,255,255)"><span style="line-height:initial">>Crispin </span></div><br clear="all"><br>-- <br><div class="gmail_signature"><div dir="ltr"><div><div dir="ltr"><div><div dir="ltr">Julien Winter<br>Cobourg, ON, CANADA<br></div></div></div></div></div></div>
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