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Dear Julien</div>
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The increase in draft is not related to the increase in height only, but to the weighted average temperature and height. It is weighted for its distribution vertically. When you don't know what that is, you have to use an average. </div>
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I invite you to see what the calculation is compared with the Draft Calculator written by Nigel available on the Stoves website. </div>
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It will generate a velocity in the riser if you input the combustion parameters. Note that it has to be the fuel composition of what is burning, not what is put in initially so if it is making char you subtract that from the elemental analysis. It will then
calculate the stoichiometric volume which is then factored for excess air. </div>
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The velocity through the holes is not linear with draft (at all) and depends on the shape of the entrance. As it becomes turbulent across the whole diameter the resistance to increased flow is strong. </div>
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Julien you are one again working at an advanced level of analysis that requires a combustion analyser and a scale to get the understanding you seek. Looking and recording notes is simply not enough at this point. Many things are not visible. </div>
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In the absence of gas measurements you can use a spreadsheet for some things, but we can't guess our way to perfection. </div>
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This conversation has been very helpful for investigation of the influence of changing a single physical dimension. The only easy and available addition that is cheap is to put a scale under the stove so you can see the burn rate. </div>
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To capture the mass in real time get a scale with a COM port and get a program for the computer (needs one COM port). The program is available from me. Using it you can see and record the real time burn rate. Then the effect of changes in the dimensions can
be tracked. </div>
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You guys are doing interesting things. Thanks for sharing. </div>
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Regards </div>
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Crispin </div>
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<div>Hi Stovers;</div>
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<div>Paul Anderson proposed the following</div>
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<div>"About height, whether of riser or of chimney: I recommend<br>
some simple experiments when your unit is running. Have some different<br>
lengths of extensions that can be easily placed (not even attached) on top<br>
of the current unit. The additional draft will pull more secondary air<br>
and make the flame be LOWER as long as there is control over the entry of<br>
primary air. But if the primary air is free flowing, the primary air will<br>
eventually "catch up" with the increased draft, and the flame will become<br>
higher than before. If that is not what you observe, please let me know<br>
so I can adjust my thinking."</div>
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<div>According to the formula for draft found on Wikipedia,</div>
<div><a href="https://en.wikipedia.org/wiki/Chimney#Chimney_draught_or_draft">https://en.wikipedia.org/wiki/Chimney#Chimney_draught_or_draft</a></div>
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<div>increasing the riser from 10 to 20 cm could result in a potential 1.41 times increase in the volumetric flow rate of air, regardless of flame temperature. (According to the formula, increasing flame temperate will increase the flow rate, but not the ratio
of flows at 20cm / 10cm)<br>
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<div>Since gasification rate increased 1.16 in my experiment rather than 1.41 times, that suggests that doubling the chimney height should result in a 'drawing' a greater proportion of secondary air.</div>
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<div>Therefore, the lower flame that Paul hypothesizes is quit possible.</div>
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<div>I ran the test, using the 2.7% aperture grate. It was hard to say if the flame was lower or not immediately after placing the 10 cm riser extension on top of the built-in 10 riser. Basically, NO CHANGE.</div>
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<div>One possibility is that the 1.41 potential increase in flow is not being met, because to resistance to air flow through the secondary air holes. Thus the fuel/air ratio of the flame doesn't change that much. An gap, rather than holes for secondary air
may be more responsive to an increase in draft (but I have found problems with gaps, so I prefer holes).</div>
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<div>I am running these tests with the TLUD inside a 0.4 m diameter x 0.6 m tall garbage can with the bottom cut out. That reduces air turbulence. If this TLUD was used in the open, it would have to include shielding around air intakes to reduce the effect
of wind. All the same, the burner doesn't blow out easily.</div>
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<div>All the best,</div>
<div>Julien.</div>
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<div dir="ltr">Julien Winter<br>
Cobourg, ON, CANADA<br>
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