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Dear Kirk</div>
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The direct measurement of the draft needs a reasonably accurate meter. A combustion analyser can usually report in 0.01 millibars and on the devices you checked it will read between 3 and 20, maybe. </div>
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There is a draft calculator on the Stoves website written by Nigel Pemberton-Pigott with places for up to 8 stages in series, both up or down-drafting. It calculates the total. </div>
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In the case of the chimney 'twice as tall' the reality does not match the estimated 'twice as much' draft. It is always less than double because of additional heat loss in the added section. </div>
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BTW you are thinking a lot of right thoughts. Where the draft is applied and in what form can create excellent combustion conditions with very simple architecture. </div>
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Regards </div>
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Crispin </div>
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<p class="x_MsoNormal" style="margin:0in 0in 10pt"><b style=""><span style="line-height:115%; font-size:14pt"><font face="Calibri">Thoughts and questions about Buoyancy
</font></span></b></p>
<p class="x_MsoNormal" style="margin:0in 0in 0pt"><span style="line-height:115%; font-size:14pt"><font face="Calibri">I would like to pose some thoughts and questions to the group about buoyancy.<span style="">
</span>I have been trying to think this through for some time and would like to get some input.<b style=""></b></font></span></p>
<p class="x_MsoNormal" style="margin:0in 0in 0pt"><span style="line-height:115%; font-size:14pt"><font face="Calibri"><span style="">
</span>I used a manometer with attached probe to test my TLUD for gas pressure variations, measuring to .01 inch of water column, hoping to learn more about what goes on inside the stove.<span style="">
</span>I found no variations in pressure. The stove was at atmospheric pressure throughout.<span style="">
</span>I could not confirm any pressure variations within the functioning stove.<span style=""> Can anyone confirm or disprove the results of this test? Because I found no pressure variations</span>, I had to find a mechanism which could move the gasses without
pressure difference.<span style=""> </span>I am looking at buoyancy.<span style="">
</span>Heavy gasses fall and displace light gasses which rise.<span style=""> </span>
This does not mean that light gasses have “lift” as in anti-gravity.<span style="">
</span>They are simply pulled down with less force by gravity than heavier gasses.<span style="">
</span>A light weight gas, like helium or fire gasses, placed in a vacuum chamber will fall, not rise.<span style=""> D</span>o heavy gasses push the light weight gasses up?<span style="">
</span>I am thinking of it as a field of gravity where heavier gasses are pulled more by gravity than light gasses, and so heavy gasses are pulled closer to the center of gravity than light gasses (ie down).<span style="">
</span>So buoyancy can only occur in a gravitational field and is an effect of gravity.<span style="">
</span>But then in a state of equilibrium the light gasses are sitting on top of the heavy gasses and thus must be exerting a force on the heavy gasses, and vise versa the heavy gasses must be pushing up on the light gasses.<span style="">
</span>This means pressure, and it would show up in the overall atmospheric pressure and so would not be seen in my manometer tests, since both ends of the manometer are subject to it.<span style="">
</span>The question I raise is “Does the air push up on the fire gases in the non-equilibrium situation inside the stove, or is it a displacement where the air flows in and the fire flows out all due to gravity?”
<span style=""> Either way it is clear that a small bubble of light weight gas in a sea of heavier gas, such as the situation with our stoves, cannot maintain its position and must rise.
</span>If the heavy gas is pushing up on the light gas, would not a pressure area have been detected by the manometer test?</font></span></p>
<p class="x_MsoNormal" style="margin:0in 0in 0pt"><span style="line-height:115%; font-size:14pt"><font face="Calibri"><span style="">
</span>I have read that the raising fire is caused by the difference in altitude, that the atmospheric pressure on the bottom of our stoves is greater than the atmospheric pressure on the top and so the fire rises.<span style="">
</span>I find it difficult to believe that the pressure difference in two feet of stove height is significant enough to drive a high power fire, when the atmospheric pressure gradually changes over ten miles of height.</font></span></p>
<p class="x_MsoNormal" style="line-height:normal; margin:0in 0in 0pt"><span style="font-size:14pt"><font face="Calibri"><span style="">
</span>Exhaust gasses rising in a chimney do NOT create a vacuum below them and thus pull air in to the stove, as the word draft implies.<span style="">
</span>The atmospheric air falls into the stove and displaces the hotter and thus lighter weight fire gasses.<span style="">
</span>Another way to think of this is to think of a tube held in a U shape.<span style="">
</span>Both ends of the tube are at the top and both are open to the atmosphere.<span style="">
</span>If one leg of the U, say the right side, is filled with heavy gas, and the left is filled with light weight gas, the heavy gas will fall around the bend and then rise up the left leg to a point of equilibrium.<span style="">
</span>This is the same as what is happening in our natural draft stoves, the heavier atmosphere rushes into the stove and displaces the lighter gasses of the fire.<span style=""> The primary air is able to rise through the fuel in the stove like the heavy
gas in the U tube is able to rise in the left leg. </span>But since the incoming air is heated by the fire and becomes buoyant, it is itself displaced by more air from the outside.<span style="">
</span>A point of equilibrium cannot be reached until the fire is extinguished and the stove is cooled.</font></span></p>
<p class="x_MsoNormal" style="line-height:normal; margin:0in 0in 0pt"><span style="font-size:14pt"><font face="Calibri"><span style="">
</span>A taller chimney creates more draft because it contains a taller column of light weight gasses, which means a taller column of heavier atmosphere outside the chimney.<span style="">
</span>This gives more height difference for gravity to act on.<span style=""> </span>
Buoyancy strength in this situation depends on the weight (density * gravity) difference between the gasses, and the column height.<span style="">
</span>A chimney two feet tall will have twice the weight difference of gas to act on as a chimney one foot tall.<span style="">
</span>Thinking in terms of electricity, stacking the volumes of gas on top of each other is like batteries in series and stacking them side by side is like batteries in parallel.<span style="">
</span>The batteries in series will double the voltage like the air volumes stacked above each other in a chimney will double the buoyancy head.</font></span></p>
<p class="x_MsoNormal" style="line-height:normal; margin:0in 0in 0pt"><span style="font-size:14pt"><font face="Calibri"><span style="">
</span>A fire temperature of 1600 F with air temperature at 50 F to 100 F creates considerable difference in density and weight between the fire and the air.<span style="">
</span>A hot air balloon rises at only 250 F, or a temperature difference of 200 F with the air, about 1/8<sup>th</sup> of the difference the fire in our stoves creates.<span style="">
</span>The density of the gasses in the fire is about 1/4<sup>th</sup> that of the atmosphere.<span style="">
</span>One could expect to see a very lively fire based on buoyancy as the driving force.<span style="">
</span>This is my conclusion, buoyancy is the driving force in our natural draft stoves, and most likely in an open fire as well.<span style="">
</span>I do not know how this will effect stove design, but it does help me to understand how a natural draft stove operates.</font></span></p>
<p class="x_MsoNormal" style="line-height:normal; margin:0in 0in 0pt"><span style="font-size:14pt"><font face="Calibri"></font></span> </p>
<p class="x_MsoNormal" style="line-height:normal; margin:0in 0in 0pt"><span style="font-size:14pt"><font face="Calibri">Kirk</font></span></p>
<p class="x_MsoNormal" style="line-height:normal; margin:0in 0in 0pt"><span style="font-size:14pt"><font face="Calibri">Santa Rosa, CA. USA</font></span></p>
<p class="x_MsoNormal" style="margin:0in 0in 10pt"><span style="line-height:115%; font-size:14pt"><font face="Calibri"><span style="">
</span><span style=""> </span></font></span></p>
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