<html><head></head><body dir="auto" lang="en-GB" style="background-color: rgb(255, 255, 255); line-height: initial;"> <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 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><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 id="_originalContent" style=""><div><br>On Jan 11, 2016, at 8:47 PM, Julien Winter <<a href="mailto:winter.julien@gmail.com">winter.julien@gmail.com</a>> wrote:<br><br></div><blockquote type="cite"><div><div dir="ltr"><div>Hello Paul;<br><br>If I understand your description of Kevin Adair's burner, lowering an hourglass restriction to the level of a concentrator would directed secondary downward against the upward flow of the woodgas. We would no longer have a cross current burner. We would have a counter current burner. A simple demonstration can be seen here: <a href="https://youtu.be/RnVaHpLZg8U" target="_blank">https://youtu.be/RnVaHpLZg8U</a><br><br>If we have a counter current burner, then there would be good turbulent mixing of air and gas at the base of the gas flame. That is where most of the work of the burner would be done. <br><br></div><div>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. If an hourglass constriction functioned the same way (and since we have a counter current burner, the conditions are not the same) it could add some stability to the performance of a counter current burner. In an open riser, a counter current burner can create a strong draft of primary air, and cause an excessive gasification rates.<br><br></div><div>I have measured soot deposits on pots when a using a concentrator ring that I did not see with more open burners. However, a concentrator has sharp edges that can create turbulence and flame stress that may not occur with a smoother restriction, like an hourglass.<br><br></div><div>I will have more to say about counter current burners in a few days.<br></div><div><br></div><div><br></div><div>Cheers,<br></div><div>Julien.<br></div><div><br><br></div><div><div><br>-- <br><div><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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