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Post by patamos on Dec 20, 2017 22:59:17 GMT -8
One of the key factors with bells is that the flue gasses are able to slow down enough for the forces of convection and relative buoyancy to take over. When gasses are not moving quickly along a surface the friction of laminar flow is reduced, enabling more effective heat transfer into the mass.
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Post by coastalrocketeer on Dec 23, 2017 18:36:29 GMT -8
Yes. Cross section of the bell, as it appears to the flowing gasses is critical. That area for gas flow must be large enough, and thus slowing of gasses, sufficient to allow stratification or you only have a down draft channel, rather than a bell... As in the barrel in an RMH.
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Post by Orange on Dec 24, 2017 0:15:51 GMT -8
yes, but only slow down gasses to the point you don't get smoke back.
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Post by coastalrocketeer on Dec 24, 2017 1:37:59 GMT -8
yes, but only slow down gasses to the point you don't get smoke back. A well designed bell does not slow down gas flow into or out of the system... It still flows at the same velocity through combustion areas, and exhaust, depending on current burn state/intensity, and warmth of chimney and riser, etc... It is BECAUSE of the larger CSA for free gas flow in the bell that velocity drops... But it only drops in that "wide space" of the bell... A bigger bell will slow gas velocity more, but only INSIDE the bell where that CSA is much larger than system CSA. It can also be beneficial to create a larger than system CSA entrance in to the bell, as the system CSA opening may cause gas streams to disrupt the stratification of the bell if the speed is not reduced before entering the bell... The larger your bell in respect to system CSA, the less this matters, I would imagine...
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Post by patamos on Dec 24, 2017 9:49:52 GMT -8
Yes, the velocity drop acts something like the water in a fast flowing stream loses velocity when it enters a deep pool. The volume flow rate of incoming stream and outgoing stream are much the same.
Another issue to consider with bell shapes is short cutting of gasses between the inlet and exhaust port. Norbert Senf (at MHA) did some interesting experiments with a tall narrow bell (2008 or 09 I think) and noticed that with a strong chimney draft the gasses jetted straight from the low intake port to the low exhaust port. The draw of the chimney was stronger than the draw/power of buoyancy. This issues was resolved by adding a vertical baffle immediately after the intake port to direct the gasses initially upwards. The study also got me to thinking that the need for a low intake port is not crucial. The discernment between a downdraft chamber and a bell chamber is somewhat arbitrary and contingent upon flow rates relative to ISA and CSA. In other words, if a downdraft chamber has a large enough volume for the gasses to hang out ('eddy out' in river talk) then it is acting as a bell. In contra-flow masonry heaters with only 1 or 2 very large downdraft chambers this effect is happening somewhat... So for these reasons, i am inclined to believe the defining factor of the bell effect is reduction of friction between gasses and heat-exchanger wall surface.
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Post by pinhead on Jan 2, 2018 7:39:53 GMT -8
Yes, the velocity drop acts something like the water in a fast flowing stream loses velocity when it enters a deep pool. The volume flow rate of incoming stream and outgoing stream are much the same. Another issue to consider with bell shapes is short cutting of gasses between the inlet and exhaust port. Norbert Senf (at MHA) did some interesting experiments with a tall narrow bell (2008 or 09 I think) and noticed that with a strong chimney draft the gasses jetted straight from the low intake port to the low exhaust port. The draw of the chimney was stronger than the draw/power of buoyancy. This issues was resolved by adding a vertical baffle immediately after the intake port to direct the gasses initially upwards. The study also got me to thinking that the need for a low intake port is not crucial. The discernment between a downdraft chamber and a bell chamber is somewhat arbitrary and contingent upon flow rates relative to ISA and CSA. In other words, if a downdraft chamber has a large enough volume for the gasses to hang out ('eddy out' in river talk) then it is acting as a bell. In contra-flow masonry heaters with only 1 or 2 very large downdraft chambers this effect is happening somewhat... So for these reasons, i am inclined to believe the defining factor of the bell effect is reduction of friction between gasses and heat-exchanger wall surface. I agree: A low inlet isn't required for a bell to function. In both of my bell systems, the secondary bell inlet is in the roof of the bell and is the entire width of the barrel. This maintains the lowest velocity possible within the bell, which maximizes stratification. The only disadvantage - if you can call it that - is this allows hot gasses to transfer from the secondary bell back to the barrel once the velocity is low enough, i.e. during and after the coaling phase; this configuration makes the entire heat extraction system system act as one large bell as opposed to two separate bells.
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Post by Orange on Jan 17, 2018 6:22:55 GMT -8
how about these metal chimney things: they extract a lot of heat and don't look too bad?
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Post by esbjornaneer on Jan 17, 2018 6:31:46 GMT -8
While the fire is burning when the chimney temps are going down to 30-50C after the 1hour long fire .
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