On Tuesday 31 August 2010 17:40:02 Neil T wrote: > On 29 Aug 2010 at 15:57, andrew heggie wrote: > > When playing with smaller stoves (without the benefit of flue gas > > analyser) I formed the opinion that the heat exchange to a Kelly > > kettle must be inhibited by this effect. I experimented by putting > > some "fins" in the central tube to disrupt the vortex but never came > > to any conclusion. > > The vortex or swirl effect is usually very mild in a Kelly Kettle and > the thing about it I noted was the way in which the flames seemed to > stick to the tapered wall, whereas putting it on a TLUD created a > centred flame (no vortex).
Neil I was referring to the effect when I used my Kelly kettle as the "load" on a stove which I built with a tangential air inlet, not using the Kelly with its firebox as supplied. So the vortex was deliberately pronounced. Consider that the hot gas is rising in a vortex, the walls of the kelly are <100C and the flue gases ~800C, so the hot gases are light and to the middle, the cold gases next to the metal are cool and dense and stay to the outside. This is the opposite to a conventional pan with a skirt. The problem is that if the air blast is strong then it accentuates the stratification whilst creating turbulence, so the combustion is good but dilution air escapes. With a downward angled air flow into a closed container of fuel unlike how Tom Miles describes the dual concentric flow, I saw all the flow rotating in the direction of the incoming tangential air with only the entrained central flame showing a gentle opposite precession. I broke the swirl by taking the flue from near the centre and out horizontally. From the way the material in the centre of the fire pyrolyses and that on the outer fringe turns to ash it was plain primary air was not surviving to the middle of the vortex and it was only being consumed as it rose and mixed with the offgas. The link to the 1986 patent Tom showed is interesting, I couldn't quite see what was novel about it but the points in the preamble are much as I have seen for a similar suspension burner. These devices don't like wet fuel and certainly don't like a lot of fresh fuel dumped in them. Wetter fuel can be burned by preheating the air and I did this my inducing the vortex on the outside of the metal, thus allowing the air to reach ~250C before it reached the fuel+offgas. A suspension burner I later found was one made of cement for burning rice hulls, it was designed by the NRI ( a UK quango that seems to have morphed into various bodies to do with our Oversaes Development Agency). It had an interesting feed metering mechanism whereby the rice hulls were allowed to drop out of the bottom-less cylinder held a small distance above a rotating disc, on the same radius of the dis was the suction inlet to a paddle wheel centrifugal fan, which injected the hulls and air tangentially into the combustion chamber. The fuelling being controlled by the speed of disc rotation. Quite recently at a contaminated soils seminar at the medway university I met one of the guys who worked on the NRI charcoal and heating project and he was involved in these schemes but it was only a short chat during a break and I lost track of him. AJH _______________________________________________ Stoves mailing list [email protected] http://listserv.repp.org/mailman/listinfo/stoves_listserv.repp.org http://stoves.bioenergylists.org http://info.bioenergylists.org
