Dear Alex I base my answer on the experiences I had trying different flame holders on the FSP stove, the same one you saw close up.
The fact that Paul's wider spaced wire screen did not work is relevant. Until the flow is interrupted enough to 'retain' the flame, the effect is not pronounced. Above a critical threshhold, the effect of a BB becomes evident, that is, when the flow is impeded. I found that a thin plate with lots of small holes punched through with a plasma torch also worked but that the CO was still not zero. The cause I suspected was the cooling of the flame (flamelets) as they passed through the small holes. The wire gets hot enough to actually assist the ignition. The visibility is an issue - a CO flame is not necessarily visible. Remember the FLOX discussion we had a while back. There is CO at the screen that is not there 50mm higher up even when there is no flame visible. The thin wire grate used on the downdraft coal stove at SeTAR employs the same idea of assisting the ignition of smoke and CO with a hot wire, kept hot by radiant heat from the flame directly underneath. It not however the BB effect. Just the ignition. The propane heaters with a ceramic element do pretty much the same thing (turning hot gas into radiant energy) without a bluff body by having hundreds of very short flames coming through the ceramic plate which gets red hot. Regards Crispin in Johannesburg - summer! ------Original Message------ From: Alex English Hi Crispin and Paul, Bluff bodies (BB) are refered to in fluid dynamics generally for which combustion of gases is a subset. The definition of a BB "a solid obstruction in a fluid stream, having a broad flattened front and providing a shelter for small scale turbulence and zones of low velocity; a stability assister." (from the North American Combustion Handbook (Vol 2)) So does a screen qualifies as a bluff body? It can be a flame holder as can a single wire in the path. We have a screen burner on our propane refrigerator but the flame is down sream of the screen. I don't think the screen glowed. Paul's burner appears to have good flame holding at the holes in the burner head. He mentions the screen protects these flames from the wind and that combustion is below the screen. So in this case it is not clear that there is any combustion happening in or on the outer surface of the screen although it could be invisible to our eyes. So you could say that there are two screens, the holes in the burner head being the first screen, the BB perhaps, and the second is simply a radiator/wind screen. The small pressure drop across the screen also helps distribute the flow evenly. All in all, great result= great design. Alex On 04/03/2012 8:26 AM, Crispin Pemberton-Pigott wrote: > Paul, what you are describing are the effects of a 'bluff body'. It is why they are sometimes used above a flame. > Crispin _______________________________________________ Stoves mailing list to Send a Message to the list, use the email address [email protected] to UNSUBSCRIBE or Change your List Settings use the web page http://lists.bioenergylists.org/mailman/listinfo/stoves_lists.bioenergylists.org for more Biomass Cooking Stoves, News and Information see our web site: http://www.bioenergylists.org/
