Hi, Sorry if this has already been discussed, in which case I missed it. What do the outermost holes in silent burners do? These are in the top flange/rim outside of the caps. Sometimes they are drilled holes, sometimes pierced semicircular slots with the ‘half moons’ bent downwards, sometimes pointing clockwise and sometimes anti-clockwise. Occasionally I have come across (older?) burners with no holes/piercing at all. Small flame buds often form below the holes, but I can’t imagine the heat generated from them contributes much towards vaporisation.
Hi @Twoberth I have always thought that these vent holes are there to ensure good air flow and swirl to the combustion area, and thus improved combustion. Generally the correct fuel to air mixture is set under the burner caps and then burns outside the cap and partly within the skirt. Certainly the earlier deep-skirted silent burners seem to manage OK without vent holes around their base, but all the shorter skirts seem to have multiple vents, although they vary in diameter beween burners and manufacturers. I used to think that air flowed upwards, into the skirt. However at high power I too have seem small flamelets extending downwards from these vent holes. Perhaps air only flow upwards through the vents at lower power levels? I would be interested in other’s thoughts. Best Regards, Kerophile.
@kerophile, thanks for the quick reply. I go along with the swirl in the skirt, as it explains the shape of the more common pierced and bent holes which would promote a swirl. With simmering flames the pressure below the skirt may be higher and air flow upward through the vents causing the swirl. With full power the reverse may be true and the swirl caused by some of the flammable and exhaust gases exiting down the vents. The question now is what does the swirl do? It isnt large enough to affect the shape of the flamelets which protrude radially and straight from the outer cap holes, but it must be significant enough to help anchor the flamelets. I read somewhere that when making an air/propane burner for a cylindrical melting furnace, the burner will not work outside the furnace (the flame blows out as the air is increased), but it does work inside the furnace if it enters from the side and is angled towards the inner wall and so producing a swirling action which helps to anchor the flame on the end of the burner. Back to silent stove burners. Maybe the early deep skirts produced their own swirl, and the holes only became necessary with shallower skirts. We need a silent burner that can talk!
Hi, It would be good to have comments from @presscall as he did a lot of work in understanding flame behaviour in No.4 type stoves. One additional effect of the skirt vents would be in handling fuel-rich flames, such as might occur when jets become worn. The extra oxygen and swirl from the vents should allow excess fuel to be burnt and avoid the soot and smell of unburnt fuel. Best Regards, Kerophile.
@kerophile Well, we had discussions back in the day, you, me, BernieDawg et al, so I’d not presume to be an authority but I think the preceding comments have summed it up already. As has been said, the holes in the burner skirt ventilate the annular trough the skirt forms, enabling proper combustion of the flamelets after they’ve exited the burner cap. Think of the effect of stifling an automobile exhaust. In short time the primary combustion is adversely affected. I’ve managed to illustrate this with some photos. First I created a flattened wire ring to block the burner skirt holes - at least partially. Stove/burner is a Primus No.4. Normal combustion without the ring in place. ... and with the ring in place Side view. Without the ring. ... and with it in place Bear in mind that the ring I formed and loosely sat in the burner skirt could not be expected to seal the ventilation holes, but only partially masked them. Still sufficiently I think you’ll agree however to show the disruptive effect on exhaust of the flames and consequently on combustion. Total blanking off of the holes may be expected to create a more pronounced disruption of combustion. John
@Twoberth thank you for this intelligent question! They are becoming rare in this forum. The skirt of the silent burner is the most misunderstood part of the kerosene-pressure stove. It's function is often misinterpreted as a kind of wind-shield for the flames. In fact it's something quite different. The skirt collects energy from the flames and conducts it to the lower parts of the burner where a remarkable amount of energy is necessary to vaporize the fuel. This is a large PRIMUS No. 25, from 1932 with two silent and two roarer burners, standard size and large. Compare the standard size silent burner on the left and the one with a large skirt. These two burners have the same dimensions of inner cap, outer cap, mixing tube, jet-orifice and burner tubes. The only difference is the dimension of the skirt! But the power output of the larger stove is at least 150%, if the smaller is taken as 100%. I did not use the large stove more than a few times, because I don't cook goulash for ten hungry outdoor men very often. In reality I think the output is rather at 200% than at 150% of the usual burner. But when I measured the power output, it was not at full power. The designers of the silent burner had the problem to design a skirt which is capable to catch enough heath from the flame-tips without disturbing the flow of the gas-air mixture. This is made more complex by the fact that in real cooking the gas stream speed is quite different at full power or at simmering. The flamelets not only have a different length, but also the flame-tips are at different places. The older solution was to make a deep skirt which is narrow for the simmering-size flamelets and wide and high for the full speed flames. This shape can be optimized for full speed quite well, but not so good for simmering. The disadvantage of a large skirt is, it can easily be bent or may catch over-boiling soup or milk. Another solution was, to make the skirt smaller and try to keep the flames in a middle position by supply of additional air through the holes in the skirt-bottom. High gas speed would take in more air through the holes, so a dynamic equilibrium would keep the flame size in a middle range and the flame-tips near the skirt. To improve this additional air intake, the holes were made in the shape of half moons, which gives the gas-flow a twist tangential to the skirt. Also a way to make sure the hottest gases flow along the skirt. Perfect gas-stream under a pot of 7 litres, not full power. Best Regards Radler
Hi @Radler I agree with your view that it is good to have such an intelligent question on CCS. I found your response very interesting and well argued, However i do have a problem with your statement that a standard silent burner, such as used on a No.5 style stove, has the same cap dimensions as the burner used on the No.25 stove: https://classiccampstoves.com/threads/1930s-primus-burners-overview.21383/ I provided some detailed dimensions for some sient burner caps several years ago: Do you have any comments? Best Regards, Kerophile.
Hi John @presscall . Great presentation/photos illustrating the function of the vent holes in the skirt of silent burners. They certainly seem to improve the stability and mixture contol of the flame pattern. Thanks for taking the time to do the experiment. Best Regards, George.
Seems to depend on the manufacturer too primus seem to favour holes with a small flap attached optimus like plain holes only difference ive noticed with mine is the primus #5 takes very little coaxing to underburn but the optimus with its plainer design doesnt though it really likes a long preheat to run properly op 111/7 it is i,m thinking they may help shape the flame too keep it aimed upward too
@kerophile Well observed! But the large burner on my PRIMUS No.25 is not the Original. I got the stove with this large burner, a second, standard size burner, and a Optimus toolbox. The burner with the large skirt has no marking on it at all, but is well made and of European standard. I think it was made after WWII. All the dimensions are the same as standard at silent burners, except the size of the skirt. The outer cap is marked as a PRIMUS No. 4205 PRIMUS METAL. The inner cap has also standard size, but the mixing tube is fixed to the inner cap, not to the burner. The burner has no mixing tube. The example shows quite clearly, the dimensions of standard burners are able to cope with a gas-stream and power-output much higher than standard burners ever reach. Their limiting factor is the amount of fuel, which can be vaporized. And this is dependant of the shape and size of the skirt. Of course a burner with a higher output is not "better". The power output must be suitable for the purpose. Cooking rice for two people or making hot water for a group of fifteen men is not the same thing. Best Regards Radler
Hi @Radler . Your explanation makes it clearer. Thank you. I understand what you mean about the mixing tube being fixed to the inner cap, although I had only seen this on the inner cap of a Primus No. 4140 or 4142 burner mounted on a Pr. No10 stove. https://classiccampstoves.com/threads/primus-no-10-1939.5446/ Best Regards, Kerophile.
Hi, @presscall shows some change in flame pattern when covering the skirt holes in the No 4 burner experiments, so clearly the skirt holes have an effect. However there are lots of silent burner caps available that work on 123’s, 96’s, etc. which have no skirt holes. My curiosity says that more research on my part is needed (literature and experiment). I will keep you informed if I find anything new.
You’re referring to converter caps that sit in a roarer burner bell as a substitute for a flame plate. There’s no pronounced shrouding of the flame outlets in that arrangement.
OK maybe I used two bad examples above, but the converter caps also work on other stoves with pronounced shrouds - Optimus Polaris, Phoebus 725.
In my Radius 5 stove burner has just 6 quite large vent holes on burner skirt. If there is too much pressure in tank small flames appear on outer side of burner skirt on those holes. Time to time some of those small flames goes off and non burned kero&air mix escapes to air. Indoors that stove is not nice to use.
