Metal Mesh / Mantle Mechanism

Try this.

Thank you for the link.

I think this link gets more at the cause. I've been wondering if each tiny whole acts as an a stove orifice does. Still though I'm not ready to lay this one to bed, what could be the reason behind this. It makes more sense when you think of a wall (or endcap of a pipe) with only 1 small orifice. But when you have hundreds of said orifices next to each other, basically forming a subdivided larger pathway. Why is it that the flame cannot travel through.

Further Speculation. Maybe, combustion cannot continue to take place when confined on 4/6 of its surface area is dampered? Maybe breaking the flame up like that increases the surface area of the combusting gas, increasing the burn rate and consuming the fuel relatively instantly.

The metal mesh quenches the flame. The temperature on the out side will be below the combustion temperature. On a domestic gas stove if the flame was on for a long time and the stove switched off and relit at once usually the flame will strike through. If the burner is allowed to cool slightly, the flame can be relit safely.

In the mantle lamps the situation is different. The gas mantle is made with ceramic materials (Cerium and Thorium oxides) that give off bright visible light when heated by a flame which itself is nearly colourless.

http://en.wikipedia.org/wiki/Gas_mantle

There is also a relationship between the gas velocity and the holding of the flame on the mantle. At velocities well above normal operating values the flame will lift off the mantle.

The mesh sack is to increase light output. They were looking for ways to make the gas lamps the ones that use to light or steets brighter without increased fuel cost. The lamp is made so that combustion takes place just inside the walls of the mesh sack. So that it heats the sack up to high temperature. Today's mantle are made of synthetic cloths which have been soaked in mainly thorium nitrate. Other metals are added to give the mesh strength after the cloth is burned off. When heated thorium gives most of its light off in the visible spectrum a white light.

So if I have a mantle that is hot enough to glow I wouldn't recommend venturing in to a room filled with gas with it.

I think you're perhaps mixing the function of a lamp mantle and a flame arrestor. The wire mesh (Davy lamp) works great for methane, but I would never try it for acetylene. You have to look at the combustion properties of the different gases. The other problem is when the lamp is being extinguished; you have a brief millisecond there when the mesh quits working.

yes I'm aware of the that the mantles now days are probably more important for lighting purposes. The chemicals soaked into the mantle supposedly are chosen based on the amount of light they emit when heated.

The mesh sack though was originally conceived for the "flame arrestor" function which is what I'm interested in. I think the Davey's lamp principle works for propane as well. The pop. science article i was reading was discussing using it on both natural gas and propane stove tops, as a safety precaution.

I'm interested in what TVP45 said about the danger when extinguishing. I could understand if you extinguished it by snuffing it out, but the more likely method, of cutting off the gas supply doesn't seem like it would lend itself to increasing the escape chance. Also one article I found suggested that the mesh hood, was also used for oil buring lanterns as well, which is important because it shows that the mechanism isn't related to vapor pressure or any sort of gas property, it it explicitly limited to a open flame.

Another example a colleague I work with was able to provide. When working with a bunsen burner. people commonly use a mesh as a platform to set your beaker on. If you remove the beaker though, the relatively wide gaped mesh still acts as a barrier that the flames do not penetrate. Very interesting

Cutting off the gas supply is a problem. I suppose a Davy lamp would work OK on propane as well. There are now dozens of commercial units available, all of them rated for specific gases, or groups of gases.

I don't understand your comment about "explicitly limited to a open flame". If you are not familiar with combustion, I suggest you don't actually try any of these.

"I don't understand your comment about "explicitly limited to a open flame". If you are not familiar with combustion, I suggest you don't actually try any of these."

I mentioned the oil lamp flame because I imagine the conditions in a low pressure flame quite different from those in a the flame coming off a high pressure orifice. Yes I'm aware its not the liquid oil burning so much as the vapor coming off the liquid. but I think you'll admit the flames are quite different from a pressurized "torch flame."

A further discussion point is the davey lamps were used as indicators of methane levels because in high methane environments the methane would diffuse the mesh and noticably affect the flame color. However even in these conditiosn the flame typically would not spread outside the mesh.

I appreciate any input on the subject. I am curious because I am not a combustion expert, nor a chemist. I'm actually a molecular biologist. Knowing this, feedback on the subject instead of its author would be more helpful. Thank you

"Another example a colleague I work with was able to provide. When working with a Bunsen burner. people commonly use a mesh as a platform to set your beaker on. If you remove the beaker though, the relatively wide gaped mesh still acts as a barrier that the flames do not penetrate. Very interesting"

I have a similar observation with a metal mesh over a Bunsen burner from back in high school chemistry class (c1956!). You can light the fuel above the mesh, and it will not travel thru the mesh to the burner; in other words, there is no flame between the burner and the mesh.

Theoretically:

The concept of ignition must be considered as well as the physical properties of the combustible material. Before a flame will be initiated, some mechanism of ignition must occur, an event to initiate the chemical reaction between the gas and oxygen must occur. Even if this happening on a microscopic scale . . . until such time as sufficient quantities of the material are drawn together a flame will not develop. This can be spontaneous ignition simply due to the chemical characteristics between the combustible material and oxygen. When a source of heat only is taken into a room where a combustible gas is present that heat source may not be sufficient to initiate the reaction between the gas and oxygen. This is due to the ability of the atmosphere in the room to absorb the heat by conduction, convection and radiation (the three mechanisms of heat transfer) and no ignition occurs. That does not mean that an ignition source other than the lamp (as a carried in heat source) can not inititate the combustion, sometimes static electricity is sufficient.

ahh the auto-ignition temperature. Thats a really good point I had not considered. That makes a little more sense to me. So in a theoretical situation if a flame was burning below or even just at the autoignition temperature, the contact with the mesh might disperse enough heat to lower the temperature just below the autoignition point of the surrounding gases.

Anyone know if there is a direct mathmatical relationship between the autoiginition temperature and the expected burn temperature (this might be too variant on the size of the flame, not sure).

Another concept that fits into the picture is the way a flame interacts with a membrane. As your example of a bunson burner and a wire mesh. If you study the various components of an open flame you will see that it consists of a interace between the combustible gas and the oxygen, followed by a combustion zone. Within the combustion zone, as more and more chemical action occurs, the amount of heat generated is increased and the flame changes color from blue to white, the white tip on the bunson burner is the point of highest temperature. From the white tip the color is principally blue, indicating a superheated gases. That color is the combustion products produced and when a "contaminate gas", such as a chlorinated hydrocarbon (freon) is present the flame will indicate its presence by changing color since the combustion products are now different.

Consider the insertion of a wire mesh such as the beaker sets on. That essentially disperses the flame, increasing the surface area of the combination of reactants, natural gas & oxygen is now spread over much more surface area, more surface area for reactants means better combustion, and the flame now is no longer able to "propogate" through the screen. Additionally, even when the flame is traveling down a pipe, the insertion of a screen will have a similar effect and will act as a flame suppressor and prevent the flame continuing to travel down the pipe, as in a ventilation duct or up a hose that contains acetylene gas.

In the case of a lamp mantel, the flame is inside of the mantel, and is thus not an "exposed" flame. The container represented by the mantel entirely contains the flame and thus the mantel becomes a heated surface that is essentially a heat source. Yes, it is white, but that is primarily due to the addition of the thorium or other rare-earth that produces a spectrum of light when under electon excitation, very similar to bremsstrahlung radiation (as in x-ray) generation. Without the thorium, the color would be a red-to-blue and that would not produce a significant amount of visible light.

Gasoline burning marine engines have a flame arrestor screen, to keep a backfire from igniting engine room vapors. Like you mentioned about the acetylene hose safety flash arrestors, it's just a screen. There's something about the flame 'front' can't pass a screen. Vapors can pass the screen and get ignited by something else, though.

As far as I know, early gas lamps just burned a gas flame, like a candle. They only put out so much light. Then Coleman comes out with this thorium mantle trick, and the light output is greatly increased. A working mantle is just ash, delicate as a spiderweb. Somehow it is like a catalyst or something. This whole paragraph is just my theory.

If you're after heat, I think another approach is used. The screen is going to be there to get the most heat and efficient combustion safely out of the gas flow. I remember the room gas heaters with the ceramic screens. They must have been pretty efficient, because they did not need an exhaust pipe. Some fuel burners will emit carbon monoxide; some are safer, it's an interesting science.

That was good! You explain things very well; I wish I had your skills at that.

kyzine, thank you for the post, I agree with TVP that you phrased it really well.

Thank you all, I think I have a better idea of whats going on now. This subject was really fascinating for me.

Hi Kyzine,

I think you have given a very good answer to this really interesting question, thank you.

GA to you Sir

I know it makes no difference as you already have a thumbs up, but I had to say this and give the points to a great reply to the OP.

Thanks BB for the assurance _{and insurance}

Consider also that many metals and metal oxides, are to some degree catalysts. Iron is an important catalyst in the production of ammonia. Thus, even a small contribution from catalytic action at the flame front can tend to stabilize the flame.

Hi con-science,

Thank you for asking such a really interesting question, at least to me!

You appear to have had the initial helping hand by 'PWS' so I must give that member a mention. Well down.

The second and what I think is a very good answer is from Kyzine who threw some extra detail in there I didn't know, and I know eveything!................ The last couple of words in the previous sentence are a fib! .

Why I like Gas lighting is because it also keeps you warm. And lets out a really comfortable light. Not too bright or harsh, so you go "aarrrhh" or need to cover your eyes each time the light is lit from dark. My Nan had gas lighting and she had some beautiful shades on hers.

Thank once again.

Some excellent answers.

An additional factor, is that the mesh forces the flame to burn behind it. It does this by spreading the heat out and increasing turbulence in it's immediate vicinity. the hot mesh itself ensures ignition of any combustible mixture in contact with it or close to it.

This complete combustion at or behind the mesh means that a small area beyond the mesh is loaded with combustion by-products and little oxygen, hence is incombustible.

Any movement of the flame front into this area leads to extinction of the flame there and an area which can radiate heat away thereby giving a cooled area in the tiny zone of no combustion, damping any tendency to ignite the rest of the mixture. Even if the screen itself becomes hot enough to do so, this small area of incombustible mixture adjacent to the screen would prevent the spread.

Sorry, this is a clumsy description of a fairly complex phenomenon.

Some great answers in here :) Wire mesh Woven mesh