Electrical conductivity of flames (OT?)

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   ...Jim Thompson

"OK genius, what's your answer" (in the voice of Kayla from Firefly. :^)

There's also a big thermal gradient from the nozzle to the probe.

Seems like the electrons would like to flow along the thermal gradient, from hot to cold.

George H.

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Larkin can always be counted on to provide some snotty (but useless) remark. ...Jim Thompson

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So much for "nailing" it.

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I remember that. I think I was in HS too. As I recall there were 2 electrodes and HV was required but I don't recall how high. Was there a DC bias too? The low end response was limited by the length of the flame between the electrodes but the high end was way up there. It was also supposed to work as a microphone too. Art

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The interesting part of electrical conductivity through gases usually turns out to be what's happening at the surfaces of the electrodes.

At the positive electrode it's usually electron capture. At the negative electrode you've got to have something going on that ejects electrons from the electrode surface.

Mostly this is positive ion bombardment. If you get up into the arc regime, the surface gets hot enough to deform and in an electric field it deforms into a bed of spikes, with the tips of the spikes sharp enough to give you thermally assisted field-emission. Keeping a glow discharge going with positive ion bombardment needs a lot more voltage drop up against the anode than does an arc.

The pointy electrode is going to get to arc discharge conditions a lot faster than a smooth spherical electrode. That might do your rectification.

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Bill Sloman, Sydney

The therrmopile is certainly standard for many older water heaters. But the application is the pilot is already on, and you are just detecting when the pilot light goes out. If the pilot goes out, the thermopile can no longer hold open the relay for the gas.

Obviously over the years the safety features have become more complicated. Look at a Takagi water heater if you want to see a complicate control system. Not only does it detect if there is a flame, but it can check the quality of the flame to insure the heater is burning the fuel correctly.

Takagi makes good stuff if you are looking for a tankless heater. They don't spend a lot of money on mass marketing.

Back to flame detection, it seems counterintuitive, but some flame detection schemes look for UV light.

George Herold schrieb:

Hello,

here is another application of currents through a flame:

A generator without moving parts. It is possible also using thermocouples, but the efficency is poor.

Bye

On a sunny day (Thu, 4 Apr 2013 18:20:48 -0400) it happened "tm" wrote in :

Small thermocouples I have used heat up in a few ms. Do not know how long those will live inside a flame, Optical detection?

I wonder how much UV is involved. Flames make a lot of UV, and UV flame detector tubes use the photoemission off a metalic surface to initiate a gas discharge.

--

John Larkin                  Highland Technology Inc 
www.highlandtechnology.com   jlarkin at highlandtechnology dot com    

Precision electronic instrumentation 
Picosecond-resolution Digital Delay and Pulse generators 
Custom timing and laser controllers 
Photonics and fiberoptic TTL data links 
VME  analog, thermocouple, LVDT, synchro, tachometer 
Multichannel arbitrary waveform generators

I don't think this has anything to do with thermionic emission, it is an io nized plasma phenomenon. The charged particles arrange themselves to mainta in some kind of space charge neutrality by developing a spatial potential, and this potential has to be overcome before the plasma conducts. Since the potential has a fixed orientation, the plasma ends up being a rectifier. T he patent requirement about relative areas of the probes probably has to do with establishing the geometry and orientation of the plasma potential. Th e electrical properties of flames and their resulting plasmas have been stu died extensively since the early 1900s but I don't find anything on the rec tification effect until 1966 when computers became available to numerically solve the multitude of equations representing all the physics.

Artemus schrieb:

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Reinhard

AFAIUI, the maximum delay requirement will depend on the amount of gas that can escape in that time, and what kind of damage it could do. Lot of systems use thermopiles to hold a gas valve open, many (most?) consumer gas appliances these days use flame rectification ("flame rod"), and many big systems use UV optical means ("Fireye" is one brand). There's typically a purge cycle before a restart so even if some gas has accumulated it doesn't tend to go bang. But if gas is leaking out slowly over a long period of time, eventually it will probably find a flame, and if it's in an enclosed volume you could have a big and destructive bang.

Best regards, Spehro Pefhany

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Hi Bill, This is a plasma which (I think) makes it different. I don't see any problem getting the current into and out of the probe since it's sitting right in the flame. On the other side I'm not sure what happens right at the burner interface, ions have to get from the flame/plasma to the burner. (?)

Someone in a different group found this thesis online.

I'm not sure I buy the mechanism, but it does talk about more resistance with higher flow rates. Which is at least consistent with my 'electrons against the stream' hypothesis.

Say I don?t know plasmas at all, can I assume that most of the current is carried by the electrons and essentially ignore the ionized atoms/ molecules?

George H.

ionized plasma phenomenon. The charged particles arrange themselves to main tain some kind of space charge neutrality by developing a spatial potential , and this potential has to be overcome before the plasma conducts. Since t he potential has a fixed orientation, the plasma ends up being a rectifier.

Fred, can you explain a bit more where the 'built in' potential comes from?

The patent requirement about relative areas of the probes probably has to do with establishing the geometry and orientation of the plasma potential. The electrical properties of flames and their resulting plasmas have been studied extensively since the early 1900s but I don't find anything on the rectification effect until 1966 when computers became available to numerically solve the multitude of equations representing all the physics.

Yeah I found a nice paper by R.A. Wilson in Rev. Mod. Physics (3)

156-189 (1931) (but no rectification discussion.) Do you have a reference for one of the more recent papers that talks about the space charge and where the potentail comes from.

George H.

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Hi Bud, Yeah it appears the conduction is better with the rod/probe positive. But I think almost all the current will be carried by the electrons not matter which direction the current is flowing in. (My plasma physics is weak.... but for a given electric field there will be some drift velocity for both the ionized atoms and electrons. If we assume that they have the same collision rate, then the electrons are going to get moving faster between collisions due to their light mass.)

George H.

There is ionized gas at the flame rod. I think there is conduction in both directions, but higher conduction with the rod positive. My guess is that is because the electrons are lighter, thus more mobile, and more electrons can reach the rod than ions.

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Thompson? Attempting irony? Your remark about rectification was 100% useless, repeating what had just been said with no hint of knowledge or of information; that was *my* point.

We already assumed that you wouldn't know the Physics, so that was obvious, too.

Try saying something useful, or intelligent, or maybe even informed, now and then.

--

John Larkin         Highland Technology, Inc 

jlarkin at highlandtechnology dot com 
http://www.highlandtechnology.com 

Precision electronic instrumentation 
Picosecond-resolution Digital Delay and Pulse generators 
Custom laser drivers and controllers 
Photonics and fiberoptic TTL data links 
VME thermocouple, LVDT, synchro   acquisition and simulation

discusses the Debye potential. The paper I found was 1966 British Royal something another, Rectification properties spherical probe in plasma , something or another, can't find it now...