I just read Cecil’s article on What exactly is fire. According to the article fire is a hot glowing gas. Since it is physical matter I wondered if it could conduct electricity. I know that our atmosphere (a mixture of gases) can conduct electricity, since lightning and spark plugs exist, but what about fire. Is there an agreed upon electrical resistance, or would it be just like trying to arc through air.
Yes, although I expect it’s not going to have the same sort of resistance/etc properties you’d expect from a short piece of copper wire.
AFAIK, some pilot light detectors in gas-fired heating equipment work by the conductivity of the flame.
That is the coolest effin thing I have ever seen. That is going to the top of my project list. I just have to find the plans.
Yes. Within the flame front, the chemical reaction generates all kinds of ion activity, which presents the opportunity for electrical conduction.
When I was in grad school I did research on an internal combustion engine. One aspect of my work involved measuring the progress of the flame front across the combustion chamber. The cylinder head was fitted with a number of ionization probes, each of which detected the passage of the flame at its location. Each probe was a wire tip exposed to the combustion chamber, but insulated from the surrounding cylinder head metal by a thin ceramic tube. The wire tip was biased to 100 volts, and when the flame arrived at the probe tip, the ions created an electrically conductive path from the probe tip to the surrounding head material, allowing electrical current to flow from the probe to the cylinder head. So when we saw a spike in current flow from the probe, we knew the flame had just gone by.
This same flame detection technology is used on many residential furnaces to detect ignition of the main gas burner. If no flame is detected within X seconds after the furnace’s computer opens the gas valve, the computer will shut off the gas and wait for a while (for unburned gas to clear out) before reopening the gas valve and attempting ignition again. After a certain number of ignition attempts without detecting a flame, the computer assumes there’s a real problem and shuts the furnace down altogether.
The atmosphere really doesn’t like to conduct electricity, although if you crank up the voltage enough, you can ionize it and make it conduct electricity. That’s why lightning and spark plugs work: voltage builds and builds until SNAP you get a sudden electrical breakdown of the air between point A and point B that dramatically reduces resistance and lets current flow. In the atmosphere, you need to produce a voltage of about 20,000 volts per inch of distance to cause the air to ionize and begin conducting. At higher pressures, the voltage-per-distance requirement is even higher; inside an engine, where the compression stroke produces pressures of 20-30 atmopheres (before ignition), it may take 30,000 volts or more to produce a spark, even though the spark plug’s electrode gap is only a tiny fraction of an inch.
What is so special about the flame front that causes ion activity? Does it work with other hydrocarbons like diesel, kerosene or paper? (BTW i work as a mechanic so engine talk is right up my ally)
To the above can a flame complete a simple circuit, say to light a lightbulb?
I think it will take a chemist to explain in great detail, but my understanding is that even a seemingly simple bulk chemical reaction (e.g. combustion of methane, CH[sub]4[/sub], in air) has an astounding array of intermediate sub-reactions, many of which produce ions and free electrons as various intermediate compounds form and are torn apart.
My grad school work was with propane as a fuel, but a colleague used similar instrumentation on a gasoline-fueld engine. Knowing that ionization flame detectors are used in residential furnaces, I can confirm that the technique also works with natural gas. It seems likely that it would work with any hydrocarbon fuel; since paper also has hydrogen and carbon in it, I’ll wager it works there too, although the flame front is a lot murkier there.
Since the ions within the flame front form a conductive path, yes, they can complete a simple circuit, and this is exactly the thing that makes an ionization flame detector work. Could it pass enough current to light a lightbulb? Depends on the lightbulb, and on the flame detector. Regarding the ion probes I used back in grad school, with a back-of-the-envelope calculation I estimated the resistance of the ion path during flame passage to be a few hundred megaohms. With a lot of voltage, and a large-surface-area ion probe, maybe you could maybe move enough current to light a small LED. You would have better luck using the ionization signal as an input to a computer/controller, or to a solid-state relay, which in turn controls a separate circuit that powers a suitably bright light.
Nah, it’s a resistor. Needs low current, high voltage. Maybe it can light an LED or a neon sign.
In those “flame speaker” projects, usually they have you add a wick made of fiberglass, and wet it with salt solution. If flame glows sodium-yellow or copper-green, you’ve injected plenty of ions and increased conductivity.
They do indeed, it’s called the flame rectification principle (page 11 of this document outlines it). I used to design such gas ignitor systems, and I was told that no-one really knew what the underlying mechanism was, just that it was reliable enough to be allowed under the relevant safety standards. I could have been misinformed, it seems that an anion/cation imbalance within the flame would be the most obvious explanation.
Anyhoo, the electrical model of a relatively clean gas flame looks like a high resistance (about a few hundred million ohms, give or take quite a lot) in series with a perfect diode. If you apply an AC voltage to the ignitor electrode the flame rectification effect produces a small DC bias, and this can be detected quite easily. When no flame is present, the DC component is zero.
However, if you’re talking about arcing a high voltage within a flame then the electrical model will be quite different. Here the crucial parameter will be the arcing voltage (usually expressed in V/m), and this will depend upon the gas mix, the ionisation level, and the gas pressure. Below the arcing threshold the electrical analogue of a flame is the resistor/diode model as described above, but during arcing the high level of ionisation means the model now looks like more of a short circuit, or at least quite a low resistance. Spark plug manufacturers know all about this sort of thing.