Talk:Flame rectification

Underlying rectification mechanism

The reference Möllberg, Andreas (2005) does not allow one to conclude what the underlying mechanism of the rectification is, because it deals exclusively with electrodes that are asymmetric in several simultaneous respects, due to the one being the burner, and the the other an electrode in the flame:

• The electrode sizes are substantially different
• The one is always downstream in the gas flow
• The burner is always at one extreme of the flame (the base)
• The two electrodes are in chemically different parts of the flame
• The two electrodes are most probably at substantially different temperatures

I have not looked at the other two references. However, one should guard against the assumption that carrier mobility is responsible for the effect, as this has little explanatory power unless combined with some other asymmetry. In as article such as this, it would be safest to leave out the explanation altogether, and simply document that the effect is used for various purposes. Speculation on what the real underlying effect is, a likely candidate might be primarily the different chemical environments of the electrodes (different concentrations of ionisation or even ion species at the plasma-electrode interface). Two factors would contribute to this: the different state of combustion at various parts of the flame, and different temperatures. Factors such as the velocity flow vector, electrode size, electrode material and electron–ion mobility differences seem to me unlikely to play a substantial role. It might be as simple as the rectification effect of a vacuum tube: a hot electrode could release electrons more readily into a plasma than a cold one, the plasma acts as a conductive path, and free electrons might combine unimpeded with a colder electrode. However, there will be competing effects, such as ionization and de-ionization occurring at the plasma–electrode interface, where the ions rather than free electrons become the dominant current carriers at one or both interfaces, akin to electrolysis. — Quondum 20:27, 30 June 2013 (UTC)

I agree completely. You could also add to the list that some of the sources suggest that one of the electrodes should be specially coated to make the effect most effective. I've changed the language of the intro to the article to "The effect is commonly described as being caused by..." to reflect this. Given the fairly comprehensive literature on other aspects of plasma physics, it's remarkable that a component of millions of gas appliances worldwide operates by physical principles which are apparently not documented in the academic literature. -- The Anome (talk) 19:17, 3 July 2013 (UTC)

How it really works

I've read a number of articles online describing how flame rectification allegedly works in gas furnace ignition systems, and for the most part they're all wrong. It's best to think of it as a thermionic emitter system, i.e., the same thing that goes on in a vacuum tube rectifier. A high alternating voltage is applied to the igniter electrode, which produces an electric arc in front of a jet of gas and air. The jet ignites and the flame heats the electrode to red or orange heat, while another electrode, typically of much heavier construction and made of stainless steel or other metal that resists oxidation and connector to the electrical ground of the furnace cabinet, remains relatively cool. The red-hot electrode becomes a net emitter of electrons (cathode), while the cooler electrode collects the emitted electrons (anode). As long as the high alternating voltage continues to be applied to the igniter electrodes, there will be a larger net electron flow from the hot electrode to the cool electrode, on the order of microamperes to milliamperes. This DC bias in the electric arc can be detected with appropriate circuitry and used to prove that there's a flame. If the DC bias disappears, we can assume the flame has gone out and the gas valve is then closed to prevent an explosion hazard. To understand flame rectification at a visceral level, it helps if one has worked with vacuum tube amplifiers, but since the advent of transistors, most engineering schools have neglected this technology for the last 40 years. — Quicksilver^{T @} 05:47, 28 September 2013 (UTC)