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In this video someone filled a syringe with oxyhydrogen (2:1 hydrogen-oxygen mixture obtained with a "brown gas generator", an efficient electrolyzer).

The mixture at the tip of the needle was lit and is burning, which is understandable. What I don't understand though is why the flame doesn't travel up the needle and into the syringe.

Normally burning process is restricted by oxygen supply (which in order is restricted by products of burning, like carbon dioxide replacing the oxygen.) Only thanks to thermal updraft more oxygen is supplied, sustaining the process.

Now in this case there is no such restriction - we don't need external oxygen supply - as demonstrated in the video, the flame can even burn under water! So why won't it burn inside the needle of the syringe? Why doesn't it spread into the syringe through the needle?

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Looking at the video, it's hard to say because they don't show the whole syringe, but my guess is the operator is pushing on the plunger to force the mixture out of the needle. A free flame has a propagation speed that depends on several parameters. But for the most part, it's on the order of a few meters per second. Because of how thin the needle is relative to the plunger, it would not take much pushing to have the velocity of the gas leaving the needle exceed the propagation speed of the flame (which prevents it from traveling upstream into the needle).

This is based mostly on the observation of how far the flame is jetting out when it's in the air. That's a long flame, and there's very little deflection upwards like one would expect in a flame like a candle (where there is no forcing). Rather, it looks more like a flame in an acetylene torch where the back pressure of the gas prevents the flame from traveling into the nozzle.

It is possible in some situations that the flame might create a suction effect and basically feed itself by sucking gases into it. However, syringe plungers take quite a bit of force to move and I seriously doubt a flame that small could create enough suction to pull the plunger down and sustain itself that way.

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  • $\begingroup$ In one hand, yes, gas speed in the needle can be pretty high, on the other hand, oxyhydrogen is an explosive mix, so the propagation speed is more than "a few meters per second", so I'll wait for some more voices to confirm. (OTOH, I believe it's possible the oxygen content in the mix may be lower, some of oxygen lost to oxidation of the electrodes in the generator.) $\endgroup$ – SF. Feb 5 '15 at 2:23
  • $\begingroup$ @SF. A discussion of what causes a deflagration-to-detonation event is beyond what we can discuss here in the comments, but premixed H2-O2 flames are common. Particularly in rocket motors. Detonation is not always the end result. There are countless papers for the flame speed, here is one example where H2-O2 flames are around $6 m/s$ for laminar flame speed. H2-Air flames are usually about half that, or $3 m/s$. $\endgroup$ – tpg2114 Feb 5 '15 at 2:38
  • $\begingroup$ And note, those flame speeds are for stoichiometric ratios close to $\phi = 1.2$ and atmospheric pressures. $\endgroup$ – tpg2114 Feb 5 '15 at 2:43
  • $\begingroup$ The flame speed is not very impressive until pressure goes way up, as in a H2 O2 rocket nozzle, which in slow motion you can see does not start instantly. The pressure has to build. I once watched the flame of "brown gas" move back up a glass tube tot he plastic bag I was using for a container. If was not fast, but faster than I reacted and there was a bang like balloon popping - reaction sped up as the bag pressure increased. I'm sure there is also something about the reaction rate in a small pipe similar to flow in a small pipe. $\endgroup$ – C. Towne Springer Feb 5 '15 at 3:50
  • $\begingroup$ @C.TowneSpringer Precisely. A freely propagating H2-O2 flame is certainly cool looking, but hardly the explosion most people associate with hydrogen. $\endgroup$ – tpg2114 Feb 5 '15 at 3:53
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Prediction of the quenching effect of various surface geometries

www.sciencedirect.com/science/article/pii/S0082078455801002

The pipe walls are cold, and take part of the flame energy.If they are too close to the center of the flame, the catalitic reactions ocur near the walls but not the ignition, that could happen down the stream. So there is a critical orifice size given a stream velocity below which, gases are too cold to ignite because pipe walls take too much energy and cold gases down fast enought. That is the reason why H2+O2 cant explode then.

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