Why doesn't oxyhydrogen flame travel through a narrow pipe? 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?
 A: 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.
A: 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.
