What happens when two flames (whether from dragons or blowtorches) meet head on? Many films make it appears as a front where the two flames repel each other, but my intuition is that, in fact, the two flames would continue as two waves on the surface of a lake.
In short, does anyone have sufficient scientific skills (or experience) to describe what happens when two flames of the same intensity impact directly opposite one another?
Can a dragon block a fire attack with its own fire?
Flames are not waves, which can cross each other without perturbation.
Flames are basically flows of particles (the combustible) which are undergoing an energetic, exothermic reactions with the comburent (usually air Oxygen).
When the particle flows cross each other they will influence their respective motion, with a "simple" vectorial summation of their velocities, executed according to the principles of conservation of momentum and energy.
Fire is not "a thing" itself, but rather the rapid, exothermic oxidation of some material, normally gas (most or all liquids, that seem to burn are actually evaporating from the heat and only the gas burns).
So if two streams of fire meet, they will in most cases behave, like two colliding streams of gas. Unfortunately I do not seem to be able to find a simulation of this. But here, you can see two streams of water collide. Gas will mostly look similar, but there will be more mixing of the two streams and more turbulences, resulting in a bigger cloud where they meet.
[edit: added from comment]
If angle, strength and surface are no exact match, but roughly right, it will still result in a lot of turbulences and most parts of the streams will disperse. But it quickly gets unpredictable and there may be small clouds of flame that reach one of the participants or cause collateral damage. If you watch the video, you will see, that the streams are not exactly the same. As a result, in the first 3 seconds the right source is hit by a blob of the left stream. After that, the streams reach an equilibrium. But this is partly because the sources are perfectly still.
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This of course only works, if both streams are roughly the same size and power and directly colliding. If one is bigger or has more pressure or they hit each other at an angle, it starts to get complicated.
But in basic principle, yes, you can block a stream of fire with another stream of fire.
Fire happens when some sort of fuel undergoes combustion. With dragons this is usually presumed to be a flammable gas they are exhaling. The danger of such a fire comes from the combustion heating the gases that result from the combustion and other nearby gases to such a high temperature that they'll burn you or set you on fire if they touch you. So if you want to block a dragon's fire blast, you need to prevent that stream of very hot gases from touching you.
A simple experiment would be to have two people try to blow smoke into each other's faces at the same time. That should be a very rough approximation of what would happen. I suspect that the two streams actually would cancel out and become a cloud of smoke in the middle.
Well since fire has no physical substance they should carry on through each other like, "two waves on the surface of a lake." However fire consumes oxygen so passing through where fire had just burn oxygen, they should both die immediately if in a confined space, while if they are outside, imaginable when two fire breathing creatures meet, then the fire should just lose some or most of its size, with anther possibility being that they will combine into one flame (it would depend on the speed of the fire). However if the fire was being created and maintained by a liquid fuel the effect would be near identical to that of which when water meets water head on.
