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Consider this experiment:

https://www.youtube.com/watch?v=1BxQd6AGYiI&app=desktop

A pot of highly-flammable liquid burns with a small flame if placed on an extensive flat surface (observation A). Place to half-tubes around it which let in air in a (predominantly) circular fashion, let's say clockwise (seen from the flame). Now the flame is much higher (observation B) and it rotates like a tornado (observation C).

I would explain this in a turbulent vs laminar flow context. If a burning liquid is placed on an extensive flat surface, the flame is impeded by the turbulences it creates above itself. The air cannot rise fast enough to produce a decently long flame (explanation of A). Now, if you impose a directional air flow and this flow is laminar, then the flow velocity should be much larger and the flame is higher (observation C). The 'tornado' effect is just a side-effect of this type of rotating laminar flow (obs B).

Do you agree? What are your ideas?

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  • $\begingroup$ I wouldn't call a flame in air laminar flow... $\endgroup$ – G. Bergeron Dec 20 '16 at 11:35
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I don't think we need to invoke laminar-turbulent transition to explain this phenomenon.

On the extensive flat surface, the flame is heating all the air around. The flame rises because of the difference of temperature which leads to a difference of air density (look at a flame in microgravity, here for example). Hot air goes up, gives a vertical flame. Cooler air comes from the bottom to constantly feed the flame.

By confining the flame in the tube, only the air in the tube is heated. It still goes up but, without a opening, no cool air comes in and so no flow from bottom to top to help the flame to grow.

When he opens the confinement a bit, an air flow is driven by the pressure difference due to the temperature difference. He puts the two parts in a way that air comes tangentially so it forces to shape like this but the vortex would have developed anyway as in a sink. You can monitor the flame's height by changing the opening size. The entering air will have a different velocity in order to keep the flow rate constant.

I think he would have get the exact same result by just drilling one or more holes at the bottom of the glass vase.

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