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Thermodynamics/The effects of heating a vertical pipe


I have a question regarding a phenomena that I observed when a set of unusual circumstances came about in relation to a heated object. I will try and describe this as best I can but I was unable to take accurate measurements at the time so all measurements are approximate.

A piece of thin walled steel galvanised pipe 15cm in dia and 2.4m long standing vertically with both ends unrestricted. The bottom edge of the pipe was insulated from the remainder by a piece of insulating material about 60cm square and 15mm thick. The whole length of the inside of the pipe was unrestricted including top and bottom. The free air temperature was 35 C.

Over a period of approx 90 minutes the temperature of the whole length of the pipe was gradually and evenly raised to just over 100 C with the free air remaining at 35 C.

Upon achieving maximum temperature the volume of air entering and leaving the pipe was very substantially more than could have been possible from convection alone and it was almost impossible to cap the end of the pipe due to the high air pressure.

My theory is that as the pipe gradually heated up, air was drawn in at the bottom. To start with the speed of this air increased with temperature then it reduced the surface temperature of the inside of the pipe by a fraction of a degree for a fraction of a second. This caused the air below this point to slow up momentarily as the temperature fell. However the air below this point did not slow up due to inertia and pressed against the column of air above it causing it to speed up.

At its maximum this was happening hundreds of times a second generating an ever higher air speed inside the pipe to a point governed by the final temperature of the pipe.

This leads to my questions (a) Is it possible that my theory is correct? (b) Is it possible to calculate the measurements of the pipe and the temperatures required to achieve this effect? (c) might it be possible for me to recreate this effect?

Thank you.

Hi Bob,

In essense what happens is hot air in the pipe expands and therefore becomes less dense and rises up the pipe drawing cold air in at the base. If the conditions are right the speed will reach the RESONANCE frequency of the pipe. There is an excellent article on Acoustic Resonance on Wikipeadia. That you should read.

I hope this helps.
Best wishes,


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Thermodynamics, chemistry, chemical reactions, kinetics, chemical reaction safety, dust explosion technology, static electricity. General science.


Over 40 years experience as a practicing thermochemist in industry. Head of the fire and explosion laboratory of a major European chemical company (Ciba-Geigy). Now retired.

Institute of Chemical Engineers. Royal Society of Chemistry.

Chartered Chemist, Member of the Royal Society of Chemistry (C.Chem MRSC). Msc Sheffield.

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