I would like to get an explanations of an observed physical phenomena.
When a metal tube is placed vertically and heated and the air inside rises, the speed of the air moving through the tube continues to increase greatly when the tube reaches a certain temperature, despite no further increase in temperature.
I would like to understand why.
A little background:
Many years ago I was working on a construction project in the Kalahari Desert when I saw something unusual. In the works yard. There was a length of straight galvanised thin wall steel pipe standing vertically, about 15 cm dia and about 2.4 m long, that was making a very unusual noise like a jet engine. I went closer and saw that the bottom of the pipe rested on the corner of a tin box and was in the shade but the rest of the pipe was in the hot sun (45 degrees in the shade) and I was unable to touch it due to its high temperature. The noise was being generated by air that was being drawn up into the bottom of the pipe and then pushed out of the top at extremely high speed.
The only reason that I could see for this was that I was observing some sort of ram or standing wave effect and the air rising up the inside of the pipe had accelerated due to this. As the sun went down and the temperature dropped the phenomenon got less and less until it stopped. The same thing happened each day when the pipe got hot.
Best Answer
The effect you describe is really just the "draw" of a chimney - hot air rises because it is less dense than cooler air. The weight of the column of air inside the chimney is this less than the weight of the column of air it displaced (Archimedes' principle) and it experiences an upwards force. Again, because air is light you only need a little bit of force to accelerate it.
According to the calculator found at http://engineeringtoolbox.com/natural-draught-ventilation-d_122.html , for the dimensions you gave, and assuming the air inside the tube is heated uniformly to 45 C (with an ambient of 25 C - I just picked a number) the air flow velocity would be 1.8 m/s. In itself that might not be "roaring", but I suspect that your pipe acted a bit like an organ pipe - with a lot of resonances. That resonance leads to an amplification of the sound of rushing air, and that is why the pipe made such a lot of noise.
With the dimensions given, the air inside could quickly get up to temperature. Total air flow per hour would be about 120 m3 which would require about 2.5 MJ to heat by 25 C. The area exposed to sunlight was about 1/3 of a square meter meaning it would receive a few 100 W of sunlight. And 100 W for an hour would give 3.6 MJ. The numbers add up (are of the right order of magnitude).