Abstract
Abstract The article presents an experimental and numerical investigation of unsteady-state natural convection in air in a horizontal cylindrical tube with constant heat generation on the wall. After the start of heating, a linear increase in the wall temperature with time is established. The ends of the tube are open. The flow structure is observed by using the smoke visualization technique, and the temperature field is investigated with the aid of an optical Mach-Zehnder interferometer. In the process of the experiment, the Rayleigh number increases with tube heating from zero to the maximum values, which vary from 8·4 × 103 at the specific heat flux on the tube wall, qw = 4·96 kW/m2, to 1·2 × 105 at qw = 17·1 kW/ m2. The original free-convective flow had a two-vortex structure. With further heating, a stepwise transition to a four-vortex structure is observed, which persists up to the maximum Rayleigh number attained.
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