Abstract
Experimental results are presented with respect to the influence of various surface roughness on the natural convection of water and spindle oil along a vertical cylinder. Here are introduced the roughness of repeated-ribs of 0.5 mm height and 6.4, 12.8, 25.6 spacing-height ratio, that of dispersed-protrusions of 0.5 mm height, and that of dense-pyramids of 1.0 mm height. The local heat-transfer coefficients in the turbulent region, which are evaluated based on the base area of the cylinder, increase slightly in the case of water and decrease slightly in the case of oil. The magnitudes of the variations, however, are at most 10 per cent of the values in the case of the smooth surface. The upper limit of the laminar region is also not affected so much by the roughness. Some considerations on these results are presented in reference to the results of measurements made on temperature profiles in the boundary layer and rising velocities of vortex-pairs and turbulent lumps, beside those of observation done on the fluid motion in the boundary layer and those hitherto reported on the case of forced convection.
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