Effect of pitch-to-diameter ratio on the natural convection heat transfer of two vertically aligned horizontal cylinders

Natural convection heat transfer experiments were conducted for two parallel horizontal cylinders using various pitch-to-diameter ratios (P/D) from 1.02 to 9 for the Prandtl numbers between 2014 and 8334 and Rayleigh numbers between 7.3×107 and 4.5×1010. Based upon analogy concept, mass transfer rate were measured instead of heat transfer rates by measuring the limiting current of the cathodic deposition of copper from acidified copper sulfate solution. The mass transfer rates for the lower cylinders were unaffected by the presence of the upper cylinders, and agreed well with the prediction from existing heat transfer correlations developed from a single horizontal cylinder. The Nusselt number ratios of the upper to lower cylinders increased with P/D. The ratios were less than 1 at P/D values less than about 1.5 for laminar flows, but the ratios were almost 1 at a P/D very close to 1 for turbulent flows. The variation of the ratios with P/D becomes steep with higher Prandtl numbers, which is confirmed by numerical simulations using the FLUENT program. The plating pattern that appeared on the surface of the cylinder revealed local mixed convection heat transfer. The area covered by thin lines, denoting the flow separation on top of the upper cylinder, increased for laminar but decreased for turbulent flow due to the laminarization phenomena in turbulent mixed convection.

► It extended the natural convection phenomena of double cylinders to turbulent.
► Using a short analogous electroplating system, Ra of order 10 was achieved.
► Copper plating patterns revealed the flow separation and local heat transfer.
► Laminarization at the top of the upper cylinder for turbulent was visualized.
► The effects of the Pr were identified using FLUENT program.