Experimental investigation on free convection from a horizontal cylinder beneath an adiabatic ceiling

Steady two-dimensional free convection heat transfer from a horizontal, isothermal cylinder located underneath an adiabatic ceiling is studied experimentally using a Mach–Zehnder interferometer. Effects of the ratio of cylinder spacing from the adiabatic ceiling to its diameter (L/D) on heat transfer from the cylinder are investigated for various Rayleigh numbers. Experiments are performed for L/D = 0.1, 0.3, 0.5, 0.7, 1.0, 1.5 and 2.4, and the Rayleigh number ranging from 1000 to 40,000. For each experiment thermal and flow fields are illustrated. Results indicate that the adiabatic ceiling has no influence on free convection heat transfer from the cylinder for L/D ⩾ 1.5. But by reducing this ratio from 1.5 to 0.5, Nusselt number decreases considerably. For close spacing of (L/D < 0.5), average Nusselt number again increases due to the vortex formation at the top of the cylinder where its effect is present as a sharp increase in the magnitude of local Nusselt number around circumferential position 130 ⩽ θ ⩽ 180. Flow visualization by smoke is done to exhibit the existence and the location of the vortices. A correlation based on the experimental data for the average Nusselt number of the cylinder as a function of Ra and L/D is presented in the aforementioned ranges.