Natural convection heat transfer from two horizontal cylinders at high Rayleigh numbers

This paper presents an experimental investigation conducted on two evenly heated vertically arranged horizontal cylinders with a diameter of 54 mm in water. The change in Nusselt number on the upper cylinder compared to a single cylinder is presented for seven Rayleigh numbers ranging from 1.82E7 to 2.55E8 for vertical separation distances S = 1.5D, 2D, 3D, 4D, and 5D. The results show that the average Nusselt number on the upper cylinder increases for 32 out of the 35 investigated cases. A statistical description of the ensemble averaged velocity field above the upper cylinder is also presented for Rayleigh numbers 1.82E7 and 5.18E7 for S = 2D and 4D. A comparison of the buoyant plume above the upper cylinder with the plume above a single cylinder showed that the flow characteristics change considerably. The plume is significantly wider and the velocity fluctuations are about two times larger than for a single cylinder. The total kinetic energy level above the second cylinder is larger than above a single cylinder. Far downstream the plume above the second cylinder approaches a Gaussian profile associated with turbulent planar plumes.