Experimental study of free convection from a pair of vertical arrays of horizontal cylinders at very low Rayleigh numbers

Steady laminar free convection from a pair of vertical arrays of equally-spaced uniformly heated horizontal cylinders set in free air is studied experimentally. A specifically developed experimental facility is used to perform heat flux and temperature measurements and thermal field visualization by means of Schlieren technique. Experiments are performed for pairs of tube-arrays consisting of five circular cylinders, for center-to-center horizontal and vertical spacing ranging from 2 to 145 cylinder-diameters and from 4 to 12 cylinder-diameters respectively, and for values of the Rayleigh number based on the cylinder-diameter in the range between 2.4 and 11.9. It is found that any cylinder may exhibit either enhanced or reduced Nusselt numbers with respect to the case of single array, depending on its location in the array, on the geometry of the array, as well as on the Rayleigh number. The visualization has allowed to relate a number of observed effects to the fact that side-by-side placement of the arrays produces a lateral air suction into the space between them.