Experimental Free Convection From Piping in District Heating Utilidors

Experiments were performed to study the two-dimensional natural convection heat transfer from two heated isothermal horizontal cylinders to an isothermal-cooled rectangular enclosure. The experiments were designed to simulate the heat transfer encountered in underground heat distribution systems where steam and condensate lines are routed through underground or in-ground corridors (utilidors) from a central plant. The steam supply and condensate return lines were simulated with two copper cylinders. The fluid between the cylinders and enclosure was distilled water to simulate the Rayleigh number range encountered with air in actual utilidors. Results were obtained for the overall heat transfer coefficient between the two cylinders and the enclosure. The data was correlated over a Rayleigh number, Ra_L, range of 2.1 × 10⁸ to 4.8 × 10⁹ representative of the Rayleigh number, based upon a hypothetical gap width, in a typical utilidor exposed to extreme enclosure to piping temperature differential boundary conditions. The corresponding Nusselt numbers, Nu_L, ranged from 21 to 59 when both cylinders were heated for water as the intermediate fluid. Corresponding heat transfer coefficients calculated for the utilidor case with air as the intermediate fluid were found to be smaller compared to some other correlations for concentric cylinders.