Heat Transfer From Spheres in the Naturally Turbulent, Outdoor Environment

Abstract

Heat transfer coefficients from spheres were measured in a naturally turbulent, outdoor environment and compared to measurements made in wind tunnels at various turbulent intensities. The experiments were performed using spheres of three different diameters placed at different heights relative to the ground surface. The time-average values of Nusselt number and Reynolds number were obtained over 5-min time periods. Over the Reynolds number range 2000–35,000, the value of the Nusselt number obtained outdoors was up to 2.2 times the values determined in low turbulent intensity wind tunnels. The data, averaged for each sphere at each height, showed an average enhancement of the Nusselt number that decreased with height from 1.8 to 1.1. The heat transfer enhancement was found to be essentially independent of Reynolds number and correlated with the ratio of height above the surface to the sphere diameter. Standard micrometeorological theory was used to estimate the turbulence intensity as a function of height above the ground. The enhancement in heat transfer was found to be mainly correlated with turbulence intensity. The results for all but the lowest placed spheres are in substantial agreement with results using artificially induced turbulence.