UNSTEADY HEAT AND MASS TRANSFER FROM SPHERES

This paper reports theoretical and experimental results on unsteady heat and mass transfer from small spherical bodies for small Strouhal numbers. Experiments are done in a stream consisting of steady and unsteady components. Unsteady flows are generated by an acoustic resonance in a tube and fluctuate purely sinusoidally. In heat-transfer experiments in an air stream several spherical thermistors are used and the time-averaged Nusselt number is shown to be well correlated, both experimentally and theoretically, by means of Reynolds number with time-averaged velocity as its characteristic velocity and the ratio of velocity amplitude of unsteady component to steady component. Unsteady mass-transfer experiments are done by evaporation of small droplets of water and prophyl-alcohol in an air stream. The results are found to be represented by a similar correlating relation to that for heat transfer. A resonant oscillation of deformation of a droplet is observed due to surface tension and during this resonance the Sherwood number is found to increase proportionally to the unsteady component of the stream. © 1969.