International Journal of Heat and Mass Transfer, Vol.39, No.9, 1815-1826, 1996
Convective Heat-Transfer of Rotating Spheres and Spheroids with Nonuniform Surface Temperatures
The latitudinal and overall convective heat transfer of relating spheres and spheroids with axis ratios of 0.50 less than or equal to alpha less than or equal to 1.0 and limited thermal diffusivity and, thus, non-uniform surface temperatures was experimentally investigated in a wind tunnel over the range of Reynolds numbers 1.1 x 10(4) < Re < 5.2 x 10(4). The surface temperature was remotely measured with an "AGEMA 800" thermal imaging system during particle cooling in the tunnel. A numerical technique was then used to calculate the time-evolution of the temperature distribution within the particle and the convective heat transfer coefficients at the surface. The results indicate how the convective heat transfer of rotating particles varies with latitude. the rotation rate (equivalent to Strouhal numbers 0 less than or equal to Sr less than or equal to 0.06) and the position of rotation axis. Within experimental error, these factors do not influence the rotal heat transfer. However, they will be of importance in two-component, three-phase systems where deposition of substances, chemical reactions including release of latent heat, radiation and other processes are specific to site and local temperature. The present study, directed towards the local transfer rates controlling hailstone growth, provides the basic method of how to approach such complex situations. The overall Nusselt number, determined by integrating latitudinal heat transfer over the whole particle surface, was parameterized as a function of Reynolds number and axis ratio. Good agreement was found between this parameterization and direct overall measurements by other authors, indicating that thermal diffusivity changes from copper and aluminum to ice have a negligible effect.