CONVECTIVE HEAT-TRANSFER OF ROTATING SPHERES AND SPHEROIDS WITH NONUNIFORM SURFACE TEMPERATURES

The latitudinal and overall convective heat transfer of relating spher es 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, n on-uniform surface temperatures was experimentally investigated in a w ind 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 t he temperature distribution within the particle and the convective hea t transfer coefficients at the surface. The results indicate how the c onvective heat transfer of rotating particles varies with latitude. th e rotation rate (equivalent to Strouhal numbers 0 less than or equal t o Sr less than or equal to 0.06) and the position of rotation axis. Wi thin 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 incl uding release of latent heat, radiation and other processes are specif ic to site and local temperature. The present study, directed towards the local transfer rates controlling hailstone growth, provides the ba sic method of how to approach such complex situations. The overall Nus selt number, determined by integrating latitudinal heat transfer over the whole particle surface, was parameterized as a function of Reynold s number and axis ratio. Good agreement was found between this paramet erization and direct overall measurements by other authors, indicating that thermal diffusivity changes from copper and aluminum to ice have a negligible effect.