An experiment is to be designed to measure free convection heat-transfer coefficients from spheres by preheating aluminum spheres of various diameters to an initial temperature and then measuring the temperature response as each sphere cools in room air. Because of the low value of the Biot number the sphere may be assumed to behave as a lumped capacity. The sphere is also blackened so that the radiation loss from the outer surface will be given by qrad = σ Asurf (T4 −T4surr), where the temperatures are in degrees Kelvin. From information in this chapter, anticipate the cooling curve behavior for 5-mm-, 25-mm-, and 50-mm-diameter aluminum spheres cooling from 230oC in room air at 20oC. How often would you advise reading the temperatures of the spheres and room? What range of Rayleigh numbers would you expect to observe in these experiments? Can you suggest a way to correlate the data in terms of the significant dimensionless groups?