Heat transfer from small objects in microgravity: Experiments and analysis

Heat transfer over a sub-millimeter spheroidal solid is of interest in many engineering processes. One important mechanism of heat transfer in the above processes is natural convection which leads to heat transfer rates many times larger than that of pure conduction. Despite the huge literature devoted to natural convection heat transfer rates over spheres (and to a smaller extent over spheroids) there is not a generally accepted correlation especially for small Rayleigh numbers. Existing correlations for external geometries predict a progressively increasing contribution of natural convection to heat transfer with respect to gravity (starting from zero gravity). To test the validity of these correlations, experiments are performed for the estimation of heat transfer rates at low gravity. Heat pulses are given to a miniature thermistor with a nearly spheroidal shape immersed in a liquid and its thermal response is registered during heating in parabolic flights. The contribution of natural convection to heat transfer is undoubtedly estimated from runs in which acceleration varies from 0 to 1.8 g. Surprisingly enough, the experiments showed that the Rayleigh number must take a minimum value before non-negligible effect of natural convection on heat transfer appears (existence of a threshold Rayleigh number). In the absence of natural convection (below Rathr) the experimental thermal response curves can be successfully described by approximating solutions of the transient heat conduction equation for the spheroidal geometry of the thermistor. Apparently, additional research is needed regarding the natural convection around sub-millimeter objects for small Rayleigh numbers.