A Simple Instructional Experiment on Unsteady Heat Conduction That Quantifies the Existing Competition between Natural Convection and Radiation

Abstract

The goal of this paper was to examine the unsteady cooling of a hot small sphere that was suspended from the ceiling of a large room by a wire. Theoretical analyses and experimental observations have demonstrated that the magnitudes of the convective heat transfer coefficients in natural convective air flows around bodies are small. In all likelihood, equivalent radiative transfer coefficients have comparable magnitudes, particularly if a metallic surface is nearly black, to deliver a high value of the total hemispherical emissivity of the surface of a body. Adopting the lumped model, the variations of temperature with time were estimated based on two mechanisms: (a) nonlinear natural convection, (b) surface radiation, and (c) nonlinear natural convection coupled with surface radiation. Employing an initial temperature of 550 C, the temperature evolution of the sphere was predicted analytically and numerically and also measured experimentally. The experimental temperature versus time data gathered have convincingly demonstrated that when the mechanisms of nonlinear natural convection and surface radiation are brought together, the numerically-determined temperatures went down markedly, coming close to the measured temperatures.

Keywords

sphere unsteady heat conduction nonlinear natural convection surface radiation

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