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Published online by Cambridge University Press: 05 June 2012
Principle
The rate at which heat is transferred from the surface of a heated object will become equivalent to the rate at which heat is generated within the object when steady-state conditions have been attained. If the associated conduction and convection heat-transfer effects can be eliminated, radiation heat transfer will become the only means of transferring the heat.
Object
It is an easy matter to model the heat-exchange process that takes place between a heated sphere and a spherical shell in which it is concentrically located because of the simplicity of the geometry. If the heat flowing along the support by which the heated sphere is suspended is negligible and most of the air has been extracted from the space between the heated sphere and the spherical shell, radiation heat transfer becomes the only mechanism capable of transferring the heat generated within the sphere. The purpose of this experiment is to determine the emissivity of a copper surface by substituting the temperatures of the heated sphere and the glass shell corresponding to steady-state conditions into the radiation heat-transfer equation and performing an analysis that yields the value of emissivity.
Background
Figure 23.1 depicts a copper sphere of radius Rcs suspended within a spherical glass jar of radius Rgj. The sphere is attached to the cover plate by a thin-walled stainless-steel tube which prevents any significant heat transfer by conduction.
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