In a material processing experiment conducted aboard the space shuttle, a coated niobium sphere of 10-min diameter is removed from a furnace at 900°C and cooled to a temperature of 300°C. Although properties of the niobium vary over this temperature range, constant values may be assumed to a reasonable approximation, with
(a) If cooling is implemented in a large evacuated chamber whose walls are at 25°C, determine the time required to reach the final temperature if the coating is polished and has an emissivity of £ = 0.1. How long would it take if the coating is oxidized and
(b) To reduce the time required for cooling, consideration is given to immersion of the sphere in an inert gas stream for which
(c) Considering the effect of both radiation and convection, what is the time required for cooling if
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Introduction to Heat Transfer
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