An apparatus commonly used for measuring the reflec-tivity
Chapter 12, Problem 12.92(choose chapter or problem)
An apparatus commonly used for measuring the reflec-tivity of materials is shown below. A water- cooledsample, of 30-mm diameter and temperature Ts?300 K, is mounted flush with the inner surface of alarge enclosure. The walls of the enclosure are grayand diffuse with an emissivity of 0.8 and a uniformtemperature Tf?1000 K. A small aperture is locatedat the bottom of the enclosure to permit sighting of thesample or the enclosure wall. The spectral reflectivityof an opaque, diffuse sample material is as shown.The heat transfer coefficient for convection betweenthe sample and the air within the cavity, which is alsoat 1000 K, is h?10 W/m2?K. (a) Calculate the absorptivity of the sample.(b) Calculate the emissivity of the sample.(c) Determine the heat removal rate (W) by thecoolant.(d) The ratio of the radiation in the A direction to thatin the B direction will give the reflectivity of thesample. Briefly explain why this is so
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