A deep cavity of 50-mm diameter approximates a blackbody and is maintained at 250°C while exposed to solar irradiation of 800 W/m 2 and surroundings and ambient air at 25°C. A thin window of spectral transmissivity and reflectivity 0.9 and 0, respectively, for the spectral range 0.2 to 4?m is placed over the cavity opening. In the spectral range beyond 4?m, the window behaves as an opaque, diffuse, gray body of emissivity 0.95. Assuming that the convection coefficient on the upper surface of the window is 10 W/m2 ? K, determine the temperature of the window and the power required to maintain the cavity at 250°C.

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Gg = 800 W/m? 25°C -T, Window D= 50 mm