A sphere (k = 185 W/m · K, a = 7.25 X 10-5 m2/s) of 30-mm diameter whose surface is diffuse and gray with an emissivity of 0.8 is placed in a large oven whose walls are of uniform temperature at 600 K.
The temperature of the air in the oven is 400 K, and the convection heat transfer coefficient between the sphere and the oven air is 15 W/m2 · K.
(a) Determine the net heat transfer to the sphere when its temperature is 300 K.
(b) What will be the steady-state temperature of the sphere?
(c) How long will it take for the sphere, initially at 300 K, to come within 20 K of the steady-state temperature?
(a) For a black surface at 60°C, determine the emis- sive power for the spectral region 9-12 µ,m.
(b) Calculate the radiant power (W) received by the thermograph in the same range (9-12 µ,m) when viewing, in a normal direction, a small black wall area, 200 mm2, at Ts = 60°C. The solid angle w subtended by the aperture of the thermograph when viewed from the target is 0.001 sr.
(c) Determine the radiant power (W) received by the thermograph for the same wall area (200 mm2) and solid angle (0.001 sr) when the wall is a gray, opaque, diffuse material at Ts = 60°C with emissiv- ity 0.7 and the surroundings are black at Tsur = 23°C.