Two concentric spheres of diameter D = 0.8 m and D2 T1 = 410 K and T2 = 300 K. = 1.2 m are separated by an air space and have surface temperatures of (a) If the surfaces are black, what is the net rate of radiation exchange between the spheres, in W? 912 = W (b) What is the net rate of radiation exchange between the surfaces if they are diffuse and gray with & = 0.5 and ɛ2 = 0.05, in W? 912 = W (c) What is the net rate of radiation exchange if D2 is increased to 20 m, with ɛ2 = 0.05, ɛj = 0.5, and D = 0.8 m, in W? 912 = i W

Two concentric spheres of diameter D = 0.8 m and D2 T1 = 410 K and T2 = 300 K. = 1.2 m are separated by an air space and have surface temperatures of (a) If the surfaces are black, what is the net rate of radiation exchange between the spheres, in W? 912 = W (b) What is the net rate of radiation exchange between the surfaces if they are diffuse and gray with & = 0.5 and ɛ2 = 0.05, in W? 912 = W (c) What is the net rate of radiation exchange if D2 is increased to 20 m, with ɛ2 = 0.05, ɛj = 0.5, and D = 0.8 m, in W? 912 = i W

Principles of Heat Transfer (Activate Learning with these NEW titles from Engineering!)

8th Edition

ISBN:9781305387102

Author:Kreith, Frank; Manglik, Raj M.

Publisher:Kreith, Frank; Manglik, Raj M.

Chapter11: Heat Transfer By Radiation

Section: Chapter Questions

Problem 11.49P

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A long, horizontal, cylindrical steel reactor, 1 m in diameter, has a surface temperature of 300ºC. The emissivity of the steel is 0.6, and the heat transfer coefficient for natural convection is 5 W m−2 K−1 . Heat is lost by convection to the air at 15ºC, and also by radiation to the surroundings, which can be considered to be a black body at 15ºC. a) Calculate the total heat loss per metre length of the reactor, and the proportions lost by convection and radiation b) The reactor is then insulated with a thin layer of insulation material to reduce the total heat loss to one-tenth of its original value. This causes the surface temperature of the steel to rise to 400ºC. The thermal conductivity of the insulation is 0.01 W m−1 K−1 , and its surface emissivity is 0.2. Show that the resulting surface temperature of the insulation is about 89ºC, and calculate the thickness of insulation required, stating any assumptions made. Specifically need help with part b