13.72 An opaque, diffuse, gray (200 mm x 200 mm) plate with an emissivity of o.8 is placed over the opening of a furnaceand is known to be at 400 K at a certain instant. The bottom of the furnace, having the same dimensions as the plate, isblack and operates at 1000 K. The sidewalls of the furnace are well insulated. The top of the plate is exposed to ambient airwith a convection coefficient of 25 W/m² . K and to large surroundings. The air and surroundings are each at 300 K.Ty = 300 K- T = 300 Kh = 25 W/m2 .KsurAirPlate, 400 K, ɛ = 0.8-Insulated sidewalls200 mmFurnace bottom,1000 K(a) Evaluate the net rate of radiative heat transfer to the bottom surface of the plate.(b) If the plate has mass and specific heat of 2 kg and 900 J/kg · K, respectively, what will be the change in temperature ofthe plate with time, dT „/ dt? Assume convection to the bottom surface of the plate to be negligible.

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Answered: 13.72 An opaque, diffuse, gray (200 mm…

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 11.65P

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11.31 A large slab of steel 0.1 m thick contains a 0.1 -m-di- ameter circular hole whose axis is normal to the surface. Considering the sides of the hole to be black, specify the rate of radiative heat loss from the hole. The plate is at 811 K, and the surroundings are at 300 K.
Two large parallel plates with surface conditions approximating those of a blackbody are maintained at 816C and 260C, respectively. Determine the rate of heat transfer by radiation between the plates in W/m2 and the radiative heat transfer coefficient in W/m2K.
1.26 Repeat Problem 1.25 but assume that the surface of the storage vessel has an absorbance (equal to the emittance) of 0.1. Then determine the rate of evaporation of the liquid oxygen in kilograms per second and pounds per hour, assuming that convection can be neglected. The heat of vaporization of oxygen at –183°C is .
A tungsten filament is heated to 2700 K. At what wavelength is the maximum amount of radiation emitted? What fraction of the total energy is in the visible range (0.4to0.75m)? Assume that the filament radiates as a graybody.
Determine the total average hemispherical emissivity and the emissive power of a surface that has a spectral hemispherical emissivity of 0.8 at wavelengths less than 1.5m, 0.6 at wavelengths from 1.5to2.5m, and 0.4 at wavelengths longer than 2.5m. The surface temperature is 1111 K.
11.41 Determine the steady-state temperatures of two radiation shields placed in the evacuated space between two infinite planes at temperatures of 555 K and 278 K. The emissivity of all surfaces is 0.8.
Question No.13: a) What is a gray body? Derive the expression for radiation heat exchange between two gray surfaces connected by single refractory surface. b) A 60mm thick plate with a circular hole of 30mm diameter along the thickness is maintained at uniform temperature of 277°C. Find the loss of energy to the surroundings at 20°C, assuming that the two ends of the hole to be as parallel discs and the metallic surfaces and surroundings have black body characteristics.
A space satellite in the shape of a sphere is traveling in outer space, where its surface temperature is held at 284.65 K. The sphere "sees" only outer space, which can be considered as a black body with a temperature of 0 K. The polished surface of the sphere has an emissivity of 0.1. Calculate the heat loss per m2 by radiation.

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