3-9 A diffuse surface at 750 K has a hemispherical spectral emissivity that can be approximated by the solid line shown: 0.8 0.7 0.55 Ex(A) 0.45 0.2 0 1 4 5 7 10 λ, μ 8 (a) What is the hemispherical total emissive power of the surface? What is the total intensity emitted in a direction 45° from the normal to the surface? ' (b) What percentage of the total emitted energy is in the wavelength range 4 << 7 μm? How does this compare with the percentage emitted in this wavelength range by a gray body at 750 K with € = 0.35? Answer: (a) 9801 W/m², 3120 W/(m² sr); (b) 54.25%, 39.05%

3-9 A diffuse surface at 750 K has a hemispherical spectral emissivity that can be approximated by the solid line shown: 0.8 0.7 0.55 Ex(A) 0.45 0.2 0 1 4 5 7 10 λ, μ 8 (a) What is the hemispherical total emissive power of the surface? What is the total intensity emitted in a direction 45° from the normal to the surface? ' (b) What percentage of the total emitted energy is in the wavelength range 4 << 7 μm? How does this compare with the percentage emitted in this wavelength range by a gray body at 750 K with € = 0.35? Answer: (a) 9801 W/m², 3120 W/(m² sr); (b) 54.25%, 39.05%

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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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.
11.68 Two infinitely large, black, plane surfaces are 0.3 m apart, and the space between them is filled by an isothermal gas mixture at 811 K and atmospheric pressure. The gas mixture consists of by volume. If one of the surfaces is maintained at 278 K and the other at 1390 K, calculate (a) the effective emissivity of the gas at its temperature, (b) the effective absorptivity of the gas to radiation from the 1390 K surface, (c) the effective absorptivity of the gas to radiation from the 278 K surface, and (d) the net rate of heat transfer to the gas per square meter of surface area.
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.
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 rate of radiant heat emission in watts per square meter from a blackbody at (a) 15C, (b) 600C, and (c) 5700C.
1.25 A spherical vessel, 0.3 m in diameter, is located in a large room whose walls are at 27°C (see sketch). If the vessel is used to store liquid oxygen at –183°C and both the surface of the storage vessel and the walls of the room are black, calculate the rate of heat transfer by radiation to the liquid oxygen in watts and in Btu/h.
Describe and compare the modes of heat loss through the single-pane and double-pane window assemblies shown in the sketch below.
Which of the following is(are) true regarding the solar gain on windows? A. The solar gain due to north-facing windows is greater than the solar gain due to south-facing windows. B. The solar gain due to east-facing windows is greater than the solar gain due to west-facing windows. C. The solar gain due to west-facing windows is greaterthan the solar gain due to east-facing windows. D. The solar gain due to south-facing windows is greaterthan the solar gain due to north-facing windows.
Use integration to verify the formula given in Table 9.2 for Ixy of a half parabolic complement.
The varying intensity of the sun's energy that reaches the earth is due in part to the A. solar constant. B. declination angle. C. lunar constant. D. solar design strategy.
1.15 A thermocouple (0.8-mm-diameter wire) used to measure the temperature of the quiescent gas in a furnace gives a reading of . It is known, however, that the rate of radiant heat flow per meter length from the hotter furnace walls to the thermocouple wire is 1.1 W/m and the convection heat transfer coefficient between the wire and the gas is K. With this information, estimate the true gas temperature. State your assumptions and indicate the equations used.