Problem 4Emissivity/Absorptivity0.830.6་A white paint that can be used for passive radiative (b) 1cooling under the Sun. Provided the followingsimplified spectral absorptivity/emissivity plot (α)or &) on the right, calculate the net radiative heatflux qrad from a uniform-temperature paintedsurface at Ts=300 K. The Sun's irradiation on Earthis Gsun 1380 W/m² and its blackbody temperatureis Tsun = 5800 K. Ignore the radiation from theatmosphere (surroundings).0.20°35DOWavelength (um)

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Answered: Problem 4 Emissivity/Absorptivity 0.8…

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.
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 .
6.4 Two large diffuse parallel plates are maintained at temperatures T₁ = 1400 K and T₂ = 700 K. The plates are made from the same metal, and their spectral emissivities as a function of wavelength, &, are approximated as shown by two constant values joined by a linear decrease with wavelength. Compute the net radiant energy flux being transferred from plate 1 to plate 2. What is the energy flux if both plates are assumed gray with an approximate average emissiv- ity of 0.5 applied over the entire spectral range? 91 T₁ = 1400 K, Ex T₂ = 700 K, EX 92-91 0.85 Ex 0.15 0 Answer: q₁106,850 W/m²; 91.gray = 68,070 W/m². 2 λ (μm) 7
An engineered passive radiative cooler coating is placed under the Sun. Provided the following simplified spectral emissivity/absorptivity plot below, calculate the total diffuse emissivity and absorptivity if its uniform surface temperature is a Ts=20°C. Assume the Sun's irradiation onk Earth is Gsun=1380 W/m2 and its blackbody temperature is Tsun= 5800K. Ignore the radiation from the atmosphere (surroundings).
4. The surface temperature of a planet is T, and the measured temperature is Tm (i.e. the temper- ature measured at the top-of-atmosphere based on the upwards flux Ft there). Assume radiative equilibrium. (a) Show that the infrared optical depth of the planet's surface 7 is related to the surface temperature T, and the measured temperature Tm by Calculate the infrared optical depth at the surfaces of (a) Earth, where T, = 300 K and Tm = 250 K. (b) Venus, where T, = 750 K and Tm = 230 K. (c) Mars, where T, = 240 K and T, = 220 K.
A typical car's exterior consists of a thin layer of silica (SiO2) over an opaque painted metal panel. Silica is transparent in the visible wavelengths but offers high reflectance in the near- to mid- infrared wavelengths. The plot on the next page depicts the diffuse spectral reflectivity (pa) of the car's surface: Spectral reflectivity, P₂ 0.8 0.6 0.4 ལ 0.2 0 0.1 1 1 10 Wavelength, λ(μm) 100 If the car's exterior temperature is T₁ = 77°C, determine both the total absorptivity (a) and the total emissivity (a) of the silica-covered panel. Assume that the Sun's temperature is Tsun = 5800 K.
11.19 A long rod, with a diameter of 3 mm, is placed in a vacuum parallel to a large and flat heated surface. The other side of the rod is exposed to vacuum chamber walls at 300 K. Assume that the heated surface and the surface of the rod are both gray with equal emissivities of 0.7. The rod has a volumetric heat capacity of pc, = 6.7 × 106J m‍³ K¨¹. Calculate the heating rate of the rod at 335 K, 500 K, 700 K, and 800 K. Rod Heated surface at 830 K
A proposed method for generating electricity from solar irradiation is to concentrate the irradiation into a cavity that is placed within a large container of a salt with a high melting temperature. If all heat losses are neglected, part of the solar irradiation entering the cavity is used to melt the salt while the remainder is used to power a Rankine cycle. (The salt is melted during the day and is resolidified at night in order to generate electricity around the clock.) 9R = Est-3.45 MW i Salt Tsalt = 1000°C Mirror MW qR Consider conditions for which the solar power entering the cavity is asol = 7.10 MW and the time rate of change of energy stored in the salt is Est = 3.45 MW. For a cavity opening of diameter D, = 1 m, determine the rate of heat transfer to the Rankine cycle, qr, in MW. The temperature of the salt is maintained at its melting point, Tsalt = Tm= 1000°C. Neglect heat loss by convection and irradiation from the surroundings. Sun Heliostats
Two concentric spheres of diameter D = 0.7 m and D2 = 1.2 m are separated by an air space and have surface temperatures of T = 400 K and T, = 300 K. (a) If the surfaces are black, what is the net rate of radiation exchange between the spheres, in W? 912 = (b) What is the net rate of radiation exchange between the surfaces if they are diffuse and gray with & = 0.5 and &2 = 0.05, in W? 912 = W (c) What is the net rate of radiation exchange if D2 is increased to 20 m, with &2 = 0.05, €1 = 0.5, and D1 = 0.7 m, in W? 912 = W (d) What is the net rate of radiation exchange if the larger sphere behaves as a black body (82 = 1.0) and with &1 = 0.5, D2 = 20 m, and D = 0.7 m, in W? 912 = W
A thin, disk-shaped silicon wafer of diameter D=20 cm on a production line must be maintained at a temperature of 100 deg C. The wafer loses heat to the room by convection and radiation from its upper surface, while heat is supplied at a constant flux from below. The surrounding air is at 20 deg C, while all surrounding surfaces (which can be treated as blackbodies) can be approximated to be isothermal at a temperature of 15 deg C. The wafer-to-air heat transfer coefficient is 30 W/m2-K and the emissivity of the wafer’s surface (which can be approximated to be gray) is 0.85. How much heat (in W) must be supplied to the wafer?
3. A star has a surface temperature of 5,500K. Determine the radiative heat transfer received by planet Z in Petawatts with diameter of 2100 km, temperature of surface of 300K and mean absorptivity of 0.8. Radiative energy loss due to radiation dispersion on outer space is 99%.

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