12-41 Consider a circular grill whose diameter is 0.3 m. The bottom of the grill is covered with hot coal bricks at 1100 K, while the wire mesh on top of the grill is covered with steaks initially at 5°C. The distance between the coal bricks and the steaks is 0.20 m. Treating both the steaks and the coal bricks as blackbodies, determine the initial rate of radiation heat transfer from the coal bricks to the steaks. Also, determine the initial rate of radiation heat transfer to the steaks if the side opening of the grill is covered by aluminum foil, which can be approx-

12-41 Consider a circular grill whose diameter is 0.3 m. The bottom of the grill is covered with hot coal bricks at 1100 K, while the wire mesh on top of the grill is covered with steaks initially at 5°C. The distance between the coal bricks and the steaks is 0.20 m. Treating both the steaks and the coal bricks as blackbodies, determine the initial rate of radiation heat transfer from the coal bricks to the steaks. Also, determine the initial rate of radiation heat transfer to the steaks if the side opening of the grill is covered by aluminum foil, which can be approx-

Elements Of Electromagnetics

7th Edition

ISBN:9780190698614

Author:Sadiku, Matthew N. O.

Publisher:Sadiku, Matthew N. O.

ChapterMA: Math Assessment

Section: Chapter Questions

Problem 1.1MA

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A surface gains 500 W/m2 heat at an equilibrium temperature of 30 °C and it is subjected to a simultaneous heat loss to environment by convection and to surroundings by radiation. The convection conditions are 10 °C and 20 W/m2 K. If the surface assumed to be black what should be the surrounding temperature to sustain this physical condition?
Earth absorbs solar energy and radiates infrared energy. The intensity of the solar radiation incident on earth is J = 1350 Wm-2, also known as the solar constant. Assume earth’s surface (ground) temperature to be uniform at Ts, and that the ground and atmosphere are black (emissivity = 1) for infrared radiation. The radius of the earth is 6.378 x 106 m. The diagram shows the ground at the surface temperature Ts and the atmosphere, represented as a thin black layer, at temperature Ta . Suppose the atmosphere absorbs 100% of the infrared radiation emitted by the ground. Assume that the ground absorbs 47.5% of the incident solar energy, and that the atmosphere absorbs 17.5% of the incident solar energy (for a total of 65% absorbed by the planet). Calculate the "steady state” numerical values of the earth’s ground temperature Ts and the atmospheric temperature Ta taking into account the “greenhouse effect” of atmospheric infrared absorption and emission described above.
A thin vertical copper plate is subjected to a uniform heat flux of 1000 W/m2 on one side, while the other side is exposed to ambient surrounding at 5°C. The surface of the plate is oxidized black and can be treated as a blackbody. The heat transfer coefficient due to natural convection on the plate surface is 5.3 W/m2?K. Determine the surface temperature of the plate. Discuss the contribution of the net radiation heat transfer on the total heat loss from the plate.
2_5190874259647370... A cylindrical shape is with diameter of 1.2m and length of 1.2m. The top surface has an emissivity of 0.7 and temperature 500 K. The bottom surface has an emissivity of o.5 and temperature 650 K. The side surface has an emissivity of 0.4. Determine the shape factors for all surfaces. E 0.0 L=650 K II >
A furnace is located next to a dense array of cryogenic fluid piping. The ice-covered piping approximates a plane surface with an average temperature of Tp = 0°C and a total, hemispherical emissivity of εp = 0.56. That is, the piping can be replaced by a flat surface with the given properties. The furnace wall has a temperature of Tf = 247°C and a total, hemispherical emissivity of εf = 0.86. To minimize the heat loading on the regirgeration equipment and piping, a reflective aluminum radiation shield with a total, hemispherical emissivity of εs = 0.13 on each of its sides is installed between the pipiing and the furnace wall as shown. Assume that all surfaces are diffuse and gray and treat the problem as one-dimensional (surfaces form infinite parallel plates). What is the heat load on on the refrigeration unit? That is, what is the net radiative power at the surface which approximates the dense array of ice-cold piping, qrad,p in Watts?
13-37 A row of tubes, equally spaced at a distance that is twice the diameter of the tubes, is positioned between two large parallel plates. The surface temperature of the tubes is constant at 10°C and the top and bottom plates are at constant temperatures of 100°C and 350°C, respectively. If the surfaces behave as blackbodies, determine the net radiation heat flux leaving the bottom plate. FIGURE P13–37 T3= 100°C T2 = 10°C T = 350°C
A 30 x 30 cm door in a brick kiln is at a distance of 3.5 m from a wall 12 m wide and 5 m high. The door is so located that a normal from the center of the door strikes the wall midway from the ends and 1.5 m above the floor. The opening into the kiln may be considered a black body; the wall is painted flat paint (Ɛ = 0.46). The temperature inside the kiln is 1 400 C and the wall temperature is 60 C. Determine (a) the configuration factor F12 and (b) the net radiant heat exchange between the opening and the wall when the fire door is open?
A one-dimensional plane wall is exposed to convective and radiative conditions at x = 0. The ambient and surrounding temperatures are T, = 20°C and Tsur = 40°C, respectively. The convection heat transfer coefficient is h = 20 W/m2-K, and the absorptivity of the exposed surface is a = 0.78. Determine the convective and radiative heat fluxes to the wall at x = 0, both in W/m2, if the wall surface temperature is T, = 29°C. Assume the exposed wall surface is gray, and the surroundings are large. qlony i W/m2 g" i W/m? drad
A furnace is located next to a dense array of cryogenic fluid piping. The ice-covered piping approximates a plane surface with an average temperature of Tp = 0°C and a total, hemispherical emissivity of εp = 0.53. That is, the piping can be replaced by a flat surface with the given properties. The furnace wall has a temperature of Tf = 206°C and a total, hemispherical emissivity of εf = 0.83. To minimize the heat loading on the regirgeration equipment and piping, a reflective aluminum radiation shield with a total, hemispherical emissivity of εs = 0.14 on each of its sides is installed between the pipiing and the furnace wall as shown. Assume that all surfaces are diffuse and gray and treat the problem as one-dimensional (surfaces form infinite parallel plates). What is the heat load on on the refrigeration unit? That is, what is the net radiative power at the surface which approximates the dense array of ice-cold piping, qrad,p in Watts?
This experiment is conducted to determine the emissivity of a certain material. A long cylindrical rod of diameter D₁ = 0.01 m is coated with this new material and is placed in an evacuated long cylindrical enclosure of diameter D₂ = 0.1 m and emissivity 2 = 0.95, which is cooled externally and maintained at a temperature of 200 K at all times. The rod is heated by passing the electric current through it. When steady operating conditions are reached, it is observed that the rod is dissipating electric power at a rate of 16 W per unit of its length, and its surface temperature is 600 K. Based on these measurements, determine the emissivity of the coating on the rod. The emissivity of the coating on the rod is 0.1165
A spherical instrumentation device with a diameter of 102.16 mm has an emissivity of 0.32. It is contained in a large space simulation chamber whose temperature is preserved at 64.1 K. If its daily operation is restricted to the temperature range 50.6
Two very large parallel plates are kept at uniform temperatures T1=800 K and T2=500 K, and have emissivities ε1=0.2 y ε2=0.7, respectively, as shown in the figure. Determine the net rate of radiation heat transfer between the two surfaces per unit surface area of the plates.