Thermal Radiation Heat Transfer 6th Edition Howell Solutions Manual.pdf

RADIATION HEAT TRANSFER
b b

,Objectives
• Define view factor, and understand its importance in radiation heat tran
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sfer calculations b




• Develop view factor relations, and calculate the unknown view fac
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tors in an enclosure by using these relations
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• Calculate radiation heat transfer between black surfaces
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• Determine radiation heat transfer between diffuse and gray surfaces in
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an enclosure using the concept of radiosity
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• Obtain relations for net rate of radiation heat transfer between the surfa
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ces of a two- b b b



zone enclosure, including two large parallel plates, two long concentri
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c cylinders, and two concentric spheres
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• Quantify the effect of radiation shields on the reduction of radiation he
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at transfer between two surfaces, and become aware of the importance
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of radiation effect in temperature measurements
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2

,THE VIEW FACTOR b b




• View factor is a purely geometric quantity
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and is independent of the surface properties
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and temperature. b




• It is also called the shape factor, configurati
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on factor, and angle factor.
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• The view factor based on the assumption th
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at the surfaces are diffuse emitters and diffu
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se reflectors is called the diffuse view factor
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, and the view factor based on the assumptio
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n that the surfaces are diffuse emitters but sp
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Radiation heat exchange between surfa
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ces depends on the orientation of the su
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ecular reflectors is called the specular view
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rfaces relative to each other, and this de
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factor.
pendence on orientation is accounted f
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or by the view factor.
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Fij the fraction of the radiation leaving
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surface i that strikes surface j directly
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b



The view factor ranges between 0 and 1.
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3

, To develop a general expression for the view factor,
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consider two differential surfaces dA 1 and dA2 on tb b b b b b b b b



wo arbitrarily oriented surfaces A1 and A2, Respecti
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vely. The distance between dA1 and dA2 is r, and the
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angles between the normals of the surfaces and the li
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ne that connects dA1 and dA2 are θ1 and θ2, respecti
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b
b
b
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vely. Surface 1 emits and reflects radiation diffusely
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in all directions with a constant intensity of I1, and t
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he solid angle subtended by dA2 when viewed by d
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b
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A1 is dω21. b
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The rate at which radiation leaves dA1 in the directio
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Geometry for the determinati
n of θ1 is I1cosθ1dA1.
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b b b
b
on of the view factor between
Noting that dω21 = dA2 cosθ2 /r2, the portion of
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two surfaces
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this radiation that strikes dA2 is :
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The total rate at which radiation leaves dA1 (via emission and reflection) in all directions is the r
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adiosity (which is J1 = πI1) times the surface area,
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