By A. J. Ede
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Extra info for An Introduction to Heat Transfer Principles and Calculations
Emissivities are much lower, and vary considerably with wavelength; radiation is not diffuse, and reflected radiation is largely governed by the ordinary laws of optics ("specu lar" reflection). 1, they must be used with caution. 1. Emissivities of substances The following are to be regarded as rough, average values of the total hemi spherical emissivity at moderate temperatures. They may also be used as absorptivities for radiation from bodies of moderate temperature. Emissivity (per cent) Substance Emissivity (per cent) White paper 90-95 Aluminium, polished 4-6 Asbestos 90-95 Aluminium, oxidized 10-20 White paint 80-95 Copper, polished 2-5 Black paint 80-95 Copper, oxidized 60-70 Aluminium paint 30-70 Chromium, polished White brick 80-90 Cast iron, bright 20-25 Rough red brick 85-95 Cast iron, oxidized 60-70 Glass 90-95 Steel, polished Wood 80-95 Steel plate, oxidized 60-80 Soot 95-98 Steel plate, rough 85-95 95 Tin plate, polished 3-10 Substance Water 8-10 5-15 Further data on emissivities will be found in Refs.
1. See also Refs. 34, 39. 1. Shape factors System Plane slab, thickness d, area A Hollow cylinder, radii r\ and ri 0*2 > n ) , length / Hollow rectangular box, internal dimensions Λ, b, c, wall thickness d; total internal surface area A\, total external surface area At. Case 1: all ß, b, c are between d/5 and 2d S Ajd 2πΙ logefo/n) 2 -Xab+bc+cd) a +2'16(a+b+c) + l'2d 2 -(ab+bc+cä) Case 2: one internal dimension, say c, less than d/5 4l-86(a+b)+0-35 Case 3: two internal dimensions, say b and c, less than d/5 2·78α logio(^2Mi) Case 4: all three internal dimensions less than d/5 ^^\AXA2) Another series of shape factors concerns the conduction of heat from a buried object to the surface of the ground (or any similar situation).
7)) was given for estimating the heat exchange between a body and a black surface which surrounds it. A similar expression, though now only approxi mate, can be given for the heat exchange between a gas at tem perature Tl and a black enclosure at T2: [H/L 2 T] Ψ=σ(ε1Γ14-α1)2Γ24). 24) 38 INTRODUCTION TO HEAT TRANSFER Here εί is the effective emissivity of the gas at temperature Tl9 and a1>2 is the effective absorptivity at 7\ for radiation emitted from a surface at T2. To find εΐ9 it is necessary to calculate the effective path length as already indicated, multiply it by the pressure, and with this parameter and the temperature 7\ find the corresponding effective emissivity from a chart appropriate to the gas in question.
An Introduction to Heat Transfer Principles and Calculations by A. J. Ede