tailieunhanh - Heat Transfer Handbook part 61

Heat Transfer Handbook part 61. The Heat Transfer Handbook provides succinct hard data, formulas, and specifications for the critical aspects of heat transfer, offering a reliable, hands-on resource for solving day-to-day issues across a variety of applications. | RADIATIVE PROPERTIES OF SOLIDS AND LIQUIDS 593 Figure Spectral directional reflectance of blackened glass at room temperature. From Brandenberg 1963. other hand the emittance of amorphous solids solids without a crystal lattice tends to be independent of temperature. Effects of Surface Conditions Up to this point the present discussion of radiative properties has assumed that the material is pure and homogeneous and that its surface is isotropic and optically smooth. Very few real material surfaces come close to this idealization. In usually hostile industrial environments even an initially ideal material will have its surface composition and quality altered Heating of the material may be accompanied by strong oxidation or other chemical reaction producing an opaque surface layer of a material quite different from the substrate. Similar statements can be made about materials exposed to corrosive atmospheres for extended periods of time. In addition very few surfaces have an optically smooth finish when new exposing them to heat and or corrosive atmospheres is generally accompanied by further roughening of the surface finish. Surface Roughness A surface is optically smooth if the average length scale of surface roughness is much less than the wavelength of the electromagnetic wave. Therefore a surface that appears rough in visible light X m may well be optically smooth in the intermediate infrared X 50 m . This difference is the primary reason why results from electromagnetic wave theory cease to be valid for very short wavelengths. 594 THERMAL RADIATION The character of roughness may be very different from surface to surface depending on the material method of manufacture and surface preparation and classification of this character is difficult. A common measure of surface roughness is given by the root-mean-square rms roughness om. The rms roughness can be measured readily with a profilometer a sharp stylus that traverses the surface recording the .