Chapter 7 - Radiative Exchange Between Nonideal Surfaces

Abstract

In Chapter 6 we saw that, in certain situations, the directional nature of the reflectance of surfaces can strongly influence radiative heat transfer rates. This effect occurs particularly in open configurations, in enclosures with long channels, or in applications with collimated irradiation. Since real surfaces are neither diffuse nor specular reflectors, the actual directional behavior may have substantial impact, as we saw from the data in Fig. 6-17. We also noted that solar collectors did not appear to perform very well because, in our gray analysis, the reradiation losses were rather large. However, experience has shown that reradiation losses can be reduced substantially if selective surfaces (i.e., strongly nongray surfaces) are used for the collector plates. Apparently, there are a substantial number of applications for which our idealized treatment (gray, diffuse—i.e., direction-independent—absorptance and emittance, gray and diffuse or specular reflectance) is not sufficiently accurate.