2. Total Hemispherical Emissivity

In this chapter, we will deal with transient cooling of vertically hanged hot special steel plates by radiation and natural convection heat transfer. In engineering calculations of radiation heat transfer, a real surface emitting thermal radiation at a temperature T_s has a total emissive power less than the total emissive power of a black surface at T_s, namely less than σ× T⁴_s. The ratio of a real surface’s emissive power to a black surface’s emissive power at the same T_s is called the emissivity of a surface. Emissivity of a surface is a function of surface temperature, wavelength and direction of radiation emission. In engineering calculation, we deal with total hemispherical emissivity, ε_Ts, which is the emissivity of a surface that is averaged over all wavelengths and hemispherical directions and which is only a function of surface temperature as given in Equation 2-1.