Heat Transfer: Glossary Glossary blackbody A body with a surface emissivity of 1. Such a body will emit all of the thermal radiation it can (as described by theory), and will absorb 100% of the thermal radiation striking it. Most physical objects have surface emissivities less than 1 and hence do not have blackbody surface properties. density, r The amount of mass per unit volume. In heat transfer problems, the density works with the specific heat to determine how much energy a body can store per unit increase in temperature. Its units are kg/m3. emissive power The heat per unit time (and per unit area) emitted by an object. For a blackbody, this is given by the Stefan-Boltzmann relation s*T4 graybody A body that emits only a fraction of the thermal energy emitted by an equivalent blackbody. By definition, a graybody has a surface emissitivy less than 1, and a surface reflectivity greater than zero. heat flux, q The rate of heat flowing past a reference datum. Its units are W/m2. internal energy, e A measure of the internal energy stored within a material per unit volume. For most heat transfer problems, this energy consists just of thermal energy. The amount of thermal energy stored in a body is manifested by its temperature. radiation viewfactor, F12 The fraction of thermal energy leaving the surface of object 1 and reaching the surface of object 2, determined entirely from geometrical considerations. Stated in other words, F12 is the fraction of object 2 visible from the surface of object 1, and ranges from zero to 1. This quantity is also known as the Radiation Shape Factor. Its units are dimensionless. rate of heatgeneration, qgen A function of position that describes the rate of heat generation within a body. Typically, this new heat must be conducted to the body boundaries and removed via convection and/or radiation heat transfer. Its units are W/m3. specific heat, c A material property that indicates the amount of energy a body stores for each degree increase in temperature, on a per unit mass basis. Its units are J/kg-K. Stefan-Boltzmannconstant, s Constant of proportionality used in radiation heat transfer, whose value is 5.669 x 10-8 W/m2-K4. For a blackbody, the heat flux emitted is given by the product of s and the absolute temperature to the fourth power. surface emissitivy, e The relative emissive power of a body compared to that of an ideal blackbody. In other words, the fraction of thermal radiation emitted compared to the amount emitted if the body were a blackbody. By definition, a blackbody has a surface emissivity of 1. The emissivity is also equal to the absorption coefficient, or the fraction of any thermal energy incident on a body that is absorbed. thermal conductivity, k A material property that describes the rate at which heat flows within a body for a given temperature difference. Its units are W/m-k. thermal diffusivity, a A material property that describes the rate at which heat diffuses through a body. It is a function of the body's thermal conductivity and its specific heat. A high thermal conductivity will increase the body's thermal diffusivity, as heat will be able to conduct across the body quickly. Conversely, a high specific heat will lower the body's thermal diffusivity, since heat is preferentially stored as internal energy within the body instead of being conducted through it. Its units are m2/s.