| 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 view factor, 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 heat generation, 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-Boltzmann constant, 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. |
| Formula Home | |
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Heat Transfer |
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Overview |
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Conduction |
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Convection |
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Forced Convection |
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Free Convection |
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Radiation |
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Glossary |
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Applications |
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Lam. Flow Over Plate |
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Two Body Radiation |
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Resources |
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Bibliography |
Glossary
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