Enclosure: An open or closed enclosure in a radiation problem is a set of surfaces radiating to each other. You can have many enclosures, with surfaces radiating to each other. The program uses the definition of an enclosure to calculate view factors amongst surfaces belonging to an enclosure. Each open enclosure can have its own space temperature or space node which radiates to the ambient temperature.

Radiating Surfaces: An open or closed enclosure can consist of many surfaces radiating to each other. Each radiating surface has an emissivity and a direction of radiation assigned to it. The Emissivity for a surface can be a function of temperature.

View Factors: To compute radiation exchange between any two surfaces, you calculate the fraction of the radiation leaving surface i which is intercepted by surface j. This fraction is known as the view factor, form factor, or shape factor. You can calculate view factors using the hidden/non-hidden method for 2D and 3D problems or the Hemicube method for 3D problems.

Emissivity: Emissivity is a surface radiative property defined as the ratio of the radiation emitted by the surface to the radiation emitted by a black body at the same temperature. The program restricts radiation exchange between surfaces to gray-diffuse surfaces. The word gray signifies that emissivity and absorptivity of the surface do not depend on wavelength (either can depend on temperature). The word diffuse signifies that emissivity and absorptivity do not depend on direction. For a gray diffuse surface, emissivity = absorptivity and emissivity + reflectivity = 1. Note that a black body surface has a unit emissivity. If your analysis includes radiation, you must specify emissivity using MP,EMIS or EMIS.

Stefan-Boltzmann Constant: Stefan-Boltzmann constant provides the proportionality constant between the radiative heat flux and the forth power of temperature in the radiation model. The units for the constant depends on the absolute temperature units used in the model.

Temperature Offset: The unit of temperature plays an important role in radiation analysis. You can perform radiation calculations in absolute temperature units. If the model is defined in terms of degrees Fahrenheit or degrees Celsius, you must specify a temperature offset. The temperature offset is 460° for the Fahrenheit system and 273° for the Celsius system.

Space Temperature: For an open enclosure problem, the program requires specification of a space temperature for energy balance to the ambient. Each enclosure can have its own space temperature.

Space Node: For an open enclosure problem, if the ambient is another body in the model, you can use the temperature of a space node to represent the free-space ambient temperature

Radiosity Solver: The radiosity solver method accounts for the heat exchange between radiating bodies by solving for the outgoing radiative flux for each surface, when the surface temperatures for all surfaces are known. The surface fluxes provide boundary conditions to the finite element model for the conduction process analysis. When new surface temperatures are computed, due to either a new time step or iteration cycle, new surface flux conditions are found by repeating the process. The surface temperatures used in the computation must be uniform over each surface facet to satisfy the conditions of the radiation model.