What is the view factor (F12)?

What is the view factor (F12)?

• A. The geometric view factor between surface 1 and surface 2
• B. The radiation heat transfer coefficient between surfaces
• C. The emissivity of surface 1
• D. The overall heat transfer coefficient between surfaces

Answer: (A)

F12 is the geometric view factor between surface 1 and surface 2: the fraction of radiation leaving surface 1 that reaches surface 2 directly. It depends only on geometry—areas, relative orientation, distance, and any occlusions—not on material properties like emissivity or on heat transfer coefficients. It ranges from 0 to 1 and reflects how much of the emitted radiation from one surface is geometrically able to strike the other. A useful property is reciprocity: A1 times F12 equals A2 times F21, linking the view factors in both directions. In a closed enclosure, the sum of view factors from a given surface to all other surfaces equals 1. In radiation calculations, F12 is used with the surfaces’ temperatures and emissivities to determine how much energy is exchanged. Emissivity, radiative heat transfer coefficients, and overall heat transfer coefficients involve surface properties and resistances rather than the pure geometric fraction of exchanged radiation.

F12 is the geometric view factor between surface 1 and surface 2: the fraction of radiation leaving surface 1 that reaches surface 2 directly. It depends only on geometry—areas, relative orientation, distance, and any occlusions—not on material properties like emissivity or on heat transfer coefficients. It ranges from 0 to 1 and reflects how much of the emitted radiation from one surface is geometrically able to strike the other.

A useful property is reciprocity: A1 times F12 equals A2 times F21, linking the view factors in both directions. In a closed enclosure, the sum of view factors from a given surface to all other surfaces equals 1. In radiation calculations, F12 is used with the surfaces’ temperatures and emissivities to determine how much energy is exchanged. Emissivity, radiative heat transfer coefficients, and overall heat transfer coefficients involve surface properties and resistances rather than the pure geometric fraction of exchanged radiation.