13 - Radiation Heat Transfer Flashcards
Assumptions for radiation heat transfer
Uniform
Isothermal
Diffuse
Medium has no impact
Define view factor
Purely geometric quantity and is independent of the surface properties and temperature
(also called ‘shape factor’, ‘configuration factor’ and ‘angle factor’)
Define diffuse view factor
View factor based on the assumption that the surfaces are diffuse emitters and diffuse reflectors
Define specular view factor
View factor based on the assumption that the surfaces are diffuse emitters but specular reflectors
Summation rule
Sum of the view factors from a surface i of an enclosure to all surfaces of the enclosure, including to itself, must equal unity
Superposition rule
The view factor from a surface i to a surface j is equal to the sum of the view factors from surface i to the parts of surface j
Symmetry rule
Two (or more) surfaces that possess symmetry about a third surface will have identical view factors from that surface
Define radiosity
Total radiation energy leaving a surface per unit time and per unit area
When are some surfaces modelled as being adiabatic?
If their back sides are well insulated and net heat transfer through them is zero