How do you find the view factor of radiation?
View factor between elementary areas
- The dimensionless view factor Fij between two elementary areas, Ai and Aj , satisfies the relation.
- where Rij is the distance between the two areas and ϕi,ϕj ϕ i , ϕ j are the angles between Rij and the normals to the surfaces of the areas (Figure 2).
What is the view factor F12?
The view factor F12 is the fraction of energy exiting an isothermal, opaque, and diffuse surface 1 (by emission or reflection), that directly impinges on surface 2 (to be absorbed, reflected, or transmitted). View factors depend only on geometry.
What is meant by view factor and why is it so important in calculation of radiation heat transfer?
In simpler terms, the view factor measures how well one surface can see another surface. View factors are purely geometrical parameters and are independent of the physical surface properties and temperature. Radiation heat transfer between surfaces depends on the orientation of the surfaces.
How do you calculate radiative heat transfer?
The rate of heat transfer by emitted radiation is determined by the Stefan-Boltzmann law of radiation: Qt=σeAT4 Q t = σ e A T 4 , where σ = 5.67 × 10−8 J/s · m2 · K4 is the Stefan-Boltzmann constant, A is the surface area of the object, and T is its absolute temperature in kelvin.
What does the view factor represent?
The view factor is the degree to which heat carried by radiation can be passed between two surfaces. The view factor is the fraction of radiation leaving one surface which is intercepted by a second surface. The intensity of the emitted radiation depends on the view factor of the surface relative to the sky.
What material absorbs radiation the best?
Electromagnetic radiation X-ray and gamma radiation are best absorbed by atoms with heavy nuclei; the heavier the nucleus, the better the absorption. In some special applications, depleted uranium or thorium are used, but lead is much more common; several cm are often required.
What does the view factor represents when the view factor from a surface to itself not zero?
When the view factor from a surface to itself not zero. The view factor represents the fraction of radiative energy that is diffused from one surface element and strikes the other surface element directly with no intervening reflections.
How is heat transfer by radiation?
Radiation is the transfer of heat by means of electromagnetic waves. The transfer of heat by radiation involves the carrying of energy from an origin to the space surrounding it. The energy is carried by electromagnetic waves and does not involve the movement or the interaction of matter.
How much heat is lost through the roof?
Give or take, about 25% of the heat produced by your boiler will escape through the roof of your home. About 35% of the heat will escape through the walls and through gaps, in and around windows and doors, and about 10% of heat will disappear through the floor.
What are some examples of radiation heat transfer?
Heat transfer by radiation occurs when microwaves, infrared radiation, visible light, or another form of electromagnetic radiation is emitted or absorbed. An obvious example is the warming of the Earth by the Sun. A less obvious example is thermal radiation from the human body.
What does the view factor represent when the view factor from a surface to itself is zero?
What does the view factor represents? When the view factor from a surface to itself not zero. The view factor represents the fraction of radiative energy that is diffused from one surface element and strikes the other surface element directly with no intervening reflections.
How can you determine the view factor F12 when the view factor F21 and the surface areas are given?
The view factors for the heat exchange between the top and bottom surfaces, F12 and F21, are the same because the areas of both surfaces are the same; F12 = F21. We can find this common view factor from Figure 13-5 with L1/D = L2/D = 1 since all sides of the cube are the same.
Which is the view factor of radiative heat transfer?
In radiative heat transfer, a view factor,, is the proportion of the radiation which leaves surface
How is the view factor used in radiation analysis?
The view factor has proven to be very useful in radiation analysis because it allows us to express the fraction of radiation leaving a surface that strikes an- other surface in terms of the orientation of these two surfaces relative to each other.
What are the view factors of isothermal radiation?
12is the fraction of energy exiting an isothermal, opaque, and diffuse surface 1 (by F emission or reflection), that directly impinges on surface 2 (to be absorbed, reflected, or transmitted). View factors depend only on geometry. Some view factors having an analytical expression are compiled below. We will use the subindices inF
What are the problems of radiation heat transfer?
Many problems encountered in practice involve geometries of constant cross section such as channels and ducts that are very long in one direction relative to the other directions. Such geometries can conveniently be considered to be two-dimensional, since any radiation interaction through their end surfaces will be negligible.