Radiation heat transfer or simply called as thermal radiation occurs without the necessity of any medium even in vacuum space. Or, when a medium is available, the transfer process does not depend on the medium itself. The radiation will be transferred limitless if there is no obstruction. A classic example of this method, heat from the sun is mostly transferred through this method to the Earth.
How exactly can the heat be transferred without any medium? The answer is that heat is transferred as an electromagnetic wave which is similar to light.
In radiation, there are several factors that affect the amount of heat transferred:
- Temperature difference
For radiation, temperature shall be power by four and subtract. Thus, radiation heat transfer is more significant when heat source has a very high temperature. As the opposite, radiation heat transfer can be omitted when the temperature difference is small.
Emissivity is the effectiveness of a surface to emit thermal radiation. The value is a ratio of thermal radiation from a surface to thermal radiation from an ideal black body. The maximum value for emissivity is 1 and the minimum value is 0. Any material has value in between is called as a gray body.
- View factor
View factor is a factor that characterized arrangement of each related surface. It is the proportion of the radiation which leaves one surface that strikes surface B. It can be different when the surface is parallel or not. The surface shape also affects this factor.
Thermal radiation rate can be expressed by following equation:
qr = Heat transfer rate (Watt, Btu/h)
s = Boltzman constant (5,669 x 10-8 W/m2.K4, 0.1714 x 10-8 Btu/h.ft2.0R4 )
T = Radiation temperature (oC, K)
hr = Radiation heat transfer coefficient (W/m2 K, Btu/h ft2 0F)
F12 = View factor between surfaces