Heat Transfer: Radiation

Heat Transfer Through Radiation

• Radiation is the heat transfer by electromagnetic waves such as ultra violet, infrared and visible light radiation.
• The heat current (power) due to radiation can be expressed as Stefan-Boltzmann Law:

• $A$ is the surface area of the object
• $e$ is called emissivity and shows how close our object is to an ideal radiating surface
• $e$ ranges between 0 and 1 and is dimensionless
• The standard unit for $H$ is $\frac{J}{s}=watt$
• $\sigma$ is Stefan – Boltzmann Constant and is: $\sigma=5.6704 \times10^{-8}\ \frac{w}{m^2\cdot K^4}$
• $e$ for a completely black surface is 1 and has smaller values for lighter surfaces
• Radiation intensity on the surface of the radiating object is: $\frac{H}{A}=e\sigma T^4$
• The standard unit for intensity is $\frac{J}{s\cdot m^2}=\frac{watt}{m^2}$
• Radiation intensity at distance d from the center of a radiating object is:

Absorption

• A body can absorb the radiation from its environment. Only part of incident radiation to the surface of an object is absorbed and the rest is reflected.
• The radiation power absorbed by a body is proportional to its effective surface area if the rays of radiation are perpendicular to the surface

• For a sphere with radius R, the effective surface area is $\pi R^2$
• $H_{net}$ is the net heat flow of an object and is defined as:

• Where $H_{in}$ is the heat absorbed by radiation and $H_{out}$ is the heat radiated by the same object
• The Albedo factor determines the fraction of incident radiation reflected from the surface of the object
• (1-Albedo) is the fraction of incident radiation absorbed by the object