Radiation heat transfer is the transfer of heat from one body to another body in the form of electromagnetic waves. Most of these radiations lie in the infrared region.
Unlike Conduction and Convection, radiation heat does not require any medium. All objects above absolute zero temperature emit radiation energy. Don’t miss this article on how does Contact less Infrared Thermometer Works?
Radiation Heat is more efficient in vacuum because any media between heat transfer bodies can absorb electromagnetic waves. A good example of radiation heat is the transfer of heat from sun to earth due to electromagnetic waves through vacuum space. But when the sun radiation enters the earth atmosphere, their impact starts reducing because of the presence of air and other particles.
All objects (solid, liquid or gas) emit electromagnetic radiation because of thermal agitation inside their molecules. Rate of heat flux emitted by a body depends on body temperature and type of surface (color/finish etc.)
Heat Transfer by Radiation Examples
Following are the examples of heat transfer by radiations:
- Heat transfer from sun to earth.
- Room heating radiators.
- The light emitted by an incandescent lamp or fireplace.
- Cooking food in the microwave.
- In the greenhouse effect, radiation from the sun comes through the glass to the plants.
- Thermal radiation from the human body that is used to measure human skin temperature using Infrared thermometers.
Radiation heat Transfer Equation : Stefan-Boltzmann Law
Stefan-Boltzmann Law of thermal radiation states that the rate of radiation heat transfer per unit area in black body is directly proportional to the fourth power of the body temperature.
But in reality no body is perfectly a black body. Therefore emissivity coefficient is used to consider this impact.
Radiation Heat Transfer = ε σ T⁴ A
T = absolute temperature in Kelvin
A = area of the emitting body in square meter
ε = emissivity coefficient of the body
σ = Stefan Boltzmann Constant = 5.6703 x 10⁸ W/m² K⁴
What is a Black Body ?
A Black body absorbs all electromagnetic radiation in all frequency ranges falling on it and in thermal equilibrium black body emits all the radiations. Therefore black body is a perfect radiator.
But in the real world black bodies do not exist. Real bodies emit radiations at much lower rate than a black body. In other words, real bodies are not perfect emitters. Therefore emissivity (ε) is used to include the impact of real surfaces on radiation emission. Emissivity for black body is “one”.
What is Emissivity?
Emissivity defines a material ability to emit absorbed energy and returns to its normal temperature.
Mathematically emissivity is equal to the ratio of the emission from a real surface to the emission from a perfect black surface at the same temperature and wavelength. Emissivity value for a body varies from 0 to 1. For example Human skin has a high value of emissivity 0.98.
Any surface with emissivity equal to “zero” is a perfect reflector. Whereas surfaces with emissivity equal to “one” are perfect emitters. Emissivity for a material depends on material and surface properties. For example, reflective surfaces have low emissivity whereas rough and oxidized material surfaces have high emissivity.
Below table shows emissivity value for commonly used materials.
Radiation Heat Transfer Calculator
You can use this calculator to calculate rate of radiation heat if body surface area, temperature and emissivity values are known.
To sum up, Radiation-heat-transfer in grey bodies depends on outside temperature, Body temperature and surface finish. Higher the value of emissivity, Higher will be the rate of heat transfer. Don’t miss this article on various modes of heat transfer.