What Is Radiation Heat Transfer

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Radiation heat transfer is basically the energy transfer via electromagnetic waves. Before going into details about radiation heat transfer, it is essential to understand the radiations first.

What Are Radiations?

Radiations are defined as the electromagnetic waves having wavelength of 0.1 to 100 microns, which doesn’t require any medium to travel. Radiation waves are classified into two types; ionizing radiations and non ionizing radiations. Ionizing radiations have the tendency (because of having sufficient energy) to ionize an atom. Non-ionizing radiations cannot ionize an atom (heat waves, radio waves and light waves are the examples of non-ionizing radiations).  Hence, in physics on nuclear engineering, we deal with the ionizing radiations, while here; non-ionizing radiations are of our main interest.

 Image: Salvatore Vuono

What Is Radiation Heat Transfer?

According to the quantum theory, radiations consist of energy packets (named as photons), that has no rest mass and move at the velocity of light. So, radiation heat transfer basically deals with the exchange or transfer of that energy between the bodies.  Every object, having temperature greater then absolute zero (0 K) emits radiations. Mostly, the solids are considered as the radiation emitters, because the energy emitted by the fluid particles is usually absorbed by the nearby molecules, and thus this energy cannot reach the surface. These emissions are directly proportional to the temperature of the body; the higher the temperature, higher will be the radiations emissions.

Hence:

Ever object, above absolute zero temperature emits energy carrying electromagnetic radiations. When these radiations fall on the other object, some energy is transferred from the radiation waves to the object. This transferred energy is known as the radiation heat transfer.

Emissivity:

Emissivity is the tendency of an object to release electromagnetic radiations per unit area and per unit time. In order to calculate the radiation emissive power, we assume an ideal surface, which can absorb and radiate all wavelength radiations. This ideal surface is named as the black body, or the ideal radiator. However, the heat flux of the real surface is less than that of the black body. According to Stefan Boltzmann’s law:

E = ԑσ T_s⁴

Where:

E             =             Emissive power of real surface;  (W/m²)

ԑ             =             Radiative property of the surface.

σ             =             Stefan Boltzmann constant;          (5.67 x 10^-8 W/m².K⁴)

T_s            =             Absolute temperature;                   (K)

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What Is Heat Transfer

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Heat transfer is considered as one of the most basic discipline of chemical engineering & technology. It generally concerns with generation, consumption and conversion of heat energy in the system. Heat transfer; it self plays a very important role in process industries, and it is always better for the process industries, to optimize their heat transfer processes [may include furnaces, evaporators, distillation units, dryers, reaction vessels etc] by the selection of proper heat exchangers, preventing heat losses and controlling the heat flow rate.

Heat transfer under normal conditions always flows from a hotter region to a cooler region, i.e. heat transfer follows the temperature gradient between the two systems [or system and surrounding], until a thermal equilibrium is maintained between the two systems.  However, according to the “Clausius statement of second law of thermodynamics”, if the work is done on the system, the heat can flow from a colder region to a hotter region. This Clausius statement is the basic principle behind the working of refrigerators.

Now how to calculate heat transfer :

\[Q=UAΔT\]

Where,

Q = transfer of heat per unit time
A = heat transfer area
ΔT = temperature difference between two systems

This is the most basic expression to define heat transfer, there are several other heat transfer formulas derived from this expression. Picture at the right side is taken from images by dan.

There are three forms of heat transfer:

1) Conduction

Conduction is generally considered as the heat transfer phenomena for the solids, however conduction can also occur in fluids too [in microscopic level, like diffusion phenomena]. In solids, Conduction is the result of transfer of vibration energy from one molecule to other, while in fluids, it occurs in addition as a result of transfer of kinetic energy. Conduction follows the Fourier Law of Heat Conduction:

\[Q=-kAΔT\]

Where,

Q = transfer of heat per unit time
k = conductive heat transfer coefficient
A  = heat transfer area
ΔT = temperature difference between two systems

  The thermal conductivity units in SI system is W/mK.

2) Convection

Convective heat transfer occurs when the heat is transferred from a solid surface to a moving fluid owing to the temperature difference between the solid and the fluid. Convection follows the Newton’s Cooling Law of Heat Convection:

\[Q=hA(T-T_{'})\]

Where,

Q = transfer of heat per unit time
h = convection heat transfer coefficient
A  = heat transfer area
T = temperature of fluid
T'= temperature of solid

3) Radiation

All materials radiate thermal energy in the form of electromagnetic waves. So radiation is the transfer of heat by the emission of electromagnetic waves. When they fall on the body, they may partially be reflected, transmitted or absorbed. Radiation is that fraction which falls and absorbed by the body.

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