Radiation Heat Transfer

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Presentation transcript:

Radiation Heat Transfer

Introduction Conduction and convection heat transfer require the presence of a medium while radiation does not. All objects with temperatures above absolute zero radiate energy. Energy is transferred by electromagnetic waves. Electromagnetic waves travel at the speed of light in perfect vacuum (~ 3x108 m/s).

Electromagnetic waves are characterized by their frequency () or wave length ():  = c/ (c: speed of wave in that medium) c = c0/n (n: refractive index, c0 is the speed of light) Frequency is the number of oscillations per second. It is independent of the medium, it only depends on the source. Electromagnetic waves consists of photones which has the energy e,

Electromagnetic spectrum and thermal radiation The type of electromagnetic radiation that is related to heat transfer is the thermal radiation as a result of energy transitions of molecules, atoms, and electrons. Temperature is a measure of these activities. Thermal radiation is also defined as the portion of electromagnetic spectrum that extends from 0.1 to 100 m (it includes visible, infrared, and part of the ultraviolet radiation)

Blackbody A blackbody is defined as a perfect emitter and absorber of radiation. The maximum emitting body. Absorbs all incident radiation. Emits radiation uniformly in all direction.

The radiation flux incident on a surface is called irradiation and is denoted by G. The fraction of irradiation absorbed by the surface is called absortivity. The fraction of irradiance reflected by the surface is called reflectivity. The fraction of irradiation transmitted by the surface is called transmissivity.

The View Factor Radiation heat transfer between surfaces depends on orientation of the surfaces relative to each other. To account for the effects of orientation on radiation, define the view factor which is geometric quantity.

The view factor from surface i to surface j is denoted by Fij=the fraction of radiation leaving surface i that strikes surface j directly. The reciprocity relation, allows the calculation of a view factor from the knowledge of the other.

View factors when i=j

The summation rule

Symmetry rule

Radiation Heat Transfer: Blackbody Radiation exchange between 2 black bodies:  : A negative value for indicates that radiation is from surface 2 to 1.

Enclosure of N surfaces Consider the enclosure of N black surfaces maintained at specified temperatures. The net radiation from any surface i to other surfaces is: A negative value for Q indicates a net radiation to the surface. The net heat transfer from a surface to itself is zero.