Radiation Heat TransferDefinition: Transfer of energy across a system bounded by means of an electromagnetic mechanism that is caused solely by a temperature difference.

Physical Mechanism: Whenever a charged particle undergoes acceleration, energy possessed by the particle is converted into a form of energy known as electromagnetic radiation.

The electromagnetic radiation include: gamma rays, x-rays, ultraviolet radiation, visible light, infrared, microwave. Those that are produced by vibrational and rotational movements of atoms and molecules or change in the atomic energy levels that are indicated by temperature: THERMAL RADIATION

THERMAL RADIATION Heat transfer represents the exchange of thermal radiation between bodies at different temperatures, with each body:

1. Converting internal energy into outflowing electromagnetic waves.2. Absorbing incoming electromagnetic waves which are converted to

internal energy.

Electromagnetic SpectrumElectromagnetic waves are characterized by a frequency and propagation velocity (= speed of light)

Speed of light in vacuum Co = 3*108 m/s, index of fraction n = Co/C (for gases n = 1, for water n = 1.5)Wave length: = C/.

Radiation propagation velocity and are functions of (medium) While the frequency is a function of:

1. Radiating source.2. medium or substance through which it is

transmitted.

Electromagnetic waves can also be described as discrete packets of energy known as quanta or photons (introduced by Max Planck in quantum theory)

h: Planck constant (Thermal radiation range

for : 0.1 to 1000 m)

Thermal Radiation PropertiesThey depend on:

1. Surface emission properties (absorption, reflection and transmission)2. Medium properties.

Surface emission properties:The rate of thermal radiation is fn(Ts, nature of the surface, or frequency )

Total emissive power (Thermal radiation Flux): Thermal radiation emitted over all wavelengths into the entire hemispherical space above a surface.

Surface that emits maximum possible radiation at any given temperature is called a BLACK BODY

Stefan Boltzmann law where : Stefan Boltzmann constant = 5.67*10-8 W/m2 K4.

Note: A perfect blackbody does not exist. It can be approached by painting a surface with lampblack paint

For a non-blackbody: where is the emissivity (0 to 1) fn of surface nature, T

For metals: << polished surface >> oxidized or anodized surface

Subtotal emissive power: E0-Thermal radiation emitted over a range of wavelengths (0 to ) into the entire hemispherical space above a surface.

Fig. 5-4 shows E0- for a blackbody at different temperatures. (Fig. 5-5 is a generalization of this one)

Monochromatic Emissive Power: EThermal radiation flux emitted per unit wavelength

d.

Relations:

as >>> (Total emissive power)

Blackbody thermal radiationCan also be calculated based on the quantum theory

where C1 = 3.743 * 108 W m4/m2

C2 = 1.439 * 104 m K

Fig 5-6: Eb starts from zero at low to a peak then falls at higher. The peak in Ebincreases and shifts to a shorter as T increases. The value of l at which the peak is found is given by Wein’s law.

This change is related to color change for heat treated steel (dull red-bright red – bright yellow – glowing white at 1300 oC)Example 5-1: Fraction of solar energy in the visible light (0.38-0.76 m)

Thermal Radiation from Real Surfaces

Figure 5-7 shows Blackbody, polished copper, anodized Al monochromatic power E.Monochromatic emissivity: = E/Ebdepends on the surface only

Figure 5-8 and 5-9 for E.

Relation between

If Graybody

Surface Irradiation PropertiesIrradiation G: Incoming thermal radiation flux

G = G + G + G. / G1 = . Most solids are opaque ( = 0)

absorptivity, reflectivity, transmissivity. (define)

The concept of a blackbody is important. It provides a standard upon which performance of a real surface is compared.

are fn(, TR, characteristics of receiving surface). Figure 5-13 shows their variation versus T for different surfaces.

Study examples 5-4 (cavity blackbody) and 5-5 Kirshoff’s law.

Monochromatic Irradiation Properties1 = .

In longer wavalength cases, glass (and polyurethane) is opaque, then most of the energy are absorbed and reflected, non-graybody characteristics are responsible for GREENHOUSE EFFECT. [Glass is transparent for short wavelength radiation (high T source) but opaque for long wavelength radiation (low T source)]