Radiation Heat Transfer P M V Subbarao Professor

Radiation Heat Transfer P M V Subbarao Professor Mechanical Engineering Department Primitive and Powerful Mode…. .

Radiation Based Manufacturing Processes source The frequency of microwave oven is 2. 45 GHz (2450 MHz). And the frequency of Broadcasting TV is around 12 GHz.

Principle of Microwave Heating

Microwave That can Heat !!! Vibration of 2450 million times plus and minus to be replaced per second.

Source of Electro-magnetic Radiation • Any system with temperature above absolute zero (0 K) emits electromagnetic radiation. • In a simplified picture, radiation comes from the constantly changing electromagnetic fields of the oscillating atoms. • The two prominent characters of the wave are the wavelength (λ) and frequency (ν). • The amount of radiation emitted by a body depends on its temperature, and is proportional to T 4. • As the temperature of the object increases, the amount of radiation emitted increases very rapidly. • The emitted radiation will travel at the speed of light until it is absorbed by another system. • The absorbing medium can be gas, liquid, or solid.

Electromagnetic Spectrum

A Thermodynamic System as A Donor • For gases and semitransparent solids, emission is a volumetric phenomenon. • In most solids and liquids the radiation emitted from interior molecules is strongly absorbed by adjoining molecules. • Only the surface molecules can emit radiation.

Hemispherical Surface Emission The radiation emitted by a body is spatially distributed:

Radiation Laws • The average or bulk properties of electromagnetic radiation interacting with system are systematized in a simple set of rules called radiation laws. • These laws apply when the system being studied is what physicists call a blackbody radiator. • Generally, blackbody conditions apply when the radiator has very weak interaction with the surrounding environment and can be considered to be in a state of equilibrium. • Although stars do not satisfy perfectly the conditions to be blackbody radiators, they do to a sufficiently good approximation that it is useful to view stars as approximate blackbody radiators.

Planck Radiation Law • The primary law governing blackbody radiation is the Planck Radiation Law. • This law quantifies the intensity of radiation emitted by unit surface area into a fixed direction (solid angle) from the blackbody as a function of wavelength for a fixed temperature. • The Planck Law can be expressed through the following equation. h = 6. 625 X 10 -27 erg-sec (Planck Constant) K = 1. 38 X 10 -16 erg/K (Boltzmann Constant) C = Speed of light in vacuum

The Planck Law suggests a distribution that; peaks at a certain wavelength, the peak shifts to shorter wavelengths for higher temperatures. The area under the curve grows rapidly with increasing temperature.

Shifting Peak Nature of Radiation

Wein’s Displacement Law: • At any given wavelength, the black body monochromatic emissive power increases with temperature. • The wavelength lmax of the peak decreases as the temperature increases. • The wavelength at which the monochromatic emissive power is a maximum is found by setting the derivative of previous Equation with respect to l.

Wien law for three different stars

Region Some Blackbody Temperatures Wavelength Energy Blackbody Temperature (centimeters) (e. V) (K) Radio > 10 < 10 -5 < 0. 03 Microwave 10 - 0. 01 10 -5 - 0. 01 0. 03 - 30 Infrared 0. 01 - 7 x 10 -5 0. 01 - 2 30 - 4100 Visible 7 x 10 -5 - 4 x 10 -5 2 -3 4100 - 7300 Ultraviolet 4 x 10 -5 - 10 -7 3 - 103 7300 - 3 x 106 X-Rays 10 -7 - 10 -9 103 - 105 3 x 106 - 3 x 108 Gamma Rays < 10 -9 > 105 > 3 x 108

The Magnetron : An artificial Microwave Emitter The microwave radiation of microwave ovens and some radar applications is produced by a device called a magnetron.

Stefan-Boltzmann Law • The maximum emissive power at a given temperature is the black body emissive power (Eb). • Integrating this over all wavelengths gives Eb.

Emissivity • A black body is an ideal emitter. • The energy emitted by any real surface is less than the energy emitted by a black body at the same temperature. • At a defined temperature, a black body has the highest monochromatic emissive power at all wavelengths. • The ratio of the monochromatic emissive power El to the monochromatic blackbody emissive power Ebl at the same temperature is the spectral hemispherical emissivity of the surface. The total (hemispherical emissive power is, then, given byë

Define average (hemisherical) emissivity, at a defined temperature Here, e can be interpreted as either the emissivity of a system, which is wavelength independent, or as the average emissivity of a surface at that temperature. A surface whose properties are independent of the wavelength is known as a gray surface. The emissive power of a real surface is given by