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Chapter 5 Radiation Heat Transfer.pptx

Chapter 5 Radiation Heat Transfer.pptx - BKF2422 Heat...

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BKF2422 Heat Transfer Chapter 4: Principles of Steady-state Heat Transfer in Radiation
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Topic Outcomes It is expected that students will be able to: Utilize the basic equation of radiation for black and gray bodies. Solve problems related to combined radiation and convection heat transfer mechanism. Apply view factors to determine radiation heat transfer rate for various geometries. Analyze and solve problems for radiation heat transfer involves absorbing gases medium.
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Content Introduction to Radiation Heat Transfer Mechanism of radiation heat transfer Combined radiation and convection heat transfer
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Radiation Heat Transfer Radiation heat transfer does not required a medium to  transfer energy (work in a vaccum). Radiation heat transfer is the transfer of heat by  electromagnetic radiation (vary with wavelength). In the electromagnetic spectrum, only thermal radiation is  pertinent to heat transfer where it affects the thermal state of  matter.  Radiation heat transfer occurs when large temperature  different exist.
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Mechanism of Radiant Heat Transfer Composed of 3 steps: 1)Thermal energy from hot source is converted to energy of electromagnetic radiation waves. 2)Waves travel through space in straight lines and strike a cold object. 3)Waves are absorbed by the body and converted back to thermal energy.
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When thermal radiation falls upon a body, - part is absorbed in form of heat (absorptivity, = fraction absorbed) - part is reflected back to space (reflectivity, = fraction reflected) - part is transmitted through the body (transmissivity, = fraction transmitted) Absorptivity and Black bodies 1
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- Opaque body,   = 0                                           τ Absorptivity and Black bodies (cont.) When a body is  opaque  it will  not transmit  any of the  incident radiation,  * Opaque = non transparent When a body is black body, it will  absorb all  incident  radiation from all directions  without reflecting   and  transmitting  it  - Black bodies,   = 0,  τ ρ  = 0                                               1 1
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Emissivity, measure of how much radiation is emitted from the object For black body, ε = 1 (emits the maximum energy) Kirchoff’s law:- For any black or non black solid surface at same surface temperature, T 1 absorptivity = emissivity Radiation from a body and emissivity body black of power emissive total surface a of power emissive total , Emissivity B E E 1 1
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Stefan-Boltzmann law:- Basic equation for radiation heat transfer from a perfect black body. (ε = 1.0) where, Stefan-Boltzmann constant, σ = 5.676 x 10 -8 (W/m 2 .K 4 ), A = surface area of body (m 2 ), T = temperature of black body (K) For non black body, (ε < 1.0) Substances that have emissivities <1.0 are called
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