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BKF2422 Heat Transfer CHAPTER 4: PRINCIPLES OF STEADY-STATE HEAT TRANSFER IN RADIATION

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. Analyse and solve problems for radiation heat transfer involves absorbing gases medium.

Part 1: Content Introduction to Radiation Heat Transfer ◦ Mechanism of radiation heat transfer ◦ Combined radiation and convection heat transfer

Mechanism of heat transfer:

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.

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.

Absorptivity and Black bodies When thermal radiation falls upon a body, -

part is absorbed in form of heat (absorptivity,  = fraction absorbed)

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part is reflected back to space (reflectivity,  = fraction reflected)

-

part is transmitted through the body

(transmissivity, = fraction transmitted)

    1

Absorptivity and Black bodies (cont.) • When a body is opaque it will not transmit any of the incident radiation, - Opaque body, τ = 0     1 • When a body is black body, it will absorb all incident radiation from all directions without reflecting and transmitting it - Black bodies, τ = 0, ρ = 0   1

* Opaque = non transparent

Radiation from a body and emissivity Emissivity,  measure of how much radiation is emitted from the object Emissivity,  

E total emissive power of a surface  E B total emissive power of black body

For black body, ε = 1 (emits the maximum energy) Kirchoff’s law:- For any black or non black solid surface at same surface temperature, T1 absorptivity = emissivity

1   1

Radiation from a body and emissivity (cont.) Stefan-Boltzmann law:Basic equation for radiation heat transfer from a perfect black body. (ε = 1.0)

q  A T

4

where, Stefan-Boltzmann constant, σ = 5.676 x 10-8 (W/m2.K4), A = surface area of body (m2), T = temperature of black body (K) For non black body, (ε < 1.0)

q  AT 4

Substances that have emissivities