U of U CHEN 3453 - Lecture 02 - Fundamentals of Convection and Radiation (6 pages)

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Lecture 02 - Fundamentals of Convection and Radiation



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Lecture 02 - Fundamentals of Convection and Radiation

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Lecture Notes


Pages:
6
School:
University of Utah
Course:
Chen 3453 - Heat Transfer

Unformatted text preview:

CH EN 3453 Heat Transfer Fundamentals of Convection and Radiation Reminders Homework 1 due Friday at 4 00 p m Minus 50 if not in by 4 00 Solutions to 2 5 are at www chen3453 com Help session today 4 30 p m in MEB 2325 Conflicts with that time Come with questions Convection Heat transfer due to the combined influence of bulk and random motion for fluid flow over a surface Requires the presence of temperature variations in the material medium Figure 1 1 b Convection Convective Heat Transfer Rate Relation of convection to flow over a surface and development of velocity and thermal boundary layers Newton s law of cooling Eq 1 3a h Convection heat transfer coefficient W m2 K Types of Convective Heat Transfer Heat Transfer Coefficient h Example Convection The case of a power transistor which is of length L 10 mm and diameter D 12 mm is cooled by an air stream of temperature T 25 C Under conditions for which the air maintains an average convection coefficient of h 100 W m2 K on the surface of the case what is the maximum allowable power dissipation if the surface temp is not to exceed 85 C Radiation Figure 1 1 c Radiation Energy that is emitted by matter due to changes in the electron configurations of its atoms or molecules and is transported as electromagnetic waves or photons Transport does not require a material medium and occurs most efficiently in a vacuum Radiation Radiative Heat Transfer Rate Heat transfer at a gas surface interface involves radiation emission from the surface and may also involve the absorption of radiation incident from the surroundings irradiation G as well as convection if Ts T Energy outflow due to emission E Emissive power W m2 Surface emissivity 0 1 Eb Emissive power of a blackbody the perfect emitter Stefan Boltzmann constant 5 67 x 10 8 W m2 K4 Energy absorption due to irradiation Gabs Absorbed incident radiation W m2 Surface absorptivity 0 1 G Irradiation W m2 Example Radiation A spherical interplanetary probe of 0 5 m diameter contains



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