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Basics of Heat and Mass Transfer

Introduction to the topic by summarizing the content of the first 5 weeks of course 11121 at DTU*Byg, Denmark.
by

Kasper Skårhøj

on 9 March 2011

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Transcript of Basics of Heat and Mass Transfer

Heat and Mass Transfer Definition of "Heat": - amount of thermal energy in object
Unit of heat: Q [J] = Joule
Unit of heat flow: Q' [W] = Watt = J/s Thermal energy is the average kinetic energy of vibrating atoms and free electrons in the object: Examples in latent heat in buildings: Transport law: Examples of radiation: Transport law: Examples of convection: Transport law: Examples of conduction: Transport law: Examples of advection Transport law: Solar
Wall Transfer of material results in transfer of the stored sensible heat Transmission of atomic kinetic energy gives sensible heat transfer Local micro-conduction and -advection between surface and fluid Each body at temperature > 0 K emits electromagnetic radiation.
Electromagnetic radiation exchange yields sensible heat transfer.
Radiation transfer only between surfaces (air taken transparent) in steady state Thermal Resistance Conduction Convection Electrical current: U=I*R
Heat current: T=Q'*R Boundary Layer Velocity Thermal typically, transitions of velocity and temperature occur within a narrow layer near the surface Natural or forced convection Natural: density differences (temperature, concentration, …)
Forced: pressure differences (wind, HVAC equipment, …) Laminar or turbulent Properties Relations Black body

Grey body View factor Heat flows in the direction of decreasing temperature. James Prescott Joule (1818-1889) is a British physicist born in Salford, Lancashire, England. Joule is best known for his work on the conversion of electrical and mechanical energy into heat and the first law of thermodynamics. The energy unit joule(J) is named after him. The Joule's law of electric heating that he formulated states that the rate of heat production in a conducting wire is proportional to the product of the resistance of the wire and the square of the electric current.Through his experiments, Joule has demonstrated the mechanical equivalence of heat, i.e., the conversion of mechanical energy into an equivalent amount of thermal energy, which laid the foundation for the conservation of energy principle. Joule, together with William Thomson (later Lord Kelvin), discovered the temperature drop of a substance during free expansion, a phenomenon known as the Joule-Thomson effect, which forms the foundation of the operation of the common vapor-compression refrigeration and air conditioning systems.
(Heat and Mass Transfer, Cengel, pg. 5) Joule Basics [J]=[W/s] Steady State Independence of time Basic Forms of Transfer Heat balances are defined for "control surfaces" or "volumes": Electromagnetic Spectrum Resistance
Networks Conduction Convection Radiation (and convection) Serial and Parallel
Resistance Radiation Mixed Convection Heat Balances
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