Note for Heat Transfer - HT by Tariku Negash

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Debre Markos University Mechanical Engineering Department HEAT TRANSFER (MEng 3121) Chapter 1 INTROUCTION TO HEAT TRANSFER Prepared and presented by: Tariku Negash E-mail: thismuch2015@gmail.com Lecture at Mechanical Engineering Department Institute of Technology, Debre Markos University, Debre Markos, Ethiopia 10/28/2017 Heat Transfer 1

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1.1 Definition of Heat and Mass Transfer a. Heat Transfer Flow of energy due solely to a temperature difference ▪ from 2nd Law of Thermodynamics, heat flows in direction of decreasing temperature ▪ heat energy can be transported through a solid, liquid, gas, or vacuum. b. Mass Transfer ➢ Mass transfer is the net movement of mass from one location, usually meaning stream, phase, fraction or component, to another. Or ➢ From the region of high concentration to the lower concentration. Examples: Evaporation of water from a pond to the atmosphere. 2 Heat Transfer 10/28/2017

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1.2 MODES OF HEAT TRANSFER Heat can be transferred in three different modes: Conduction, Convection, & Radiation. A. Conduction: The transfer of energy from the more energetic particles of a substance to the adjacent less energetic ones as a result of interactions between the particles. ➢ In solids, it is due to the combination of vibrations of the molecules in a lattice and the energy transport by free electrons (i.e. solids in metallic form). ➢ In gases and liquids, conduction is due to the collisions and diffusion of the molecules during their random motion. 3 Heat Transfer 10/28/2017

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The rate of heat conduction through a plane layer is proportional to the temperature difference across the layer and the heat transfer area, but is inversely proportional to the thickness of the layer. Where:K = Thermal conductivity, : A measure of the ability of a material to conduct heat. dT/dx = Temperature gradient : The slope of the temperature curve on a T-x diagram. 4 Heat Transfer 10/28/2017