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HEAT TRANSFER TERMINOLOGY
Heat Flux

Thermodynamics Heat Transfer and Fluid Flow Volume 2 of 3
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HEAT TRANSFER TERMINOLOGY Heat Transfer Heat and Work Distinction should also be made between the energy terms heat and work.  Both represent energy in transition.   Work is  the transfer of  energy resulting  from a force  acting through  a distance. Heat is energy transferred as the result of a temperature difference.   Neither heat nor work are thermodynamic properties of a system.  Heat can be transferred into or out of a system and work can be done on or by a system, but a system cannot contain or store either heat or work.   Heat into a system and work out of a system are considered positive quantities. When a temperature difference exists across a boundary, the Second Law of Thermodynamics indicates the natural flow of energy is from the hotter body to the colder body.  The Second Law of Thermodynamics denies the possibility of ever completely converting into work all the heat supplied to a system operating in a cycle.   The Second Law of Thermodynamics, described by Max Planck in 1903, states that: It  is  impossible  to  construct  an  engine  that  will  work  in  a  complete  cycle  and produce  no  other  effect  except  the  raising  of  a  weight  and  the  cooling  of  a reservoir. The second law says that if you draw heat from a reservoir to raise a weight, lowering the weight will not generate enough heat to return the reservoir to its original temperature, and eventually the cycle will stop.  If two blocks of metal at different temperatures are thermally insulated from their surroundings and are brought into contact with each other the heat will flow from the hotter to the colder.   Eventually the two blocks will reach the same temperature, and heat transfer will cease.   Energy has not been lost, but instead some energy has been transferred from one block to another. Modes of Transferring Heat Heat is always transferred when a temperature difference exists between two bodies.   There are three basic modes of heat transfer: Conduction  involves the transfer of heat by the interactions of atoms or molecules of a material through which the heat is being transferred. Convection  involves  the  transfer  of  heat  by  the  mixing  and  motion  of  macroscopic portions of a fluid. Radiation,  or  radiant  heat  transfer,  involves  the  transfer  of  heat  by  electromagnetic radiation that arises due to the temperature of a body. The three modes of heat transfer will be discussed in greater detail in the subsequent chapters of this module. HT-02 Page 2 Rev. 0







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