Thermal_Performance_Measures

Thermal_Performance_Measures - Thermal Performance Measures...

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Thermal Performance Measures A general performance measure is expressed as: result desired achive input to Required result Desirded Measure e Performanc = Heat engine/power cycles: A schematic representation of a heat engine operating in a cycle: The heat engine is a power cycle where the desired result of the cycle is the work transfer of energy to the surroundings during each cycle. The power cycle receives heat transfer of energy into the system from some hot body (hot thermal reservoir) in the amount Q in , and rejects heat transfer of energy out in the amount Q out to some cold body (cold thermal reservoir). Clearly, in a power cycle Q in > Q out . The desired result is W cycle , and the required input is Q in . The corresponding performance measure is called the thermal efficiency and is represented by the symbol η . in cycle Q W = An alternative form is expressed as: Hot thermal reservoir at T H Cold thermal reservoir at T C HEAT ENGINE T C T H Q H = Q in Q C = Q out W NET = W cycle = Q in – Q out

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in out in out in Q Q Q Q Q = = 1 η Thus, the thermal efficiency represents the extent to which the energy added by heat to the system Q in is converted into a net work output W cycle . Since energy is conserved (first law of thermodynamics), the thermal efficiency can never by greater than one. In actual power cycles the efficiency is always less than one. Not all energy added to the system by heat energy is converted into work; a portion of the
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This note was uploaded on 07/21/2011 for the course EML 3100 taught by Professor Staff during the Summer '10 term at FSU.

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Thermal_Performance_Measures - Thermal Performance Measures...

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