THERMODYNAMICS EML 3100
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This 5 page Class Notes was uploaded by Aric Jast MD on Thursday September 17, 2015. The Class Notes belongs to EML 3100 at Florida State University taught by Leonard Van Dommelen in Fall. Since its upload, it has received 75 views. For similar materials see /class/205479/eml-3100-florida-state-university in Engineering Mechanical at Florida State University.
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Date Created: 09/17/15
Thermal Performance Measures A general performance measure is expressed as Desirded result Performance Measure 2 Required 1nput to achive des1red result Heat enginepower cycles A schematic representation of a heat engine operating in a cycle Hot thermal reservoir at TH w i Cold thermal reservoir at Tc 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 and rejects heat transfer of energy out in the amount Qout to some cold body cold thermal reservoir Clearly in a power cycle Qin gt Qom The desired result is chcle and the required input is Q The corresponding performance measure is called the thermal ef ciency and is represented by the symbol 77 chcle Qin An alternative form is expressed as QinQout 1 77 Qm Qm Thus the thermal efficiency represents the extent to which the energy added by heat to the system Q is converted into a net work output chcle Since energy is conserved rst 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 input energy must be rejected to a cold bodyreservoir by heat transfer The second law of thermodynamics will provide a maximum efficiency must be less than one for cycles operating between two reservoirs Refrigeration and heat pump cycles A refrigeration and heat pump cycles are power cycles in reverse A schematic diagram of refrigeration and heat pump cycles is shown below Hot thermal reservoir at TH In the refrigeration and heat pump cycles the energy transfer into the system by heat transfer Q is from a cold bodyreservoir and the energy transfer out of the system by heat transfer Qom is to a hot bodyreservoir This transfer of heat energy is accomplished by a net work input chcle to the system Note that refrigeration and heat pump cycles differ only in their objectives The objective of a refrigeration cycle is to cool a refrigerated space or maintain a temperature of a house below the temperature of its surroundings The objective of a heat pump is to maintain the temperature within a house above the temperature of the surroundings Since the refrigerator and heat pump cycles have different objectives their performance measures called coef cients of performance are defined differently Refrigeration Cycles The desired result is the energy transfer to the system by heat transfer from the cold bodyreservoir Qin required input to achieve this result is the net work transfer into the system chcle Let the symbol for the refrigeration coefficient of performance be thus or alternatively Qin Qout Qin In a refrigerator Qom is rejected to the room where the refrigerator is located and chcle is usually provided by the electric motor that drives the refrigerator compressor Note that Q represents the heat transfer in to the system and out of the refrigerated space in order to maintain the desired cool space at a temperature below the room temperature To maintain this desired cool temperature Qin must balance the heat gain by the cool space and its contents from the relatively warm room and the frequent opening and closing of the refrigerator door Heat Pump Cycles The desired result is the energy transfer from the system by heat transfer from the hot bodyreservoir Qom required output to achieve this result is the net work transfer into the system chcle Let the symbol for the heat pump coefficient of performance be y thus Qout W cycle 7 or alternatively Qout 7 Qout Qin In a heat pump Q is obtained from the surrounding atmosphere and chle is usually provided by electricity Note that Qom represents the heat transfer out of the system and in to the heated space in order to maintain the desired warm space at a temperature above the ambient outdoor temperature To maintain this desired indoor warm temperature Qom must balance the heat loss by the house to the surrounding cool atmosphere Note that the value of y is never less than one Maximum performance measures for cycles operating between two reservoirs The Carnot power cycle is the most efficient power cycle Since it is an internally and externally reversible cycle its efficiency will always be greater than any irreversible cycle operating between the same two reservoirs Note that in the Carnot cycle we showed that the ratio of the heat transfers is related to the absolute temperatures of the corresponding thermal reservoirs QHTH QC TC Qin Qout QH TH Qc Tc QC Qout Cold thermal reservoir at Tc To obtain the maximum performance measure we replace the heat ratios by the corresponding absolute temperature ratios in the previously defined performance measures Power cycles Since a refrigeratorheat pump is a Carnot cycle in reverse the same maximum performance formulation also applies Thus we may write Refrigeration cycles Heat Pump cycles
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