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# Unit 5 - Engi2800(Section I Engineering Thermodynamics I...

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10/05/09 http://jjl.me.dal.ca/Engi2800 1 J.M. Chuang Dalhousie University Dept. of Mechanical Engineering Last Modified: October 5, 2009 Unit #5 (Chapter 5, Textbook) The First Law of Thermodynamics Engi2800 (Section I) Engineering Thermodynamics I

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10/05/09 http://jjl.me.dal.ca/Engi2800 2 Contents Thermodynamic Cycle First Law of Thermodynamics Thermodynamics 1 st Law for a CM Undergoing a Cycle Thermodynamics 1 st Law for a Change in State of a CM Undergoing a Cycle Heat Engines and Refrigerators/Heat Pumps Performance Measure Brief Summary of 1 st Law of Thermodynamics Internal Energy – A Thermodynamic Property Enthalpy – A Thermodynamic Property Specific Heat Internal Energy, Enthalpy and Specific Heat of Ideal Gas Internal Energy, Enthalpy and Specific Heat of Liquid and Solid Summary
10/05/09 http://jjl.me.dal.ca/Engi2800 3 Thermodynamic Cycle A thermodynamic cycle is a series of thermodynamic process transferring heat and work, while varying pressure, temperature, and other state variables, eventually returning a system to its initial state. State properties depend only on the thermodynamic state and cumulative variation of such properties add up to zero. Path quantities, such as heat and work are process dependent, and cumulative heat and work are non-zero. The first law of thermodynamics dictates that the net heat input is equal to the net work output over any cycle. Thermodynamic cycles often use quasistatic processes to model the workings of actual devices. quasistatic process is a thermodynamic process that happens infinitely slowly. A thermodynamic cycle is a closed loop on a diagram. The area enclosed by the loop is the work done by the process: P - V Q in Q out W Cyclic integral Cycle: Initial state = final state W net ; out = Q net ; in = Q in ¡ Q out W net ; out = I P dV; Isobaric (constant pressure) Isometric (constant volume) Isothermal (constant pressure) Adiabatic (no heat transfer)

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10/05/09 http://jjl.me.dal.ca/Engi2800 4 Thermodynamics 1 st Law for a Control Mass Undergoing a Cycle (1) A cycle with two processes. 1 st process: work is done by the paddle (work done by lowing the weight) 2 nd process: system returns to its initial state (transferring heat from the system until the cycle has been completed) 1 st law of Thermodynamics: During any cycle a system (control mass) undergoes, the cyclic integral of the heat is proportional to the cyclic integral of the work, i.e. Since For N individual processes making up a cycle: I ±Q = I ±W Cyclic integral of heat transfer Cyclic integral of the work E initial = E ¯nal = ) ¢ E cycle = 0 N X i =1 Q i = N X i =1 W i = ) Q in ¡ Q out = W in ¡ W out Q cycle or Q net W cycle or W net
10/05/09 http://jjl.me.dal.ca/Engi2800 5 Thermodynamics 1 st Law for a Control Mass Undergoing a Cycle (2) If cycle is traversed clockwise: If cycle is traversed anti-clockwise: Power cycle Heat supplied to system Work done by system Refrigerator/Heat pump cycle Work supplied to system

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Unit 5 - Engi2800(Section I Engineering Thermodynamics I...

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