PRINCIPLES OF NAVAL ENGINEERING 



An isobaric state change involves changes of 

 enthalpy. One equation which has frequent appli- 

 cation to isobaric state changes is written as 



^"iu^p'-h-h 



where 



(q, 9) = heat transferred between state 1 and 

 ^ state 2, with subscript p indicating 

 constant pressure 



h- = enthalpy of working fluid at state 1 



h„ . enthalpy of working fluid at state 2 



ISOMETRIC STATE CHANGES. - A state 

 change is said to be isometric when the volume 

 (and the specific volume) of the working fluid is 

 maintained constant. In other words, an isomet- 

 ric change is a constant-volume change. Isomet- 

 ric changes involve changes in internal energy, 

 in accordance with the equation 



^ = "2-"l 



where 



a = heat transferred, with subscript v 

 indicating constant volume 



u^ ; specific internal energy of working 

 substance at state 1 



u„ - specific internal energy of working 

 substance at state 2 



ISOTHERMAL STATE CHANGES.-An iso- 

 thermal change is one in which the temperature 

 of the working fluid remains constant throughout 

 the change. 



ISENTHALPIC STATE CHANGES. -When the 

 enthalpy of the working fluid does not change 

 during the process, the change is said to be isen- 

 thalpic. Throttling processes are basically isen- 

 thalpic— that is, h^ = h„. 



ISENTROPIC STATE CHANGES.-An isen- 

 tropic state change is one in which there is no 

 change in the property known as entropy . The 

 significance of entropy and of isentropic state 

 changes is discussed in a later section of this 

 chapter. 



ADIABATIC STATE CHANGES.-An adiabatic 

 state change is one which occurs in such a way 

 that there is no transfer of heat to or from the 

 system while the process is occurring. In many 

 real processes, adiabatic changes are produced 

 by performing the process rapidly. Since heat 

 transfer is relatively slow, any rapidly per- 

 formed process can approach being adiabatic. 

 Compression and expansionof working fluids are 

 frequently achieved adiabatically. For an adia- 

 batic process, the energy equation may be written 

 as 



U2-Ui = W 



where 



U^ z internal energy of working fluid at 

 state 1 



U„ = internal energy of working fluid at 

 state 2 



W = work performed on or by the work- 

 ing fluid 



In words, we may say that the net change of 

 internal energy is equal to the work performed 

 in an adiabatic process. The work term may be 

 either positive or negative, depending upon 

 whether work is done on the working substance, 

 as in compression, or by the working substance, 

 as in expansion. 



THERMODYNAMIC CYCLES 



A thermodynamic cycle is a recurring series 

 of thermodynamic processes through which an 

 effect is produced by the transformation or re- 

 distribution of energy. In other words, a cycle 

 is a series of processes repeated over and over 

 again in the same order. 



All thermodynamic cycles may be classified 

 as being open cycles or closed cycles. An open 

 cycle is one in which the working fluid is taken 

 in, used, and then discarded. A closed cycle is 

 one in which the working fluid never leaves the 

 cycle, except through accidental leakage; in- 

 stead, the working fluid undergoes a series of 

 processes which are of such a nature that the 

 fluid is returned periodically to its initial state 

 and is then used again. 



The open cycle is exemplified by the internal 

 combustion engine, in which atmospheric air 

 supplies the oxygen for combustion and in which 



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