Principles of naval engineering - Historic Naval Ships Association

PRINCIPLES OF NAVAL ENGINEERINGAn isobaric state change involves changes ofenthalpy. One equation which has frequent applicationto isobaric state changes is written aswhere^"iu^p'-h-h(q, 9) = heat transferred between state 1 and^ state 2, with subscript p indicatingconstant pressureh- = enthalpy of working fluid at state 1h„ . enthalpy of working fluid at state 2ISOMETRIC STATE CHANGES. - A statechange is said to be isometric when the volume(and the specific volume) of the working fluid ismaintained constant. In other words, an isometricchange is a constant-volume change. Isometricchanges involve changes in internal energy,in accordance with the equationwhere^ ="2-"la = heat transferred, with subscript vindicating constant volumeu^ ; specific internal energy of workingsubstance at state 1u„ - specific internal energy of workingsubstance at state 2ISOTHERMAL STATE CHANGES.-An isothermalchange is one in which the temperatureof the working fluid remains constant throughoutthe change.ISENTHALPIC STATE CHANGES. -When theenthalpy of the working fluid does not changeduring the process, the change is said to be isenthalpic.Throttling processes are basically isenthalpic—that is, h^ = h„.ISENTROPICSTATE CHANGES.-An isentropicstate change is one in which there is nochange in the property known as entropy . Thesignificance of entropy and of isentropic statechanges is discussed in a later section of thischapter.ADIABATIC STATE CHANGES.-An adiabaticstate change is one which occurs in such a waythat there is no transfer of heat to or from thesystem while the process is occurring. In manyreal processes, adiabatic changes are producedby performing the process rapidly. Since heattransfer is relatively slow, any rapidly performedprocess can approach being adiabatic.Compression and expansionof working fluids arefrequently achieved adiabatically. For an adiabaticprocess, the energy equation may be writtenaswhereU2-Ui =U^ z internal energy of working fluid atstate 1U„ = internal energy of working fluid atstate 2W =Wwork performed on or by the workingfluidIn words, we may say that the net change ofinternal energy is equal to the work performedin an adiabatic process. The work term may beeither positive or negative, depending uponwhether work is done on the working substance,as in compression, or by the working substance,as in expansion.THERMODYNAMIC CYCLESA thermodynamic cycle is a recurring seriesof thermodynamic processes through which aneffect is produced by the transformation or redistributionof energy. In other words, a cycleis a series of processes repeated over and overagain in the same order.All thermodynamic cycles may be classifiedas being open cycles or closed cycles. An opencycle is one in which the working fluid is takenin, used, and then discarded. A closed cycle isone in which the working fluid never leaves thecycle, except through accidental leakage; instead,the working fluid undergoes a series ofprocesses which are of such a nature that thefluid is returned periodically to its initial stateand is then used again.The open cycle is exemplified by the internalcombustion engine, in which atmospheric airsupplies the oxygen for combustion and in which178