功熵与功(火用)

Work entropy and work exergy

类似于克劳修斯采用由3个等温过程和3个绝热过程组成的可逆热转功循环,建立热向功转换与热量从高温向低温转换的等价定律,并导出克劳修斯等式和熵的方法.本文采用由3个等压过程和3个等容过程组成的可逆功转热循环,建立了功向热转换与功量从高压向低压转换的等价定律,导出了逆循环等式和功熵,并推广至任意可逆的逆循环.系统的体积就是功熵,它是逆循环的核心物理量.类似于(火用)的定义和表达式,定义了新的物理量——功(火用),并借助于逆p-V循环导得了它的表达式.而后阐明了功熵和功(火用)的物理意义,并与热熵和热(火用)的物理意义进行对比.最后介绍了它们在可逆过程性能分析中的应用.

Clausius proposed a reversible thermodynamic cycle made up of three isothermal processes and three adiabatic processes to establish the theorem of the equivalence of the transformation of heat to work and the transformation of heat at a higher temperature to a lower temperature.Then,he derived the Clausius equality and the concept of entropy(later called thermal entropy to prevent confusion).Similarly,this study uses a reversible reversed thermodynamic cycle made up of three isobaric processes and three isochoric processes to establish the theorem of the equivalence of the transformation of work to heat and the transformation of work at a higher pressure to a lower pressure,and then derives a corresponding equality for a reversed thermodynamic cycle and the concept of work entropy.The work entropy is shown to be equal to the system volume which is the core physical quantity of a reversed thermodynamic cycle.Although this is not a new quantity,the work entropy gives the volume a new physical interpretation corresponding to thermal entropy.Next,by analogy with the definition of exergy(later called thermal exergy to prevent confusion),the work exergy is defined as the maximum heat output that can be transformed from the work in a system at pressure p though a reversed p-V cycle under ambient pressure.Furthermore,the work exergy of the working medium is defined as the sum of the heat that the working medium outputs though a series of reversed p-V cycles and the heat that the working medium outputs directly as the temperature varies,as the working medium changes reversibly from any state to the state in balance with the environment.The physical meanings of the work entropy and the work exergy are illustrated with comparisons to the thermal entropy and the thermal exergy.When it changes reversibly from any state to the state in balance with the environment,the working medium with more thermal entropy will have more unavailable energy T0(ST–ST0)and,thus,less thermal exergy,if the system outputs work through a reversible thermodynamic cycle.In contrast,the working medium with more work entropy will have less unavailable energy p0(V0–V)and,thus,more work exergy,if the system outputs heat through a reversible reversed thermodynamic cycle.Thus,the work entropy can measure the unavailable energy of the working medium while the work exergy can measure the available energy of the working medium.The'pressure-work entropy diagram'can be used to intuitively analyze a reversed thermodynamic cycle.Finally,a least action principle is proposed for reversible thermodynamic processes.When the heat-to-work action,ΔS/Q reaches a minimum,the optimal heat-to-work process is obtained that is the combination of an isothermal process and an adiabatic process.When the work-to-heat action,ΔV/W reaches a maximum,the optimal work-to-heat process is obtained that is the combination of an isobaric process and an isochoric process.