同济大学：《工程热力学》课程电子教案（讲稿）Chapter 5 The Second Law of Thermodynamics

2013-3-6 Chapter 5 The Second Law of Thermodynamics Learning Outcomes Demonstrate understanding of key concepts related to the second law of thermodynamics,including alternative statements of the second law,the internally reversible process,and the Kelvin temperature scale. List several important irreversibilities. 1

2013-3-6 1 Chapter 5 The Second Law of Thermodynamics Learning Outcomes ►Demonstrate understanding of key concepts related to the second law of thermodynamics, including alternative statements of the second law, the internally reversible process, and the Kelvin temperature scale. ►List several important irreversibilities

2013-3-6 Learning Outcomes,cont. Assess the performance of power cycles and refrigeration and heat pump cycles using,as appropriate,the corollaries of Secs.5.6.2 and 5.7.2,together with Egs 5.9-5.11. Describe the Carnot cycle. Interpret the Clausius inequality as expressed by Eq.5.13. Aspects of the Second Law of Thermodynamics From conservation of mass and energy principles,mass and energy cannot be created or destroyed. For a process,conservation of mass and energy principles indicate the disposition of mass and energy but do not infer whether the process can actually occur. The second law of thermodynamics provides the guiding principle for whether a process can occur. 2

2013-3-6 2 Learning Outcomes, cont. ►Assess the performance of power cycles and refrigeration and heat pump cycles using, as appropriate, the corollaries of Secs. 5.6.2 and 5.7.2, together with Eqs. 5.9-5.11. ►Describe the Carnot cycle. ►Interpret the Clausius inequality as expressed by Eq. 5.13. Aspects of the Second Law of Thermodynamics ►From conservation of mass and energy principles, mass and energy cannot be created or destroyed. ►For a process, conservation of mass and energy principles indicate the disposition of mass and energy but do not infer whether the process can actually occur. ►The second law of thermodynamics provides the guiding principle for whether a process can occur

2013-3-6 Aspects of the Second Law of Thermodynamics The second law of thermodynamics has many aspects,which at first may appear different in kind from those of conservation of mass and energy principles.Among these aspects are: predicting the direction of processes establishing conditions for equilibrium determining the best theoretical performance of cycles,engines,and other devices. level. Aspects of the Second Law of Thermodynamics Other aspects of the second law include: defining a temperature scale independent of the properties of any thermometric substance. developing means for evaluating properties such as u and h in terms of properties that are more readily obtained experimentally. Scientists and engineers have found additional uses of the second law and deductions from itit also has been used in philosophy,economics,and other disciplines far removed from engineering thermodvnamics. 3

2013-3-6 3 Aspects of the Second Law of Thermodynamics ►predicting the direction of processes. ►establishing conditions for equilibrium. ►determining the best theoretical performance of cycles, engines, and other devices. ►evaluating quantitatively the factors that preclude attainment of the best theoretical performance level. The second law of thermodynamics has many aspects, which at first may appear different in kind from those of conservation of mass and energy principles. Among these aspects are: Aspects of the Second Law of Thermodynamics ►defining a temperature scale independent of the properties of any thermometric substance. ►developing means for evaluating properties such as u and h in terms of properties that are more readily obtained experimentally. Scientists and engineers have found additional uses of the second law and deductions from it. It also has been used in philosophy, economics, and other disciplines far removed from engineering thermodynamics. Other aspects of the second law include:

2013-3-6 Second Law of Thermodynamics Alternative Statements There is no simple statement that captures all aspects of the second law.Several alternative formulations of the second law are found in the technical literature.Three prominent ones are: Clausius Statement Kelvin-Planck Statement Entropy Statement Second Law of Thermodynamics Alternative Statements The focus of Chapter 5 is on the Clausius and Kelvin-Planck statements. The Entropy statement is developed and applied in Chapter 6. Like every physical law,the basis of the second law of thermodynamics is experimental evidence.While the three forms given are not directly demonstrable in the laboratory, deductions from them can be verified experimentally,and this infers the validity of the second law statements. 4

2013-3-6 4 Second Law of Thermodynamics Alternative Statements ►Clausius Statement ►Kelvin-Planck Statement ►Entropy Statement There is no simple statement that captures all aspects of the second law. Several alternative formulations of the second law are found in the technical literature. Three prominent ones are: Second Law of Thermodynamics Alternative Statements ►The focus of Chapter 5 is on the Clausius and Kelvin-Planck statements. ►The Entropy statement is developed and applied in Chapter 6. ►Like every physical law, the basis of the second law of thermodynamics is experimental evidence. While the three forms given are not directly demonstrable in the laboratory, deductions from them can be verified experimentally, and this infers the validity of the second law statements

2013-3-6 Clausius Statement of the Second Law It is impossible for any system to operate in such a way that the sole result would be an energy transfer by heat from a cooler to a hotter body. 20 Hot Thermal Reservoir A thermal reservoir is a system that always remains at constant temperature even though energy is added or removed by heat transfer. Such a system is approximated by the earth's atmosphere,lakes and oceans,and a large block of a solid such as copper. 5

2013-3-6 5 Clausius Statement of the Second Law It is impossible for any system to operate in such a way that the sole result would be an energy transfer by heat from a cooler to a hotter body. Thermal Reservoir ►A thermal reservoir is a system that always remains at constant temperature even though energy is added or removed by heat transfer. ►Such a system is approximated by the earth’s atmosphere, lakes and oceans, and a large block of a solid such as copper

2013-3-6 Kelvin-Planck Statement of the Second Law It is impossible for any system to operate in a thermodynamic cycle and deliver a net amount of energy by work to its surroundings while receiving energy by heat transfer from a single thermal reservoir. Entropy Statement of the Second Law Mass and energy are familiar examples of extensive properties used in thermodynamics Entropy is another important extensive property. How entropy is evaluated and applied is detailed in Chapter 6. Unlike mass and energy,which are conserved, entropy is produced within systems whenever non-idealities such as friction are present. The Entropy Statement is: It is impossible for any system to operate in a way that entropy is destroyed. 6

2013-3-6 6 Kelvin-Planck Statement of the Second Law It is impossible for any system to operate in a thermodynamic cycle and deliver a net amount of energy by work to its surroundings while receiving energy by heat transfer from a single thermal reservoir. Entropy Statement of the Second Law ►Mass and energy are familiar examples of extensive properties used in thermodynamics. ►Entropy is another important extensive property. How entropy is evaluated and applied is detailed in Chapter 6. ►Unlike mass and energy, which are conserved, entropy is produced within systems whenever non-idealities such as friction are present. ►The Entropy Statement is: It is impossible for any system to operate in a way that entropy is destroyed

2013-3-6 Irreversibilities One of the important uses of the second law of thermodynamics in engineering is to determine the best theoretical performance of systems. By comparing actual performance with best theoretical performance,insights often can be had about the potential for improved performance. Best theoretical performance is evaluated in terms of idealized processes. Actual processes are distinguishable from such idealized processes by the presence of non- idealities-called irreversibilities. Irreversibilities Commonly Encountered in Engineering Practice Heat transfer through a finite temperature difference Unrestrained expansion of a gas or liquid to a lower pressure Spontaneous chemical reaction Spontaneous mixing of matter at different compositions or states Friction-sliding friction as well as friction in the flow of fluids

2013-3-6 7 Irreversibilities ►One of the important uses of the second law of thermodynamics in engineering is to determine the best theoretical performance of systems. ►By comparing actual performance with best theoretical performance, insights often can be had about the potential for improved performance. ►Best theoretical performance is evaluated in terms of idealized processes. ►Actual processes are distinguishable from such idealized processes by the presence of non￾idealities – called irreversibilities. Irreversibilities Commonly Encountered in Engineering Practice ►Heat transfer through a finite temperature difference ►Unrestrained expansion of a gas or liquid to a lower pressure ►Spontaneous chemical reaction ►Spontaneous mixing of matter at different compositions or states ►Friction – sliding friction as well as friction in the flow of fluids

2013-3-6 Irreversibilities Commonly Encountered in Engineering Practice Electric current flow through a resistance Magnetization or polarization with hysteresis Inelastic deformation All actual processes involve effects such as those listed,including naturally occurring processes and ones involving devices we construct-from the simplest mechanisms to the largest industrial plants. Irreversible and Reversible Processes During a process of a system, irreversibilities may be present: within the system,or within its surroundings(usually the immediate surroundings),or within both the system and its surroundings. 8

2013-3-6 8 Irreversibilities Commonly Encountered in Engineering Practice ►Electric current flow through a resistance ►Magnetization or polarization with hysteresis ►Inelastic deformation All actual processes involve effects such as those listed, including naturally occurring processes and ones involving devices we construct – from the simplest mechanisms to the largest industrial plants. Irreversible and Reversible Processes ►within the system, or ►within its surroundings (usually the immediate surroundings), or ►within both the system and its surroundings. During a process of a system, irreversibilities may be present:

2013-3-6 Irreversible and Reversible Processes A process is irreversible when irreversibilities are present within the system and/or its surroundings. All actual processes are irreversible A process is reversible when no irreversibilities are present within the system and its surroundings. This type of process is fully idealized. Irreversible and Reversible Processes A process is internally reversible when no irreversibilities are present within the system. Irreversibilities may be present within the surroundings,however. An internally reversible process is a quasiequilibrium process(see Sec.2.2.5). 9

2013-3-6 9 Irreversible and Reversible Processes ►A process is irreversible when irreversibilities are present within the system and/or its surroundings. All actual processes are irreversible. ►A process is reversible when no irreversibilities are present within the system and its surroundings. This type of process is fully idealized. Irreversible and Reversible Processes ►A process is internally reversible when no irreversibilities are present within the system. Irreversibilities may be present within the surroundings, however. An internally reversible process is a quasiequilibrium process (see Sec. 2.2.5)

2013-3-6 Example:Internally Reversible Process Water contained within a piston-cylinder evaporates from saturated liquid to saturated vapor at 100C.As the water evaporates,it passes through a sequence of equilibrium states while there is heat transfer to the water from hot gases at 500C. For a system enclosing the water there are no internal irreversibilities,but Such spontaneous heat transfer is an irreversibility in its surroundings:an external irreversibility Analytical Form of the Kelvin-Planck Statement For any system undergoing a thermodynamic cycle while exchanging energy by heat transfer with a single thermal reservoir,the net work, + can be only negative or zero- never positive: NO! Wee≤0 <0:Internal irreversibilities present =0:No internal irreversibilities (Eq.5.3) 10

2013-3-6 10 Example: Internally Reversible Process Water contained within a piston-cylinder evaporates from saturated liquid to saturated vapor at 100oC. As the water evaporates, it passes through a sequence of equilibrium states while there is heat transfer to the water from hot gases at 500oC. ►Such spontaneous heat transfer is an irreversibility in its surroundings: an external irreversibility. ►For a system enclosing the water there are no internal irreversibilities, but Analytical Form of the Kelvin-Planck Statement For any system undergoing a thermodynamic cycle while exchanging energy by heat transfer with a single thermal reservoir, the net work, Wcycle, can be only negative or zero – never positive: Wcycle ≤ 0 < 0: Internal irreversibilities present = 0: No internal irreversibilities single reservoir (Eq. 5.3) NO!