。2 Figure 1-8 Indication of quasi-equilibrium process w several small weights whose total mass is the same as the original one are used in place of the heavy weight. The process is repeated, the small weights are not removed totally but only one piece of weight, the system is allowed to restore balance before another piece of weight is removed. The state changes until it reaches a new equilibrium state. Repeat the process until the last piece of small weight is removed. And the system will reach its final state. During this process, there are several equilibrium states, states a, b, c, etc, between the initial and final states, as shown in Fig. 1-8 by the equilibrium. If the mass of the small weight is infinitesimal, mechanical equilibrium of the system can be maintained all the time. This process is a quasi-equilibrium process. The path of this process can be hown on the state diagram with a continuous solid line In order to fulfill i-equilibrium process, it is necessary to maintain the mechanical equilibrium within a system, not only the internal equilibrium, but also in equilibrium with the surroundings. The conclusion can be extended to heat transfer and chemical reactions, etc. Thermal equilibrium and chemical equilibrium require no driving force a quasi-equilibrium process can be viewed as a sufficiently slow process that allows the system to adjust itself internally so that the properties in one part of the system do not change any faster than those at other parts. A working producing device operating on quasi-equilibrium processes can deliver the most work. Although a quasi-equilibrium process is an idealized process and can not represent actual process completely. However, in engineering practice, many actual processes can closely approximate it. They can be modeled as quasi-equilibrium with negligible error. As quasi-equilibrium state is easy to be analyzed, it is chosen to serves as a base case with which actual processes can 1. 4. 2 Reversible process Reversible process is defined as a process that can be reversed without leaving any trace on the Firstly, reversible process must be a quasi-equilibrium process. Otherwise the driving force will be irreversibility in the process. For example, when two objects at different temperatures are in contact with each other, heat is transferred from the high temperature object to the low temperature one, until they reach thermal equilibrium. The high temperature body can not be heated up by retrieving the heat it transferred to the low temperature one. Thus, it is an irreversible process. The two bodies can not restore to their initial states only when some work is done on them. Therefore, they leave some effect on the surroundings so that the surroundings do not return to their original states Secondly, there is no any irreversible dissipation effect(The work loss, which is converted into heat by friction, resistance, magnetism, etc, is called dissipation loss )involved in reversible process
21 Figure 1-8 Indication of quasi-equilibrium process Now several small weights whose total mass is the same as the original one are used in place of the heavy weight. The process is repeated, the small weights are not removed totally but only one piece of weight, the system is allowed to restore balance before another piece of weight is removed. The state changes until it reaches a new equilibrium state. Repeat the process until the last piece of small weight is removed. And the system will reach its final state. During this process, there are several equilibrium states, states a, b, c, etc, between the initial and final states, as shown in Fig. 1-8 by the equilibrium. If the mass of the small weight is infinitesimal, mechanical equilibrium of the system can be maintained all the time. This process is a quasi-equilibrium process. The path of this process can be shown on the state diagram with a continuous solid line. In order to fulfill a quasi-equilibrium process, it is necessary to maintain the mechanical equilibrium within a system, not only the internal equilibrium, but also in equilibrium with the surroundings. The conclusion can be extended to heat transfer and chemical reactions, etc. Thermal equilibrium and chemical equilibrium require no driving force. A quasi-equilibrium process can be viewed as a sufficiently slow process that allows the system to adjust itself internally so that the properties in one part of the system do not change any faster than those at other parts. A working producing device operating on quasi-equilibrium processes can deliver the most work. Although a quasi-equilibrium process is an idealized process and can not represent actual process completely. However, in engineering practice, many actual processes can closely approximate it. They can be modeled as quasi-equilibrium with negligible error. As quasi-equilibrium state is easy to be analyzed, it is chosen to serves as a base case with which actual processes can compare. 1.4.2 Reversible Process Reversible process is defined as a process that can be reversed without leaving any trace on the surroundings. Processes that are not reversible are called irreversible processes. For a reversible process, not only the system but also the surroundings can return to their original state if the process is reversed. Firstly, reversible process must be a quasi-equilibrium process. Otherwise the driving force will be irreversibility in the process. For example, when two objects at different temperatures are in contact with each other, heat is transferred from the high temperature object to the low temperature one, until they reach thermal equilibrium. The high temperature body can not be heated up by retrieving the heat it transferred to the low temperature one. Thus, it is an irreversible process. The two bodies can not restore to their initial states only when some work is done on them. Therefore, they leave some effect on the surroundings so that the surroundings do not return to their original states. Secondly, there is no any irreversible dissipation effect (The work loss, which is converted into heat by friction, resistance, magnetism, etc, is called dissipation loss.) involved in reversible process
Actually, irreversible process can be discriminated easily. The process is irreversible if only volves the following factors, such as temperature difference, unrestrained expansion, mixing of two fluids, throttling process, electric resistance, inelastic deformation of solids, and chemical reactions, etc A reversible process involves none of these Reversible processes are idealized processes, and they can be approached but never reached in reality. You may be wondering, then, why we are bothering with such fictitious processes. As reversible processes are easy to analyze, they play an important role in thermodynamics theory and in practical applications. Reversible process is a basic concept in thermodynamics Example 1-21 A piston-cylinder device initially contains 0.4 m gas at 0.5 MPa and 80C. The gas expands to 0.8 m in such a way that pl=const. Assuming the process is reversible, determine the work done during this process 【 Solution】 As the process is reversible Based on the conditions given, we have hen W=," p-dv=pl, in. W=0.5××04×ln×10-3=13863kJ Discussion: The positive sign indicates that the work is done by a system 1.5 A Thermody namic Cycle a thermodynamic cycle occurs when a sy stem undergoes several processes and returns to its initial state at the end of the process. That is the initial and final states are identical for a cycle. Ir engineering, it is necessary to go through a cycle to obtain work continuously According to the function of a cycle, it can be categorized as power cycle and refrigeration cycle 1.5.1 Power Cycle The devices or systems used to produce a net power output are often called engines. Heat engines are designed for the purpose of converting thermal energy to work. And the thermodynamic cycles they operate on are called power cycle For heat engines, the working fluid receives heat g from a high-temperature source, and then expands to deliver work. To complete the cycle, there must be a compressible process. In a cycle, only part of the heat absorbed can be converted to the work that is called net work net The other part of the heat absorbed 0, will be rejected to a low-temperature sink. Thus, the net work of a system is also qual to the net heat transfer to the system That is, the net work output during a cycle is equal to the net heat input, only part of the heat transferred to the heat engine is converted into work and the other part is rejected as heat in order to complete the cycle The fraction of the heat input that is converted to the net work output is a measure of the erformance of a heat engine and is called the thermal efficiency n. It indicates how well an energy conversion is, and can be expressed as
22 Actually, irreversible process can be discriminated easily. The process is irreversible if only it involves the following factors, such as temperature difference, unrestrained expansion, mixing of two fluids, throttling process, electric resistance, inelastic deformation of solids, and chemical reactions, etc. A reversible process involves none of these. Reversible processes are idealized processes, and they can be approached but never reached in reality. You may be wondering, then, why we are bothering with such fictitious processes. As reversible processes are easy to analyze, they play an important role in thermodynamics theory and in practical applications. Reversible process is a basic concept in thermodynamics. 【Example 1-2】A piston-cylinder device initially contains 0.4 m3 gas at 0.5 MPa and 80℃. The gas expands to 0.8 m3 in such a way that pV const = . Assuming the process is reversible, determine the work done during this process. 【Solution】As the process is reversible, 2 1 d V V W p V = Based on the conditions given, we have 1 1 pV p V = Then 2 1 1 1 2 1 1 1 d ln V V pV V W V pV V V = = 0.8 3 0.5 0.4 ln 10 138.63 kJ 0.4 W − = = Discussion: The positive sign indicates that the work is done by a system. 1.5 A Thermodynamic Cycle A thermodynamic cycle occurs when a system undergoes several processes and returns to its initial state at the end of the process. That is, the initial and final states are identical for a cycle. In engineering, it is necessary to go through a cycle to obtain work continuously. According to the function of a cycle, it can be categorized as power cycle and refrigeration cycle. 1.5.1 Power Cycle The devices or systems used to produce a net power output are often called engines. Heat engines are designed for the purpose of converting thermal energy to work. And the thermodynamic cycles they operate on are called power cycle. For heat engines, the working fluid receives heat Q1 from a high-temperature source, and then expands to deliver work. To complete the cycle, there must be a compressible process. In a cycle, only part of the heat absorbed can be converted to the work that is called net work Wnet . The other part of the heat absorbed Q2 will be rejected to a low-temperature sink. Thus, the net work of a system is also equal to the net heat transfer to the system W Q Q net 1 2 = − (1-10) That is, the net work output during a cycle is equal to the net heat input, only part of the heat transferred to the heat engine is converted into work and the other part is rejected as heat in order to complete the cycle. The fraction of the heat input that is converted to the net work output is a measure of the performance of a heat engine and is called the thermal efficiency t . It indicates how well an energy conversion is, and can be expressed as:
Thermal efficienc Net work Total heat inpu =9=g=1-g (1-11) g11 The thermal efficiency is usually used to evaluate the thermal economy of a power cycle 1.5.2 The Refrigeration Cycle The cycles on which the device operates reversed from power cycle called the refrigeration cycle, such as refrigerators and heat pumps. They consume some work and transfer heat from a lower temperature region to a high temperature one A d wror =Q-g2 where @, is the magnitude of the heat removed from the refrigerated space at temperatureT::2, is the magnitude of the heat rejected to the warmer space at temperature,. Wnet is the net work input to the cycle and it is a necessary term Both refrigerators and heat pumps operate on refrigeration cycles, while they just differ in their objectives. The objective of a refrigerator is to maintain the refrigerated space at a low temperature by removing heat from it. Discharging the heat to a higher-temperature medium is merely a necessary part of the operation, not the purpose. The objective of a heat pump, however, is to maintain a heated space from well water or cold outdoor air in winter, and supplying the heat to a warmer indoor alr ce, such as at a high temperature. This is accomplished by absorbing heat from a low-temperature sour The performance of refrigerators and heat pumps is expressed in terms of the coeffici performance(COP), defined COP= Desired output The performance of refrigerators can be expressed as g22 The performance of heat pumps can be expressed as COP e Q (1-13) A comparison of Eq (1-17)and Eq (1-18 )reveals that COP=COP+ It indicates that COPr can be less, greater or equal to l, however, COPp is always greater than
23 Net work output Thermal efficiency= Total heat input or net 1 2 2 t 1 1 1 1 W Q Q Q Q Q Q − = = = − (1-11) The thermal efficiency is usually used to evaluate the thermal economy of a power cycle. 1.5.2 The Refrigeration Cycle The cycles on which the device operates reversed from power cycle called the refrigeration cycle, such as refrigerators and heat pumps. They consume some work and transfer heat from a lower temperature region to a high temperature one. W Q Q net 1 2 = − where Q2 is the magnitude of the heat removed from the refrigerated space at temperature T2 ; Q1 is the magnitude of the heat rejected to the warmer space at temperature T1 . Wnet is the net work input to the cycle and it is a necessary term. Both refrigerators and heat pumps operate on refrigeration cycles, while they just differ in their objectives. The objective of a refrigerator is to maintain the refrigerated space at a low temperature by removing heat from it. Discharging the heat to a higher-temperature medium is merely a necessary part of the operation, not the purpose. The objective of a heat pump, however, is to maintain a heated space at a high temperature. This is accomplished by absorbing heat from a low-temperature source, such as from well water or cold outdoor air in winter, and supplying the heat to a warmer indoor air. The performance of refrigerators and heat pumps is expressed in terms of the coefficient of performance (COP), defined as Desired output COP= Required input The performance of refrigerators can be expressed as 2 2 R net 1 2 COP Q Q W Q Q = = − (1-12) The performance of heat pumps can be expressed as 1 1 HP net 1 2 COP Q Q W Q Q = = − (1-13) A comparison of Eq. (1-17) and Eq. (1-18) reveals that COP 1 HP R = + COP It indicates that COPR can be less, greater or equal to 1, however, COPHP is always greater than 1
Chapter 2 The First Law of Thermodynamics Energy Conservation Principle is one of the most fundamental laws in nature. It states that energ be neither created nor destroyed during a process; it can only change forms, or it can be transfe from one body to another. However, the total amount of energy remains constant. That is, energy al ways conserved. It provides a sound basis for studying the relationships among the various forms of energy and energy interactions As an example, consider the heating of water in a boiler. If energy of 15 k is transferred to water by heat from a heating element and 3 kJ of it is lost from the water to the surroundings, the increase in energy of the water will be equal to the net heat transferred to the water, which is 12 k. As another example, consider a well-insulated room heated by an electric heater. As a result of electrical work added, the energy of the room will increase. Since the room is adiabatic and there is not any heat transfer to or from the surroundings, the electrical work done on the room must be equal to the increase in the energy of the room The first law of thermodynamics mainly states that the quantity of energy is conserved during energy transformation from thermal energy to other forms of energy. Energy balance for any system undergoing any kind of process can be expressed compactly as total total energy change in the total entering the system leaving the svstem energy of the system That is, the net change(increase or decrease) in the energy of the system during a process is equal to the difference between the total energy entering and leaving the system It is called general energy balance relation. It is very helpful to understand various forms of energy and to recognize the forms of energy transfer for the successful use of this relation to solve engineering problems 2.1 Total Energy of a System Energy can exist in numerous forms such as internal(sensible, latent, chemical, and nuclear), kinetic potential, electric, and magnetic energies, and their sum constitutes the total energy e of a system. The
24 Chapter 2 The First Law of Thermodynamics Energy Conservation Principle is one of the most fundamental laws in nature. It states that energy can be neither created nor destroyed during a process; it can only change forms, or it can be transferred from one body to another. However, the total amount of energy remains constant. That is, energy is always conserved. It provides a sound basis for studying the relationships among the various forms of energy and energy interactions. As an example, consider the heating of water in a boiler. If energy of 15 kJ is transferred to water by heat from a heating element and 3 kJ of it is lost from the water to the surroundings, the increase in energy of the water will be equal to the net heat transferred to the water, which is 12 kJ. As another example, consider a well-insulated room heated by an electric heater. As a result of electrical work added, the energy of the room will increase. Since the room is adiabatic and there is not any heat transfer to or from the surroundings, the electrical work done on the room must be equal to the increase in the energy of the room. The first law of thermodynamics mainly states that the quantity of energy is conserved during energy transformation from thermal energy to other forms of energy. Energy balance for any system undergoing any kind of process can be expressed compactly as That is, the net change (increase or decrease) in the energy of the system during a process is equal to the difference between the total energy entering and leaving the system. It is called general energy balance relation. It is very helpful to understand various forms of energy and to recognize the forms of energy transfer for the successful use of this relation to solve engineering problems 2.1 Total Energy of a System Energy can exist in numerous forms such as internal (sensible, latent, chemical, and nuclear), kinetic, potential, electric, and magnetic energies, and their sum constitutes the total energy E of a system. The = total energy entering the system total energy leaving the system _ change in the total energy of the system system
various forms of energy that make up the total energy of a system include two groups: microscopic and macroscopic. The microscopic forms of energy are those related to the molecular structure of a system and the degree of the molecular activity, and they are independent of outside reference frames. The sum of all the microscopic forms of energy is called the thermodynamic energy(internal energy)of a system. The macroscopic forms of energy are those a system possesses as a whole with respect to some outside reference frame, such as kinetic and potential energies 2.1. 1 Thermodynamic(Internal) Energy (1)The internal kinetic energy For a macroscopic sample of matter at rest, on a microscopic level the atoms composing the sample are in continual, random motion, such as translation, rotation, vibration. As a result, the sample of matter at rest possesses microscopic kinetic energies, which we call the internal kinetic energy. The atomic kinetic energies are all positive, scalar numbers. Since this molecular motion is primarily a function of temperature, the internal energy is sometimes called"thermal energy. As temperature rises, the atomic kinetic energy increases (2)The internal potential energ As the atoms move, they stretch or compress the bonds holding them together. Molecules in a hardly feel any force from other molecules far away. But when two molecules approach closely potential energy rises rapidly, causing them to repel one another and move apart again. At any instant, the sample has a microscopic, internal potential energy, which is the sum of all potential energy contributions describing the interactions between the atoms or molecules (3)Chemical energy and nuclear energy Chemical energy is energy possessed by a substance caused by the arrangement of atoms composing the molecules. It is part of the internal energy. It is essentially the energy required to break chemical bonds. If a chemical reaction occurs which breaks bonds of one type and forms bonds of another type, chemical energy may go up or down Nuclear(atomic) energy is energy possessed by the system from the cohesive forces holding protons and neutrons together as the atoms nucleus The internal energy includes all of the kinetic energy motion of the atoms of the system, and all of the potential energy associated with all possible interactions between the atoms. The internal energy is denoted as U. On a unit mass basis, it is called specific internal energy and denoted by u. It has the unit J/kg or kJ/kg The internal energy, u, which arises from molecular motion, depends only on the state of the system and is a function of the state of the system. It is a point function, that is, u=u(p, T),or u=u(p, v), or u=u(v, T). It does not depend on how a system reaches that state. It has exact differentials and a small change in internal energy is denoted by dU or du using the symbol d 2.1. 2 Potential and Kinetic Energy The macroscopic forms of energy are those a system possesses as a whole with respect to some outside reference frame, such as kinetic and potential energies (1)Kinetic energy The energy that a system possesses as a result of its macroscopic motion relative to some reference frame is called kinetic energy, denoted by Ek. When all parts of a system move with the same velocity the kinetic energy can be expressed as E
25 various forms of energy that make up the total energy of a system include two groups: microscopic and macroscopic. The microscopic forms of energy are those related to the molecular structure of a system and the degree of the molecular activity, and they are independent of outside reference frames. The sum of all the microscopic forms of energy is called the thermodynamic energy (internal energy) of a system. The macroscopic forms of energy are those a system possesses as a whole with respect to some outside reference frame, such as kinetic and potential energies. 2.1.1 Thermodynamic (Internal) Energy (1) The internal kinetic energy For a macroscopic sample of matter at rest, on a microscopic level the atoms composing the sample are in continual, random motion, such as translation, rotation, vibration. As a result, the sample of matter at rest possesses microscopic kinetic energies, which we call the internal kinetic energy. The atomic kinetic energies are all positive, scalar numbers. Since this molecular motion is primarily a function of temperature, the internal energy is sometimes called “thermal energy.” As temperature rises, the atomic kinetic energy increases. (2) The internal potential energy As the atoms move, they stretch or compress the bonds holding them together. Molecules in a gas hardly feel any force from other molecules far away. But when two molecules approach closely the potential energy rises rapidly, causing them to repel one another and move apart again. At any instant, the sample has a microscopic, internal potential energy, which is the sum of all potential energy contributions describing the interactions between the atoms or molecules. (3) Chemical energy and nuclear energy Chemical energy is energy possessed by a substance caused by the arrangement of atoms composing the molecules. It is part of the internal energy. It is essentially the energy required to break chemical bonds. If a chemical reaction occurs which breaks bonds of one type and forms bonds of another type, chemical energy may go up or down. Nuclear (atomic) energy is energy possessed by the system from the cohesive forces holding protons and neutrons together as the atom’s nucleus The internal energy includes all of the kinetic energy associated with the atomic-level, random motion of the atoms of the system, and all of the potential energy associated with all possible interactions between the atoms. The internal energy is denoted as U. On a unit mass basis, it is called specific internal energy and denoted by u . It has the unit J/kg or kJ/kg. The internal energy, u , which arises from molecular motion, depends only on the state of the system and is a function of the state of the system. It is a point function, that is, u = u( p,T) , or u = u( p,v) , or u = u(v,T) . It does not depend on how a system reaches that state. It has exact differentials and a small change in internal energy is denoted by dU or du using the symbol d . 2.1.2 Potential and Kinetic Energy The macroscopic forms of energy are those a system possesses as a whole with respect to some outside reference frame, such as kinetic and potential energies. (1) Kinetic energy The energy that a system possesses as a result of its macroscopic motion relative to some reference frame is called kinetic energy, denoted by Ek . When all parts of a system move with the same velocity, the kinetic energy can be expressed as 2 k 2 1 E = mc