ME 200 Thermodynamics I Spring 2016 Lecture 21: Introducing the Second Law, Irreversibilities Yong Li Shanghai Jiao Tong University Institute of Refrigeration and Cryogenics 800 Dong Chuan Road Shanghai, 200240, P. R. China Email : liyo@sjtu.edu.cn Phone: 86-21-34206056; Fax: 86-21-34206056 1.1
Last lecture» Steady Flow Devices Lecture Contents» System Integration Apply mass and energy balance for a combination of steady-flow devices relevant to practical systems Analyze each component independently by making suitable assumptions This lecture---second Law of Thermodynamics Reversible Process and Irreversible Process Thermal Energy Reservoir Kelvin-Planck Statement Clausius Statement 1.2
First Law of Thermodynamics Why Second Law?» Energy is conserved when converting from one form into other» Considers only energy change in quantity Several alternative, yet Second Law of Thermodynamics equivalent, formulations» Predicting direction and possibility for process» Establishing conditions for equilibrium» Providing means for measuring the quality of energy» Determining theoretical limits (the best theoretical performance) of engineering devices» Evaluating quantitatively the factors that preclude the attainment of the best theoretical performance level.» Defining a temperature scale independent of the properties of any substance.» Developing means for evaluating properties such as u and h 1.3
Second Law: Direction Consider three processes» Spontaneous heat transfer T» Spontaneous expansion P» Free falling mass Z Can the reverse process occur spontaneously without any change in the surroundings? 1.4
Second Law: Quality of Energy Consider spontaneous expansion in two ways» To the atmosphere» Through a turbine Where does the energy go? How accessible is it?» As the level of disorganization in the system increases, the energy becomes less accessible and potential to do useful work is lost» Organized energy can be converted into disorganized energy 1.5
Second Law: Performance Limits What is the most work that can be extracted from engines? What is the most efficient refrigerator?» Maximum efficiency occurs when energy is used to the fullest potential or» Optimum performance results when processes occur in» Real processes are inherently irreversible owing to heat loss, friction etc. The basis of the second law of thermodynamics is experimental evidence. 1.6
CONCEPTS Thermal Energy Reservoir Thermal Energy Reservoir ::: A body with a relatively large thermal energy capacity that supplies or absorbs finite amount of heat without any change in its temperature e.g. atmosphere, oceans, two-phase systems Source ::: a reservoir that supplies energy in the form of heat to thermodynamic systems Sink ::: a reservoir that absorbs energy in the form of heat from thermodynamic systems 1.7
Statements of the Second Law I Clausius (C) 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. 1.8
Statements of the Second Law II Kelvin Planck (K-P) 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..» Analytical form of the K-P statement 1.9
Demonstration the Equivalence of the two statements The violation of each statement implies the violation of the other. A violation of C statement» Cold reservoir Hot reservoir (Q c )» Hot reservoir cold reservoir (Q H =W+Q c )» Hot reservoir Q H -Q c W In combined system Implies a violation of the K P statement. 1.10
CONCEPTS Perpetual Motion Machine PMM ::: a device that violates the laws of Thermodynamics. There are three different types» Perpetual-Motion Machines of the 1st Kind: devices that create mass, energy, or work from nothing. They violate the 1st Law or the mass conservation principle or both» Perpetual-Motion Machines of the 2nd Kind: devices that violate the 2nd Law. It is usually easiest to see that they violate the Kelvin- Planck Statement.» Perpetual-Motion Machines of the 3rd Kind: devices that produce no work, have no friction, and run indefinitely. 1.11
CONCEPTS Irreversible Process Irreversible Process ::: the system and all parts of its surroundings cannot be exactly restored to their respective initial states after the process has occurred. Reversible Process ::: the system and surroundings can be returned to their initial states. All actual processes are irreversible; we try to minimize factors causing irreversibility to optimize performance A system that has undergone an irreversible process is not necessarily precluded from being restored to its initial state. 1.12
CONCEPTS Irreversibility Irreversibility causes decrease in the optimum performance for a given device owing to various reasons» Friction sliding friction as well as friction in the flow of fluids» Inelastic deformation» Unrestrained expansion of a gas or liquid to a lower pressure» Heat transfer through a finite temperature difference ( T)» Spontaneous mixing of matter at different compositions or states» Spontaneous chemical reaction» Electric current flow through a resistance» Magnetization or polarization with hysteresis Internal Irreversibilities ::: those that occur within the system. External Irreversibilities ::: those that occur within the surroundings, often the immediate surroundings. 1.13
CONCEPTS Internally Reversible Process In an Irreversible Process, irreversibilities are present within the system, its surroundings, or both. Reversible Process The process in which there are no internal or external irreversibilities. Internally Reversible Process ::: The process in which there are no irreversibilities within the system.» The boundary that is at one temperature and the surroundings at another. heat transfer. Irreversibilities may be located within the surroundings.» At every intermediate state of an internally reversible process of a closed system, all intensive properties are uniform throughout each phase present.» No internal irreversibilities are present within a thermal reservoir. 1.14