Introduction and Basic Concepts - Slides | MAE 321, Study notes of Thermodynamics

Download Introduction and Basic Concepts - Slides | MAE 321 and more Study notes Thermodynamics in PDF only on Docsity! 1 1 MAE 321 Applied Thermodynamics Thermodynamics: An Engineering Approach, 6th edition by Yunus A. Çengel and Michael A. Boles The Following Slides Are From the Instructor’s Section of the McGraw Hill Web Site and Have Been Modified for This Course January 12, 14, & 16, 2009 Lectures Chapter 1: Introduction and Basic Concepts 3 IMPORTANCE OF DIMENSIONS AND UNITS • Any physical quantity can be characterized by dimensions. • The magnitudes assigned to the dimensions are called units. • Some basic dimensions such as mass m, length L, time t, and temperature T are selected as primary or fundamental dimensions, while others such as velocity V, energy E, and volume V are expressed in terms of the primary dimensions and are called secondary dimensions, or derived dimensions. • Metric SI system: A simple and logical system based on a decimal relationship between the various units. • English system: It has no apparent systematic numerical base, and various units in this system are related to each other rather arbitrarily. 2 4 SYSTEMS AND CONTROL VOLUMES • System: A quantity of matter or a region in space chosen for study. • Surroundings: The mass or region outside the system • Boundary: The real or imaginary surface that separates the system from its surroundings. • The boundary of a system can be fixed or movable. • Systems may be considered to be closed or open. • Closed system (Control mass): A fixed amount of mass, and no mass can cross its boundary. 5 • Open system (control volume): A properly selected region in space. • It usually encloses a device that involves mass flow such as a compressor, turbine, or nozzle. • Both mass and energy can cross the boundary of a control volume. • Control surface: The boundaries of a control volume. It can be real or imaginary. An open system (a control volume) with one inlet and one exit. 6 PROPERTIES OF A SYSTEM • Property: Any characteristic of a system. • Some familiar properties are pressure P, temperature T, volume V, and mass m. • Properties are considered to be either intensive or extensive. • Intensive properties: Those that are independent of the mass of a system, such as temperature, pressure, and density. • Extensive properties: Those whose values depend on the size— or extent—of the system. • Specific properties: Extensive properties per unit mass. Criterion to differentiate intensive and extensive properties. 5 13 Temperature Scales • All temperature scales are based on some easily reproducible states such as the freezing and boiling points of water: the ice point and the steam point. • Ice point: A mixture of ice and water that is in equilibrium with air saturated with vapor at 1 atm pressure (0°C or 32°F). • Steam point: A mixture of liquid water and water vapor (with no air) in equilibrium at 1 atm pressure (100°C or 212°F). • Celsius scale: in SI unit system • Fahrenheit scale: in English unit system • Thermodynamic temperature scale: A temperature scale that is independent of the properties of any substance. • Kelvin scale (SI) Rankine scale (E) • A temperature scale nearly identical to the Kelvin scale is the ideal-gas temperature scale. The temperatures on this scale are measured using a constant-volume gas thermometer. P versus T plots of the experimental data obtained from a constant- volume gas thermometer using four different gases at different (but low) pressures. A constant-volume gas thermometer would read -273.15°C at absolute zero pressure. 14 Comparison of temperature scales. • The reference temperature in the original Kelvin scale was the ice point, 273.15 K, which is the temperature at which water freezes (or ice melts). • The reference point was changed to a much more precisely reproducible point, the triple point of water (the state at which all three phases of water coexist in equilibrium), which is assigned the value 273.16 K. Comparison of magnitudes of various temperature units. 15 PRESSURE The normal stress (or “pressure”) on the feet of a chubby person is much greater than on the feet of a slim person. Some basic pressure gages. Pressure: A normal force exerted by a fluid per unit area 68 kg 136 kg Afeet=300cm2 0.23 kgf/cm2 0.46 kgf/cm2 P=68/300=0.23 kgf/cm2 6 16 • Absolute pressure: The actual pressure at a given position. It is measured relative to absolute vacuum (i.e., absolute zero pressure). • Gage pressure: The difference between the absolute pressure and the local atmospheric pressure. Most pressure-measuring devices are calibrated to read zero in the atmosphere, and so they indicate gage pressure. • Vacuum pressures: Pressures below atmospheric pressure. Throughout this text, the pressure P will denote absolute pressure unless specified otherwise. 17 Variation of Pressure with Depth Free-body diagram of a rectangular fluid element in equilibrium. The pressure of a fluid at rest increases with depth (as a result of added weight). When the variation of density with elevation is known 18 In a room filled with a gas, the variation of pressure with height is negligible. Pressure in a liquid at rest increases linearly with distance from the free surface. The pressure is the same at all points on a horizontal plane in a given fluid regardless of geometry, provided that the points are interconnected by the same fluid. 7 19 Pascal’s law: The pressure applied to a confined fluid increases the pressure throughout by the same amount. Lifting of a large weight by a small force by the application of Pascal’s law. The area ratio A2/A1 is called the ideal mechanical advantage of the hydraulic lift. 20 The Manometer In stacked-up fluid layers, the pressure change across a fluid layer of density ρ and height h is ρgh. Measuring the pressure drop across a flow section or a flow device by a differential manometer. The basic manometer. It is commonly used to measure small and moderate pressure differences. A manometer contains one or more fluids such as mercury, water, alcohol, or oil. 21 Other Pressure Measurement Devices Various types of Bourdon tubes used to measure pressure. • Bourdon tube: Consists of a hollow metal tube bent like a hook whose end is closed and connected to a dial indicator needle. • Pressure transducers: Use various techniques to convert the pressure effect to an electrical effect such as a change in voltage, resistance, or capacitance. • Pressure transducers are smaller and faster, and they can be more sensitive, reliable, and precise than their mechanical counterparts. • Strain-gage pressure transducers: Work by having a diaphragm deflect between two chambers open to the pressure inputs. • Piezoelectric transducers: Also called solid-state pressure transducers, work on the principle that an electric potential is generated in a crystalline substance when it is subjected to mechanical pressure. 10 28 ENERGY TRANSFER BY HEAT Energy can cross the boundaries of a closed system in the form of heat and work. Temperature difference is the driving force for heat transfer. The larger the temperature difference, the higher is the rate of heat transfer. Heat: The form of energy that is transferred between two systems (or a system and its surroundings) by virtue of a temperature difference. 29 Energy is recognized as heat transfer only as it crosses the system boundary. During an adiabatic process, a system exchanges no heat with its surroundings. Heat transfer per unit mass Amount of heat transfer when heat transfer rate changes with time Amount of heat transfer when heat transfer rate is constant 30 ENERGY TRANSFER BY WORK Specifying the directions of heat and work. Work done per unit mass Power is the work done per unit time (kW) • Work: The energy transfer associated with a force acting through a distance. – A rising piston, a rotating shaft, and an electric wire crossing the system boundaries are all associated with work interactions • Formal sign convention: Heat transfer to a system and work done by a system are positive; heat transfer from a system and work done on a system are negative. • Alternative to sign convention is to use the subscripts in and out to indicate direction. This is the primary approach in this text. 11 31 Heat vs. Work • Both are recognized at the boundaries of a system as they cross the boundaries. That is, both heat and work are boundary phenomena. • Systems possess energy, but not heat or work. • Both are associated with a process, not a state. • Unlike properties, heat or work has no meaning at a state. • Both are path functions (i.e., their magnitudes depend on the path followed during a process as well as the end states). Properties are point functions; but heat and work are path functions (their magnitudes depend on the path followed). Properties are point functions have exact differentials (d ). Path functions have inexact differentials (δ ) 32 Electrical Work Electrical power in terms of resistance R, current I, and potential difference V. Electrical work Electrical power When potential difference and current change with time When potential difference and current remain constant 33 MECHANICAL FORMS OF WORK • There are two requirements for a work interaction between a system and its surroundings to exist: – there must be a force acting on the boundary. – the boundary must move. The work done is proportional to the force applied (F) and the distance traveled (s). Work = Force × Distance When force is not constant 12 34 Shaft Work The power transmitted through the shaft is the shaft work done per unit time Spring Work Elongation of a spring under the influence of a force. 35 THE FIRST LAW OF THERMODYNAMICS • The first law of thermodynamics (the conservation of energy principle) provides a sound basis for studying the relationships among the various forms of energy and energy interactions. • The first law states that energy can be neither created nor destroyed during a process; it can only change forms. • The First Law: For all adiabatic processes between two specified states of a closed system, the net work done is the same regardless of the nature of the closed system and the details of the process. Energy cannot be created or destroyed; it can only change forms. The increase in the energy of a potato in an oven is equal to the amount of heat transferred to it. 36 Energy Balance The net change (increase or decrease) in the total energy of the system during a process is equal to the difference between the total energy entering and the total energy leaving the system during that process. The work (boundary) done on an adiabatic system is equal to the increase in the energy of the system. The energy change of a system during a process is equal to the net work and heat transfer between the system and its surroundings. 15 43 Ozone and Smog • Smog: Made up mostly of ground-level ozone (O3), but it also contains numerous other chemicals, including carbon monoxide (CO), particulate matter such as soot and dust, volatile organic compounds (VOCs) such as benzene, butane, and other hydrocarbons. • Hydrocarbons and nitrogen oxides react in the presence of sunlight on hot calm days to form ground-level ozone. • Ozone irritates eyes and damages the air sacs in the lungs where oxygen and carbon dioxide are exchanged, causing eventual hardening of this soft and spongy tissue. • It also causes shortness of breath, wheezing, fatigue, headaches, and nausea, and aggravates respiratory problems such as asthma. Ground-level ozone, which is the primary component of smog, forms when HC and NOx react in the presence of sunlight in hot calm days. • The other serious pollutant in smog is carbon monoxide, which is a colorless, odorless, poisonous gas. • It is mostly emitted by motor vehicles. • It deprives the body’s organs from getting enough oxygen by binding with the red blood cells that would otherwise carry oxygen. It is fatal at high levels. • Suspended particulate matter such as dust and soot are emitted by vehicles and industrial facilities. Such particles irritate the eyes and the lungs. 44 Acid Rain • The sulfur in the fuel reacts with oxygen to form sulfur dioxide (SO2), which is an air pollutant. • The main source of SO2 is the electric power plants that burn high-sulfur coal. • Motor vehicles also contribute to SO2 emissions since gasoline and diesel fuel also contain small amounts of sulfur. Sulfuric acid and nitric acid are formed when sulfur oxides and nitric oxides react with water vapor and other chemicals high in the atmosphere in the presence of sunlight. • The sulfur oxides and nitric oxides react with water vapor and other chemicals high in the atmosphere in the presence of sunlight to form sulfuric and nitric acids. • The acids formed usually dissolve in the suspended water droplets in clouds or fog. • These acid-laden droplets, which can be as acidic as lemon juice, are washed from the air on to the soil by rain or snow. This is known as acid rain. 45 The Greenhouse Effect: Global Warming The greenhouse effect on earth. • Greenhouse effect: Glass allows the solar radiation to enter freely but blocks the infrared radiation emitted by the interior surfaces. This causes a rise in the interior temperature as a result of the thermal energy buildup in a space (i.e., car). • The surface of the earth, which warms up during the day as a result of the absorption of solar energy, cools down at night by radiating part of its energy into deep space as infrared radiation. • Carbon dioxide (CO2), water vapor, and trace amounts of some other gases such as methane and nitrogen oxides act like a blanket and keep the earth warm at night by blocking the heat radiated from the earth. The result is global warming. • These gases are called “greenhouse gases,” with CO2 being the primary component. • CO2 is produced by the burning of fossil fuels such as coal, oil, and natural gas. 16 End of Chapter 2 Chapter 3: Properties of Pure Substances 48 PURE SUBSTANCE • Pure substance: A substance that has a fixed chemical composition throughout. • Air is a mixture of several gases, but it is considered to be a pure substance. Nitrogen and gaseous air are pure substances. A mixture of liquid and gaseous water is a pure substance, but a mixture of liquid and gaseous air is not. 17 49 PHASE-CHANGE PROCESSES OF PURE SUBSTANCES • Compressed liquid (subcooled liquid): A substance that it is not about to vaporize. • Saturated liquid: A liquid that is about to vaporize. At 1 atm and 20°C, water exists in the liquid phase (compressed liquid). At 1 atm pressure and 100°C, water exists as a liquid that is ready to vaporize (saturated liquid). 50 • Saturated liquid–vapor mixture: The state at which the liquid and vapor phases coexist in equilibrium. • Saturated vapor: A vapor that is about to condense. • Superheated vapor: A vapor that is not about to condense (i.e., not a saturated vapor). As more heat is transferred, part of the saturated liquid vaporizes (saturated liquid– vapor mixture). At 1 atm pressure, the temperature remains constant at 100°C until the last drop of liquid is vaporized (saturated vapor). As more heat is transferred, the temperature of the vapor starts to rise (superheated vapor). 51 T-v diagram for the heating process of water at constant pressure. If the entire process between state 1 and 5 described in the figure is reversed by cooling the water while maintaining the pressure at the same value, the water will go back to state 1, retracing the same path, and in so doing, the amount of heat released will exactly match the amount of heat added during the heating process. 20 58 Extending the Diagrams to Include the Solid Phase P-v diagram of a substance that contracts on freezing. P-v diagram of a substance that expands on freezing (such as water). At triple-point pressure and temperature, a substance exists in three phases in equilibrium. For water, Ttp = 0.01°C Ptp = 0.6117 kPa 59 Sublimation: Passing from the solid phase directly into the vapor phase. At low pressures (below the triple-point value), solids evaporate without melting first (sublimation). P-T diagram of pure substances. Phase Diagram 60 P-v-T surface of a substance that contracts on freezing. P-v-T surface of a substance that expands on freezing (like water). The P-v-T surfaces present a great deal of information at once, but in a thermodynamic analysis it is more convenient to work with two-dimensional diagrams, such as the P-v and T-v diagrams. 21 61 PROPERTY TABLES • For most substances, the relationships among thermodynamic properties are too complex to be expressed by simple equations. • Therefore, properties are frequently presented in the form of tables. • Some thermodynamic properties can be measured easily, but others cannot and are calculated by using the relations between them and measurable properties. • The results of these measurements and calculations are presented in tables in a convenient format. Enthalpy—A Combination Property The combination u + Pv is frequently encountered in the analysis of control volumes. The product pressure × volume has energy units. 62 Saturated Liquid and Saturated Vapor States • Table A–4: Saturation properties of water under temperature. • Table A–5: Saturation properties of water under pressure. A partial list of Table A–4. Enthalpy of vaporization, hfg (Latent heat of vaporization): The amount of energy needed to vaporize a unit mass of saturated liquid at a given temperature or pressure. 63 Examples: Saturated liquid and saturated vapor states of water on T-v and P-v diagrams. 22 64 Saturated Liquid–Vapor Mixture Quality, x : The ratio of the mass of vapor to the total mass of the mixture. Quality is between 0 and 1 0: sat. liquid, 1: sat. vapor. The properties of the saturated liquid are the same whether it exists alone or in a mixture with saturated vapor. The relative amounts of liquid and vapor phases in a saturated mixture are specified by the quality x. A two-phase system can be treated as a homogeneous mixture for convenience. Temperature and pressure are dependent properties for a mixture. 65 Quality is related to the horizontal distances on P-v and T-v diagrams. The v value of a saturated liquid– vapor mixture lies between the vf and vg values at the specified T or P. y v, u, or h. 66 Examples: Saturated liquid-vapor mixture states on T-v and P-v diagrams. 25 73Comparison of Z factors for various gases. Gases deviate from the ideal-gas behavior the most in the neighborhood of the critical point. Reduced temperature Reduced pressure Pseudo-reduced specific volume Z can also be determined from a knowledge of PR and vR. 74 OTHER EQUATIONS OF STATE Several equations have been proposed to represent the P-v-T behavior of substances accurately over a larger region with no limitations. Van der Waals Equation of State Critical isotherm of a pure substance has an inflection point at the critical state. This model includes two effects not considered in the ideal-gas model: the intermolecular attraction forces and the volume occupied by the molecules themselves. The accuracy of the van der Waals equation of state is often inadequate. 75 Beattie-Bridgeman Equation of State The constants are given in Table 3–4 for various substances. It is known to be reasonably accurate for densities up to about 0.8ρcr. Benedict-Webb-Rubin Equation of State The constants are given in Table 3–4. This equation can handle substances at densities up to about 2.5 ρcr. Virial Equation of State The coefficients a(T), b(T), c(T), and so on, that are functions of temperature alone are called virial coefficients. 26 End of Chapter 3 Chapter 4: Energy Analysis of Closed Systems 78 MOVING BOUNDARY WORK Moving boundary work (P dV work): The expansion and compression work in a piston-cylinder device. The work associated with a moving boundary is called boundary work.A gas does a differential amount of work δWb as it forces the piston to move by a differential amount ds. Quasi-equilibrium process: A process during which the system remains nearly in equilibrium at all times. Wb is positive → for expansion Wb is negative → for compression 27 79 The area under the process curve on a P-V diagram represents the boundary work. The boundary work done during a process depends on the path followed as well as the end states. The net work done during a cycle is the difference between the work done by the system and the work done on the system. 80 Polytropic, Isothermal, and Isobaric Processes Polytropic process: C, n (polytropic exponent) constants Polytropic process Polytropic and for ideal gas When n = 1 (isothermal process) Schematic and P-V diagram for a polytropic process. Constant pressure (Isobaric) process What is the boundary work for a constant- volume process? 81 ENERGY BALANCE FOR CLOSED SYSTEMS Energy balance for any system undergoing any process Energy balance in the rate form The total quantities are related to the quantities per unit time is Energy balance per unit mass basis Energy balance in differential form Energy balance for a cycle 30 88 (kJ/kg) For small temperature intervals, the specific heats may be assumed to vary linearly with temperature. Internal energy and enthalpy change when specific heat is taken constant at an average value The relation Δ u = cv ΔT is valid for any kind of process, constant- volume or not. 89 1. By using the tabulated u and h data. This is the easiest and most accurate way when tables are readily available. 2. By using the cv or cp relations (Table A-2c) as a function of temperature and performing the integrations. This is very inconvenient for hand calculations but quite desirable for computerized calculations. The results obtained are very accurate. 3. By using average specific heats. This is very simple and certainly very convenient when property tables are not available. The results obtained are reasonably accurate if the temperature interval is not very large. Three ways of calculating Δu and Δh Three ways of calculating Δu. 90 Specific Heat Relations of Ideal Gases The cp of an ideal gas can be determined from a knowledge of cv and R. On a molar basis The relationship between cp, cv and R Specific heat ratio • The specific ratio varies with temperature, but this variation is very mild. • For monatomic gases (helium, argon, etc.), its value is essentially constant at 1.667. • Many diatomic gases, including air, have a specific heat ratio of about 1.4 at room temperature. dh = cpdT and du = cvdT 31 91 INTERNAL ENERGY, ENTHALPY, AND SPECIFIC HEATS OF SOLIDS AND LIQUIDS The specific volumes of incompressible substances remain constant during a process. The cv and cp values of incompressible substances are identical and are denoted by c. Incompressible substance: A substance whose specific volume (or density) is constant. Solids and liquids are incompressible substances. 92 Internal Energy Changes for Liquids and Solids Enthalpy Changes for Liquids and Solids The enthalpy of a compressed liquid A more accurate relation than End of Chapter 4 32 Chapter 5: Mass and Energy Analysis of Control Volumes 95 CONSERVATION OF MASS Mass is conserved even during chemical reactions. Conservation of mass: Mass, like energy, is a conserved property, and it cannot be created or destroyed during a process. Closed systems: The mass of the system remain constant during a process. Control volumes: Mass can cross the boundaries, and so we must keep track of the amount of mass entering and leaving the control volume. Mass m and energy E can be converted to each other according to where c is the speed of light in a vacuum, which is c = 2.9979 × 108 m/s. The mass change due to energy change is absolutely negligible. 96 Mass and Volume Flow Rates The average velocity Vavg is defined as the average speed through a cross section. The volume flow rate is the volume of fluid flowing through a cross section per unit time. Definition of average velocity Mass flow rate Volume flow rate 35 103 ENERGY ANALYSIS OF STEADY-FLOW SYSTEMS Many engineering systems such as power plants operate under steady conditions. Under steady-flow conditions, the mass and energy contents of a control volume remain constant. Under steady-flow conditions, the fluid properties at an inlet or exit remain constant (do not change with time). 104 Mass and Energy Balances for a Steady-flow Process A water heater in steady operation. Mass balance Energy Balance 105 Under steady operation, shaft work and electrical work are the only forms of work a simple compressible system may involve. Energy Balance Relations with Sign Conventions (i.e., Heat Input and Work Output are Positive) when kinetic and potential energy changes are negligible Some energy unit equivalents 36 106 SOME STEADY-FLOW ENGINEERING DEVICES A modern land-based gas turbine used for electric power production. This is a General Electric LM5000 turbine. It has a length of 6.2 m, it weighs 12.5 tons, and produces 55.2 MW at 3600 rpm with steam injection. Many engineering devices operate essentially under the same conditions for long periods of time. The components of a steam power plant (turbines, compressors, heat exchangers, and pumps), for example, operate nonstop for months before the system is shut down for maintenance. Therefore, these devices can be conveniently analyzed as steady-flow devices. At very high velocities, even small changes in velocities can cause significant changes in the kinetic energy of the fluid. 107 Nozzles and Diffusers Nozzles and diffusers are shaped so that they cause large changes in fluid velocities and thus kinetic energies. Nozzles and diffusers are commonly utilized in jet engines, rockets, spacecraft, and even garden hoses. A nozzle is a device that increases the velocity of a fluid at the expense of pressure. A diffuser is a device that increases the pressure of a fluid by slowing it down. The cross-sectional area of a nozzle decreases in the flow direction for subsonic flows and increases for supersonic flows. The reverse is true for diffusers. Energy balance for a nozzle or diffuser: 108 Turbines and Compressors Turbine drives the electric generator In steam, gas, or hydroelectric power plants. As the fluid passes through the turbine, work is done against the blades, which are attached to the shaft. As a result, the shaft rotates, and the turbine produces work. Compressors, as well as pumps and fans, are devices used to increase the pressure of a fluid. Work is supplied to these devices from an external source through a rotating shaft. A fan increases the pressure of a gas slightly and is mainly used to mobilize a gas. A compressor is capable of compressing the gas to very high pressures. Pumps work very much like compressors except that they handle liquids instead of gases. Energy balance for the compressor in this figure: 37 109 Throttling Valves Throttling valves are any kind of flow-restricting devices that cause a significant pressure drop in the fluid. What is the difference between a turbine and a throttling valve? The pressure drop in the fluid is often accompanied by a large drop in temperature, and for that reason throttling devices are commonly used in refrigeration and air- conditioning applications. The temperature of an ideal gas does not change during a throttling (h = constant) process since h = h(T). During a throttling process, the enthalpy of a fluid remains constant. But internal and flow energies may be converted to each other. Energy balance 110 Mixing Chambers In engineering applications, the section where the mixing process takes place is commonly referred to as a mixing chamber. The T-elbow of an ordinary shower serves as the mixing chamber for the hot- and the cold-water streams. Energy balance for the adiabatic mixing chamber in the figure is: 10°C 60°C 43°C 140 kPa 111 Heat Exchangers Heat exchangers are devices where two moving fluid streams exchange heat without mixing. Heat exchangers are widely used in various industries, and they come in various designs. A heat exchanger can be as simple as two concentric pipes. Mass and energy balances for the adiabatic heat exchanger in the figure is: The heat transfer associated with a heat exchanger may be zero or nonzero depending on how the control volume is selected.