Introduction and Basic Concepts

Transcription

1 Objectives MAE 320- Chapter 1 Identify the unique vocabulary associated with thermodynamics through the precise definition of basic concepts. Review the metric SI and the English unit systems. Introduction and Basic Concepts Explain the basic concepts of thermodynamics such as system, state, state postulate, equilibrium, process, and cycle. Review concepts of temperature, temperature scales The content and the pictures are from the text book: Çengel, Y. A. and Boles, M. A., Thermodynamics: An Engineering Approach, McGraw-Hill, New York, 6th Ed., 2008 Review concepts of pressure, absolutepresuure and gage pressure. Thermodynamics and Energy The name thermodynamics stems from the Greek words therme (heat) and dynamis (power). Thermodynamics: The science of energy. Energy: The ability to cause changes. What is the difference between thermodynamics and kinetics? Thermodynamics and Energy What topics are covered in this course? Storage of energy - internal energy, (T) - potential energy, (gravity, h) - kinetic energy (motion, V) - chemical energy (reaction) Transfer of energy - from one form to another form - from one state to another state First law of thermodynamics - Energy cannot be created - Energy cannot be destroyed - Energy can be transferred Second law of thermodynamics - quantity - quality - direction Entropy Substances -H 2 O (ice, water, vapor) - ideal gas Thermodynamics and Energy Application Areas of Thermodynamics Conservation of energy principle: During an interaction, energy can change from one form to another but the total amount of energy remains constant. Energy cannot be created or destroyed. How to save energy bills? Which one do you choose between a heat pump and an electric heater? How does a heat pump work? What is the efficiency of the heat pump? Heat Pump Electric heater 1

2 Application Areas of Thermodynamics Application Areas of Thermodynamics Conventional car Hybrid car Double-layer glass window What is the efficiency of the hot water tank? What is the optimal temperature for the water tank? What is minimum tank capacity needed for your house? braking Pad-drum friction Heat dissipation Mechanical energy converted to heat braking Electric generator battery accelerating electric motor Mechanical energy converted to electric energy, converted to mechanical energy Application Areas of Thermodynamics How Hybrid car works Hybrid SUV-3D animation ZoI&feature=related Concepts of Thermodynamics The second law of thermodynamics: It asserts that energy has quality as well as quantity, and actual processes occur in the direction of decreasing quality of energy. Classical thermodynamics: A macroscopic approach to the study of thermodynamics that does not require a knowledge of the behavior of individual particles. It provides a direct and easy way to the solution of engineering problems and it is used in this text. Statistical thermodynamics: A microscopic approach, based on the average behavior of large groups of individual particles. It is used in this text only in the supporting role. Conservation of energy principle for the human body. Heat flows in the direction of decreasing temperature. 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, velocity V, energy E, and volume V are expressed in terms of the primary dimensions and are called secondary dimensions, or derived dimensions. Review SI units SI Primary units (base units) Quantity Name of Unit Symbol Length (l) meter m Mass (m) kilogram kg Time (t) second s Electrical current (I) ampere A Thermodynamic temperature (T) Kelvin K Amount of substance mole mol 2

3 Dimensions and Units English system: It has no apparent systematic numerical base, and various units in this system are related to each other rather arbitrarily. Density and Specific Gravity Density is mass per unit volume: Density: SI unit system will be adopted in our course: SI system has a systematic numerical base (clear physical meaning). SI system is a decimal system (x10). Better for communication. All the industrial country use the SI system. Only USA is employing a dual-unit system. However, all the car manufactures use the SI system. You need a (wrench) kit for metric bolts instead English sockets Easy to remember specific volume is volume per unit mass: Specific volume: (m 3 /kg) Density and Specific Gravity Specific gravity: The ratio of the density of a substance to the density of some standard substance at a specified temperature (usually water at 4 C). Dimension and Units In SI, one newton (1 N) is the force required to cause a mass of one kilogram (1 Kg) to accelerate at a rate of one meter per second squared (1 m/s 2 ) Specific weight: The weight of a unit volume of a substance. mg m γ S = = g = ρg V V In the English system, the force unit, 1 lbf, is the force required to cause a mass of lbm to accelerate at a rate of one foot per second squared (1 ft/s 2 ) Dimension and Units In daily life, people usually use weight to express mass. In science, weight is a force: W = mg Where m is the mass, g is the local gravitational acceleration, g=9.807 m/s 2 or g = ft/s 2 at sea level and 45 o latitude. On the earth, at sea level, a mass of 1 kg weighs 9.8 N W = mg = 1 kg x 9.8 m/s 2 = 9.8 kg.m/s 2 = 9.8 N On the moon, a mass of 1 kg weighs 1.63 N W = mg = 1 kg x 1.63 m/s 2 = 1.63 kg.m/s 2 = 1.63 N Unit Conversion Ratios All equations must be dimensionally homogeneous. To be dimensionally homogeneous, all the terms in an equation must have the same unit system. All non-primary units (secondary units) can be formed by combinations of primary units. Force units, for example, can be expressed as They can also be expressed more conveniently as unity conversion ratios as Mass does not change with location, but weight does Unity conversion ratios are identically equal to 1 and are unitless, and thus such ratios (or their inverses) can be inserted conveniently into any calculation to properly convert units. 3

4 Dimension and Units How much is the weight of a mass of 1 lbm? Apply Newton s second law: Dimension and Units Work, a form of energy: Work (W) = Force Distance 1 J = 1 N m In SI system: 1 lbm= kg and g =9.8 m/s 2 W = mg = kg x 9.8 m/s 2 = 4.45 kg.m/s 2 = 4.45 N 1 cal = J 1Btu= kj power = W/t Unit: J/s or watt, w A mass of 1 lbm weighs 4.45 N specific energy (e), e=e/m (J/kg) Review SI units Derived Units Quantity Description Symbol Expression in other SI units Area (A) square meter m 2 m 2 Volume (V) cubic meter m 3 m 3 velocity (or speed) (V) meters per second m/s m s -1 Acceleration (a) density (ρ), ρ=m/v specific volume (v), v=v/m meter per second squared kilogram per cubic meter cubic meter per kilogram m/s 2 m s -2 Expression in SI base units kg/m 3 kg m -3 m 3 /kg m 3 kg -1 specific gravity (SG), SG=ρ/ρ H2O Dimensionless force (F), F=ma Newton N kg m/s² m kg s -2 Review SI units Derived Units Quantity Description Symbol Expression in other SI units Expression in terms of SI base units Weight (W), W=mg Newton N kg m/s² m kg s -2 pressure (P), P=F/A pascal Pa N/m 2 m -1 kg s -2 work (energy heat) )(W), W=F*d N m Joule J m 2 kg s -2 specific energy (e), e=e/m joule per kilogram J/kg m 2 s -2 energy density joule per cubic meter J/m 3 m -1 kg s -2 power Watt, W J/s m 2 kg s -3 Continuum Matter is made up of atoms that are widely spaced in the gas phase. disregard the atomic nature of a substance and view it as a continuous, homogeneous matter with no holes, that is, a continuum. The continuum idealization allows us to treat properties as point functions and to assume the properties vary continually in space with no jump discontinuities. This idealization is valid as long as the size of the system is large relative to the space between the molecules. In this course we will limit our consideration to continuum. Despite the large gaps between molecules, a substance can be treated as a continuum because of the very large number of molecules even in an extremely small volume. 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. 4

5 Systems and Control Volumes Closed system (Control mass): A fixed amount of mass, and no mass can cross its boundary. Isolated system: in such a close system, even energy is not allowed to cross the boundary Coke Can Demonstrate Concept of system, boundary and surroundings Demonstrate closed system Demonstrate open system Systems and Control Volumes 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. In an open system, the volume can vary with time! Properties of 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. An open system (a control volume) with one inlet and one exit. Criterion to differentiate intensive and extensive properties. State and Equilibrium Thermodynamics deals with equilibrium states. Equilibrium: A state of balance. there are no unbalanced potentials (or driving forces) within the system. Thermal equilibrium: If the temperature is the same throughout the entire system. The Zeroth Law of Thermodynamics The zeroth law of thermodynamics: If two bodies are in thermal equilibrium with a third body, they are also in thermal equilibrium with each other. By replacing the third body with a thermometer, the zeroth law can be restated as two bodies are in thermal equilibrium if both have the same temperature reading even if they are not in contact. A closed system reaching thermal equilibrium. Two bodies reaching thermal equilibrium after being brought into contact in an isolated enclosure. 5

6 State and Equilibrium Thermodynamics deals with equilibrium states. Mechanical equilibrium: If there is no change in pressure at any point of the system with time. Chemical equilibrium: If the chemical composition of a system does not change with time, that is, no chemical reactions occur. Phase equilibrium: If a system involves two phases and when the mass of each phase reaches an equilibrium level and stays there. a phase is a region of space (a thermodynamic system), throughout which all physical properties of a material are essentially uniform. State and Equilibrium State: a certain condition that can be completely described a set of properties A system at two different states. A closed system reaching thermal equilibrium. The State Postulate The number of properties required to fix the state of a system is given by the state postulate: The state of a simple compressible system is completely specified by two independent, intensive properties. Simple compressible system: If a system involves no electrical, magnetic, gravitational, motion, and surface tension effects. The state of nitrogen is fixed by two independent, intensive properties. Processes and Cycles Process: Any change that a system undergoes from one equilibrium state to another. Path: The series of states through which a system passes during a process. To describe a process completely, one should specify the initial and final states, as well as the path it follows, and the interactions with the surroundings. Process diagrams plotted with thermodynamic properties as coordinates are use to visualize the processes. Some common properties as coordinates are temperature T, pressure P, and volume V (or specific volume v). The prefix iso- is often used to designate a process for which a particular property remains constant. Cycle: A process during which the initial and final states are identical. Processes and Cycles Isothermal process: A process during which the temperature T remains constant. Isobaric process: A process during which the pressure P remains constant. t Isochoric (or isometric) process: A process during which the specific volume v remains constant. Processes and Cycles Quasi-static or quasi-equilibrium process: When a process proceeds in such a manner that the system remains infinitesimally close to an equilibrium state at all times. The P-V diagram of a compression process. 6

7 The Steady-Flow Process The term steady implies no change with time. The opposite of steady is unsteady, or transient. During a steady-flow process, fluid properties within the control volume may change with position but not with time. Under steady-flow conditions, the mass and energy contents of a control volume remain constant. Steady-flow process: A process during which a fluid flows through a control volume steadily. Steady-flow conditions can be closely approximated by devices that are intended for continuous operation such as turbines, pumps, boilers, condensers, and heat exchangers or power plants or refrigeration systems. Temperature Scales 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, ice point=0 C, steam point=100 C Fahrenheit scale: in English unit system, ice point= 32 F, steam point 212 F. T ( o F) =1.8 T( o C) +32 Rankine scale: T (R) = T( o F) Thermodynamic temperature scale: A temperature scale that is independent of the properties of any substance. Thermodynamic temperature scale is the Kelvin scale (SI); T (K) = T ( C) (K) Temperature Scales Thermodynamic temperature scale Temperature Scales The International temperature Scale of 1990 (ITS-90): Tools for temperature measurement: Ice point: 0 o C ( K) Alcohol Thermometer Steam point: o C Thermocouple Atomic-resolution STM image of reconstructed Si(111)- (7 7), Frame size: 31nm STM (scanning tunneling microscope) Temperature arises from the random submicroscopic vibrations of the particle constituents of matter. These motions comprise the kinetic energy in a substance 0 K = o C Thermodynamic temperature at 0 K, absolute zero, is the temperature at which the particle constituents of matter are as close as possible to complete rest Infrared sensor Pressure Pressure: A compressive force per unit area 68 kg 136 kg Gage Pressure and Vacuum Pressure 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 pressure: Pressures below atmospheric pressure. Some basic pressure gages. A feet =300cm kgf/cm kgf/cm 2 P=68/300=0.23 kgf/cm 2 The normal stress (or pressure ) on the feet of a chubby person is much greater than on the feet of a slim person. Throughout this text, the pressure P will denote absolute pressure unless specified otherwise. 7

8 Gauge Pressure and Vacuum Pressure A pressure gauge connected to a tank displays 75 kpa in Morgantown (the atmosphere pressure is 100 kpa in Morgantown WV). After the tank was moved to Denver, CO, the pressure gauge shows a pressure of 92 kpa. What is the atmosphere pressure in Denver? 75 kpa Pascal s Law Pascal s law: The pressure applied to a confined fluid increases the pressure throughout by the same amount. P gauge = P abs -P atm P gauge, morgantown = P abs P atm, morgantown 75 KPa = P abs 100 kpa P abs 100 kpa in Morgantown The area ratio A 2 /A 1 is called the ideal mechanical advantage of the hydraulic lift. P abs = 175 kpa 92 kpa P gauge, denver = P abs P atm, denver 92 kpa = 175 kpa P atm, denver P atm, denver = 83 kpa P abs??? kpa in Denver Lifting of a large weight by a small force by the application of Pascal s law. Variation of Pressure with Depth = 0 : P2 ΔxΔy P1 ΔxΔy mg = 0 F z P ΔxΔy PΔxΔy ρδxδyδxδzg 0 P 2 1 = 2 P1 ρδzg = 0 Δx Δy Pascal s Law P 2 1 P = ρgh In a room filled with a gas, the variation of pressure with height is negligible. Δz If take Point 1 at the free surface of the liquid Where P 1 =P atm. (1). Pressure in a liquid at rest is independent of the shape or cross section of the container. (2). It changes with the vertical distance by remains in other directions. Free-body diagram of a rectangular fluid element in equilibrium. 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. The 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. Pascal s Law P atm P 2 = P atm + ρgh P gas = P 1 = P 2 P gas P gas = P 1 = P 2 = P atm + ρgh P 1 The basic manometer P 2 The gage pressure measured by manometer: P gage = ρgh In stacked-up fluid layers, the pressure change across a fluid layer of density ρ and height h is ρgh. 8

9 The Manometer The Manometer Measuring the pressure drop across a flow section or a flow device by a differential manometer. P A = P B P 1 + ρ 1 g(a+h)= P 2 + ρ 1 ga + ρ 2 gh P A P B A B C Alternatively D F Measure pressure with multi-fluid manometer Other Pressure Measurement Devices 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. t 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. Various types of Bourdon tubes used to measure pressure. The Barometer and Atmospheric Pressure Atmospheric pressure is measured by a device called a barometer; thus, the atmospheric pressure is often referred to as the barometric pressure. A frequently used pressure unit is the standard atmosphere, which is defined as the pressure produced by a column of mercury 760 mm in height at 0 C (ρ Hg = 13,595 kg/m 3 ) under standard gravitational acceleration (g = m/s 2 ). The basic barometer. The length or the cross-sectional area of the tube has no effect on the height of the fluid column of a barometer, provided that the tube diameter is large enough to avoid surface tension (capillary) effects. Dimension and Units All equations must be dimensionally homogeneous. To be dimensionally homogeneous, all the terms in an equation must have the same unit system. On a day when the barometer reads 760 Torr, a tire pressure gage reads 204 kpa. What is the absolute pressure in Pascals in the tire? P = P + P = = kpa? abs atm gauge Pa Pabs = Patm + Pgauge = ( 760torr) Pa = Pa torr 1 mmhg = 1 torr The Barometer and Atmospheric Pressure The barometer (with a diameter 3 mm in diameter) reads 10 inch H 2 O. The density of water is 1 g/cm 3. What is this pressure in kilopascals? Attention: the pressure reading is independent on the diameter of the barometer. 3 2 P = ρ gh = (1g / cm )(9.8m / s )(10inch) (unit?) P 1 10 ρgh = [ ( = 2 = 2489 Pa = kpa kg 2 ](9.8m/ s )( m) 3 ) m 9

10 Summary Importance of dimensions and units Some SI and English units, Dimensional homogeneity, Unity conversion ratios Systems and control volumes Properties of a system Density and specific gravity State and equilibrium The state postulate The state postulate Processes and cycles The steady-flow process Temperature and the zeroth law of thermodynamics Temperature scales Pressure Variation of pressure with depth The manometer and the atmospheric pressure 10