ENS 207 Introduction to Energy Systems Week 2 Introductory Concepts and Definitions in Engineering Thermodynamics • Thermal-fluid sciences – Thermodynamics – Heat Transfer – Fluid Mechanics • In this course we will mostly learn about thermodynamics principles for energy systems analysis purposes, but we will also discuss some fundamentals from heat transfer and fluid dynamics • This week is about basic thermodynamic concepts and definitions Learning Outcomes of This Lecture ►Demonstrate understanding of several fundamental concepts used in thermodynamic analysis ► Including closed system, control volume, boundary and surroundings, property, state, process, the distinction between extensive and intensive properties, and equilibrium. ►Reading Suggestion: Chapter 1 from Moran and Shapiro Thermodynamics is the science that deals with the relationship of heat and mechanical energy and conversion of one into other. • Thermodynamics is involved whenever there is an interaction between energy and matter – It is also the basis of the study of materials, chemical reactions and plasmas >> not our focus in this course • Thermodynamics can be regarded as a generalization of an enormous body of evidence. Some application areas of thermodynamics: Defining Systems • System (Dictionary Definition): – A set of things working together as parts of a mechanism or an interconnecting network; a complex whole. e.g. energy system: a system that consists of many components, devices, rules working together and primarily designed to supply energy services to end users. Thermodynamic System ►System: whatever we want to study (analyze). • A quantity of matter or a region in its space that we choose to study which is separated from its surroundings by a fictional or real boundary. ►Surroundings: everything external to the system. ►Boundary: distinguishes system from its surroundings. • can be fictional or real • can be fixed or moving interactions between a system and its surroundings, which take place across the boundary, play an important part in engineering thermodynamics. Boundary System Surroundings Closed System (Control Mass) ►A system that always contains the same matter. ►No transfer of mass across its boundary can occur. – Volume does not have to be constant. ►Isolated system: special type of closed system that does not interact in any way with its surroundings. Gas in a piston-cylinder assembly. When the valves are closed, the gas is a closed system. Control Volume (Open System) ►A given region of space through which mass flows. ►Mass may cross the boundary of a control volume. Example: Gas in a piston-cylinder assembly in an internal combustion engine When the valves are closed (compression and power stages below), the gas is a closed system. When the intakes are open (intake and exhaust stages) the gas is an open system. Courtesy: M.Bahrami, SFU • We select or define systems for our analysis purposes • Selection is arbitrary but: – Should be careful when selecting it. Alternatives are possible, selection depends heavily on the convenience in the subsequent analysis. • Some thermodynamics relations that are applicable to closed and open systems are different. Thus, it is extremely important to recognize the type of system we have before start analyzing it. Where should I put my boundary if: • the condition of the air entering and exiting the compressor is known, • and the objective is to determine the electric power input? Where should I put my boundary if: • I know the electrical power input and want to calculate how much time it will take to fill in the tank with a known volume. Fig_1-5 Macroscopic and Microscopic Views ►Systems can be described from the macroscopic and microscopic points of view. ►The microscopic approach aims to characterize by statistical means the average behavior of the particles making up a system and use this information to describe the overall behavior of the system. ►The macroscopic approach describes system behavior in terms of the gross effects of the particles making up the system – specifically, effects that can be measured by instruments such a pressure gages and thermometers. ►Engineering thermodynamics predominately uses the Note: We wil be mostly using macroscopic macroscopic approach. approach in this class! Property ►To describe a system and predict its behavior requires knowledge of its properties and how those properties are related. ►A macroscopic characteristic of a system to which a numerical value can be assigned at a given time without knowledge of the previous behavior of the system. ►For the system shown, examples include: ►Mass ►Volume ►Pressure ►Temperature Gas State ►The condition of a system as described by its properties. ►Example: The state of the system shown is described by p, V, T,…. ►The state often can be specified by providing the values of a subset of its properties. All other properties can be determined in terms of these few. State: p, V, T, … Gas Process ►A transformation from one state to another. ►When any of the properties of a system changes, the state changes, and the system is said to have undergone a process. ►Example: Since V2 > V1, at least one property value changed, and the gas has undergone a process from State 1 to State 2. State 1: p1, V1, T1, … Gas State 2: p2, V2, T2, … Gas Cycle A thermodynamic cycle consist of a sequence of processes in which the system goes back to its initial state. State 1: p1, V1, T1, … Gas State 2: p2, V2, T2, … Process 1-2 Gas State 3: p3, V3, T3, … Process 3-1 10/5/16 Gas ETM Process 2-3 20 Extensive Property ►Depends on the size or extent of a system. ►Examples: mass, volume, energy. ►Its value for an overall system is the sum of its values for the parts into which the system is divided. Intensive Property ►Independent of the size or extent of a system. ►Examples: temperature. ►Its value is not additive as for extensive properties. ►May vary from place to place within the system at any moment – function of both position and time. Mass Volume Temperature Pressure Density Steady State • If a system exhibits the same values of its properties at two state, it is in the same state at these times. • A system is said to be at steady state if none of its properties change with time. If the system is not in steady state, it is in transient state. During transient operation, properties change with time. Equilibrium ►In a thermodynamic sense, a generalized description of equilibrium can be given as: ►No unbalanced potentials or drivers that promote a change of state. ►The state of a system in equilibrium remains unchanged for all time. ►For a system to be in equilibrium, it must be in thermal, mechanical, phase and chemical equilibrium. Equilibrium ►Criteria for these four types of equilibrium will be considered in the following weeks. For now, we can think of testing to see if a system is in thermodynamic equilibrium as follows: • Isolate the system from its surroundings and watch for changes in its observable properties. • If there are no changes, we conclude that the system was in equilibrium at the moment it was isolated. The system can be said to be at equilibrium state. Equilibrium ►When a system is isolated, it does not interact with its surroundings; however, its state can change as a consequence of spontaneous events occurring internally as its intensive properties such as temperature and pressure tend toward uniform values. When all such changes cease, the system is at an equilibrium state. ►Why do we care about equilibrium? • Equilibrium states and processes from one equilibrium state to another equilibrium state play important roles in thermodynamic analysis. • Conservation of energy principle relies on the idea that equilibrium states exist at the beginning and end of a process, even though during the process the system may be far from equilibrium. • Three measurable intensive properties that are particularly important in engineering thermodynamics: – Specific volume – Pressure – Temperature Density (r) and Specific Volume (v) ►Density is mass per unit volume. 𝜌= ! " ►Density is an intensive property that may vary from point to point. ►SI units are (kg/m3). Density (r) and Specific Volume (v) ►Specific volume is the reciprocal of density: v = 1/r . ►Specific volume is volume per unit mass. ►Specific volume is an intensive property that may vary from point to point. ►SI units are (m3/kg). Specific volume is usually preferred for thermodynamic analysis whereas the density is more commonly used in fluid mechanics and heat transfer. Pressure (p) ►Consider a small area A passing through a point in a fluid at rest. ►The fluid on one side of the area exerts a compressive force that is normal to the area, Fnormal. An equal but oppositely directed force is exerted on the area by the fluid on the other side. ►The pressure is defined as: 𝑝= !!"#$%& " Pressure Units ►SI unit of pressure is the pascal: 1 pascal = 1 N/m2 ►Multiples of the pascal are frequently used: ►1 kPa = 103 N/m2 ►1 MPa = 106 N/m2 ►Other common units: ►1 bar = 105 Pa ►1 atm = 101325 Pa = 101.325 kPa=1.01325 bars Absolute Pressure ►Absolute pressure: Pressure with respect to the zero pressure of a complete vacuum. ►Absolute pressure must be used in thermodynamic relations. ►Pressure-measuring devices often indicate the difference between the absolute pressure of a system and the absolute pressure of the atmosphere outside the measuring device. Gage and Vacuum Pressure ►When system pressure is greater than atmospheric pressure, the term gage pressure is used. p(gage) = p(absolute) – patm(absolute) ►When atmospheric pressure is greater than system pressure, the term vacuum pressure is used. p(vacuum) = patm(absolute) – p(absolute) Example: Automobile tire pressure ►Common pressure reading on automobile tire pressure gauges: 32 psi ►1 psi = 6894.76 Pa = 0.068 atm ►This common reading from tires is gage pressure, i.e. it shows how much higher the pressure in the tire with respect to atmospheric pressure. ►At a location where the atmospheric pressure is 14.7 psi (~1 atm), the absolute pressure in the tire is 46.7 psi (~3.18 atm). Absolute pressure must be used in thermodynamic relations. Temperature (T) ►If two blocks (one warmer than the other) are brought into contact and isolated from their surroundings, they would interact thermally with changes in observable properties. ►When all changes in observable properties cease, the two blocks are in thermal equilibrium. ►Temperature is a physical property that determines whether the two objects are in thermal equilibrium. Thermometers ►Any object with at least one measurable property that changes as its temperature changes can be used as a thermometer. ►Such a property is called a thermometric property. ►The substance that exhibits changes in the thermometric property is known as a thermometric substance. Thermometers ►Example: Liquid-in-glass thermometer ►Consists of glass capillary tube connected to a bulb filled with liquid and sealed at the other end. Space above liquid is occupied by vapor of liquid or an inert gas. ►As temperature increases, liquid expands in volume and rises in the capillary. The length (L) of the liquid in the capillary depends on the temperature. ►The liquid is the thermometric substance. ►L is the thermometric property. ►Other types of thermometers: ►Thermocouples ►Thermistors ►Radiation thermometers and optical pyrometers Temperature Scales ►Kelvin scale: An absolute thermodynamic temperature scale whose unit of temperature is the kelvin (K); an SI base unit for temperature. ►Celsius scale (oC): T(oC) = T(K) – 273.15 Prefix iso- is used to designate a process for which a particular property is constant. • Isothermal: is a process during which the temperature remains constant • Isobaric: is a process during which the pressure remains constant • Isometric: is process during which the specific volume remains constant. Closed System Energy Balance ►Energy is an extensive property that includes the kinetic and gravitational potential energy of engineering mechanics. ►For closed systems, energy is transferred in and out across the system boundary by two means only: by work and by heat. ►Energy is conserved. This is the first law of thermodynamics. Closed System Energy Balance ►The closed system energy balance states: The change in the amount of energy contained within a closed system during some time interval Net amount of energy transferred in and out across the system boundary by heat and work during the time interval ►We now consider several aspects of the energy balance, including what is meant by energy change and energy transfer. Change in Energy of a System In engineering thermodynamics the change in energy of a system is composed of three contributions: ►Kinetic energy ►Gravitational potential energy ►Internal energy Change in Kinetic Energy ►The change in kinetic energy is associated with the motion of the system as a whole relative to an external coordinate frame such as the surface of the earth. ►For a system of mass m the change in kinetic energy from state 1 to state 2 is ( 1 2 2 m V V DKE = KE2 – KE1 = 2 1 2 ) (Eq. 2.5) where ►V1 and V2 denote the initial and final velocity magnitudes. ►The symbol D denotes: final value minus initial value. Change in Gravitational Potential Energy ►The change in gravitational potential energy is associated with the position of the system in the earth’s gravitational field. ►For a system of mass m the change in potential energy from state 1 to state 2 is DPE = PE2 – PE1 = mg(z2 – z1) (Eq. 2.10) where ►z1 and z2 denote the initial and final elevations relative to the surface of the earth, respectively. ►g is the acceleration of gravity. Energy Transfer by Work ►Energy can be transferred to and from closed systems by two means only: ►Work ►Heat ►You have studied work in mechanics and those concepts are retained in the study of thermodynamics. However, thermodynamics deals with phenomena not included within the scope of mechanics, and this requires a broader interpretation of work. Mechanical Work ►The work W done by, or on, a system evaluated in terms of macroscopically observable forces and displacements is: #" 𝑊 = $ 𝑭 & 𝑑𝒔 (Eq. 2.12) #! ►This definition of work is important and we will use this for boundary work. However, thermodynamics deals with phenomena not included within the scope of mechanics, and this requires a broader interpretation of work. Work is not a property #" 𝑊 = $ 𝑭 & 𝑑𝒔 (Eq. 2.12) #! ►To evaluate this integral, we need to know the relationship between force F and displacement (i.e. F=f(s)) ►The value of W depends on the details of the interactions taking place between the system and surroundings during a process and not just the initial and final states of the system. ►The notion of work at a state has no meaning, so the value of this integral should never be indicated as W2 − W1. Work is not a property ►The differential of work, W, is said to be inexact because, in general, the following integral cannot be evaluated without specifying the details of the process: % $ 𝛿𝑊 = 𝑊 $ ►On the other hand, the differential of a property is said to be exact because the change in a property between two particular states depends in no way on the details of the process linking the two states. Example: Volume % $ 𝑑𝑉 = 𝑉% − 𝑉$ $ Sign Convention for Work ►The symbol W denotes an amount of energy transferred across the boundary of a system by work. ►Since engineering thermodynamics is often concerned with internal combustion engines, turbines, and electric generators whose purpose is to do work, it is convenient to regard the work done by a system as positive. ►W > 0: work done by the system ►W < 0: work done on the system The same sign convention is used for the rate of energy transfer by work – called power, denoted by W . Illustrations of Work ►When a spring is compressed, energy is transferred to the spring by work. ►When a gas in a closed vessel is stirred, energy is transferred to the gas by work. ►When a battery is charged electrically, energy is transferred to the battery contents by work. ►The first two examples of work are encompassed by mechanics. The third example is an example of the broader interpretation of work encountered in thermodynamics. Modeling Expansion and Compression Work ►A case having many practical applications is a gas (or liquid) undergoing an expansion (or compression) process while confined in a pistoncylinder assembly. ►During the process, the gas exerts a normal force on the piston, F = pA , where p is the pressure at the interface between the gas and piston and A is the area of the piston face. Example: Gas in a piston-cylinder assembly in an internal combustion engine When the valves are closed (compression and power stages below), the gas is a closed system. When the intakes are open (intake and exhaust stages) the gas is an open system. Modeling Expansion and Compression Work ►From mechanics, the work done by the gas as the piston face moves from x1 to x2 is given by ò W = Fdx = ò pAdx ►Since the product Adx = dV , where V is the volume of the gas, this becomes W= ò V2 pdV (Eq. 2.17) V1 ►For a compression, dV is negative and so is the value of the integral, in keeping with the sign convention for work. Modeling Expansion and Compression Work ►To perform the integral of Eq. 2.17 requires a relationship between gas pressure at the interface between the gas and piston and the total gas volume. ►During an actual expansion of a gas such a relationship may be difficult, or even impossible, to obtain owing to non-equilibrium effects during the process – for example, effects related to combustion in the cylinder of an automobile engine. ►In most such applications, the work value can be obtained only by experiment. Modeling Expansion and Compression Work ►Eq. 2.17 can be applied to evaluate the work of idealized processes during which the pressure p in the integrand is the pressure of the entire quantity of the gas undergoing the process and not only the pressure at the piston face. ►For this we imagine the gas undergoes a sequence of equilibrium states during the process. Such an idealized expansion (or compression) is called a quasiequilibrium process. Modeling Expansion and Compression Work ►Since non-equilibrium effects are invariably present during actual expansions (and compressions), the work determined with quasiequilibrium modeling can at best approximate the actual work of an expansion (or compression) between given end states. Modeling Expansion and Compression Work ►In a quasiequilibrium expansion, the gas moves along a pressure-volume curve, or path, as shown. ►Applying Eq. 2.17, the work done by the gas on the piston is given by the area under the curve of pressure versus volume. Modeling Expansion and Compression Work ►When the pressure-volume relation required by Eq. 2.17 to evaluate work in a quasiequilibrium expansion (or compression) is expressed as an equation, the evaluation of expansion or compression work can be simplified. ►An example is a quasiequilibrium process described by pVn = constant , where n is a constant. This is called a polytropic process. ►For the case n = 1, pV = constant and Eq. 2.17 gives æ V2 ö W = (constant ) lnçç ÷÷ è V1 ø where constant = p1V1 = p2V2. Example Problem, 2.1 from textbook Problem (Example problem 2.1 from Moran) A gas in a piston–cylinder assembly undergoes an expansion process for which the relationship between pressure and volume is given by 𝑝𝑉 ! = 𝑐𝑜𝑛𝑠𝑡𝑎𝑛𝑡 The initial pressure is 3 bar, the initial volume is 0.1 m3, and the final volume is 0.2 m3. Determine the work for the process, in kJ, if (a) n=1.5, (b) n =1.0, and (c) n = 0.
