Unit A-1: List of Subjects

Transcription

1 ES312 Energy Transfer Fundamentals Unit A: Fundamental Concepts ROAD MAP... A-1: Introduction to Thermodynamics A-2: Engineering Properties Unit A-1: List of Subjects What is Thermodynamics? First and Second Law of Thermodynamics Definition of Terminology in Thermodynamics Thermodynamic Process and Cycle Fundamental Concept of Continuum Mechanics Basic Engineering Unit System

2 PAGE 1 of 9 What s Thermodynamics? The term thermodynamics stems from the Greek words theme (heat) and dynamis (motion) Thermodynamics is both a branch of physics and engineering science Science v.s. Engineering... Scientists are interested in gaining a fundamental understanding of the physical behavior Engineers are interested in studying systems and how they interact with their surroundings OBJECTIVES OF PART I (THERMODYNAMICS): Understand the basic principles and theories of thermodynamics Understand the first and second laws of thermodynamics Prepare for the fundamentals of heat transfer analysis in computer simulations in part II/III OBJECTIVES OF PART II/III (ANSYS HEAT TRANSFER ANALYSIS): Learn fundamentals of heat transfer analysis in commercial software (ANSYS) ANSYS SEMESTER COURSE PROJECT A simple heat transfer analysis will be performed, using ANSYS Project proposal will be due at the beginning of part III Final project presentation will be on the day of the scheduled final exam Final project report will be due by the end of the day of the final presentation

3 PAGE 2 of 9 First and Second Law of Thermodynamics Conservation of energy principle 1st Law of Thermodynamics 2nd Law of Thermodynamics Energy cannot be created or destroyed: it can only change forms (the 1 st Law of Thermodynamics) CONSERVATION OF ENERGY PRINCIPLE First Law of Thermodynamics: energy cannot be created or destroyed: energy is conserved, it can only change forms Second Law of Thermodynamics: energy has quality, means that the actual thermodynamic processes occur in the direction of decreasing quality of energy LAWS OF THERMODYNAMICS: POKER-PLAYER S ANALOGY (Bob Riggins, Rice University) The universe is the House, the great Casino. The great dealer, who controls the deck, always need to take His percentage; so that in the long run the player is broke and his chip (energy) is dissipated into the void (and unrecoverable). You can t win (you can t even break-even) and you can t get out of the game OTHER LAWS OF THERMODYNAMICS Zeroth Law of Thermodynamics: if two systems are in thermal equilibrium respectively with a third system, they must be in thermal equilibrium with each other (this law helps define the motion of temperature) Third Law of Thermodynamics: the entropy of a system approaches a constant value as the temperature approaches absolute zero: the entropy of a system at absolute zero is typically close to zero

4 PAGE 3 of 9 Definition of Terminology in Thermodynamics System, surroundings, and boundary The design of many engineering systems, such as this solar hot water system, involves thermodynamics A system at 2 different states SYSTEM OF THERMODYNAMICS System: an object of focus or attention, enclosed by surroundings (boundaries) Control Mass (CM): typically a closed system, defined by a fixed amount of mass in space (a system without convection or flow ) Control Volume (CV): typically an open system, defined by a fixed volume in space (a system with convection or flow ) ENGINEERING PROPERTIES Property: characteristics that can be measured or quantified Extensive properties: properties that depends on the size of the system Energy is an extensive property Intensive properties: properties that are independent to the size of the system Energy per unit mass (energy density) is an intensive property Properties are somewhat inter-related and a set of few properties can specify others by these relations STATE OF THE SYSTEM The state can often be specified by providing the values of a subset of the properties State of the system can be defined by a set of particular properties of a system

5 PAGE 4 of 9 Thermodynamic Process and Cycle The P-V diagram of a compression process Quasi-equilibrium and non-quasi-equilibrium compression processes EQUILIBRIUM Equilibrium implies a state of balance: a system in equilibrium experiences no changes when it is isolated from its surroundings A system is in thermal equilibrium if the temperature is the same throughout the system A system is in mechanical equilibrium if there is no change in pressure at any point of the system with time A system is in phase equilibrium if a multi-phase system s mass of each phase does not change with time A system is in chemical equilibrium if its chemical composition does not change with time PROCESS Process is any change that a system undergoes from one equilibrium state to another: the series of states through which a system passes during a process is called process path Quasi-static or quasi-equilibrium process: a sufficiently slow process that allows the system to adjust itself internally so that properties in one part of the system do not change any faster than those at other parts

6 PAGE 5 of 9 Fundamental Concept of Continuum Mechanics Despite the large gaps between molecules, a substance can be treated as a continuum because of the very large number of molecules even if extremely small volume Mass cannot cross the boundaries of a closed system, but energy can (control mass) A closed system with a moving boundary Volume in the space is fixed, and mass and energy can move across boundaries (control volume) CONTINUUM ASSUMPTION Substances are made up of atoms that are, in reality, widely spaced in gas phase; however, it is convenient to disregard the atomic nature of a substance and view it as continuous and homogeneous matter with no imperfections (continuum) The engineering mechanics, based on this continuum assumption is called continuum mechanics: Statics, Fluid Mechanics, Solid Mechanics, and Thermodynamics... are all continuum mechanics CONTROL MASS (CM) ANALYSIS Often referred as closed system Collection of a matter of fixed amount (mass) that we focus our attention CONTROLVOLUME (CV) ANALYSIS Often referred as open system A fixed region in space (volume) that allows flow in and out of the region

7 PAGE 6 of 9 Basic Engineering Unit System Definition of Force Weight of a unit mass (at sea-level) SI (INTERNATIONAL STANDARD) UNITS Basic units for mass, length, and time: kilogram (kg), meter (m), and second (s) Force (weight) unit: Newton (N), where: 1 N = (1 kg)(1 m/s 2 ) Temperature unit: Celsius (C) / Kelvin (K), where: K = C US CUSTOMARY (ENGLISH) UNITS Basic units for mass, length, and time: slug, foot (ft), and second (s) Force (weight) unit: pound (lb), where: 1 lb = (1 slug)(1 ft/s 2 ) Temperature unit: Fahrenheit (F) / Rankine (R), where: R = F NON-STANDARD UNITS Pound mass (lbm): weight of one pound mass on the earth s surface (gravity is 32.2 ft/s 2 ) is equal to one pound force (lb) Kilogram force (kgf): weight of one kilogram on the earth s surface (gravity is 9.8 m/s 2 ) is equal to one kilogram force (kgf) UNIT CONVERSION Non-standard units cannot be mixed up against standard units (important) Convert non-standard units into standard units: 1 slug = 32.2 lbm and 9.8 N = 1 kgf

8 PAGE 7 of 9 EXERCISE A-1-1 (Do-It-Yourself) A tank is filled with oil (density is 850 kg/m 3 ). If the volume of the tank is 2m 3, determine the amount of mass (in kg ) in the tank. Solution The volume of an oil tank is given. The mass of oil is to be determined. Assumptions Oil is an incompressible substance and thus its density is constant. Analysis Oil tank 3 Given the density and volume of oil: 850 kg/m and V = 2 m 3 The mass is density times volume, therefore: m V 3 3 Therefore, m 850 kg/m 2 m 1,700 kg

9 PAGE 8 of 9 EXERCISE A-1-2 (Do-It-Yourself) Applying appropriate unit conversions, show that 1 lbm weighs 1 lbf on earth, under the gravity of 32.2 ft/s 2. A mass of 1lbm weighs 1 lbf on earth, under standard gravity (at sea-level) Solution A mass of 1 lbm is subjected to standard earth gravity. Its weight in lb (lbf) is to be determined. Assumptions 2 Standard sea-level condition ( g 32.2 ft/s ). Analysis Applying Newton s second law, the weight (force) can be calculated. 1 slug 2 W mg 1 lbm 32.2 ft/s 1 lb (this is pound force or lbf ) 32.2 lbm

10 PAGE 9 of 9 Determine the mass and the weight of air (both in kilograms ) contained in a room (dimension: 6 m 6 m 8 m). Assume that the density of air is 1.16 kg/m 3. Assumptions The density of air is constant throughout the room. Properties 3 The density of air is given: 1.16 kg/m Analysis The mass of air in the room is: m V 3 3 Therefore, m 1.16 kg/m 668 m kg Weight of air in the room is: 2 W mg kg9.8 m/s 3,274 N In kilograms : 1 kgf 3,274 N kgf 9.8 N