ME1521 ME 152 Thermodynamics G.A. Kallio Dept. of Mechanical Engineering, Mechatronic Engineering & Manufacturing Technology California State University, Chico

ME1522 Basic Concepts & Definitions Reading: Cengel & Boles, Chapter 1

ME1523 Introduction Thermodynamics - science that deals with energy, matter, and the laws governing their interaction –general: all engineering systems involve energy and matter –fundamental: based upon primitive concepts (two primary laws) –employs a unique vocabulary based upon precise definitions –initially, it appears formal and abstract, but its significance and application will eventually be seen

ME1524 Introduction, cont. Classical Thermodynamics - macroscopic approach that deals with large systems, e.g., engines, power plants, refrigerators, etc.; studied and used by engineers Statistical Thermodynamics - microscopic approach that deals with the structure and properties of matter on an atomic/molecular level; studied and used by physicists and chemists

ME1525 Primary Laws of Thermodynamics First Law of Thermodynamics - quantitative conservation of energy principle; energy cannot be created nor destroyed Second Law of Thermodynamics - places qualitative restrictions on energy-related processes, e.g., direction of heat transfer, maximum performance of power plants

ME1526 Thermodynamic Applications See Figure 1-5 and class overhead slides

ME1527 Dimensions & Units

ME1528 Basic Thermodynamic Definitions System - quantity of matter or region of space chosen for study Surroundings - mass or region outside of system Boundary - real or imaginary surface that separates system from surroundings Closed System (Control Mass) - a fixed quantity of mass that can only experience energy transfer (no mass can enter or leave); an isolated system is a special case where no mass or energy transfer is allowed

ME1529 Basic Thermodynamic Definitions, cont. Control Volume (Open System) - region of space that can experience both energy and mass transfer across its boundary Property - a characteristic of a system that can be defined without knowledge of the system’s history Extensive Property - property that is dependent on system size Intensive Property - property that is independent of system size

ME15210 Basic Thermodynamic Definitions, cont. State - a condition of a system that is fully described by properties Equilibrium - a state where there are no imbalances due to mechanical, thermal, chemical, or phase effects State Postulate - gives the number of properties needed to fix the state of a system Simple Compressible System - a system where external force fields are negligible (i.e., electrical, magnetic, gravitational, motion, and surface tension effects)

ME15211 Basic Thermodynamic Definitions, cont. Process - a change that a system undergoes from one equilibrium state to another; the sequence of states through which the system passes is called the process path Quasi-equilibrium Process - a sufficiently slow process that allows the system to remain infinitesimally close to equilibrium Cycle - a sequence of processes that returns the system to its initial state

ME15212 Basic Thermodynamic Definitions, cont. Isothermal Process - a process where temperature remains constant Isobaric Process - a process where pressure remains constant Isochoric Process - a process where volume or density remains constant Steady-Flow Process - a control volume process where all properties at a fixed point remain constant with respect to time

ME15213 Some Basic Thermodynamic Properties Energy Density Specific Volume Pressure Temperature

ME15214 Energy Energy is an extensive property of a system; it is the capacity to do work or cause change –can be stored –can be transferred –can be transformed –is always conserved Types of Energy –mechanical, kinetic, potential, thermal, electric, magnetic, chemical, nuclear, latent, et al.

ME15215 Energy, cont. Macroscopic energy - forms of energy that a system possesses as a whole w.r.t. some external reference frame, e.g., kinetic and potential energies Microscopic energy - forms of energy related to the molecular and atomic structure of a system; the sum of all microscopic forms of energy is known as internal energy (U)

ME15216 Energy, cont. System energy can be stored as –Kinetic energy, KE = ½m V 2 e.g., throwing a ball –Gravitational potential energy, PE = mgz e.g., raising a dumbbell –Internal energy, U = ? e.g., heating the air in a room In the absence of electric, magnetic, and surface tension effects, the total energy (E) of a system is E = U + KE + PE

ME15217 Energy, cont. Energy can be only be transferred across a system boundary by –work interactions, due to a force acting through some distance –heat transfer, due to a temperature difference –mass flow, due to fluid flow into or out of a control volume Energy can be transformed in many ways, e.g., –chemical-electrical (battery) –electrical-thermal (resistor) –potential-kinetic (dropping a rock) –nuclear-thermal (nuclear reactor)

ME15218 Density and Specific Volume Density (kg/m 3 ), Specific Volume (m 3 /kg), –Specific Gravity

ME15219 Pressure Fluid Pressure (N/m 2 ) Other units: 1 pascal (Pa) = 1 N/m 2 1 kPa = 10 3 N/m 2 1 bar = 10 5 N/m 2 1 MPa = 10 6 N/m 2 1 atm = kPa = lbf/in 2 (psi)

ME15220 Pressure, cont. Absolute pressure - total pressure experienced by a fluid Gage pressure or vacuum pressure- difference between absolute pressure and atmospheric pressure (usually indicated by a measuring device): P gage = P abs - P atm P vac = P atm - P abs

ME15221 Pressure, cont. Pressure variation with depth: Pascal’s principle: a force applied to a confined fluid increases the pressure throughout by the same amount; since F = PA, mechanical advantage can be developed

ME15222 Pressure Measurement Manometer – gravimetric device based upon liquid level deflection in a tube Bourdon tube – elliptical cross-section tube coil that straightens under under influence of gas pressure Mercury barometer – evacuated glass tube with open end submerged in mercury to measure atmospheric pressure Pressure transducer – converts pressure to electrical signal; i) flexible diaphragm w/strain gage ii) piezo- electric quartz crystal

ME15223 The U-tube Manometer Simple, accurate device for measuring small to moderate pressure differences Rules of manometry: –pressure change across a fluid column of height h is  gh –pressure increases in the direction of gravity –two points at the same elevation in a continuous static fluid have the same pressure (Pascal’s law)

ME15224 Temperature Temperature (ºC or K) – measure of a body’s “hotness” or “coldness” –indicative of a body’s internal energy –used to determine when a system is in thermal equilibrium, i.e., when all points have the same temperature –see zeroth law of thermodynamics, section 1-9 –unit conversions: K = ºC R = ºF ºF = 1.8 ºC + 32

ME15225 Temperature Measurement Constant-P liquid-in-glass – utilizes volume change of mercury or alcohol in a tube Constant-V gas – utilizes pressure change of hydrogen or helium Bimetallic strip – utilizes differential CTE of adjoined dissimilar metals Thermistor, RTD – utilizes electrical resistance of metals and semiconductors Thermocouple - utilizes voltage produced from dissimilar metal junctions Optical pyrometer – utilizes infrared emission spectrum