ME 152 Thermodynamics

1. ME 152Thermodynamics G.A. Kallio Dept. of Mechanical Engineering, Mechatronic Engineering & Manufacturing Technology California State University, Chico ME152

2. Basic Concepts & Definitions Reading: Cengel & Boles, Chapter 1 ME152

3. 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 ME152

4. 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 ME152

5. 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 ME152

6. Thermodynamic Applications • See Figure 1-5 and class overhead slides ME152

7. Dimensions & Units ME152

8. 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 ME152

9. 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 ME152

10. 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) ME152

11. 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 ME152

12. Basic Thermodynamic Definitions, cont. • Isothermal Process - aprocess 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 ME152

13. Some Basic Thermodynamic Properties • Energy • Density • Specific Volume • Pressure • Temperature ME152

14. 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. ME152

15. 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) ME152

16. Energy, cont. • System energy can be stored as • Kinetic energy, KE = ½mV2 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 ME152

17. 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) ME152

18. Density and Specific Volume • Density (kg/m3), • Specific Volume (m3/kg), • Specific Gravity ME152

19. Pressure • Fluid Pressure (N/m2) • Other units: 1 pascal (Pa) = 1 N/m2 1 kPa = 103 N/m2 1 bar = 105 N/m2 1 MPa = 106 N/m2 1 atm = 101.325 kPa = 14.696 lbf/in2 (psi) ME152

20. 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): Pgage = Pabs - Patm Pvac = Patm - Pabs ME152

21. 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 ME152

22. 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 ME152

23. 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) ME152

24. 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 + 273.15 R = ºF + 459.67 ºF = 1.8 ºC + 32 ME152

25. 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 ME152