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THERMODYNAMIC an introduction

THERMODYNAMIC an introduction . Closed and open systems. Forms of energy. macroscopic. microscopic. Properties of a system. Intensive properties. Theromodynamics , an engineering approach, 2 nd ed., by Yunus A. Çengel & Michael A. Boles, McGraw-Hill, Inc., 1994

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THERMODYNAMIC an introduction

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  1. THERMODYNAMICan introduction Closed and open systems Forms of energy macroscopic microscopic Properties of a system Intensive properties Theromodynamics, an engineering approach, 2nd ed., by Yunus A. Çengel & Michael A. Boles, McGraw-Hill, Inc., 1994 http://www.wikipremed.com/image_archive.php?code=010304 Extensive properties State and equilibrium Zeroth Law

  2. SYSTEMS AND CONTROL VOLUMES • System: the material in the portion of space to be analyzed (closedor open) • Boundary: A separator, real or imaginary, between system and surroundings (can be fixed or movable.) • Surroundings: exterior environment • System: the material in the portion of space to be analyzed (closedor open) • Boundary: A separator, real or imaginary, between system and surroundings (can be fixed or movable.) • Surroundings: exterior environment  U  Q, W

  3. Mass out Energy out Mass in Energy in • Closed system (Control mass): A fixed amount of mass, and no mass can cross its boundary. • 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. An open system (a control volume) with one inlet and one exit.

  4. Heat Transfer: Conduction, Convection, Radiation Mass Transfer Forms of energy Fluid Mechanics Thermodynamics: The science of energy. The name thermodynamicsstems from the Greek words therme(heat) and dynamis(power). First law 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 Combustion • In thermodynamic analysis, it is often helpful to consider the various forms of energy that make up the total energy of a system in two groups: Macroscopic and Microscopic energy

  5. The macroscopic forms of energy , are those a system possesses as a whole with respect to some outside reference frame , such as kinetic energy (K.E.)and potential energy (P.E.) Internal energy is defined above as the sum of all the microscopic forms of energy of a system such as : K.E. of the molecules  sensible energy, Phase changed  latent energy (inter-molecular forces) Bonds in a molecule  chemical (or bond) energy (combustion, catalytic electrochemical reaction) Electronic energy Bonds within the nucleus of the atom itself  nuclear energy Temperature of the system

  6. Fuel Injector Turbine Hot exhaust Diffuser Combustion Chamber Nozzle Compressor Combustion (Fuel+Air) Qin→  H↑  H →Wout Compressor do Work on air Win→  H↑ Backwork ratio • H → KE Vair, P↑ KE→  H

  7. PROPERTIES OF A 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. Criterion to differentiate intensive and extensive properties.

  8. EQUILIBRIUM Zeroth Law of thermodynamic~!! • Thermodynamics deals with equilibrium states. • Equilibrium:A state of balance. • In an equilibrium state there are no unbalanced potentials (or driving forces) within the system. • Thermal equilibrium:If the temperature is the same throughout the entire system. • Mechanical equilibrium:If there is no change in pressure at any point of the system with time. • Phase equilibrium:If a system involves two phases and when the mass of each phase reaches an equilibrium level and stays there. • Chemical equilibrium:If the chemical composition of a system does not change with time, that is, no chemical reactions occur. A system at two different states. A closed system reaching thermal equilibrium.

  9. TEMPERATURE AND 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. Two bodies reaching thermal equilibrium after being brought into contact in an isolated enclosure.

  10. Any question ?

  11. THERMODYNAMICThefirst law The 1st Law States and Processes Work done during volume changed Path between states Cyclic Processes

  12. 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 (P ,T , v ). • 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.

  13. Process 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. Quasistatic 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.

  14. Work done during volume changed

  15. Work done during volume changed

  16. Path between states m m (isometric, isovolumic)

  17. Path between states

  18. The 1st Law of Thermodynamics Another example of energy transformaiton Qin PE of falling weight  KE of paddle  Heat in water Win Either heating or stirring can raise T of the water. 1st Law of Thermodynamics : Increase in internal energy = Heatadded  Work done Thermodynamic state variable = variable independent of history. e.g., U, T, P, V, … Not Q, W, … Joule’s apparatus

  19. The 1st Law of Thermodynamics 1st Law of Thermodynamics :

  20. Path between states :Isothermal Processes Isothermal process : T = constant. For monoatomic gas e.g. He  Isothermal processes on ideal gas

  21. Example : Bubbles ! A scuba diver is 25 m down, where the pressure is 3.5 atm ( 350 kPa ). The air she exhales forms bubbles 8.0 mm in radius. How much work does each bubble do as it arises to the surface, assuming the bubbles remain at 300 K. T = const 

  22. Constant-Volume Processes & Specific Heat (Cv) Constant-volume process ( isometric, isochoric, isovolumic ) : V= constant  CV = molar specific heat at constant volume  isometric processes for all processes Ideal gas: U = U(T)  only for const-volprocesses

  23. Isobaric Processes & Specific Heat (Cp) Isobaric Process : constant P Isotherms CP = molar specific heat at constant pressure isobaric processes Ideal gas, isobaric : Ideal gas 

  24. Adiabatic Processes No heat lost Q=0 Adiabatic process: Q = 0 (Compression is always a adiabaticprocess if it is fast enough) Think it in a common sence: Pumping the handle results in what? if there is no heat lose (Q=0) 1. gas pressure increased 2. gas temperature increased adiabat, ideal gas Adiabatic: larger p

  25. Summary:

  26. Q/A m • The ideal gas law says p V = n R T, • but the adiabatic equation says p V  = const. • Which is true, • the ideal gas law , • the adiabatic equation, or • both? • Explain. m mR mR Implies reversible process no friction and equilibrium process Reversibl work!!

  27. Diesel Power Fuel ignites in a diesel engine from the heat of compression (no spark plug needed). Compression is fast enough to be adiabatic. If the ignite temperature is 500C, what compression ratio Vmax / Vmin is needed?？ Given : Air’s specific heat ratio is  = 1.4, & before the compression the air is at 20 C.

  28. Q/A : • Name the basic thermodynamic process involved when each of the following is done to a piston-cylinder system containing ideal gas, • tell also whether T, p, V, & U increase or decrease. • the piston is lock in place & a flame is applied to the bottom of the cylinder, • the cylinder is completely insulated & the piston is pushed downward, • the piston is exposed to atmospheric pressure & is free to move, while the cylinder is cooled by placing it on a block of ice. • isometric; T , p, V =const, U . • adiabatic ; T , p, V , U . • isobaric ; T , p=const, V , U .

  29. Cyclic Processes Cyclic Process : system returns to same thermodynamic state periodically. 。 A four-process cycle

  30. Example : Finding the Work done in a cycle An ideal gas with  = 1.4 occupies 4.0 L at 300 K & 100 kPa pressure. It’s compressed adiabatically to ¼ of original volume, then cooled at constant V back to 300 K, & finally allowed to expand isothermally to its original V. How much work is done on the gas? AB (adiabatic): BC (isometric): CA (isothermal): work done by gas:

  31. From 1st law of thermodynamic, We know that: • “You cannot build a perpetual motion ! Since sou cannot get more energy out than you put in(conservation of energy).” • But…… • About the efficiency: • Can we know how much work done at least we can get after putting energy into the machine? • About the direction of heat: • When you’re holding a cup of coffee , Why doesn’t your hand get colder as the coffee become hotter and hotter , It does not against with the 1st law! The 1st law is not enough to explain both questions~! We are going to the world of 2nd law

  32. Any question ?

  33. THERMODYNAMICThesecond law The 2nd Law Clasusius statements Kelvin-Planck statements Limits on performance Irreversible Carnot cycle Entropy statement We’ll miss you, Qc …

  34. (Clausius statement) no process is possible where the sole result is the removal of heat from a low-temp reservoir and the absorption of an equal amount of heat by a high temp reservoir (Kelvin-Planck) no process is possible in which heat is removed from a single reservoir w/ equiv amount of work produced Rudolf Clausius (1828-1888) Lord Kelvin (1824-1907) Max Planck (1858-1947)

  35. Heat Engine Efficiency

  36. Limits on performance An irreversible processes normally include one or more of the following processes : • Heat transfer through a finite temperature difference • Unrestrained expansion of a gas or liquid to a lower pressure • Spontaneous chemical reactions • Spontaneous mixing of matter at different compositions or states • Friction-sliding friction as well as friction in the flowing fluids • Electric current flow through a resistance • Magnetization or polarization with hystersis • Inelastic deformation

  37. Limits on performance Nicolas Léonard Sadi Carnot 1796-1832 Reversible cycle  Carnot Cycle A Carnot Cycle consists of four steps: • Isothermal expansion (in contact with the heat reservoir) • Adiabatic expansion (after the heat reservoir is removed) • Isothermal compression (in contact with the cold reservoir) • Adiabatic compression (after the cold reservoir is removed) Every processes in the cycle are reversible! How about its efficiency ~!

  38. Efficiency of a Carnot cycle

  39. Since no one can create a 0 k cold reservoir or a ∞ k heat reservoir. Carnot efficiency is a theoretical maximum and it can’t reach 100%

  40. Entropy

  41. T v.s. S diagram of Carnot cycle

  42. The 2nd law of thermodynamic • If a process occurs in an isolated (closed and adiabatic) system the entropy of the system increases for irreversible process and remains constant for reversible processes. IT NEVER DECREASES….

  43. Any question ?

  44. Cycles • A diagram can be drawn with any pair of properties • P-T • P-V (allows the net work of a cycle to be determined: W=integral of pdV • T-S (gives the net heat of a cycle; recall 2nd law which states: dsdQ/T -> Q=integral of Tds! • If you can convert some of the heat to work, you have an engine!

  45. Cycle Types • Premixed Charge – Otto Cycle, gasoline, spark-ignition engine • Non-premixed charge or stratified charge engine (compression ignition or Deisel cycles) • Gas Turbines – Brayton Cycle • Other cycles: Rankine, …

  46. Where to start: Air (ideal gas) cycles • Assume no changes in gas properties (cp, MW, g, …) due to changes in composition, temp., …called the IDEAL air cycle! • REAL cycles must consider fuel-air mixture • which is compressed, burned, expanded,… • with accompanying changes in thermodynamic • properties

  47. Premixed Charge – Otto Cycle How can we take that into calculation? We need to simplify it !

  48. Premixed Charge – Otto Cycle Expand 4 P s Simplify v Burn: Constant Volume 5 3 Blowdown v s 1 2, 6 V (cylinder volume) Compress

  49. Heat Added 4 4 P s v Expansion T 5 v s 3 v s 3 Compression 5 s v 1 2, 6 V 1,2,6 Heat Rejected S Otto Cycle

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