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Energy Conversion

Energy Conversion. CHE 450/550. Ideal Gas Basics and Heat Capacities - I. Ideal gas: a theoretical gas composed of a set of non-interacting point particles. obeys the ideal gas law: PV=nRT R is “gas constant” [R = 8.314 J·K -1 ·mol -1 ] You may see R specific =R/MW [J·K -1 ·kg -1 ]

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Energy Conversion

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  1. Energy Conversion CHE 450/550

  2. Ideal Gas Basics and Heat Capacities - I Ideal gas: • a theoretical gas composed of a set of non-interacting point particles. • obeys the ideal gas law: PV=nRT • R is “gas constant” [R = 8.314 J·K-1·mol-1] • You may see Rspecific=R/MW [J·K-1·kg-1] • At close to normal conditions most real gases behave like an ideal gas. • Various relationships written. Most useful

  3. PV and TS diagrams Some key terms: Isobar – “at the same pressure” Isochore – “at the same volume” Isotherm – “at the same temperature” Isentropic – “at the same entropy” Adiabatic – “without heat exchange (with the surroundings)” P T V S

  4. PV and TS diagrams – Isobar and Isochore Isobar – “at the same temperature” Isochore – “at the same volume” Where do those go on the PV and TS diagrams? P T V S

  5. PV and TS diagrams – Isotherm, Isentropic and Adiabatic Isotherm – “at the same temperature” Isentropic – “at the same entropy” Adiabatic – “without heat exchange (with the surroundings)” Where do those go on the PV and TS diagrams? P T V S

  6. TS diagram – Isobars with phase change

  7. Ideal Gas Basics and Heat Capacities - II Heat capacity “C” relates the change in temperature DT that occurs when an amount of heat DQ is added Usually given as per mass (specific heat capacity, c) [J.kg-1.K-1] The conditions under which heat is added play a role: • At constant volume, cV=(du/dt)V (no PV work performed during heating) • At constant pressure cP=(dh/dt)P (constant P, so as T increases, V increases: PV work performed) • A thermally perfect gas can be shown to have cP=cV+Rspecific (Sorry but it would take too long to go through the formal derivation of this)

  8. Ideal Gas Basics and Heat Capacities - III • An important quantity is k=cP/cV, • known as the “adiabatic index” or “isentropic expansion factor” (you’ll also see it written as g gamma or k kappa) • In general cV and cP are functions of kinetic energy of molecules forming a material (translational, vibrational, rotational), and intra- and intermolecular forces.

  9. Brayton Cycle http://commons.wikimedia.org/wiki/File:Brayton_cycle.svg

  10. Rankine Cycle

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