Properties, Processes

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Properties, Processes Cycles

• Two independent properties define the state
  (i.e. condition) of a thermodynamic system.
• The state of a system can change by interaction
  with its surroundings through work and heat
  transfer.
• When this change occurs in a system, it is said
  that the system has undergone a process.
• A thermodynamic cycle is a sequence of different
  processes that begins and ends at the same
  thermodynamic state.
• Some sample processes
• Isothermal process temperature is constant T
  C
• Isobaric process pressure is constant, P C
• Isentropic process entropy is constant, s C
• Ischoric/isometric process Volume is constant,
  vC
• Adiabatic process no heat transfer, Q0

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Example Simple steam power plant cycle
The properties at different states in a cycle are
often shown on a T-v P-v or other property
diagrams.
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Types of Processes

• I Quasi-Equilibrium Process (YAC 1-7,3-4
  3-4) An idealized process that takes place so
  slowly that the properties of the system are in
  equilibrium. (Also called quasi-static processes)
• Example A very slow expansion/compression of a
  gas in a piston cylinder, often called Moving
  Boundary Work. This type of work is very common
  in real systems. e.g. IC engine. For such a
  process, work done, ?W P dV.
• Work done in a quasi-equilibrium process, going
  from state 1 to 2 is given by
• Hence work is path dependant or a path function
• Many real processes behave in such a manner.
• A device producing work has maximum efficiency
  if operating via a quasi-equilibrium process.
• II Steady Flow Process

Area under the curve
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Properties Internal Energy

• E U(internal energy)KE(kinetic
  energy)PE(potential energy)
• Extensive property since it depends on the mass
  of the system, U mu, where m is the mass of the
  system, u is the specific energy of the system
  (an intensive property like temperature and
  pressure)
• Unlike KE PE, the internal energy is a form of
  energy measured on a molecular scale. It can
  consist of different modes translational kinetic
  energy of individual molecules, rotational energy
  and vibrational energies associated with
  molecules, and intermolecular forces between
  molecules. The sum of all these molecular-level
  energies is called the internal energy.
• Internal energy is a property of the substance,
  thus, its change in value between two states is
  independent of the process.

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Process B
Process A
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Properties Enthalpy

• HUPV, defined as total enthalpy
• Hmh, huPv, specific enthalpy will be an
  intensive property
• It is an important property in many situations,
  for example, the steady flow process, h uP/?
  is a measure of the combined internal energy and
  the pressure work
• It is also useful when one considers phase
  transition E.g. when liquid water vaporizes,
  its internal energy changes from uf to ug. At
  the same time, its specific volume also changes
  from vf to vg, It goes through an expansion
  process and it does work.
• Therefore, the total change of the energy will
  be from hf uf pvf to hg. Their difference
  hfghg-hf is called the latent heat of
  vaporization at the given temperature/pressure.

T
hg
Latent heathg-hfhfg
hf
v
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Properties Specific Heats (3-6 3-7)

• The state of a pure, compressible substance can
  be determined by values of two thermodynamic
  properties. Ex PP(v, T)
• u u(T,v) internal energy is a function of two
  variables T and v (or any other two independent
  properties such as P, h)
• The internal energy can be varied by altering
  these two properties

• Define constant-volume specific heat Cv (can
  sometimes be considered as heat capacity, the
  ability of a substance to absorb or store
  energy).
• Similarly, the enthalpy can be described by

• Define constant-pressure specific heat Cp

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Specific Heats (cont.)

• Both Cp and Cv can be considered as heat
  capacities but under different processes
• Their ratio k(specific heat ratio) k Cp/Cv is
  also a property of the substance.
• Special cases incompressible fluid (density and
  v is a constant)