18-05-2012, 12:11 PM
Thermodynamic cycle
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A thermodynamic cycle consists of a series of thermodynamic processes transferring heat and work, while varying
pressure, temperature, and other state variables, eventually returning a system to its initial state.[1] In the process of
going through this cycle, the system may perform work on its surroundings, thereby acting as a heat engine.
State quantities depend only on the thermodynamic state, and cumulative variation of such properties adds up to zero
during a cycle. Process quantities (or path quantities), such as heat and work are process dependent, and cumulative
heat and work are non-zero. The first law of thermodynamics dictates that the net heat input is equal to the net work
output over any cycle. The repeating nature of the process path allows for continuous operation, making the cycle an
important concept in thermodynamics. Thermodynamic cycles often use quasistatic processes to model the workings
of actual devices.
Heat and work
Two primary classes of thermodynamic cycles are power cycles and heat pump cycles. Power cycles are cycles
which convert some heat input into a mechanical work output, while heat pump cycles transfer heat from low to high
temperatures using mechanical work input. Cycles composed entirely of quasistatic processes can operate as power
or heat pump cycles by controlling the process direction. On a pressure volume diagram or temperature entropy
diagram, the clockwise and counterclockwise directions indicate power and heat pump cycles, respectively.
Power cycles
Thermodynamic power cycles are the basis for the operation of
heat engines, which supply most of the world's electric power and
run almost all motor vehicles. Power cycles can be divided
according to the type of heat engine they seek to model. The most
common cycles that model internal combustion engines are the
Otto cycle, which models gasoline engines and the Diesel cycle,
which models diesel engines. Cycles that model external
combustion engines include the Brayton cycle, which models gas
turbines, and the Rankine cycle, which models steam turbines.
Heat pump cycles
Thermodynamic heat pump cycles are the models for heat pumps and refrigerators. The difference between the two
is that heat pumps are intended to keep a place warm while refrigerators are designed to cool it. The most common
refrigeration cycle is the vapor compression cycle, which models systems using refrigerants that change phase. The
absorption refrigeration cycle is an alternative that absorbs the refrigerant in a liquid solution rather than evaporating
it. Gas refrigeration cycles include the reversed Brayton cycle and the Hampson-Linde cycle. Regeneration in gas
refrigeration allows for the liquefaction of gases.
Modelling real systems
Example of a real system modelled by an idealized process: PV and TS diagrams of a
Brayton cycle mapped to actual processes of a gas turbine engine
Thermodynamic cycles may be used to
model real devices and systems,
typically by making a series of
assumptions.[2] simplifying
assumptions are often necessary to
reduce the problem to a more
manageable form.[2]