Page 107 - Mechanical Engineers' Handbook (Volume 4)

96 Thermodynamics Fundamentals

low-temperature reservoir
() L
() max maximum
() T turbine
() C compressor
() N nozzle
() D diffuser
() 0 reference state
() 1 initial state
() 2 final state
() * moderately compressed liquid state
() slightly superheated vapor state

Definitions
Boundary: The real or imaginary surface delineating the thermodynamic system. The bound-
ary separates the system from its environment. The boundary is an unambiguously defined
surface. The boundary has zero thickness and zero volume.
Closed System: A thermodynamic system whose boundary is not crossed by mass flow.
Cycle: The special process in which the final state coincides with the initial state.
Environment: The thermodynamic system external to the thermodynamic system.
Extensive Properties: Properties whose values depend on the size of the system (e.g., mass,
volume, energy, enthalpy, entropy).
Intensive Properties: Properties whose values do not depend on the system size (e.g., pres-
sure, temperature). The collection of all intensive properties constitutes the intensive state.
Open System: A thermodynamic system whose boundary is permeable to mass flow. Open
systems (flow systems) have their own nomenclature: the thermodynamic system is usually
referred to as the control volume, the boundary of the open system is the control surface,
and the particular regions of the boundary that are crossed by mass flows are the inlet and
outlet ports.
Phase: The collection of all system elements that have the same intensive state (e.g., the
liquid droplets dispersed in a liquid–vapor mixture have the same intensive state, that is,
the same pressure, temperature, specific volume, specific entropy, etc.).
Process: The change of state from one initial state to a final state. In addition to the end
states, knowledge of the process implies knowledge of the interactions experienced by the
system while in communication with its environment (e.g., work transfer, heat transfer,
mass transfer, and entropy transfer). To know the process also means to know the path
(the history, or the succession of states) followed by the system from the initial to the
final state.
State: The condition (the being) of a thermodynamic system at a particular point in time,
as described by an ensemble of quantities called thermodynamic properties (e.g., pressure,
volume, temperature, energy, enthalpy, entropy). Thermodynamic properties are only those
quantities that do not depend on the ‘‘history’’ of the system between two different states.
Quantities that depend on the system evolution (path) between states are not thermody-
namic properties (examples of nonproperties are the work, heat, and mass transfer; the
entropy transfer; the entropy generation; and the destroyed exergy—see also the definition
of process).
Thermodynamic System: The region or the collection of matter in space selected for anal-
ysis.