Volume
State |
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A uniform control volume needs only a
single state (a unique set of state properties), called the volume
state, to describe its thermodynamic equilibrium. A mixture
of two phases (a mixture of saturated liquid and vapor, for instance) in
equilibrium, described by a single state, constitutes a uniform system
even if the phases may not be well mixed. The aggregate system properties,
such as total mass, total energy, etc., depend on the system volume. Note
that an open system can be uniform, too! As an example, the volume state
can be used to find the mass of air in a classroom, given its volume, temperature
and pressure. |
Surface
State |
 |
A uniform control surface also needs only
a single state, called the surface state,
to describe the thermodynamic equilibrium at the surface. The difference
between a surface state and a volume state lies in the system variables
- mass flow rate vs. total mass or entropy flow rate vs. total entropy,
for instance. For example, in order to evaluate the mass flow rate of steam
at a turbine inlet the surface state daemon
is more useful than the volume state daemon. |
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