First Law for open systems

Mass and energy analysis of
open systems
Open systems
• When matter crosses the boundaries of a system chosen for
thermodynamic analysis, the system is known as open system or control
volume (CV).
• Many open systems of practical interest involve fluid crossing the
boundaries through one or more inlet and outlet ports i.e. a flow process
occurring through the control volume.
Dark line encloses
Flow processes
are processes that
involve fluid “particles/elements”
entering or leaving with specified
kinetic and gravitational potential
energies through the control
Examples of open systems
Source: internet
Schematic and modeling approach for engineering open systems
Broken line is
CV chosen
The steady flow process
A process during which the fluid flows steadily through the control volume (CV)
– Flow process fluid flows through CV.
– Steady not changing with time
During a steady flow process:
– conditions (fluid properties, flow velocity, elevation) at any fixed point within the CV are
unchanging with time.
– Properties, flow velocity or elevation may change from point to point within CV
– Size, shape, mass and energy content of the CV do not change with time.
– Rate at which heat and work interactions take place with surroundings do not change with
Devices/systems which undergo steady flow process: compressors, pumps, turbines, water supply
pipes, nozzles, heat exchangers, power plants, aircraft engines etc.
ENGINEERING examples of open systems undergoing
Source: internet
• Turbine
• Compressor (related: pump,
• Heat exchanger (related terms:
mixer etc.)
• Nozzle/diffuser
• Throttling devices (related
terms: valves)
Unsteady flow process (example)
Two peculiar boundary phenomena in open systems:
• Along with mass, thermodynamic properties enter
and leave the system through the inlet/outlet ports.
• As the fluid flows on, “invisible pistons” are pushed
into the inlet ports and pushed out of the outlet ports.

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