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. V + - Dark line encloses CV Flow processes are processes that involve fluid “particles/elements” entering or leaving with specified kinetic and gravitational potential energies through the control volume CV

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 time. 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 STEADY FLOW PROCESSES Turbine Compressor (related: pump, blower) Heat exchanger (related terms: boiler/condenser/evaporator, mixer etc.) Nozzle/diffuser Throttling devices (related terms: valves) Source: internet

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.