Control volume

A specified region in space used to analyze mass, energy, and momentum flows across its boundary. Its boundary, called the control surface, may be fixed or moving.

Control surface: The boundary separating a control volume from its surroundings. Mass and energy cross this boundary in the control-volume balances.
System (thermodynamics): A specified quantity of matter or region chosen for thermodynamic analysis. Unlike a control volume, a closed system contains the same matter throughout the analysis.
Nozzle: A duct that accelerates fluid by converting pressure or enthalpy into kinetic energy. A control volume around a nozzle relates inlet and outlet flow conditions.
Lagrangian description: A description that follows individual material particles as they move. It follows a fixed mass, whereas a control-volume analysis observes a spatial region.
Reynolds transport theorem: A theorem relating changes in an extensive property of a system to changes and fluxes within a control volume. It converts system conservation laws into equations for a chosen region in space.
Open system: A system that exchanges both matter and energy with its surroundings. A control volume is the spatial framework commonly used to analyze open systems.
Turbine: A machine that extracts work from a flowing fluid by turning a rotor. Its energy balance connects fluid enthalpy changes to shaft work.
Eulerian description: A description of physical properties at fixed or specified locations in space. It supplies the spatial viewpoint used to describe fields inside a control volume.
Mass balance: An accounting equation equating changes in stored mass with mass entering and leaving a system. It tracks accumulation when fluid crosses the control surface.
Closed system: A system that exchanges energy but not matter with its surroundings. Its fixed matter differs from the changing inventory inside many control volumes.

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