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Published byMariah Griffith Modified over 6 years ago

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Mass and energy analysis of control volumes undergoing unsteady processes

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Studying unsteady systems: conservation of mass Integrating both sides V + -

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Studying unsteady systems with the conservation of mass and energy equation General energy equation with the assumption of uniform flow at inlets/outlets Time integrated form Assume further that states at the inlets and outlets are constant with time

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Calculating energy change Assuming KE and PE effects are negligible. Assuming properties are uniform with position within the CV at final and initial states (e.g. when the control volume is at thermodynamic equilibrium at The begin and end states).

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Conservation of mass and energy for an unsteady system: final usable forms V + - Assuming properties are uniform with position within the CV at final and initial states states at the inlets and outlets are constant with time KE and PE change of the CV can be neglected

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Example problem: heat transfer during the filling of an evacuated bottle (also in Tutorials) Consider a rigid and evacuated container (bottle) of volume V that is surrounded by the atmosphere (T 0, P 0 ). At some point in time, the neck valve of the bottle opens, and atmospheric air flows in. The wall of the bottle is thin and conductive enough so that the trapped air and the atmosphere eventually reach thermal equilibrium. In the end, the trapped air and the atmosphere are also in mechanical equilibrium, because the neck valve remains open. Determine the net heat interaction that takes place through the wall of the bottle during the entire filling process. Solve this problem by (a) a closed system approach (b) by an open system approach

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