Engineering Thermodynamics
A closed system is one in which

Neither mass nor energy crosses the boundaries of the system
Both energy and mass cross the boundaries of the system
Mass does not cross boundaries of the system, though energy may do so
Mass crosses the boundary but not the energy

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Engineering Thermodynamics
The efficiency of a gas turbine is given by

(Isentropic increase in temperature)/(Actual increase in temperature)
(Actual temperature drop)/(Isentropic temperature drop)
(Net work output)/(Work-done by the turbine)
(Net work output)/(Heat supplied)

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