Air is contained in a variable-load piston-cylinder device equipped with a paddle wheel. Initially, air is at 400 kPa and 17°C. The paddle wheel is now turned by an external electric motor until 72 kJ/kg of work has been transferred to air. During this process, heat is transferred to maintain a constant air temperature while allowing the gas volume to triple. Calculate the required amount of heat transfer, in kJ/kg.

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Engineering

Pooh

19 June, 15:13

Air is contained in a variable-load piston-cylinder device equipped with a paddle wheel. Initially, air is at 400 kPa and 17°C. The paddle wheel is now turned by an external electric motor until 72 kJ/kg of work has been transferred to air. During this process, heat is transferred to maintain a constant air temperature while allowing the gas volume to triple. Calculate the required amount of heat transfer, in kJ/kg.

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Rex Mueller · Answer 1

Using the formula ∆E = Ein - Eout

Qin + Win - Wout = ∆U

Where the boundary work is defined as Wout = RTinv2/v1

Wout=0.287 * 290*In3

=91.4kj/kg

Now qin = 91.4 - 75 = 16.4kj/kg

Heat transfered is 16.4kj/kg.