Question

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.

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.