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Question:

A mass of 0.2kg of saturated refrigerant-134a is contained in a piston-cylinder device at 200kPa. Initially 75% of the mass is in the liquid phase. Now heat is transferred to the refrigerant at constant pressure until the cylinder contains only vapor. Determine:

a) the work done, and

b) the total heat transfer

Assumptions:

Piston-cylinder device is frictionless, constant pressure, quasi-equilibrium, no heat lost to surroundings

a) the work done

Formula:

W = P (V2-V1) where,

V2 = Volume at state 2

V1 = Volume at state 1

P = Pressure

W = Boundary work

Properties:

P = 200 kPa

V2 = 0.2 x vg where vg = 0.099867 (gas table) = 0.0199734 m3/kg

V1 = 0.2 x (0.75 x vf + 0.25 x vg) = 0.005106345 m3/kg

Analysis:

W = P(V2-V1)

= 200 x (0.0199734 - 0.005106345)

= 3.89 kJ boundary work

b) total heat transfer

Formula:

Q = h2 - h1 where,

Q = total heat transferred

h2 = Enthalpy state 2

h1 = Enthalpy state 1

Properties:

h2 = hg = 244.46 kJ/kg (from gas tables)

h1 = .75 x hf + .25 x hg = .75 x 38.43 + .25 x 244.46 = 89.9375 kJ/kg

Analysis:

Q = h2 - h1

= 244.46 - 89.9375

= 154.52 kJ heat

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# Homework Help: Work done and total heat transfer for piston-cylinder device

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