Work done and total heat transfer for piston-cylinder device

In summary, this conversation discusses a question about a refrigerant in a piston-cylinder device, where a mass of 0.2kg is initially 75% liquid and heat is transferred at a constant pressure until it becomes all vapor. The work done is calculated using the formula W = P(V2-V1) and the total heat transfer is calculated using the formula Q = h2 - h1. Assumptions are also given for the device.
  • #1
Baartzy89
16
0
Hi everybody, this question is on an old exam paper given to us for exam prep purposes. No answers have been supplied however. Am I on the right track?

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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  • #2
I think that is all correct.
 

Related to Work done and total heat transfer for piston-cylinder device

1. What is work done in a piston-cylinder device?

The work done in a piston-cylinder device is the force multiplied by the distance moved by the piston. This work is typically measured in Joules (J) or Newton-meters (N-m).

2. How is work calculated in a piston-cylinder device?

The work done in a piston-cylinder device is calculated by multiplying the force applied to the piston by the distance the piston moves. This can be represented by the equation W = F * d, where W is work, F is force, and d is distance.

3. What is total heat transfer in a piston-cylinder device?

The total heat transfer in a piston-cylinder device is the sum of the heat added to the system and the work input or output. This is represented by the equation Q = W + U, where Q is heat, W is work, and U is internal energy.

4. How is total heat transfer calculated in a piston-cylinder device?

Total heat transfer in a piston-cylinder device is calculated by adding the amount of heat added to the system to the work input or output. This can be represented by the equation Q = W + U, where Q is total heat transfer, W is work, and U is internal energy.

5. What factors affect work done and total heat transfer in a piston-cylinder device?

The work done and total heat transfer in a piston-cylinder device can be affected by factors such as the pressure, volume, and temperature of the system. Changes in these variables can impact the amount of work and heat transferred in the device.

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