Thermodynamics concept question

[Rumpelstiltzkin]

Here's the question:
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A 0.5m3 rigid tank contains refrigerant-134a initially at 200kPa and 40% quality. Heat is now transferred to the refrigerant until the pressure reaches 800kPa. Determine (a) the mass of the refrigerant in the tank and (b) the amount of heat transferred. Also, show the process on a P-υ diagram with respect to saturation lines.
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The solution for this question was already released, and I know the working to do it, but I can't understand the concept for part (b). In the solution, the amount of heat transfer was given as Q = (mass) * (change in specific internal energy, u).

I'm confused: Why can't we use change in enthalpy, h? Doesn't the change in enthalpy also mean heat transfer? I know it has something to do with the volume being constant ("rigid tank"), but I don't see how exactly.

Would someone care to explain what's the difference between these two properties?

Thanks.

 

[Valkarie]

In this case, the volume is constant, so the change in enthalpy (h) is equal to the change in specific internal energy (u). The heat transfer can be calculated by multiplying the mass of the refrigerant with the change in internal energy. This is because the heat transfer is equal to the amount of energy that is being transferred between the system and its surroundings. Since the volume is constant, the change in enthalpy is equal to the change in internal energy.

 

[piercas]

The change in enthalpy, h, does indeed represent heat transfer in a constant pressure process. However, in this problem, the tank is rigid, meaning the volume remains constant. Therefore, the pressure and temperature will increase as heat is added, but the volume will not change. This means that the specific internal energy, u, is a more appropriate property to use in this case, as it takes into account the constant volume of the system. The change in specific internal energy, u, is equal to the change in enthalpy, h, minus the change in the product of pressure and specific volume, PΔv. This is known as the work done on the system, and in this case, it is equal to zero since the volume is constant. Therefore, the amount of heat transferred can be calculated using the change in specific internal energy, u, as shown in the solution. I hope this helps clarify the difference between these two properties in this specific scenario. Additionally, on a P-υ diagram, the process would be shown as a vertical line since the volume is constant, and it would intersect the saturation line at the final pressure and quality state.