Use of Entropy for a Control Volume in Energy Balance

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SUMMARY

Entropy is utilized in energy balance calculations for open systems due to its ability to account for irreversible processes and energy dispersal, which is crucial in applications like turbines. In a control volume with constant volume (Δv = 0), the relationship h = u + PΔv simplifies to h = u, but this does not negate the importance of entropy in analyzing energy transfers. The energy balance for steady flow systems is expressed as W = m2(h2 + ke2 + pe2) - m1(h1 + ke1 + pe1), highlighting the role of enthalpy in conjunction with kinetic and potential energy. Understanding these principles is essential for accurate energy analysis in thermodynamic systems.

PREREQUISITES
  • Understanding of thermodynamic concepts, particularly open systems
  • Familiarity with the first law of thermodynamics
  • Knowledge of energy balance equations in fluid mechanics
  • Basic principles of entropy and enthalpy
NEXT STEPS
  • Study the role of entropy in irreversible processes in thermodynamics
  • Explore the first law of thermodynamics in open systems
  • Learn about energy balance equations specific to turbines and similar systems
  • Investigate the relationship between enthalpy and internal energy in steady flow conditions
USEFUL FOR

Students and professionals in mechanical engineering, thermodynamics researchers, and anyone involved in energy systems analysis will benefit from this discussion.

Acnhduy
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Hi all,

I'm having some trouble figuring out why entropy is used instead of enthalpy for an open system.

From what I understand, an open system uses entropy to calculate internal energy. Since the control volume is constant (i.e. Δv = 0), wouldn't using : h = u + PΔv effectively be h = u? So in this case, wouldn't you use enthalpy instead of entropy since volume isn't changing?Just to confirm, in an open system like a turbine or something, with the assumption of no heat transfer and negligible potential energy, the energy balance becomes:

W = ΔU
W = m2(h2 + ke2 + pe2) - m1(h1 + ke1 + pe1)

... and with steady flow ...

w = (h2 + ke2) - (h1 + ke1)

Please let me know if I am misunderstanding something, thanks!
 
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