Enthelpy in Throttle Process (Joule–Thomson expansion)

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SUMMARY

The Joule-Thomson expansion process maintains constant enthalpy (H) due to the conservation of energy during pressure changes without heat loss. This phenomenon is characterized by the flow work equation, represented as P2V2 - P1V1, which illustrates the relationship between pressure and volume in an open system. The confusion arises from the distinction between flow work and the integral form of work done, specifically why flow work is expressed as Δ(PV) rather than ∫PdV. Understanding this derivation is crucial for grasping the principles of the first law of thermodynamics in open systems.

PREREQUISITES
  • Understanding of thermodynamic principles, particularly the first law of thermodynamics.
  • Familiarity with the concept of enthalpy and its applications in thermodynamics.
  • Knowledge of open systems and control volume analysis.
  • Basic grasp of fluid mechanics and flow work calculations.
NEXT STEPS
  • Study the derivation of the first law of thermodynamics for open systems.
  • Explore the concept of flow work in detail, focusing on its mathematical representation.
  • Investigate the Joule-Thomson effect and its applications in real-world systems.
  • Learn about enthalpy changes in various thermodynamic processes, including isenthalpic processes.
USEFUL FOR

Students and professionals in thermodynamics, mechanical engineers, and anyone involved in fluid dynamics or energy systems will benefit from this discussion.

MisterX
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Why is it written that enthalpy (##H##) is constant for Joule-Thompson expansion? It seems the essence of this process is to convert from one pressure to another with no heat loss. How does that connect with enthalpy being constant? When I learned about enthalpy it seemed to be most relevant to systems at constant pressure. However in the Joule-Thompson effect the pressure is surely changing. Sources mention that the "flow work" is ## P_2V_2 - P_1V_1##. This leads to constant ##H## by conservation of energy. I guess I don't understand why the flow work is ##\Delta(PV)## and not ## \int P dV##. Maybe it also seems strange for me to think of a fluid doing work on itself or something. Anyway, I feel confused about this.
 
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Are you familiar with the derivation of the open-system (control volume) version of the first law of thermodynamics?
 

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