About this video

• Video Title: Lec-13 Thermodynamic Property Relations-I
• Channel: nptelhrd
• Speakers: Speaker 1
• Duration: 01:00:09

Overview

This video lecture focuses on thermodynamic property relations, specifically delving into the concepts of availability (exergy) and its balance in open and closed systems. It revisits the irreversibility in natural processes and introduces the second law efficiency. The lecture then applies these concepts to analyze turbines, compressors, and heat exchangers, and concludes by introducing fundamental thermodynamic property relations and functions like Helmholtz and Gibbs.

Key takeaways

• Availability (Exergy) is Not Conserved: Unlike energy, availability is destroyed in natural processes due to irreversibility, quantified by T0 times the entropy change of the universe.
• Second Law Efficiency: This efficiency measures how closely a process approaches a reversible ideal, defined as the ratio of ideal work to actual work (for work-producing devices) or vice versa (for work-consuming devices).
• Analysis of Devices: The lecture demonstrates the application of availability and second law efficiency to turbines, compressors, and heat exchangers, calculating their respective efficiencies and irreversibilities.
• Isentropic vs. Second Law Efficiency: The distinction is clarified: second law efficiency compares actual work to ideal work between fixed states, while isentropic efficiency compares actual work to the work of a reversible adiabatic process from the same initial state.
• Introduction to Thermodynamic Potentials: The lecture introduces Helmholtz (F = u - Ts) and Gibbs (G = H - Ts) functions as important thermodynamic properties with physical significance, particularly in isothermal and isobaric processes, respectively.

Here are flashcards based on the video transcript:

Flashcard 1

• Front: What is the fundamental concept regarding availability (exergy) in natural processes?
• Back: Availability (exergy) is not conserved; it is destroyed or reduced in any natural process due to irreversibility.

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Flashcard 2

• Front: How is irreversibility related to availability destruction in a closed system?
• Back: Irreversibility is equal to the destruction of availability, which can be expressed as T0 times the entropy change of the universe.

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Flashcard 3

• Front: What is the general definition of the second law efficiency for a work-producing device like a turbine?
• Back: Second Law Efficiency = (Actual Work Output) / (Ideal Work Output or decrease in flow availability)

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Flashcard 4

• Front: What is the general definition of the second law efficiency for a work-consuming device like a compressor?
• Back: Second Law Efficiency = (Ideal Work Requirement) / (Actual Work Requirement)

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Flashcard 5

• Front: In an adiabatic turbine, what is the relationship between actual work and the decrease in flow availability?
• Back: Actual work (h1 - h2) is less than the decrease in flow availability (af1 - af2) if the process is irreversible.

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Flashcard 6

• Front: What is isentropic efficiency, and how does it differ from second law efficiency for a turbine?
• Back: Isentropic efficiency compares actual work to the work of a reversible adiabatic (isentropic) process from the same initial state. Second law efficiency compares actual work to the ideal work between the same initial and final states, considering irreversibility.

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Flashcard 7

• Front: How is the second law efficiency defined for a heat exchanger?
• Back: Second Law Efficiency = (Availability Increase in the cold fluid) / (Availability Decrease in the hot fluid)

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Flashcard 8

• Front: What is the definition of Helmholtz function (F)?
• Back: F = u - Ts

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Flashcard 9

• Front: What is the definition of Gibbs function (G)?
• Back: G = H - Ts

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Flashcard 10

• Front: What physical significance does the change in Helmholtz function (dF) represent under specific conditions?
• Back: The change in Helmholtz function represents the maximum work (reversible work) obtainable from a closed system undergoing an isothermal process where the system temperature equals the surrounding temperature.