Book cover for Thermodynamics: An Engineering Approach

Thermodynamics: An Engineering Approach

Yunus A. Cengel, Michael A. Boles

ISBN #9781259822674

9th Edition

2,694 Questions

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59,300 Students Helped

Homework Questions

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Summary
Learning Objectives
Key Concepts
Example Problems
Explanations
Common Mistakes

Summary

Chapter 9 on Gas Power Cycles provides an in-depth look at various power cycles including the Rankine, Carnot, Otto, and Diesel cycles, and their modifications such as reheat and regeneration. The integration of gas and steam cycles in combined cycle systems is explored, emphasizing improved thermal efficiency and cogeneration. The chapter also highlights the importance of second-law analysis through exergy calculations to identify irreversibilities in the cycles, thereby guiding enhancements in performance. Overall, the chapter underscores the evolving design optimizations in modern power generation systems.

Learning Objectives

1

Explain the fundamental principles of gas power cycles and their significance in energy conversion systems.

2

Describe the Rankine cycle and its modifications such as reheat and regeneration, including their impact on thermal efficiency.

3

Analyze combined cycles that integrate gas and steam power cycles for enhanced overall performance.

4

Apply second-law analysis and exergy calculations to identify irreversibilities and potential improvements in power cycles.

5

Discuss practical applications and design considerations in gas turbines, reciprocating engines, and turbojet engine modifications.

Key Concepts

CONCEPT

DEFINITION

Gas Power Cycles

Processes that convert fuel energy into mechanical work through combustion and expansion, typically found in gas turbines and reciprocating engines.

Vapor Power Cycle

A thermodynamic cycle, such as the Rankine cycle, that uses phase change of a working fluid for energy conversion.

Rankine Cycle

A model that describes the operation of steam power plants, where water is converted to steam and expanded to produce work.

Reheat and Regeneration

Modifications to the Rankine cycle which involve reheating the steam or using extracted steam to preheat feedwater, enhancing thermal efficiency and reducing fuel consumption.

Combined Cycle

A power generation system that integrates both gas and steam cycles to improve overall efficiency by utilizing waste heat from the gas turbine.

Second-Law Analysis

A method using exergy calculations to evaluate the irreversibilities within a thermodynamic system, providing insights into efficiency improvements.

Exergy

A measure of the maximum useful work possible during a process that brings the system into equilibrium with a heat reservoir.

Cogeneration

The simultaneous production of electricity and useful heat, maximizing the energy utilization from fuel.

Example Problems

Example 1

What are the air-standard assumptions?

Example 2

What is the difference between air-standard assumptions and the cold-air-standard assumptions?

Example 3

Why is the Carnot cycle not suitable as an ideal cycle for all power-producing cyclic devices?

Example 4

How does the thermal efficiency of an ideal cycle, in general, compare to that of a Carnot cycle operating between the same temperature limits?

Example 5

How are the combustion and exhaust processes modeled under the air-standard assumptions?
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Step-by-Step Explanations

QUESTION

How does the introduction of a reheat process improve the efficiency of a Rankine cycle?

STEP-BY-STEP ANSWER:

Step 1: Identify the basic Rankine cycle stages: water pressurization, heat addition, expansion in the turbine, and heat rejection.
Step 2: Introduce the reheat process where, after partial expansion, the steam is sent back to the boiler to be reheated before undergoing further expansion.
Step 3: Analyze how the effective average temperature during heat addition increases due to the reheat process.
Step 4: Explain that a higher effective temperature during heat addition reduces the fuel requirement for achieving the same energy output, thereby increasing the cycle efficiency.
Step 5: Conclude that the reheat process minimizes thermal stresses and improves the expansion efficiency in the turbine.
Final Answer: The reheat process improves Rankine cycle efficiency by increasing the effective heat addition temperature and reducing fuel consumption while minimizing turbine blade stresses.

Rankine Cycle Efficiency with Reheat

QUESTION

How can second-law analysis be utilized to identify irreversibilities in a combined cycle?

STEP-BY-STEP ANSWER:

Step 1: Define the concept of exergy and its role in measuring the usable work potential of the system.
Step 2: Identify each component of the combined cycle (gas turbine, heat recovery steam generator, steam turbine) for analysis.
Step 3: Calculate the exergy destruction in each component by comparing the actual work output to the ideal reversible work.
Step 4: Determine which process (e.g., combustion, heat transfer, expansion) exhibits the highest irreversibility.
Step 5: Suggest modifications or enhancements to reduce the identified irreversibilities.
Final Answer: Second-law analysis using exergy calculations helps pinpoint the components and processes with the greatest irreversibilities in a combined cycle, guiding engineers towards targeted improvements.

Second-Law Analysis Using Exergy

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Common Mistakes

  • Confusing the roles of the various modifications (reheat vs. regeneration) in the Rankine cycle.
  • Assuming that higher thermal efficiency always directly translates to lower fuel consumption without considering system design constraints.
  • Overlooking the significance of second-law analysis, particularly exergy destruction, in identifying inefficiencies within power cycles.
  • Misinterpreting air-standard assumptions and applying them without considering real gas effects and component deviations in actual operating cycles.