Reiner Decher
Professor (Adjunct in Civil Engineering)
Oxford University Press, 1994
ISBN 0-19-507959-0
Errata EC : click here to access errata to date noted.
With the understanding of thermodynamics over the past two centuries or so, the resultant technology has affected much of modern man's activity and accomplishments. The engines that propel society's vehicles on fields, roads, rails, sea lanes, through air, and into space attest to the ingenuity of humankind to use understanding to his or her advantage. The recent revolution in human activity has made heavy demands on heat resources and has exacted environmental costs. This work celebrates the accomplishments, expresses a concern for the environment, and is motivated by the reality that these resources will become more scarce in the future; technology will be asked to alleviate the effects of this scarcity. If one overriding point is to be made by this work, it is that demands for increased efficiency require costlier energy conversion processes. If a smooth transition toward more efficient use of thermal resources is to be made, it will require investment in the capital plant before the economic consequences of resource shortages are felt by the society using diminishing resources. This investment may well determine whether the desired changes can indeed be accomplished. A look around in the modern world clearly shows that access to power generated by machines and resources assists people with the burden of physical and economic survival, allowing time and resources for the needs of society, and enrichment, recreation, and art for its members. Planning for a day with reduced access to these resources is important, but investing for that day even more so. The transition may well lead to a reordering of the list of who is wealthy and who is less so.
This work is intended to convey an understanding of the physical processes involved in the transformation of one energy form to another. The emphasis is on a description of models of the elementary processes to allow assessment of performance potential and to allow a determination of the sensitivity to design choices. Since many energy conversion processes involve the manipulation of gaseous substances, there is a heavy emphasis on the description of fluids and of gases in particular. Energy conversion processes involve heat and work interactions between a system and its environment, friction, as well as state and property descriptions. In order to arrive at simple, understandable relations, simplifications are made that allow description at the expense of some numerical accuracy. More accurate descriptions can be made with more sophisticated computational tools. Nearly all numerical calculations presented are made with the equations developed, so that the reader can implement them on a computer and reproduce them with his or her own choice of parameters. The specific reduction to practice is generally not covered here. The reader is encouraged to consult the bibliography as well as the cited and current literature for coverage in greater depth.
The reader is assumed to be proficient in the calculus and the physical sciences, including an introduction to thermodynamics and fluid mechanics.
At the conclusion of most of the chapters and some appendices in this book, there are problems provided for the serious student. The nature of these is rather unusual in that the problems may be open-ended or may ask for functional relationships that force the development of assumptions, and seek techniques for solution. It is hoped that an inquiry will follow as to whether the conclusions drawn are physically meaningful in the context of the assumptions. This approach makes the problems more difficult for the student. Thus, before using these problems, an instructor should work out the problems to a satisfactory degree to ensure that they meet teaching goals.
Chapter outline
In the study of engines for the production of power, the student must begin with classical thermodynamics. That introduction is usually insufficient to bridge the gap between the analysis introduced there and the practical implementation. This book is an attempt to describe an approach to determining the performance of real engines for electromechanical power or propulsive thrust. Thus the approach is to describe the following elements of the broad and universally important subject of energy and power. The chapters, their principal goals, and additional background requirements for the reader are listed below.
1. Energy forms, energy utilization and resources available: physics, history, and earth's resource limitations.
2. Conversion process characterization: How good is the process and how good can it be? Review of elementary thermodynamics.
3. Characteristics of power systems: fuel consumption rate, compactness, cost, weight, vehicular versus stationary power sources. How is energy used?
4. Description of the gas used as the working medium in thermodynamic cycles. How complex does its description have to be for satisfactory description of processes?
5. Combustion: How much heat is available from a fuel and at what temperature?
6. Heat exchangers: What are the limitations of devices designed to transfer heat from one fluid to another?
7. Heat engine modeling: Irreversible thermodynamics plays an important role in determining overall engine design and performance characteristics: components and processes.
8. Otto and Diesel cycle engines are examples of fuel energy limited engine cycles which can be economically operated at part power: cycle variations can bring performance improvement.
9. Brayton cycle engines are examples of a temperature-limited cycle for power on earth and in space: design point operation and closed cycles.
10. Stirling cycles engines are examples of engines whose performance is strongly influenced and limited by the manner in which the environment of a thermodynamic system interacts with the system: thermodynamic description and performance.
11. The performance of some cycles is limited by the working fluid properties: The Rankine cycle and its modifications can be made to operate with other cycles. The MHD work interaction as a high performance turbine makes unusual demands on the cycle into which it is incorporated.
To this point the material is primarily applied thermodynamics, an extension of that normally covered in a first semester course. In Chapters 12- 15, it is assumed that the student has had an exposure to one-dimensional gas dynamics and an introduction to airfoil and wing theory.
12. Fluid kinetic energy is an energy form of interest in many internal and external flows: Convertibility of thermal energy to kinetic energy.
13. Propellers and wind turbines are mechanical means employed to add power to an unbounded fluid or extract power from it: Performance limits of such aerodynamic devices.
14. Compressors: Steady flow cycles require efficient means of raising pressure of an internal flow; description of these aerodynamic devices, their performance and limitations.
15. Turbines: power extraction from internal flows with an emphasis on the difference between compressors and turbines.
16. Part power operation of the Brayton cycle: What role do the cycle configuration and the choice and characteristics of the components (described in chapters 14 and 15) play in determining the part-power performance.
17. Energy storage for matching to user needs: Physics and limitations of practical devices.
18. Environmental impact of energy conversion: Chemical emissions and thermal and radioactive waste. Maximizing performance of power systems has consequences for the resource, system economics and the environment. Solar power is discussed here as an power resource alternative with its own set of important environmental consequences.
Not covered is the topic of nuclear power generation. This topic requires coverage of physics and engineering which is extensive and is well covered in a number of books. Omission of nuclear power from this text is not intended to imply that it is unimportant. Future recognition of the environmental and resources questions associated with fossil fuels may will serve to reestablish nuclear power to its needed place, in spite of valid concerns regarding the storage of nuclear waste and the safety of plant operations. Also not included is a discussion of so-called direct energy conversion, from heat to electricity. A companion book covering this topic, including electrochemistry (batteries and fuel cells), thermoelectric and thermionic conversion, and magnetohydrodynamics, is also available.
Comments and suggestions are welcome. Contact via e-mail. E-mail address: decher@aa.washington.edu