Thermodynamics

Chapter 9GAS POWER CYCLESTwo important areas of application for thermodynamicsare power generation and refrigeration. Both are usuallyaccomplished by systems that operate on a thermodynamiccycle. Thermodynamic cycles can be divided into twogeneral categories: power cycles, which are discussed in thischapter and Chap. 10, and refrigeration cycles, which are discussedin Chap. 11.The devices or systems used to produce a net power outputare often called engines, and the thermodynamic cycles theyoperate on are called power cycles. The devices or systemsused to produce a refrigeration effect are called refrigerators,air conditioners, or heat pumps, and the cycles they operateon are called refrigeration cycles.Thermodynamic cycles can also be categorized as gascycles and vapor cycles, depending on the phase of theworking fluid. In gas cycles, the working fluid remains in thegaseous phase throughout the entire cycle, whereas in vaporcycles the working fluid exists in the vapor phase during onepart of the cycle and in the liquid phase during another part.Thermodynamic cycles can be categorized yet anotherway: closed and open cycles. In closed cycles, the workingfluid is returned to the initial state at the end of the cycle andis recirculated. In open cycles, the working fluid is renewed atthe end of each cycle instead of being recirculated. In automobileengines, the combustion gases are exhausted andreplaced by fresh air–fuel mixture at the end of each cycle.The engine operates on a mechanical cycle, but the workingfluid does not go through a complete thermodynamic cycle.Heat engines are categorized as internal combustionand external combustion engines, depending on how theheat is supplied to the working fluid. In external combustionengines (such as steam power plants), heat is supplied to theworking fluid from an external source such as a furnace, ageothermal well, a nuclear reactor, or even the sun. In internalcombustion engines (such as automobile engines), this isdone by burning the fuel within the system boundaries. Inthis chapter, various gas power cycles are analyzed undersome simplifying assumptions.ObjectivesThe objectives of Chapter 9 are to:• Evaluate the performance of gas power cycles for which theworking fluid remains a gas throughout the entire cycle.• Develop simplifying assumptions applicable to gas powercycles.• Review the operation of reciprocating engines.• Analyze both closed and open gas power cycles.• Solve problems based on the Otto, Diesel, Stirling, andEricsson cycles.• Solve problems based on the Brayton cycle; the Braytoncycle with regeneration; and the Brayton cycle withintercooling, reheating, and regeneration.• Analyze jet-propulsion cycles.• Identify simplifying assumptions for second-law analysis ofgas power cycles.• Perform second-law analysis of gas power cycles.| 487