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Chapter09

# Chapter09 - Chapter 9 Gas Power Cycles Study Guide in...

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Chapter 9 Gas Power Cycles Study Guide in PowerPoint to accompany Thermodynamics: An Engineering Approach , 5th edition by Yunus A. Çengel and Michael A. Boles

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2 Our study of gas power cycles will involve the study of those heat engines in which the working fluid remains in the gaseous state throughout the cycle. We often study the ideal cycle in which internal irreversibilities and complexities (the actual intake of air and fuel, the actual combustion process, and the exhaust of products of combustion among others) are removed. We will be concerned with how the major parameters of the cycle affect the performance of heat engines. The performance is often measured in terms of the cycle efficiency. η th net in W Q =
3 Carnot Cycle The Carnot cycle was introduced in Chapter 5 as the most efficient heat engine that can operate between two fixed temperatures T H and T L . The Carnot cycle is described by the following four processes. Carnot Cycle Process Description 1-2 Isothermal heat addition 2-3 Isentropic expansion 3-4 Isothermal heat rejection 4-1 Isentropic compression

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4 Note the processes on both the P-v and T-s diagrams. The areas under the process curves on the P-v diagram represent the work done for closed systems. The net cycle work done is the area enclosed by the cycle on the P-v diagram. The areas under the process curves on the T-s diagram represent the heat transfer for the processes. The net heat added to the cycle is the area that is enclosed by the cycle on the T-s diagram. For a cycle we know W net = Q net ; therefore, the areas enclosed on the P-v and T-s diagrams are equal. η th Carnot L H T T , = - 1 We often use the Carnot efficiency as a means to think about ways to improve the cycle efficiency of other cycles. One of the observations about the efficiency of both ideal and actual cycles comes from the Carnot efficiency: Thermal efficiency increases with an increase in the average temperature at which heat is supplied to the system or with a decrease in the average temperature at which heat is rejected from the system.
5 Air-Standard Assumptions In our study of gas power cycles, we assume that the working fluid is air, and the air undergoes a thermodynamic cycle even though the working fluid in the actual power system does not undergo a cycle. To simplify the analysis, we approximate the cycles with the following assumptions: • The air continuously circulates in a closed loop and always behaves as an ideal gas. • All the processes that make up the cycle are internally reversible. • The combustion process is replaced by a heat-addition process from an external source. • A heat rejection process that restores the working fluid to its initial state replaces the exhaust process.

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Chapter09 - Chapter 9 Gas Power Cycles Study Guide in...

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