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The final temperature of the blocks with the engine running is less than in the absence of the engine because energy is removed from the overall system in the form of work. The entropy change of the combined system without the engine increases, but there is no change in entropy with the heat engine.

Other problems involving heat engines operating on the Carnot cycle include Nos. 4.1 -4.5, 4.11 and 4.12, and 4.15 and 4.16.

4.5 Thermodynamics of open systems

Before embarking on analyses of practical power cycles, understanding of open (or flow) systems is necessary. The laws of thermodynamics can be applied to fluids flowing through devices that change the properties of the fluid by exchanging heat and/or work with the surroundings. Examples of such devices include pumps, boilers, turbines, valves, nozzles (used for increasing fluid velocity) and orifices in pipes. Some of these devices are included in the simple steam cycle shown in Fig. 4.3. In this cycle, the components are connected in series with the working fluid circulating continuously through them. Each device in the cycle is subjected to a First-law analysis, and in some, application of the Second law provides additional information on their performance.

4.5.1 The First law for open (flow) systems

All of the devices mentioned above can be represented by the schematic open system shown in Fig. 4.9. This generalized device has a rigid casing forming a boundary m — area = Aj state: pj, Tj velocity = V

Fig. 4.9 Schematic of an open system area = Ae state: Pe, Te velocity = Ve

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Solar Panel Basics

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