Heat engines, thermodynamic laws applied

In order to determine the distributions of pressure, velocity, and temperature the principles of conservation of mass, conservation of momentum (Newton s Law) and conservation of energy (first law of Thermodynamics) are applied. These conservation principles represent empirical models of the behavior of the physical world. They do not, of course, always apply, e.g., there can be a conversion of mass into energy in some circumstances, but they are adequate for the analysis of the vast majority of engineering problems. These conservation principles lead to the so-called Continuity, Navier-Stokes and Energy equations respectively. These equations involve, beside the basic variables mentioned above, certain fluid properties, e.g., density, p viscosity, p conductivity, k and specific heat, cp. Therefore, to obtain the solution to the equations, the relations between these properties and the pressure and temperature have to be known. (Non-Newtonian fluids in which p depends on the velocity field are not considered here.) As discussed in the previous chapter, there are, however, many practical problems in which the variation of these properties across the flow field can be ignored, i.e., in which the fluid properties can be assumed to be constant in obtaining fire solution. Such solutions are termed constant... 

There were some contradictions in Carnot s work—a result of his reliance on the caloric theory—that were subsequently cleared up by Clausius. Clausius accepted Carnot s proposition that some heat must be thrown away when converting heat to work as a law of nature, something that cannot be proved or derived from something else, but as far as we have ever been able to tell describes the way the world works. He called it the second law of thermodynamics and then sought to recast it in a different, more general, form that did not apply to heat engines alone. He showed that an equivalent statement of the... 

Thermodynamic analysis of power plants seeks to characterize efficiency and identify sources of losses. First law analysis assesses performance based on energy balance equations, while second law analysis uses exergy balances and looks for locations of exergy destruction. In this section, analysis methods are developed to apply thermodynamic balance equations to analyze heat engines and power plant components. Results are summarized in Appendix B of this chapter and detailed examples are provided in Section 23.6. 

The amount of heat addition (Qh), heat rejection (QJ, and turbine work output (Wnet) in a heat engine cycle is estimated by applying the first law of thermodynamics. The thermal efficiency of a real heat engine cycle is less than the reversible Carnot cycle efficiency given by Equation 4.3. 

This text is an introduction to engineering heat transfer. The philosophy of the text is based on the development of an inductive approach, earlier introduced by the author ( Conduction Heat Transfer, 1966), to the formulation and solntion of applied problems. Since the greatest difficulty a student faces is how to formulate rather than how to solve a problem, the formulation of problems is stressed from the beginning and throughout the entire text. This is done by first noting that heat transfer rests on but goes beyond thermodynamics, and taking as a basis the well-known form of the first law of thermodynamics for a system,... 

As useful as the first law of thermodynamics is, we will see that it is not sufficient to answer some questions. First, we will consider some of the questions that the first law cannot address. We will then introduce efficiency and see how it applies to engines, which are devices that convert heat into work. The second law of thermodynamics can be expressed in terms of efficiency, so we will introduce the second law at this point. 

The student needs to understand that instead of skimming the surface of thermodynamics in a short one-semester treatment we have plowed more deeply into just a few examples of the first law in this chapter. The idea is that we have selected what we think are important illustrations with sufficient detail to prepare an interested student to elect a second semester and yet provide a good foundation for students who stop at just one semester of study and need to apply principles of thermodynamics to other disciplines. We have shown details for partial derivatives and ways to find heats of reactions, but as chemical engineers and chemistry graduate students will tell you, there is a lot more to thermodynamics, except now you have a good preparation for further study. 

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