Analysis and Optimization of Entropy Generation Rate within Solid Media


Student thesis: Doctoral Thesis

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  • Mohsen TORABI


Awarding Institution
Award date3 Nov 2015


It is well-known that the first law of thermodynamics, that is heat transfer, is silent regarding the quality of a thermal process. Heat transfer perspective has been strongly questioned after the introduction of entropy by Clausius in the middle of the 19th century. He stablished the second law of thermodynamics which is concerned about the quality of a thermal process. The second law of thermodynamics brings about an appreciable tool to investigate the sources of irreversibilities within a thermal system and, therefore, to minimize thermal disorders. Consequently, the second law of thermodynamics aims to model a thermal process with less exergy destruction which gives us more useful energy to produce work.
Although recently convective thermal systems have been studied within the framework of the second law of thermodynamics, pure conductive thermal structures or conductive-convective thermal systems have gained limited attention. This dissertation conducts comprehensive analysis regarding both first and second laws of thermodynamics on fundamental conductive geometries together with a specific conductive-convective thermal system. In almost all of the cases, either two methods have been used to tackle the equations or the provided data with the used method has been verified with available data within the literature. Since the main aim of this three years PhD program was about thermodynamics analyses, a special attention has been paid to the second law perspective. It has been tried to optimize the studied thermal system from the second law perspective, where the optimal system is available. If we can do these analyses and finally optimize a thermal system from the second thermodynamics law point of view, we are able to better use the available exergy of the system and therefore to provide more work.
The dissertation has been divided into six chapters. The first chapter covers the background and fundamental equations regarding entropy generation. The derived equations have been used in preceding chapters to be incorporated and modified in more complicated situations.
Chapter two considers entropy generation in walls with temperature-dependent internal heat generation, and convective-radiative boundary conditions. Both homogenous and functionally graded materials are thoroughly examined. Nonlinear ordinary differential equations are solved with a well-known approximate analytical technique which is known as differential transformation method. After validation with previous publications, effects of many thermophysical parameters on the temperature distribution, local and total entropy generation rates are investigated. Finally, it has been tried to minimize the total entropy generation rate by optimization of the cooling parameters.
Chapter three conducts an intensive analysis on a double-layer solid structure. Using two materials with different thermal conductivities and internal heat generations in each layer, entropy generation analysis is performed for the first time for completely conductive cylindrical multilayer structures. In this chapter, two methods have been used to solve the system of ordinary differential equations. Firstly, the general problem which consists of nonlinear ordinary differential equations has been solved with a combined analytical-numerical solution technique. For validation, a completely analytical solution has been exercised for a linear version of the problem. After confirmation, a meticulous study has been performed regarding the optimization of the thermal system form the second law of thermodynamics of point of view.
Thermal contact resistance is important consideration in meso and microscales. Chapter four considers thermal contact resistance and its effect in three fundamental multilayer structures, namely walls, cylinders and spheres. Both first and second laws analyses are performed. The solution procedure is similar to the previous chapter. After solution, the effect of thermal contact resistance will be examined and entropy generation minimization is also performed.
Chapter five performs analyses regarding entropy generation in conductive-convective media with nanoparticles. This chapter considers entropy generation in rotating circular geometries which have many applications in industries. Analytical solution for the velocity field is performed. The velocity field is used within the energy equation to produce analytical solution for the temperature field. Using heat transfer and entropy generation formulas, a comprehensive analysis regarding the temperature field, local and total entropy generation rates is conducted.
Finally, Chapter six gives conclusion and makes some important statements regarding entropy generation in solid media.