This chapter provides a specific study of the performance of thermal systems, principally heat exchangers, which are applied in several industrial applications such as chemical industry, energetic industry, industrial lasers, and so on. These thermodynamics systems were critical in transferring heat from a higher to a lower temperature fluid. They have been used for several years and are available currently for various designs. Thermodynamic properties influence the heat transfer and the performance of heat exchangers. Therefore, it is important during the design of heat exchangers to select primary the accurate operating conditions in terms of thermodynamics to provide a minimum amount of entropy generation in the system. In this study, the concept of entropy is used to analyze heat transfer processes from the thermodynamic viewpoint through the second law of thermodynamics. To assess heat exchanger performance, investigations are given for entropy generation, entropy generation number, and efficiency. These studies offer a new way to obtain well-designed heat exchangers.
Top1. Introduction
In the major industry applications, heat exchangers are extensively used such as chemical, mechanical and gas industries. They are planned to transfer energy between two or more fluids. Among the various devices, selecting the accurate heat exchanger is a complex task. In fact, the design and the choice of a heat exchanger is very important, since many elements have to be considered, such as the pressure drop, the rate of heat transfer, the efficiency…etc. In any thermal system, the enhancement of system efficiency is related to reducing losses during the process by analyzing irreversibility. As a result, the system performance is based on an investigation of the concept of the second law of thermodynamics. This concept had ample considerations in the previous investigations and also until today. It is applied in several research based on irreversibility analysis to give optimum conditions to design thermal systems and particularly balanced counter-flow heat exchangers.
(Bejan,1977a, 1977b) presented an extended study of the irreversibility process due to heat transfer and viscous effects for various flow configurations. In another researcher (Bejan,1980) he analyzed in details the irreversibility process, throw entropy generation concept, using the second law of thermodynamics and accounting for only heat transfer process. Particularly, he evaluated the entropy generation rate in a balanced counter-flow heat exchanger with zero pressure drop. More investigations of entropy generation in a counter-flow heat exchanger are found in the work of Bejan (1977c).
Moreover,(Sekulic,1986a) used irreversibility concept based on the second law of thermodynamic in a co-current and counter-current heat exchangers to analyze the optimization condition through the choice of minimum entropy generation. He also presented (Sekulic,1986b)the concept of entropy generation to evaluate the quality of heat transfer process in heat exchanger analysis. He used the quality called “Heat Exchange Reversibility Norm” (HERN), which measure the value of energy transformation of heat exchangers.
Many others researchers investigated the entropy generation on various type of heat exchangers and are found in the literature (Ordóñez (2000), Kolenda (2004), Guo (2010), Basak (2012), and Li (2013).
However, the physical performance of heat exchangers are optimized to reach maximum efficiency by optimizing many amounts of physical parameters for every application.
The use of thermodynamic analysis, for systems that involves the first and second laws of thermodynamics, allows to measure and to specify the degree of the performed processes.
On the foundation of the two laws of thermodynamics, the concept of entropy is applied to analyze the performance of heat exchangers. This study mainly aims to expose the different investigations on entropy generation to improve the efficiency of heat transfer in such devices.
Depending on the direction of the flows, two different operating modes are distinguished. Parallel and counter-flow heat exchangers. In parallel-flow (or co-current) the heating and the heated fluid flow in the same direction. In counter-flow (or counter-current) they flow in opposite directions. Therefore, the counter-flow mode gives more efficiency because the heat is distributed more uniformly across the heat exchanger and lets to extract the maximum amount of heat. This particularity allows the type of arrangement to be used more often. The degree of efficiency gained by using a counter-flow system depends on several parameters especially the flow rates and temperatures.
In this chapter, new researches based on thermodynamic optimization in terms of overall exchange coefficient as well as effectiveness and entropy generation in heat exchangers, are provided to give a comprehensive issue for better performance systems.
The current review is organized as follows: mechanisms of entropy transfer are given in section 2, where thermodynamic interaction between the system and its surrounding is treated with more details. In this section, entropy balance and entropy generation equations for various cases are presented. Application to heat exchanger counter-flow is given in section 3. In subsection 3.1 system of heat exchanger is described. Practical definitions for calculations of effectiveness and entropy generation rate are given in subsection 3.2 and 3.3 respectively. In the last part of section 3, we proposed the entropy generation number to evaluate the irreversibility process in heat exchanger. This chapter ended with diverse results of the very recent investigates that are summarized in section 4. In subsection 4.1, new results for heat exchangers using conventional fluids are exposed while results with nanofluids are presented in subsection 4.2.A conclusion is given in Section 5.