Entransy dissipation number and its application to heat exchanger performance evaluation

In general, thermal processes can be classified into two categories: heat-work conversion processes and heat transfer processes. Correspondingly, the optimization of thermal processes has to have two different criteria: the well known entropy generation minimization method and the recently proposed entransy dissipation maximization method. This study analyzes the thermal issues in a heat exchanger group, and optimizes the unit arrangements under different constraints based on a suitable optimization criterion. The result indicates that the principle of minimum entropy generation rate is valid for optimizing heat exchangers in a thermodynamic cycle with given boundary temperatures. In contrast, the entransy dissipation maximization is more suitable in heat exchanger optimizations involving only heat transfer processes. Furthermore, the entropy generation rate induced by dumping used streams into ambient surroundings has to be taken into account, except for that originating from the hot and cold-ends of heat exchangers, when using the entropy generation minimization to optimize heat exchangers undergoing a thermodynamic cycle. Heat exchangers, one of the most important devices in thermal engineering, are being used in more than 80% of energy utilization systems. Improving their effectiveness has often been regarded as the key issue in energy conservation. Consequentially, thermal engineers have developed a large number of passive and active technologies to enhance thermal performance by adding extended or rough surfaces and various inserts to enlarge the exchange surfaces or by introducing surfaces or fluid vibrations and external electric or magnetic field to expedite the process [1][2][3]. Although all of these ideas have been more or less successful in reducing energy consumption, the fundamental physics involved is still not clear. One issue is the relationship between heat exchanger effectiveness and heat *Corresponding author (email: chenqun@tsinghua.edu.cn) transfer irreversibility, noting that heat transfer is an irreversible process [4]. The crucial problem is in understanding why heat exchangers with different flow arrangements lead to diverse heat transfer performances under the same given conditions. Using the entropy generation as a measure of irreversibility for any irreversible process, Bejan [5,6] introduced the concept of irreversibility as being due to finite temperature difference as well as fluid friction in heat transfer processes and optimized the regenerative heat exchanger for a Brayton cycle heat engine based on the criterion of minimum entropy generation. Thereafter several researchers, e.g. Poulikakos [7]

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confidence: 99%

A comparison of optimization theories for energy conservation in heat exchanger groups

Chen

Wang

et al. 2011

“…The first approach can reduce costs, but possibly at the expense of sacrificing heat exchanger performance [11]. As representative of the second approach, entropy generation minimization suffers from so-called "entropy generation paradox" [8,12].…”

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confidence: 99%

“…Chen et al applied entransy dissipation theory to the volume-to-point conduction problem [15]. Guo et al defined an entransy dissipation number to evaluate heat exchanger performance that not only avoids the "entropy generation paradox" resulting from the entropy generation number, but can also characterize the overall performance of heat exchangers [12]. Xu et al [16] developed an expression of the entransy dissipation induced by flow friction under finite pressure drop in a heat exchanger.…”

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confidence: 99%

“…The present work, based on expressions of entransy dissipation from heat conduction under finite temperature differences and flow friction under finite pressure drops [14,16], and on the dimensionless method proposed by Guo et al [12], defines an overall entransy dissipation numbers. The minimum overall entransy dissipation number is then taken as an objective function.…”

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confidence: 99%

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Entransy dissipation minimization for optimization of heat exchanger design

Guo

et al. 2011

Self Cite

In this paper, by taking the water-water balanced counterflow heat exchanger as an example, the entransy dissipation theory is applied to optimizing the design of heat exchangers. Under certain conditions, the optimal duct aspect ratio is determined analytically. When the heat transfer area or the duct volume is fixed, analytical expressions of the optimal mass velocity and the minimal entransy dissipation rate are obtained. These results show that to reduce the irreversible dissipation in heat exchangers, the heat exchange area should be enlarged as much as possible, while the mass velocity should be reduced as low as possible. entransy, heat exchanger, optimization design Citation:Li X F, Guo J F, Xu M T, et al. Entransy dissipation minimization for optimization of heat exchanger design.

“…Accordingly, the temperature of the hot fluid, local entransy dissipation rate and temperature of the cold fluid in the heat exchanger can be determined. For the sake of comparison, the entransy dissipation number is adopted in the present work, and can be written as [20]:…”

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confidence: 99%

The entransy dissipation minimization principle under given heat duty and heat transfer area conditions

Guo

Cheng

2010

Self Cite

Under given heat duty and heat transfer area conditions, the equipartition of the entransy dissipation (EoED) principle, the equipartition of the temperature difference (EoTD) principle, and the equipartition of the heat flux (EoHF) principle are applied to the optimization design of a heat exchanger with a variable heat transfer coefficient. The results show that the difference between the results obtained using the EoED and EoTD principles is very small, far smaller than that between the results obtained using the EoED and EoHF principles. The correct entransy dissipation minimization principle is chosen to optimize the parameters in the hot and cold fluids in a two-fluid heat exchanger, under given heat duty and heat transfer area conditions. The results indicate that the proper choice of the two alternative fluids has an important role in the successful application of the entransy dissipation minimization principle. The fluid that could improve the total heat transfer coefficient should be chosen, or the fluid that makes the temperature profiles of the hot and cold fluids parallel and decreases the temperature difference between the hot and cold fluids after optimization simultaneously, could be the proper one. entransy dissipation, heat exchanger, equipartition of entransy dissipation, equipartition of temperature difference, equipartition of heat flux Citation:Guo J F, Xu M T, Cheng L. The entransy dissipation minimization principle under given heat duty and heat transfer area conditions. Chinese Sci Bull, 2011Bull, , 56: 2071Bull, −2076Bull, , doi: 10.1007 With the rapidly increasing price of petroleum and coal, the efficient use of energy resources has become one of the most effective ways of reducing demand on those resources. The heat exchanger as an energy utilization device is widely used in power engineering, petroleum refineries, and chemical and food industries. Hence, reducing unnecessary energy dissipation in a heat exchanger to improve its performance is an important goal. The heat transfer occurring in the heat exchange process usually involves heat conduction under a finite temperature difference, and with fluid friction and mixing. These are the typical irreversible non-equilibrium thermodynamic processes. In recent decades, the application of the second law of thermodynamics in heat exchangers has attracted a lot of attention [1].