Application of entransy dissipation extremum principle in radiative heat transfer optimization

The concepts of entransy flux and entransy dissipation in radiative heat transfer were introduced based on the analogy with heat conduction and heat convection processes. Entransy will be partially dissipated during the radiative heat transfer processes due to the irreversibility. The extremum principle of entransy dissipation was developed for optimizing radiative heat transfer processes. This principle states that for a fixed boundary temperature the radiative heat transfer is optimized when the entransy dissipation is maximized, while for a fixed boundary heat flux the radiative heat transfer process is optimized when the entransy dissipation is minimized. Finally, examples for the application of the entransy dissipation extremum principle are presented. Supported by the National Basic Research Program of China (“973” Project) (Grant No. 2007CB206901)     

Entransy functions for steady heat transfer

In this paper, the entransy functions for steady heat transfer are summarized and discussed based on the variational theory and the entransy theory. The entransy functions for steady convective heat transfer are derived for the first time. In steady heat transfer processes, it is shown that the steady distributions of heat flux and temperature(radiative thermal potential) should make the corresponding entransy functions reach their minimum values when the temperature(radiative thermal potential) or the heat flux of the boundary is given. The extremum entransy dissipation principles and the minimum entransy-dissipation-based thermal resistance principles are compared with the entransy functions. It is shown that the entransy functions can describe a steady state,but cannot directly give a way to optimize heat transfer processes, while the extremum entransy dissipation principles and the minimum entransy-dissipation-based thermal resistance principles act in an opposite way.     

Heat transfer performance evaluation of one-stream heat exchangers and one-stream heat exchanger networks

One-stream heat exchangers and one-stream heat exchanger networks are widely used in engineering. In this paper, the heat transfer performance evaluation of one-stream heat exchangers and one-stream heat exchanger networks is analyzed with the concepts of entropy generation rate, entropy generation number, revised entropy generation number, entropy resistance, entransy dissipation rate, entransy dissipation number and generalized thermal resistance. For the analyzed one-stream heat exchangers, our numerical results show that the extremum value of the entransy dissipation rate and the minimum values of the entropy resistance and the generalized thermal resistance always lead to the largest heat transfer rate or the lowest temperature of the cooled object,while the minimum values of the other parameters do not always. For the analyzed one-stream heat exchanger networks, the minimizations of entransy dissipation rate, entransy dissipation number and generalized thermal resistance always correspond to the lowest average temperature of the cooled objects, while the minimizations of the other parameters do not. Therefore, only the extremum entransy dissipation principle and the minimum generalized thermal resistance principle are always applicable for the heat transfer performance evaluation of the systems in this paper, while the applicability of the other parameters is conditional.     

对流换热可控温差场分布原则

应用火积概念导出了对流换热过程的火积耗散表达式,进一步基于火积耗散极值原理讨论了换热器N股冷、热流在不同情况下参与换热的优化。研究表明,若参与换热的任何冷、热流之间的温差可以独立调控,在总换热量一定寻求火积耗散最小或在总火积耗散一定的条件下寻求换热量最大,则整个换热系统冷、热流之间的温差分布均匀时换热最优;若参与换热的任何冷、热流体之间的换热量或火积损耗可独立确定,则换热冷热流之间的温差分别保持各自的均匀温差分布时换热最优;若在任何冷、热流之间存在可能换热的情况下,无论是总换热量一定时寻求火积耗散最小,还是总火积耗散一定时寻求换热量最大,整个换热系统不同的冷热流换热的优化温差并不是同一个常数。     

Progress of entransy analysis on the air-conditioning system in buildings

It is of great importance to improve the energy performance of the air-conditioning system for building energy conversation. Entransy provides a novel perspective to investigate the losses existing in the air-conditioning system. The progress of entransy analysis in the air-conditioning system is comprehensively investigated in the present study. Firstly missions and characteristics of the air-conditioning system are analyzed with emphasis on heat or mass transfer process. It is found that reducing the temperature difference, i.e. reducing the entransy dissipation helps to improve the performance. Entransy dissipations and thermal resistances of typical transfer processes in the air-conditioning system are presented. Characteristics of sensible heat transfer process and coupled heat and mass transfer processes are researched in terms of entransy dissipation analysis. Reasons leading to entransy dissipation are also clarified with the help of unmatched coefficient ξ. Principles for reducing the entransy dissipation and constructing a high temperature cooling system are summarized on the basis of case studies in typical handling processes. It's recommended that reducing mixing process, improving match properties are main approaches to reduce the entransy dissipation. The present analysis is beneficial to casting light on the essence of the air-conditioning system and proposing novel approaches for performance optimization.     

Constructal multidisciplinary optimization of electromagnet based on entransy dissipation minimization

Based on entransy dissipation, the mean temperature difference of solenoid (electromagnet) with high thermal conductivity material inserted is deduced, which can be taken as the fundament for heat transfer optimization using the extremum principle of entransy dissipation. Then, the electromagnet working at steady state (constant magnetic field, constant heat generating rate per unit volume) is optimized for entransy dissipation minimization (i.e. mean temperature difference minimization) with and without vo...     

Distribution optimization of circulating water in air-cooled heat exchangers for a typical indirect dry cooling system on the basis of entransy dissipation

The flow and heat transfer of air-cooled heat exchangers play important roles in the performance of indirect dry cooling systems in power plants,so it is of benefit to the design and operation of a typical indirect dry cooling system to optimize the thermo-flow characteristics of air-cooled heat exchangers.The entransy dissipation method is applied to the performance optimization of air-cooled heat exchangers in this paper.Two irreversible heat transfer processes in air-cooled heat exchangers,the heat transfer between circulating water and cooling air and the mixing of circulating water,are taken into account and analyzed by means of the entransy dissipation method.The total entransy dissipation rate,which connects the geometrical parameters of air-cooled heat exchanger sectors and the heat capacity rates of the fluids to the heat flow rate in every sector,is obtained.Based on the mathematical relation and the conditional extremum method,an optimization equation group is derived,by which the air-cooled heat exchanger with known air-side parameters is optimized,showing that the entransy dissipation based optimization approach can contribute to the distribution optimization of circulating water in air-cooled heat exchangers of a typical indirect dry cooling system.     

An entransy dissipation-based optimization principle for solar power tower plants

The entransy theory,which can be used to optimize the heat transfer network of a solar power tower system(SPTS)and improve its energy efficiency,was introduced in this paper.Firstly,the irreversibility of the heat transfer processes in a SPTS was analyzed and the total entransy dissipation equation of a SPTS was derived.Then,two types of optimization problems(reducing the total circulating flow rate or the total heat-exchanging area)of a SPTS were solved with conditional extremum model based on the formulas of total entransy dissipation.Finally,the entransy dissipation-based optimization principle was applied to a simple SPTS without re-heater and a complex SPTS with a re-heater.The results showed that under the chosen calculation conditions the minimum total thermal conductance was 19306.03 W K?1 for a SPTS without re-heater when the total heat capacity rate of heat transfer fluid(HTF)was 3200 W K?1.The minimum total thermal conductance was about 7.9%lower than the value predicted based on the typical outlet temperature of a receiver.This meant that the total heat exchange area or initial investment could be effectively reduced under the prescribed total HTF circulating flow rate.We also studied the variation trends of the two optimized results including minimum total HTF heat capacity rate and minimum total thermal conductance.The minimum total HTF heat capacity rate decreased with the given total thermal conductance,the minimum total thermal conductance decreased first and then increased with the given total HTF heat capacity rate.We also found that for a SPTS with a re-heater,the mixing temperature and the mixing position of HTF had significant effects on the two types of optimization problems.     

Constructal optimization for geometry of cavity by taking entransy dissipation minimization as objective

The entransy dissipation extremum principle provides new warranty and criterion for optimization of heat transfer. For two cases (body with heat generation and body heated externally) of a solid conducting wall with an open cavity, a dimensionless equivalent thermal resistance based on entransy dissipation definition was taken as the optimization objective to optimize the model constructal geometry. Numerical results validated the necessity and feasibility of the presented method. Comparisons of the numerical results based on minimization of dimensionless maximum thermal resistance and minimization of dimensionless equivalent thermal resistance, respectively, showed that there was no obvious difference between the two results when the volume fraction Φ occupied by cavity was small, but the difference between the two results increased with the increases of Φ and the body aspect ratio H/L for any model. The optimal cavities for bodies heated externally were more slender than those for bodies with heat generation. Heat origin had obvious effect on the global performance of heat transfer. The entransy dissipation of body heated externally increased 2–3 times than that of body with heat generation, indicating that the global performance of heat transfer weakened. The method presented herein provides some guidelines for some relevant thermal design problems.     

Application of entransy dissipation based thermal resistance to design optimization of a novel finless evaporator

Entransy has been applied and studied in a broad range of heat transfer optimizations recently. Current study proposes the entransy of evaporators to conduct the optimization of heat exchangers in heating, ventilation, air conditioning and refrigeration systems. A novel finless bare tube heat exchanger was studied using a validated heat exchanger modeling tool. The capacity based optimization and entransy dissipation based thermal resistance were used and compared. The applicability of using entransy dissipation based thermal resistance in this type of heat exchanger optimization has been discussed. It has been demonstrated that minimizing entransy dissipation and maximizing capacity are equivalent to optimizing evaporators with fixed flow rates and different when optimizing evaporators with variable flow rates and the deviation is negligible when heat exchanger capacity is small (～1 kW) and more obvious as heat exchanger capacity increases. Thus entransy dissipation based thermal resistance could be used as an alternative optimization index to capacity for evaporators with fixed flow rate and small capacity evaporators with variable flow rates and should be used individually with capacity as an optimization index for evaporators with large capacity and variable flow rates.     

基于理论的相变储能换热器传热性能分析

在理论成功应用于常规换热器的基础上,将传递效率、耗散数及基于耗散的换热器热阻应用于相变储能换热器的传热性能分析中。定义广义耗散率并由此推导出相变储能换热器蓄热、放热及总过程的传递效率及其瞬时值。确定耗散数及基于耗散的换热器热阻计算中换热量的取法。选取一种相变储能装置作为分析对象,通过理论分析绘制各主要部分温度变化趋势,进一步简化得到硅油、水的出口温度表达式,作为算例分析基础。结果表明, 传递效率的应用范围最广,可用于计算相变储能换热器蓄热、放热及总过程的(瞬时)不可逆热损失,且评价结果与传热性能相符,瞬时传递效率随蓄热时间的增加先增大后不变再增大,随放热时间的增加先减小后不变再减小; 耗散数在蓄热过程和总过程中的评价结果与传递效率一致,瞬时耗散数随蓄热时间的增加先减小后不变再减小,然而在放热过程中的应用受限。基于耗散的换热器热阻的部分评价结果与实际不符,应用限制较大。蓄热过程及总过程中,当蓄热量、取热量与蓄、放热阶段时长同步变化时, 传递效率、耗散数与基于耗散的换热器热阻几乎无变化;当装置传热性能提高时, 传递效率增大, 耗散数减小,基于耗散的换热器热阻减小;放热过程中,设置参数的变化不影响装置传热性能, 传递效率基本无变化。     

Work entransy and its applications

The entransy theory has been applied to the analyses of heat-work conversion systems. The physical meaning and the applications of work entransy are analyzed and discussed in this paper. Work entransy, which is clarified to be a process dependent quantity, is not the entransy of work, but the system entransy change accompanying work transfer. The relationship between the work entransy and the output work is set up. When the application preconditions are satisfied, larger work entransy leads to larger output work. Entransy loss, which was proposed and applied to heat work conversion processes with irreversible heat transfer, is the net entransy flow into the system and the summation of work entransy and entransy dissipation. The application preconditions of entransy loss are also discussed.     

导热过程不同优化目标的优化准则

对于不同目的导热过程存在不同的优化目标。基于传递方程和火积传递方程,分别导出了以降低作功能力耗散为目标的导热过程优化准则和以降低传热能力耗散为目标的导热过程优化准则。以降低作功能力耗散为目标的导热过程优化准则表明,在总导热能力一定的条件下,相对温度梯度分布均匀导热最优;以降低传热能力耗散为目标的导热过程优化准则表明,在总导热能力一定的条件下,温度梯度分布均匀导热最优。     

能量传递系统强化的热力学判据

审视了传热和传质强化以及能量传递过程系统优化设计方法,指出了强化能量传递过程推动力与降低能量耗散的矛盾。基于场协同原理和火用传递过程动力学理论,导出了火用传递速率与能量损耗(火用损)及其与能量传递过程推动力的热力学关系,表明了强化能量传递过程推动力与降低能量耗散矛盾的统一性。进一步给出优化设计能量传递系统的热力学判据。依此分析了热传导和对流传热单元过程的优化设计方法。结果表明,可以通过协调不同驱动力之间的关系及其驱动力的大小,实现强化能量传递和降低能耗之间的权衡。该热力学判据具有普适性,场协同原理和最小熵产原理为该判据特例。     

Discussion on the extensions of the entransy theory

In this paper, the preconditions for the extensions of the entransy theory are summarized and discussed. As the physical meaning of entransy is the “potential energy” of thermal energy, the concepts and analyses method of the entransy theory can be extended based on the viewpoint of potential energy. The extension to microeconomics is taken as an example to show how the entransy theory is extended. The concept of economic entransy is defined based on the analogy analysis between heat transfer and microeconomics, and the indirect tax is found to be the “potential energy” of goods or services that the government takes from the market. With the extension, a new viewpoint is introduced to understand and analyze the microeconomic system. If the irreversibility in nature makes the potential energy in one system can never increase automatically, the entransy decrease principle and the corresponding equilibrium criteria can further be extended. Furthermore, if the mathematical expressions of the governing equations of the analyzed physical or social phenomena in the system are the same as those in heat transfer, the principle of least action and the optimization principles in the entransy theory can be extended. More similarities between heat transfer and the other phenomena result in more extensions of the entransy theory.     

火积耗散理论在平行流热管换热器性能评价中的应用

火积耗散作为传热过程可逆性判断依据,可应用于换热器优化. 分析探究了热管换热器的温差、充液率、倾角及迎面风速对火积耗散的影响,利用焓差实验室进行平行流热管换热器性能试验. 随温差增大,火积耗散增大. 随充液率增大,火积耗散先增大后减小,温差越大,火积耗散变化幅度随充液率增大而增大. 增加倾角,火积耗散比未倾斜时小. 随迎面风速增大,火积耗散减小,且减小幅度随温差增大而增大. 相同传热量,火积耗散值最小时,为系统最优工况.     

Analyses of thermodynamic performance for the endoreversible Otto cycle with the concepts of entropy generation and entransy

In this paper,power the the and endoreversible the Otto cycle is analyzed with the entropy generation minimization objectives,and the the entransy theory.of The output power,the heat-work conversion efficiency are taken as the optimization rate,and relationships the output heat-work conversion efficiency,entransy the entropy generation and the entropy generation rate numbers,the work entransy are loss rate,The entransy loss of coefficient,the dissipation rate the entransy variation associated with discussed.applicability entropy the entropy generation minimization and the entransy theory while to the analyses is also analyzed.It is found do that smaller generation rate does not always lead to larger output our power,smaller entropy entransy generation loss numbers and not always lead to larger heat-work conversion efficiency,either.larger the In calculations,power,both larger larger rate larger entransy variation heat-work rate associated with work correspond also to that output while entransy is loss coefficient suitable results the in larger conversion developed efficiency.It is found concept of entransy dissipation not always for analyses because it was for heat transfer.     

换热器新评价标准——火积耗散均匀性系数

定义了火积耗散均匀性系数,选用倾角β=30°～70°的复合人字形板式换热器为研究对象,利用三维CFD数值模拟软件ANSYS-CFX模拟分析换热器的火积耗散率、火用损失率以及系统火积耗散均匀性.结果表明:相同流量时,β=30°的复合人字形板式换热器的火用损失率最少;相同换热量时,β=30°的复合人字形板式换热器的火积耗散率最小;相同传热单元数与有效度时,β=30°的复合人字形板式换热器的火积耗散率最小且系统火积耗散均匀性最优.     

热声热机换热器性能的分析

热声换热器热量传递的速率与效率直接影响着热声热机的性能. 　　耗散理论能更好地揭示换热器的传热优化特性,在热声换热器研究中引入　　耗散理论,针对顺流和逆流两种情况,计算了热声换热器的　　耗散热阻,并和最小熵产原理的结果进行了对比分析. 结果表明,在一定条件下,顺流比逆流情况下的不可逆损失要大;当换热器低温端流体的热容量小于高温端流体热容量时不可逆损失较小,结果最优.     

基于“火积”的喷淋室内气-水热湿交换性能分析

为研究空气饱和程度对换热效果的影响,采用数值模拟方法,计算气体沿等温线、等焓线、等含湿量线向饱和线靠近的过程中全热交换量、"火积"耗散、"火积"耗散热阻、全热交换效率的变化情况。研究发现:气体沿着这三条线逐渐向饱和线靠近时其"火积"耗散热阻始终在减小,全热交换效率在增大。