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.     

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.     

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。     

地埋管换热器渗流传热特性的研究

通过建立地埋管管群分层渗流换热模型,模拟地下渗流流动方向为30°时不同分层下地埋管管群周围土壤的温度分布,通过模拟可知地下渗流的流速在一定的范围内有助于减弱地埋管管群周围土壤中的冷积聚,使地埋管管群周围土壤温度恢复快.同时引入单位井深换热量,将纯导热模型与分层渗流模型的单位井深换热量进行对比,得到分层渗流模型的换热效率...     

基于火积耗散理论的并联热管传热特性仿真分析

根据火积耗散理论,建立了热管内气液二相介质的火积耗散率数学表达式,并将其编译成C语言程序嵌入到Fluent计算中,模拟分析并联分离式重力热管的传热特性.研究结果表明：热管传热过程中的不可逆损失主要由温差传热和相变两部分组成,在热管冷凝段,相变火积耗散数集中分布在热管顶端,约占冷凝段总火积耗散数的97.7%;此外,热管内总火积耗散数与加热功率呈正相关,随着加热功率的增加,热管内总体的不可逆程度增大.     

翅片间距对两流路双排管蒸发器换热影响分析

翅片管蒸发器换热性能直接影响制冷空调机组能效,调整翅片间距是增大换热面积、改进换热效果的有效途径。文章建立了翅片管蒸发器的流体流动简化模型,利用EVAP-COND软件模拟了外掠气流中三角风速分布工况下,不同翅片间距对两流路双排管蒸发器换热性能的影响。结果表明:翅片间距越大,蒸发器的换热量越小,翅片间距为1.75、2.00、2.25、2.50 mm时的换热量分别比1.50 mm时减小了4.4%、9.02%、14.1%和18.2%;翅片间距越大,制冷剂流量就越小,蒸发器的总换热量也就越小;蒸发器中显热比随翅片间距的增大而降低,综合考虑空气阻力损失及换热,翅片间距为2.00 mm较为合理;不同翅片间距工况下,制冷剂在不同支路每根管内吸热对外掠空气降温影响与换热量一致。     

数据机房自然冷却用泵驱动回路热管换热机组性能实验研究

设计了一种数据中心自然冷却用泵驱动回路热管换热机组,并介绍该机组的构成和工作原理,针对研制的样机搭建实验系统并较全面地进行实验研究。结果表明：当室内外温差为10 ℃时,机组能效比COP为5.88;当室内外温差为18 ℃时,机组COP可达10.41;当系统质量流量在一定范围变化时,即工质气化率在2%～50%范围内,机组换热量没有显著变化,且换热量与室内外温差近似呈线性关系;同时系统阻力越大,蒸发器进出口温差越大,显热换热量所占比例也越大。     

地铁专用间接蒸发冷却器布水方式优化

为了解决常规间接蒸发冷却器由于表面水膜均匀性、完整性差而导致换热效率低的问题,提出了两侧旋转布水间接蒸发冷却器,进行了3种布置方式下的换热性能实验,运用正交实验对影响换热器性能的因素进行研究,研究表明:开孔正对气流方向时换热器换热性能最佳,且旋转布水装置存在最佳转速76 r/min,喷水量、空气流速、冷却水流量、冷却水进口温度的增加使换热器的换热量增大,喷水温度、空气温度的升高使换热器的换热量减少,其中冷却水进口温度的改变对换热性能的影响最为显著,温度由35℃上升到39℃时,换热量提高37.62％,单位面积换热量为1.14 kW,该换热器可安装于地下车站排风坑道内,可有效地解决地铁站冷却塔安装位置难题.     

运用夹点设计法对一实际换热网络的改造

随着能源价格的不断提升,化工企业的节能降耗成为紧迫必行的任务。通过在对一实际过程的换热网络结构分析的基础上提取热、冷工艺物流,在给定的最小传热温差（20℃）限制下运用夹点设计法合成具有最大热回收的新换热网络;采用断开热负荷回路、取消热负荷较小的换热器达到减少换热器个数、简化网络的目的;由于热负荷转移造成了某些换热器传热温差违反最小传热温差限制、甚至出现传热温差为负值（违反热力学第二定律）的情况,采用能量松弛法恢复这些换热器的传热温差至给定的最小传热温差。经过调优后换热器网络总换热设备个数与原实际网络的相同,但与原实际网络相比,热、冷公用工程均节省1220kW。     

竖直串联地埋管蓄热的传热分析

为解决大温差地下蓄热的传热分析及其工程应用,基于传统的并联地热换热器地下温度场分布,围绕地埋管大温差蓄热的地源热泵复合系统,针对竖直多级串联地埋管蓄热方法,建立了其物理与数学模型,进行了多级串联地埋管换热器传热分析,得了适宜工程应用的三级串联地埋管换热器目标函数的最优解.结果表明：三级串联地埋管换热器逐级换热量比的最优解为β1 =0.357800、β2 =0.332749、β3=0.309451;定期改变循环液流动方向,能够使各级地埋管换热效率趋于均匀,提高串联地埋管换热器的总换热强度;大温差蓄热应考虑地埋管换热器的承受能力,一般PE管材换热器承受温度不宜超过80℃.     

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.     

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。     

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)     

An area method for visualizing heat-transfer imperfection of a heat exchanger network in terms of temperature–heat-flow-rate diagrams

The identification of the imperfection originating from finite-temperature-difference heat transfer is an indispensable step for both the performance analysis and the better design of a heat exchanger network(HEN) with the aim of energy saving. This study develops a convenient area method for visualizing the heat-transfer imperfection of a HEN in terms of temperature–heat flow diagrams( T-Q diagrams) by combining the composite curves that have already been used in pinch analysis and the recently developed entransy analysis. It is shown that the area between the hot and cold composite curves and the hot and cold utility lines on a T-Q diagram is just equal to the total entransy dissipation rate during the multi-stream heat transfer process occurred in a HEN, and this area can be used to graphically represent the total heat-transfer imperfection of the HEN. The increase in heat recovery or decrease in energy requirements with decreasing the minimum temperature difference, ΔT_(min), of a HEN can then be attributed to a lower entransy dissipation rate, quantitatively represented by the decrease of the area between the composite curves and the utility lines. In addition, the differences between the T-Q diagram and the pre-existing energy level–enthalpy flow diagram(Ω-H diagram) in the roles of visualizing process imperfection and designing HENs are discussed.     

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.     

重力热管蒸汽发生器热力学分析

为挖掘热管蒸汽发生器回收余热的潜能,基于耗散理论,对不考虑黏性耗散影响和考虑黏性耗散影响的两种情况,分别推导出无量纲参数（耗散数和耗散数）的表达式,并结合工程案例,分析了热管蒸汽发生器烟气侧入口处迎风面流速、热管外径及热管蒸发段长度等参数对耗散数和耗散数的影响,并给出一些有关热管蒸汽发生器设计的建议.研究表明：当不考虑黏性耗散影响时,耗散数和耗散数的变化趋势与温度耗散的变化趋势相同;当考虑黏性耗散影响时,耗散数和耗散数的变化趋势与温度耗散和压力耗散乘积的变化趋势相同,且耗散数比耗散数对迎风面流速、热管外径和热管蒸发段长度等参数变化的响应更敏感.上述方法为热管余热回收装置的热力学性能研究提供了参考.     

Generalized constructal optimization for the secondary cooling process of slab continuous casting based on entransy theory

Based on constructal theory and entransy theory,a generalized constructal optimization of a solidification heat transfer process of slab continuous casting for a specified total water flow rate in the secondary cooling zone was carried out.A complex function was taken as the optimization objective to perform the casting.The complex function was composed of the functions of the entransy dissipation and surface temperature gradient of the slab.The optimal water distribution at the sections of the secondary cooling zone were obtained.The effects of the total water flow rate in the secondary cooling zone,casting speed,superheat and water distribution on the generalized constructal optimizations of the secondary cooling process were analyzed.The results show that on comparing the optimization results obtained based on the optimal water distributions of the 8 sections in the secondary cooling zone with those based on the initial ones,the complex function and the functions of the entransy dissipation and surface temperature gradient after optimization decreased by 43.25%,5.90%and 80.60%,respectively.The quality and energy storage of the slab had obviously improved in this case.The complex function,composed of the functions of the entransy dissipation and surface temperature gradient of the slab,was a compromise between the internal and surface temperature gradients of the slab.Essentially,it is also the compromise between energy storage and quality of the slab.The"generalized constructal optimization"based on the minimum complex function can provide an optimal alternative scheme from the point of view of improving energy storage and quality for the parameter design and dynamic operation of the solidification heat transfer process of slab continuous casting.     

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.