污垢对管内层流换热性能影响的热力学分析和评价

采用热力学第二定律，分别在恒壁温和恒热流两种典型工况下分析了污垢对管内层流换热性能的影响；引入单位传热量的熵增率对污垢管道的热力学性能进行了评价；讨论了管内流体雷诺数(无污垢时)、量纲为1的入口换热温差、量纲为1的热流密度和污垢层厚度等参数对单位传热量熵增率的影响；并把结果和紊流时的对应工况进行了比较。结果可为工程上换热设备的优化设计提供依据。     

传热强化管管内熵产分析与热力性能评价

根据热力学第二定律,对传热强化管管内热力过程进行熵产分析,建立了基于流动与传热过程熵增原理的管内传热熵产方程,导出恒热流和恒壁温2种常见工况下的无因次熵产数表达式.在不同雷诺数和进口温差条件下,对2种螺旋槽管和光管进行恒壁温工况的熵产分析和热力性能评价,分析了传热和流动摩阻引起的熵产变化规律及2种不可逆损失占总熵产的份额.结果表明,熵产分析可用于评价传热强化管的综合热力性能,确定合理的运行工况、结构参数及强化换热形式,为强化管的应用评估及优化设计提供参考.     

太阳能集热器的能质分析

依据热力学第一、二定律，对平板型太阳能集热器的传热和流动过程进行了能质综合分析，提出了一项评价太阳能集热器热力学性能的指标——单位有用收益的Yong损，讨论了雷诺数、无因次入口换热温差和无因次热流密度等参数对太阳能集热器热力学性能的影响。     

管内强化换热元件综合热力性能分析及评价

针对强化换热元件的综合热力性能分析和评价问题,基于过程熵产分析的统一分析方法建立了管内综合热力性能分析方程,并以管内强化传热介绍了其应用。结果表明：该方法物理意义明确,简单易用,依据其不仅可通过参数分析获得各种方式的能量综合利用效果,而且还可依据其确定合理的流动工况参数、结构参数和合理的强化形式。图7参6     

辐射与导热复合换热熵产率传统公式质疑

以冬季太阳照射下建筑物窗户玻璃内部辐射与导热复合换热过程为例,对传统的辐射换热局部熵产率计算公式进行了质疑.通过对窗户玻璃内部复合换热过程局部熵产率的分析,结果表明传统的辐射换热局部熵产率公式与热力学第二定律相矛盾,因而是错误的.其根源在于传统的辐射换热局部熵产率公式认为辐射热流密度由局部温度梯度所驱动.事实上,由于辐射传递过程的容积效应,辐射热流与所考虑的半透明介质系统整体温度分布有关.     

考虑污垢时换热器热力学性能的评价

在分析污垢对换热器传热性能影响的基础上,在考虑污垢时采用Ｙｏｎｇ损率这一指标对换热器的热力学性能进行了评价,讨论了传热数和冷热流体热容量率比对其性能的影响,并把结果与不考虑污垢时的情况进行了比较,得到了一些有益的结论。     

管内对流换热过程的热优化分析

对管内对流换热过程的温差换热和粘性摩擦引起的熵产进行了分析和优化计算 ,对换热设备和传热技术的设计和优化组织具有一定的指导意义。     

管内经对流换热的热力经济性分析

鉴于管内换热和阻力同步增长的事实,依据Ｗｅｂｂ指标对管内强化对流换热方式下传热和流阻的综合热力性能进行了推导,得到了热力性能指标Ｑ／Ｑｓ、Ｐ／Ｐｓ和Ｆ／Ｆ与管内对流换热努氏数Ｎｕ和管内阻力系数λ之间的函数关系式。在此基础上,对螺旋槽管强化管内换热的热力性能进行了分析。     

广义不可逆布雷森循环生态学性能系数分析

以反映热机循环输出和损失之比的生态学性能系数(ECOP)为目标,用有限时间热力学理论,对广义不可逆布雷森循环进行性能分析。导出了在牛顿传热律下广义不可逆布雷森循环无因次功率、效率、无因次熵产率、无因次生态学函数和生态学性能系数的解析式;并通过数值算例得到它们之间的关系。结果表明,内不可逆性对该热机各种性能参数产生一定的影响,以ECOP为目标优化具有效率较高,熵产率较低的优势。     

螺旋槽管传热与污垢性能的实验研究

进行了螺旋槽管的传热性能实验，得出了螺旋槽管管内强制对流换热关联式。以硬度800mg／L的人工硬水作为工质，在0．24m／s，管外水浴60℃和相同管内工质入口温度的条件下，进行了螺旋槽管及其对应光管管内污垢的对比实验。结果表明，螺旋槽管有较好的传热性能，但阻垢性能却弱于光管。     

换热表面结垢对U型管蒸汽发生器传热性能的影响分析

U型管蒸汽发生器的壳侧沉积了来自二回路系统中的腐蚀产物,结垢导致热量聚积在金属换热管上,容易造成垢下热点腐蚀,危害设备安全。为了明确结垢对蒸汽发生器传热性能的影响,本研究基于仿真平台APROS建立了U型管蒸汽发生器的分布式模型,并根据已公开论文中的数据进行了模型准确性验证;推导了污垢热阻与表面换热系数之间的关系式,分析了不同结垢厚度、位置对U型管蒸汽发生器换热区域的传热管壁面温度、流体温度、传热系数、热流密度等的影响程度。研究结果表明：随着结垢程度的加剧,蒸汽发生器的换热效率不断降低,出口蒸汽品质不断下降;结垢对沸腾段换热效率的影响比对过冷段换热效率的影响更大。     

Heat transfer and entropy analysis for a transparent gas flowing in a tube

Heat transfer by forced convection and radiation in tubes is very important for high‐temperature heat exchangers, which find wide applications in power plants. In addition, the entropy analysis gives insight into the qualitative measure of the heat transfer processes. Consequently, in the present study, forced convection and radiation heat transfer in flow through a tube is considered. The wall and fluid sides temperature rise are predicted for different tube lengths. The entropy analysis is carried out and the influence of tube length and heat transfer coefficient on the volumetric entropy generation are examined. It is found that the wall temperature and the volumetric entropy generation increases as the tube length increases. The point of maximum volumetric entropy generation moves close to the tube inlet as the tube length increases. In addition, the maximum volumetric entropy generation becomes independent of tube length for high heat transfer coefficients. Copyright © 1999 John Wiley & Sons, Ltd.     

A comparative study of viscous dissipation effect on entropy generation in single-phase liquid flow in microchannels

The influence of viscous dissipation on entropy generation in fully developed forced convection for single-phase liquid flow in a circular microchannel under imposed uniform wall heat flux has been studied. In the first-law analysis, closed form solutions of the radial temperature profiles for the models with and without viscous dissipation term in the energy equation are obtained. In the second-law analysis, for different Brinkman number and dimensionless heat flux, the variations of dimensionless entropy generation and Bejan number as a function of the radial distance are investigated. The two models are compared by analyzing their relative deviations in dimensionless entropy generation and Bejan number. Comparisons are also performed for average dimensionless entropy generation and average Bejan number. Contribution of heat transfer irreversibility and fluid friction irreversibility to the deviations is analyzed and discussed. It is found that, under certain conditions, the effect of viscous dissipation on entropy generation in microchannel is significant and should not be neglected.     

螺旋翅片管束换热过程的熵产分析

对不同翅片间距Sf、管束横向节距St和管束纵向节距Sl的9组螺旋翅片管束的换热和流动过程进行了试验研究.分析了换热过程的熵产,研究了雷诺数(RP)、翅片间距、管束横向节距和管束纵向节距对管束换热熵产数NsH、流动熵产数NsF和总熵产数Ns的影响.结果表明:对不同布置方式的管束,随着Re的增加,NsH迅速减小,NsF逐渐增加,Ns先减小后增加;翅片间距对NsH影响较小,在高Re下,翅片间距增大时,NsF和Ns均明显降低;横向节距对NsH几乎没影响,但随着横向节距的增加,NsF和Ns均明显降低;管束纵向节距对NsH、NsF和Ns的影响都很小.     

空气含量和水侧污垢对凝汽器传热系数影响的数值分析

以N12180凝汽器为例,采用自行编制的程序数值计算了空气浓度和污垢对凝汽器汽侧传热系数、冷却管壁导热系数以及平均传热系数的影响,分析了空气与污垢对传热过程的影响机理。结果表明：凝汽器进口空气浓度从0增加到0．01％时,凝汽器平均传热系数降低30％;0．5mm的污垢厚度将使冷却管壁的导热系数降低98％,凝汽器平均传热系数降低85％。     

Optimal Porosity in an Air Heater Conduit Filled with a Porous Matrix

In the present study, flow and forced convective heat transfer in an air heater conduit filled with a porous matrix with a uniform constant solar heat flux has been investigated analytically, based on minimal entropy generation principle. While trying to decrease entropy generation due to heat transfer, pressure loss entropy generation increases, which indicates that an optimal porosity value exists. The influence of Reynolds number, fluid properties, constant uniform heat flux, flow, and geometry of the system on the optimum matrix porosity has been investigated. It was revealed that optimum matrix porosity values increase as Reynolds number increases. In the range of the present study, a correlation predicting optimal matrix porosity was proposed using least squares analysis.     

Entropy generation analysis of nanofluid flow in a circular tube subjected to constant wall temperature

Due to their improved thermal conductivity, nanofluids have the potential to be used as heat transfer fluids in thermal systems. However adding particles into nanofluids will increase the viscosity of the fluid flow. This demonstrates that there is a trade-off between heat transfer enhancement and viscosity. It might not be ideal to achieve a heat transfer enhancement along with a relatively high pumping power. This study presents an analytical investigation on the entropy generation of a nanofluid flow through a circular tube with a constant wall temperature. Nanofluid thermo-physical properties are obtained from literature or calculated from suitable correlations. The present study focuses on water based alumina and titanium dioxide nanofluids. Outcome of the analysis shows that titanium dioxide nanofluids offer lower total dimensionless entropy generation compared to that of alumina nanofluids. Addition of 4% titanium dioxide nanoparticles reduces the total dimensionless entropy generation by 9.7% as compared to only 6.4% reduction observed when using alumina. It is also noted that dimension configurations of the circular tube play a significant role in determining the entropy generation.     

碳钢_水热虹隙管内部强化传热机理研究

在微层蒸发模型的理论基础上,对热虹吸管内部设置分流管结构强化沸腾传热者机理分析。建立分流管强化热哐吸管内部沸腾传热模型,同时选择七种不同的分流管开孔结构,与光滑管进行了对比实验研究,寻得最佳分流管结构,并综合大量实验数据建立强化沸腾传热准数方程式。