Heat Transfer and Flow Characteristics of Water Flow with Air Injection in Polyethylene Tube Placed over Metal Rods: Influence of Pitch Length and Tube Width

A polyethylene tube can be used as a heat exchanger for a low-running-cost?temperature control system. In this system, the flow of temperature-controlled?water in the tube is used as the heat source, and the tube is placed on the ceiling of a temperature-controlled space using a metal net. Owing to this structure, the tube is deformed by its weight. This deformation has a significant influence on heat transfer and flow characteristics. Therefore, an air injection method, in which air and water are injected simultaneously into the tube, is developed for preventing the deformation of the tube. In this study, bedding metal rods were used instead of a metal net. The influence of the pitch length of the metal rods (5 - 15 cm) and the width of the polyethylene tube 15, 20, 25, 30, and 35 cm was examined experimentally. The length of the polyethylene tube was 178 cm. The air flow rate was 9.5 × 10-5 m3/s. The water flow rates were 60, 80, 100, 120, and 140 mL/min. Results show that the thermal response improved because of air injection. In particular, the temperature at steady state increased, and steady state was attained approximately 1.2 - 3 times faster with air injection than without air injection. The optimum pitch length of the metal rods and the range of the optimum width of the polyethylene tube were 8 cm and 20 - 25 cm, respectively, with and without air injection.

Flow Characteristics of Grains in a Conical Silo with a Central Decompression Tube Based on Experiments and DEM Simulations

Grains are widely present in industrial productions and processing,and are stored in silos.In the silo,auxiliary structures are added to achieve efficient production.However,little effort has been devoted to the influence of the internal structure of the silo on the granular flow.In this work,a silo with a central decompression tube is studied through experimental measurements and discrete element methods.Then,the influences of the central decompression tube on the flow behavior of grains and wall pressure are analyzed.Results show that the grains are in mass flow in the silo without a central decompression tube,while the grains are in funnel flow in the silo with a central decompression tube.Moreover,regardless of whether there is a central decompression tube in the silo,the maximum pressure appears at the top of the conical silo.In the lower part of the silo,the wall pressure of the silo with a central decompression tube is lower than that of the silo without a central decompression tube.Therefore,a silo with a central decompression tube is more conducive to grain storage and discharge than a silo without a central decompression tube.

Thermo-flow characteristics and air flow field leading of the air-cooled condenser cell in a power plant

Special A-frame geometry of the air-cooled condenser cell and the complicated flow field at the exit of the axial flow fan bring on the air mal-distribution on the surface of the finned tube bundles and the deteriorated thermo-flow performances of a condenser cell. It is of benefit to the design and operation optimization of the direct dry cooling system in a power plant to investigate the thermo-flow characteristics of the condenser cell and propose the flow leading measures of cooling air. On the basis of the representative configuration of the air-cooled condenser cell in a 600 MW direct dry cooling power plant, the computa- tional models of the air side fluid and heat flows are built, in which the actual fan blade geometric details are considered. Various flow field leading ways of cooling air are presented and the thermo-flow characteristics in the A-frame condenser cell and through the finned tube bundles are compared. Results show that the flow field leading measures can result in the increased volumetric flow rate and heat rejection, thus bringing on the improved performance of the condenser cell. The improvement of thermo-flow oerformances depends upon the geometric details of the flow guiding device.

Sand deposit-detecting method and its application in model test of sand flow

Against the background of the sand-flow foundation treatment engineering of Guangzhou Zhoutouzui variable cross-section immersed tunnel, a kind of sand deposit-detecting method was devised on the basis of full-scale model test of sand-flow method. The real-time data of sand-deposit height and radius were obtained by the self-developed sand-deposit detectors. The test results show that the detecting method is simple and has high precision. In the use of sand-flow method, the sand-carrying capability of fluid is limited, and sand particles are all transported to the sand-deposit periphery through crater, gap and chutes after the sand deposit formed. The diffusion range of the particles outside the sand-deposit does not exceed 2.0 m. Severe sorting of sand particles is not observed because of the unique oblique-layered depositing process. The temporal and spatial distributions of gap and chutes directly affect the sand-deposit expansion, and the expansion trend of the average sand-deposit radius accords with quadratic time-history curve.

Condensation heat transfer characteristics of vapor flow in vertical small-diameter tube with variable wall temperature

To explore the condensation characteristics of vapor flow inside vertical small-diameter tubes, the classical Nusselt theory is revised and an analytical model with variable tube wall temperature is established by considering the effect of surface tension exerted by condensate film bending as well as the effect of shear stress on vapor-liquid interface. The effects of various factors including tube wall temperature and gravityon flow condensation in small-diameter tubes are analyzed theoretically to show the heat transfer characteristics. Comparison with the experimental data indicates that the proposed analytical model is fit to reveal the fundamental characteristics of flow condensation heat transfer in vertical small-diameter tube.

Numerical and experimental study on the hydraulic characteristics of a vertical axial pump under various clearances of flare tube

In order to investigate the characteristics of a vertical axial flow pump under various clearances of flare tube, the bell-shaped inlet and box culvert outlet channels with flare tube are studied numerically and experimentally. Then, the cases of inlet and outlet channels with the least hydraulic loss are selected to form an integral pump system, for which both numerical simulation and experimental investigation are carried out. The numerical results agree well with the experimental data. It is shown that the clearances of the interfaces between different components of the pump system have a significant impact on the internal flow structure, turbulent entropy and hydraulic performance of the inlet and outlet channels. For the cases of normalized bottom clearance less than 0.5 and normalized top clearance larger than 0.4, the internal flow and hydraulic performance indexes of inlet and outlet channels are relatively poor. There also exists a critical clearance in either inlet or outlet channel at which the hydraulic loss reaches the maximum. The results serve as an important reference for the design as well as safe and efficient operation of the vertical axial flow pump system.

Numerical simulation of liquid falling film on horizontal circular tubes

The objective of this study is to investigate numerically the flow characteristics of falling film on horizontal circular tubes. Numerical simulations are performed using FLUENT for 2D configurations with one and two cylinders. The volume of fluid method is used to track the motion of liquid falling film and the gas-liquid interface. The effect of flow characteristics on heat and transfer coefficient may be remarkable, although it has been neglected in previous studies. The velocity distribution and the film thickness characteristics on the top tube, some special flow characteristics on the bottom tube, intertube flow modes and effect of liquid feeder height on flow characteristics have been studied. Our simulations indicate that 1） the velocity distributions of the upper and lower parts of the tube are not strictly symmetric and non- uniform, 2） the film thickness depends on flow rate and angular distributions, 3） the flow characteristics of the top tube are different from those of the bottom tube, 4） three principal and two intermediate transition modes are distinguished, and 5） the liquid feed height plays an important role on the formation of falling film. The numerical results are in a good agreement with the theoretical values by the Nusselt model and the reported results.

基于Flowmaster平台的雷达行波管多支路液冷系统流动换热分析

为研究雷达行波管的多支路散热问题,运用一维流体仿真软件Flowmaster建立雷达行波管多支路液冷系统的仿真模型,对该液冷系统的稳态流动换热过程及多泵并联"启动/停止"切换的瞬态流动过程进行仿真分析。仿真结果表明,稳态流动下调节各支路流阻特性可使各支路达到流量相同的平衡状态,从而使各支路的换热特性保持一致;多泵并联"启动/停止"切换的瞬态过程对系统的压力分布产生较大影响。从数值仿真方面解决行波管散热的多支路流动换热特性分析的问题,对雷达行波管液冷系统的设计工作有着一定的参考价值。

管壳式换热器折流板开孔流场特征及场协同分析

折流板是管壳式换热器最主要的结构之一,但流体介质容易在折流板背风侧产生一个相对静止的“流动死区”,影响换热器壳程的换热效果。因此,本文利用Fluent对换热器壳程折流板进行开孔研究,分析流体流动特征,同时利用场协同分析的方法对其进行了验证分析。结果表明:折流板开孔可有效减小换热器壳程背风区的“流动死区”,达到强化传热的效果。

基于双因素优选法和Taguchi法的涡流发生器结构及布置优化

为提高换热器性能,在雷诺数为1 000~4 400时,通过数值模拟方法研究新型双面交错排布式的涡流发生器的开孔方案及数量变化对翅片管式换热器传热和流阻特性的影响。采用双因素优选法设计涡流发生器结构,通过Taguchi法对布置位置进行优化设计,利用换热因子、摩擦因子和综合性能评价系数等指标对不同方案下翅片管式换热器的热工性能进行比较。结果表明:与单面布置涡流发生器的方式相比,双面布置方式综合性能更佳;涡流发生器开孔数量增多并不能使得摩擦因子持续下降,靠近涡流发生器尾端且仅开一个孔时综合性能较优;涡流发生器在第四排管布置与否对换热及流阻的影响不大,综合性能差异不大,但是随着涡流发生器继续减少,综合性能出现较大衰减。

基于Fluent的电铲通风除尘系统文丘里管数值模拟

电铲通风除尘系统用于保证机棚内的正压环境,它可以防止粉尘对机棚以及电气室内机电设备造成损害,同时降低其内部机电设备的工作温度,该系统性能的好坏直接影响到电铲内部相关零部件的使用寿命和整机作业效率。文丘里管是通风除尘脉冲清灰系统的核心零件,通过对其进行数值模拟分析,可得到文丘里管内部气体流动特性的变化规律。所研究结果可为文丘里管优化设计提供理论指导,并对通风除尘系统作业效果的改善具有重要工程实用价值。

开孔矩形翅片椭圆管流动与换热特性的数值研究

对电站直接空冷系统的基本换热元件矩形翅片椭圆管建立三维物理数学模型,对空气侧流动和传热性能进行数值研究．分析了不同迎面风速下翅片上无扰流孔和开有扰流孔两种情况下矩形翅片表面的局部表面传热系数分布规律,发现椭圆基管后存在的尾流区使得翅片的强化换热作用减弱。比较了扰流孔的尺寸、数目和位置对管外空气侧流动与换热的影响,结果表明:扰流孔尺寸对流动与换热存在明显影响,而扰流孔数目和位置的影响相对比较小．

结构异常管路对瓦斯爆炸传播特性的影响

在实验研究的基础上,分析了结构异常管路对瓦斯爆炸传播特性的重要影响。研究结果表明,在拐弯处的瓦斯爆炸传播过程是一个压力波、火焰、复杂流动场相互作用的过程,压力波超压、火焰传播速度迅速增大,对拐弯处的壁面破坏特别严重。弯管角度对瓦斯爆炸传播特性有很大的影响,瓦斯爆炸通过不同角度的弯管后,火焰传播速度和压力波超压值都有不同程度的变化。管道拐弯既增加了燃烧区的湍流度而加速燃烧产生能量以推动加速传播,同时也因为拐弯而增大了总阻力和热量向壁面的传递,弯角处膨胀波也会抑制瓦斯爆炸的传播。管道拐弯对瓦斯爆炸传播特性的影响取决于抑制因素和激励因素的综合作用。

炉管泄漏口喷流噪声的辐射特性

研究了管道泄漏喷流的流动特征及其噪声产生的物理机制,指出泄漏喷流的混合区域是产生噪声的主要区域,湍流射流的速度扰动是产生辐射声能量的根本原因。根据泄漏声辐射的声功率和频谱,分析给出了泄漏喷流噪声的峰值频率与喷流速度和喷口直径,以及辐射声功率与喷流速度之间的关系。对泄漏声辐射的频谱规律和指向性特征进行了实验研究,得出管道泄漏口径大小、管内气体压力(喷流速度)等参数与喷流噪声辐射的频谱分布、声压级大小和指向性等声学参量之间的关系,为炉内管道泄漏声检测技术的进一步发展提供了实验数据和声学依据。

翅片螺距对锯齿螺旋翅片换热管特性的影响

在连续螺旋翅片管基础上发展而来的锯齿螺旋翅片管具有易于制造、翅化比大,传热系数和翅片效率更高等优点,已广泛应用于各类大型气体换热设备中。为获得翅片螺距对锯齿螺旋翅片换热管特性的影响,在分析其影响机理的基础上对5个锯齿螺旋翅片管错列管束进行了实验研究。实验结果表明:在实验研究的翅片螺距范围(3.831～4.167mm),相同Re下随翅片螺距增大,管束翅侧Nu增大19%,Eu减小8%;因锯齿螺旋翅片中气流更易渗透和冲刷充分,翅片螺距大于4mm时其对翅侧Nu的影响已不明显;随气流Re增大,管束综合传热性能j/f(Colburn传热因子与摩擦因子比值)先增大后减小,在Re=7000～10000范围j/f较大;相同Re下管束j/f随翅片螺距增大而增大,在实验的翅片螺距范围内管束j/f可增大28%。将实验结果与文献中已有的相关计算关联式预测结果进行了比较。提出了包含翅片螺距、管束横向节距及纵向节距影响的锯齿螺旋翅片管束换热与阻力特性计算关联式,供相关工程设计参考。

纵向节距对锯齿螺旋翅片换热管特性影响的试验研究

锯齿螺旋翅片管是在连续螺旋翅片管基础上发展而来的强化换热管型,具有易于制造、翅化比大,换热系数和翅片效率更高等优点,在大型气体换热设备中具有广泛的应用前景。为获得管束布置结构对锯齿螺旋翅片换热管特性的影响,在分析其影响机理的基础上对9个锯齿螺旋翅片管错列管束进行试验研究,获得纵向节距S2对锯齿螺旋翅片管束换热与阻力特性的影响规律,并提出相应的关联式。试验结果表明:在相同雷诺数Re和管束横向相对节距S1/do下存在最优管束纵向相对节距S2/do使得翅侧努塞尔数Nu与综合换热性能j/f(Colburn传热因子与摩擦因子比值)最大;翅侧欧拉数Eu随S2增大而减小;在试验研究的S2/do=2.41～3.07范围,选取较优S2可使管束翅侧Nu数提高6%且Eu数稍有减小,管束j/f可相应提高约7%。

海水淡化中降膜蒸发过程的实验研究

由于横管降膜蒸发具有传热系数高、热耗低、可利用低品位能源等优点,横管降膜蒸发技术成为低温多效海水淡化技术中应用最广泛的方法。搭建了单管横管降膜试验台,观测了横管降膜蒸发的流动过程,研究了横管降膜蒸发过程中的流动特性和传热特性,归纳了管间距、喷淋密度对横管降膜蒸发过程中流动特性和传热特性的影响。实验结果表明:喷淋密度的增加,有助于传热系数的提高,横管顶部传热系数要高于底部。

空冷凝汽器椭圆翅片椭圆管束外空气的流动与传热特性

研究空冷凝汽器椭圆翅片椭圆管管束外空气的流动与传热特性,对火电站空冷岛的设计与运行具有重要意义.通过CFD模拟,获得了椭圆翅片椭圆管管束外冷却空气的流场和温度场,计算得到了空冷凝汽器冷却空气对流换热平均Nu和摩擦系数f随Re的变化规律,并采用最小二乘法拟合得到了相应的关联式.结果表明:随冷却空气流动Re的增大,Nu增大,f减小.

含尘气流中换热器管壁磨损特性的实验研究

对光滑圆管 ( O-T管 )、高频焊螺旋翅片管 ( HF-T管 )及镍基渗层钎焊螺旋翅片管 ( HF-T-NL管 )的磨损特性进行了实验研究 ,确定了各种管子的磨损量及磨损部位 ,并对其抗磨损性能进行了比较。结果表明 :由于镍基渗层极大地提高了管材的表面硬度 ,同时有效地增强了管子的抗氧化腐蚀能力 ,从而使镍基渗层钎焊螺旋翅片管的耐磨损性能远远优于其它两种换热管。使用镍基渗层钎焊螺旋翅片管可大大地提高换热设备在高温含尘气流中的工作寿命。图 6表 4参

管束间气液两相流动特性研究进展

从流型及其转变、含气率、压降和速度分布等几方面对前人在管束间气液两相流动特性的工作做了综述，归纳并探讨了最新进展与方向。