三角扰流蛇形翅片管流动换热特性数值研究

蛇形翅片管作为空冷凝汽器基本换热面,其流动与换热性能对空冷电站经济性有着重要影响,基于此,对三角扰流蛇形翅片管的气侧换热及流动特性进行了数值研究,分析了流体温度、压力沿流程方向的变化规律以及迎面风速、扰流三角个数、扰流三角尺寸等因素对蛇形翅片管换热及流动性能的影响,结果表明三角扰流部位局部阻力是沿程阻力的25%～70%,扰流三角对换热有明显的促进作用,换热系数最大提高37%.本研究可为蛇形翅片管的设计及优化提供理论参考.     

柴油机壁流式过滤体灰烬滤饼沉积流动阻力特性

根据灰烬滤饼沉积规律,结合单孔道模型,建立了壁流式过滤体灰烬滤饼沉积数学模型.该模型综合考虑了灰烬深床沉积和灰烬滤饼沉积阶段灰烬分布的影响,研究了灰烬滤饼沉积阶段过滤体流动阻力特性,包括各项流动阻力对总流动阻力的影响程度、流动阻力随灰烬分布的变化以及流动阻力随灰烬沉积量的变化.结果表明:过滤壁面流动阻力的影响程度最大,灰烬层流动阻力次之;沉积量为定值时,随灰烬堵塞段比例的增加,过滤体流动阻力逐渐降低;随灰烬沉积量的增加,过滤体流动阻力呈线性递增.计算结果与试验结果较为吻合,表明该数学模型能够较合理地描述灰烬滤饼沉积阶段流动阻力的特性.     

水平微细管内CO2流动沸腾流态研究

针对CO_2水平微细管内流动沸腾换热流态及流态转变特性进行实验研究。实验工况:热流密度(5~35 k W/m~2)、质量流率(50~600 kg/(m~2·s))、饱和温度(-40~0℃)、管径(0.5~1.5 mm)。实验表明:CO_2在微细管内实际流态分别是泡状流、弹状流、间歇流、层流、波状流、混状流、环状流和雾状流;干涸过程中的流态主要为环状流-雾状流、波状流-雾状流的过程及不稳定的环状流;通过理论计算获得CO_2微细管内流动沸腾换热流态图,流态图显示热流密度对高干度区域流态转变有显著影响,质量流率大小直接决定了换热过程所经历的流态;不同饱和温度工质热物性不同改变了流型;理论分析所采用的流态形式与实际CO2在微细管内所具有的流态类型基本一致。     

多孔介质蓄热系统动态蓄热特性试验与表征

为了解多孔介质蓄热材料热动态条件下的蓄热特性,得到表征蓄热材料的动态蓄热特性参数,在120 kW多孔介质热动态蓄热系统中,对不同孔径(2.9、4、5.5 mm)、长度(100～400 mm)的蜂窝多孔陶瓷蓄热体在不同热烟气条件下的动态蓄热特性包括蓄热速率、蓄热效率、蓄存单位热量所要克服的蓄热阻力进行了试验研究。结果表明:蓄热速率与时间的关系呈"抛物线"状,蓄热效率随蓄热时间的进行逐渐降低。相同时间下蓄热速率及单位蓄热阻损随蓄热体比表面积的增大或孔径的减小而增大,蓄热效率随蓄热体长度的增加而增大。根据试验研究和分析,采用蓄热速率、蓄热效率及单位蓄热阻损可以用来表征多孔蓄热体的动态蓄热特性。     

蓄热体余热回收换热器强化传热实验研究

利用蓄热式热热交换理论和高温空气燃烧技术的原理,在热态实验基础上建立了蜂窝陶瓷蓄热体的性能研究实验.结果表明,热效率及温度效率随换向时间的增加均呈现先上升后下降的趋势,存在一个最佳换向时间,即热效率和温度效率随着长度的增加而增大,但阻力损失也随之增大;同时存在一个最佳气体流速使蓄热体效率与经济效益达到最佳值;蓄热体的平均温度与气体出口温度均随着换向周期数的增加而升高;对于给定几何外形尺寸的蓄热体,四边形孔格结构的蓄热体具有较大的比表面积,流动性更好,具有更高的温度效率和热效率.     

气流两相流冲刷水平管束的阻力特性研究

本文对水平布置的、壳管式换热器中的气液两相流冲刷管束时的流动阻力进行了实验研究。实验段采用壳管式换热器模型，沿长度方向用三块折流板将管束分成四个冲刷流程。当气液两相流从一端进入并冲刷管束时，分别测量摩擦阻力局部阻力，并用分相流动模型进行分析，得到良好的计算关联式。图６参９     

不同角度下方形钝体绕流的特性研究

针对水流作用下三维钝体绕流的尾迹演化特性,采用大涡模拟法模型四种不同迎流角度,分析其受力情况、频率特性、分离形式和3D流动状态,揭示了钝体绕流现象的形成机理,提出了涡旋形态变化的一般规律。研究表明,随着迎流角度的增加,升力和阻力系数呈先降后升的趋势,当θ=12°时两者均达到最小值;分析了四种角度下流体在钝体周围的运动规律,结果表明其流动分离位置和回流区长度各不相同;雷诺数一定时不同角度下的绕流涡旋频率基本一致。该计算结果对类似工程设计有重要参考意义。     

气液两相流冲刷水平管束的阻力特性研究

本文对水平布置的、壳管式换热器中的气液两相流冲刷管束时的流动阻力进行了实验研究。实验段采用壳管式换热器模型，沿长度方向用三块折流板将管束分成四个冲刷流程。当气液两相流从一端进入并冲刷管束时，分别测量摩擦阻力和局部阻力，并用分相流动模型进行分析，得到良好的计算关联式。图６参９     

再生式换热器的实验研究与数值模拟

为了解再生式换热器工作特性,采用Matlab软件计算简化后的模型,对换热器内流体和固体的温度分布进行了数值模拟,并进行了实验验证.研究了蓄热体长度、比表面积、速度和蓄热体材质对换热效果的影响.结果表明,面积和流速对换热效果影响明显,而改变蓄热体材质基本无影响.     

蜂窝蓄热式空气单预热烧嘴换热特性的热态试验研究

采用热态模拟方法对蜂窝蓄热式空气单预热烧嘴的换热特性进行了试验研究，通过对试验数据的分析，得出了蓄热式烧嘴换热性能与空气流量、换向时间的关系及蓄热室内温度的分布特点等，确定了最佳换向时间、单位蓄热体体积的合适流量和蓄热室的适宜长度等。     

Reverse flow regions in three-dimensional backward-facing step flow

Laser-Doppler velocity measurements adjacent to the bounding walls of three-dimensional (3-D) backward-facing step flow are performed for the purpose of mapping the boundaries of the reverse flow regions that develop in this geometry (adjacent to the sidewalls, the flat wall and the stepped wall) as a function of the Reynolds number. The backward-facing step geometry is configured by a step height (S) of 1 cm, which is mounted in a rectangular duct having an aspect ratio (AR) of 8:1 and an expansion ratio (ER) of 2.02:1. Results are presented for a Reynolds number range between 100 and 8000, thus covering the laminar, transitional and turbulent flow regimes. The boundaries of the reverse flow regions are identified by locating the streamwise coordinates on a plane adjacent to the bounding walls where the mean streamwise velocity component is zero. The size of the reverse flow regions increases and moves further downstream in the laminar flow regime; decreases and moves upstream in the transitional flow regime; and remains almost constant or diminishes in the turbulent flow regime; as the Reynolds number increases. The spanwise distribution of the boundary line for the reverse flow region adjacent to the stepped wall develops a minimum near the sidewall in the laminar flow regime, but that minimum in the distribution disappears in the turbulent flow regime. Predictions agree well with measurements in the laminar flow regime and reasonably well in the turbulent flow regime.     

Dispersed flow film boiling of nitrogen with swirl flow

The heat-transfer characteristics of two-phase film boiling of nitrogen in tubes with tape-generated swirl flow are described. The swirl flow substantially augments the heat transfer and minimizes the thermal non-equilibrium, which is responsible for the relatively poor performance of boilers without twisted-tape inserts. Assuming thermal equilibrium, it is possible to formulate a relatively simple superposition correlation for both evaporation and superheat regions of the once-through boiler. The correlation gives good prediction of wall temperatures along the tube for the entire range of test conditions.     

Experimental investigation of stator flow in diagonal flow fan

Experimental investigations were conducted for the internal flow of the stator of the diagonal flow fan. Comer separation near the hub surface and the suction surface of the stator blade are focused on. At the design flow rate, the values of the axial velocity and the total pressure at stator outlet decrease near the suction surface at around the hub surface by the influence of the comer wall. At low flow rate of 80-90 % of the design flow rate, the comer separation between the suction surface and the hub surface can be found, which become widely spread at 80 % of the design flow rate.     

Heat transportation by oscillatory flow with steady flow component

This paper deals with heat transportation by an oscillatory flow composed of a sinusoidal oscillatory flow superimposed with a steady flow. Velocity and temperature fields, heat transportation rate, work rate, and heat transportation efficiency were investigated through numerical analysis. Results obtained elucidated that (1) the phase difference between velocity and temperature variation remained the same as that of the sinusoidal reciprocal flow without the use of a steady flow component. (2) In the upstream direction heat was mainly transported by the steady flow component and in the downstream direction transportation was mainly performed by the oscillatory flow component. (3) The heat transportation rate of the present oscillatory flow composed of both steady and oscillatory flow components was less than the arithmetic sum of the rates produced by the steady flow and the sinusoidal oscillatory flow. (4) The heat transportation rate was increased immensely by superimposing the steady flow on the sinusoidal oscillatory flow. (5) Conversely, work done by the present oscillatory flow increased only slightly. © 2006 Wiley Periodicals, Inc. Heat Trans Asian Res, 35(7): 482–500, 2006; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20130     

Experimental investigation of flow friction for liquid flow in microchannels

In this paper, investigations on the liquid flow in microchannels with different experimental methods are presented. The experiments were carried out in channels with hydraulic diameter ranging from 30 μm to 344 μm at Reynolds number ranging from 20 to 4000. Based on the experimental data collected and those available in the literature, comparisons and analysis have been carried out to evaluate the possible phenomena occurring in the liquid flow in microchannels. Results obtained show that characteristics of flow in microchannels agree with conventional behaviors predicted by Navier-Stokes equations in the region of those dimensions tested. In this paper, the detailed explanations on experimental results are discussed.     

Numerical simulation of steady flow fields in coiled flow inverter

Flatter velocity profiles and more uniform thermal environments are extremely desirous factors for improved performance in flow reactors and heat exchangers. One means of achieving it in laminar flow systems is to use mixers and flow inverters. In the present study a new device is introduced based on the flow inversion by changing the direction of centrifugal force in helically coiled tubes. The objective of the present study is to characterize flow development and temperature fields in the proposed device made up from the configurations of bent coils. The main mechanism generating the flow is the production of spatially chaotic path by changing the direction of flow using a 90° bend in helical coils (alternating Dean flow). If the direction of centrifugal force is rotated by any angle, the plane of vortex formation also rotates with the same angle. Thus in helical flow a 90° shift in the direction of centrifugal force cause a complete flow inversion. Complete flow fields and thermal fields in helical coil and bent coil configuration were studied using computational fluid dynamics software (FLUENT 6.0). The three-dimensional governing equations for momentum and energy under the laminar flow conditions were solved with a control-volume finite difference method (CVFDM) with second-order accuracy. The flow pattern obtained for the helical coil was in good agreement to those observed by the previous investigators [S.W. Jones, O.M. Thomas, H. Aref, Chaotic advection by laminar flow in twisted pipe. J. Fluid Mech. 209 (1989) 335–357; Ch. Duchene, H. Peerhossaini, P.J. Michard, On the velocity field and tracer patterns in a twisted duct flow. Phys. Fluids 7 (1995) 1307–1317]. The comparison of the flow fields and temperature fields in the helical tube and bent coil configuration are discussed. The bent coil configuration shows a 20–30% enhancement in the heat transfer due to chaotic mixing while relative pressure drop is 5–6%. The results of the present study can be used to model transport processes for developing flows in curved tubes such as chromatographic columns (less axial dispersion [A.K. Saxena, K.D.P. Nigam, Coiled configuration for flow inversion and its effect on residence time distribution. AIChE J. 30 (1984) 363–368]), Chemical reactors (narrower RTD), heat transfer devices, and some biomedical devices.     

Cross-leakage flow between adjacent flow channels in PEM fuel cells

In polymer electrolyte membrane (PEM) fuel cells, serpentine flow channels are used conventionally for effective water removal. The reactant flows along the flow channel with pressure decrease due to the frictional and minor losses as well as the reactant depletion because of electrochemical reactions in the cells. Because of the short distance between the adjacent flow channels, often in the order of 1 mm or smaller, the pressure gradient between the adjacent flow channels is very large, driving part of reactant to flow through the porous electrode backing layer (or the so-called gas diffusion layer)—this cross-leakage flow between adjacent flow channels in PEM fuel cells has been largely ignored in previous studies. In this study, the effect of cross-flow in an electrode backing layer has been investigated numerically by considering bipolar plates with single-channel serpentine flow field for both the anode and cathode side. It is found that a significant amount of reactant gas flows through the porous electrode structure, due to the pressure difference, and enters the next flow channel, in addition to a portion entering the catalyst layer for reaction. Therefore, mixing occurs between the relatively high concentration reactant stream following the flow channel and the relatively low reactant concentration stream going through the electrode. It is observed that the cross-leakage flow influences the reactant concentration at the interface between the electrode and the catalyst layer, hence the distribution of reaction rate or current density generated. In practice, this cross-leakage flow in the cathode helps drive the liquid water out of the electrode structure for effective water management, partially responsible for the good PEM fuel cell performance using the serpentine flow channels.     

Performance test and flow measurement of contra-rotating axial flow pump

An application of contra-rotating rotors has been proposed against a demand for developing higher specificspeed axial flow pump.In the present paper,the advantage and disadvantage of using contra-rotating rotorsare described in comparison with conventional type of rotor-stator,based on theoretical and experimentalinvestigations.The advantages are as follows:(1)The pump is inherently designed as smaller sized and atlower rotational speed.(2)A stable head-characteristic curve for flow rate with negative slope appears.(3)As the rear rotor rotational speed is varied as independent control of front rotor,the wider range of highperformance operation is obtained by rear rotor speed control in addition to front rotor speed control.Thedisadvantages are as follows:(1)The structure of double shaft system becomes complex.(2)The pumpperformance is inferior at over flow rate as the rear rotor loading is weakened.(3)The blade rows interac-tion from rear rotor to front rotor more strongly appears.Then the rear rotor design is a key to achievehigher pump performance.Some methods to overcome these disadvantages will be discussed in more detailstoward wider usage of contra-rotating axial flow pump in various industrial fields.     

Local flow measurements of vertical upward bubbly flow in an annulus

Local measurements of flow parameters were performed for vertical upward bubbly flows in an annulus. The annulus channel consisted of an inner rod with a diameter of 19.1 mm and an outer round tube with an inner diameter of 38.1 mm, and the hydraulic equivalent diameter was 19.1 mm. Double-sensor conductivity probe was used for measuring void fraction, interfacial area concentration, and interfacial velocity, and laser Doppler anemometer was utilized for measuring liquid velocity and turbulence intensity. A total of 20 data sets for void fraction, interfacial area concentration, and interfacial velocity were acquired consisting of five void fractions, about 0.050, 0.10, 0.15, 0.20, and 0.25, and four superficial liquid velocities, 0.272, 0.516, 1.03, and 2.08 m/s. A total of eight data sets for liquid velocity and turbulence intensity were acquired consisting of two void fractions, about 0.050, and 0.10, and four superficial liquid velocities, 0.272, 0.516, 1.03, and 2.08 m/s. The constitutive equations for distribution parameter and drift velocity in the drift-flux model, and the semi-theoretical correlation for Sauter mean diameter namely interfacial area concentration, which were proposed previously, were validated by local flow parameters obtained in the experiment using the annulus.