椭圆管与扁管管板式换热器换热性能的分析比较

本文对椭圆管与扁管管板式换热器的充分发展的周期性层流流动与换热特性进行了数值计算分析,给出了在400相似文献   

新型轴对称内翅管换热过程的数值模拟

采用数值模拟的方法,对新型整体式轴对称不等高内展翅片换热管的管内流动、换热及阻力特性进行了研究,并将模拟结果分别与实验测量值和光管经验公式计算值进行了比较,结果表明:数值模拟结果与实验测量结果和经验公式计算结果符合得较好;新型内翅片管的强化换热效果最好,其换热系数约为光管的1.67~2.27倍,为中心对称翅片管的1.17~1.35倍;内翅片管的阻力系数是光管的1.44~2.20倍,中心对称和新型内翅片管具有基本相同的阻力系数;8翅和10翅新型内翅片管的综合性能参数较优.     

骨架发热多孔介质通道内单相流阻力与换热特性数值模拟

以Fluent 6.3为平台,采用局部非热平衡模型,对紊流及紊流过渡区范围内骨架发热多孔介质竖直通道内的非达西强制对流换热进行了数值模拟。采用三维N-S方程及标准k-ε湍流模型描述多孔介质内的流动,详细研究了孔隙有效雷诺数Re(400Re2000),表面热流密度q(q=5、30和90 kW/m2)和冷却剂入口温度Tin(Tin=20、50和80℃)的变化对多孔介质流道内流动阻力及换热特性的影响。结果表明:低热流密度下,表面热流密度的变化对流动阻力和换热系数的影响很小;小球直径对换热系数的影响显著,且随着雷诺数的增加而增加;换热系数随冷却剂入口温度的增加而减小。     

槽式太阳能DSG系统集热管内强化传热的数值模拟

为了强化集热管内蒸汽换热,避免管壁温度过高,对槽式太阳能直接产生蒸汽系统(DSG)采用内螺纹管和光管时过热段的传热及阻力特性进行了数值模拟,分析了螺纹高度、螺距和雷诺数对螺纹管内换热特性和流动阻力特性的影响,并与光管的数值模拟结果进行了比较.结果表明:内螺纹管增强了管内蒸汽扰动,提高了集热管换热性能,降低了管壁温度.螺纹管的努塞尔数Nu是光管的1.6～2.4倍,且螺纹高度越高、螺距越小,换热性能越好;螺纹管的阻力系数f是光管的3.1～8.7倍,且螺纹高度越高、螺距越小,f越大.     

阵列射流冲击冷却换热特性的数值研究

运用数值计算的方法对不同流动取向的多排孔冲击射流冷却特性进行了三维模拟,并对有初始横向流的多排孔冲击射流冷却特性进行了数值研究,揭示出射流雷诺数、流动方向、初始横向流对冲击冷却传热特性的影响规律。结果表明:研究范围内,射流雷诺数越大,冲击靶面换热效果越好;冲击腔室两端都设为出口时努赛尔数峰值所对应的射流驻点区向下游偏移最小且换热效果最好;当横流雷诺数与射流雷诺数之比大于0.5之后,有横流时的冲击射流冷却局部努赛尔数比无横流时有较为显著下降。     

垂直环管内旋流换热与流动的数值模拟研究

基于垂直环管内旋流对流动边界层的扰动机理,采用数值模拟的方法研究了叶片角度、雷诺数以及进口水温对管内换热以及流动特性的影响,揭示了重力对环管内旋流流动的内在影响机制。结果表明：与水平环管相比,垂直环管的综合换热性能变化平缓,主要受到重力对压降的影响;与雷诺数相比,叶片角度对流场以及温度场的影响最显著;在雷诺数小于15 000,叶片角度为30°时管内的换热性能最佳。     

管内插入网状肋强化换热的数值研究

采用三维数值模拟方法研究了网状肋用于圆形通道的流动换热特征,分析了雷诺数Re、肋的形状、肋的网格间距、肋的直径以及流体与固体的导热系数比kf/ks对通道平均阻力系数和换热性能的影响,并引入换热性能评价指标PEC对2种网状肋进行了比较.结果表明:管内插入网状肋对流动阻力系数和换热性能有很大影响;方形肋的换热效果比圆形肋好,但其阻力较大;随着Re的减小,2种肋的降温效果均增强;肋的网格间距越小,则换热效果越好但阻力越大;随着肋直径的增大和流体与固体导热系数比kf/ks的减小,换热效果增强.     

矩形翅片椭圆管气侧流动与换热规律的数值研究

利用Fluent软件,在矩形翅片椭圆管尺寸给定的情况下,对椭圆基管偏移位置不同的椭圆矩形翅片管外空气侧层流流动与换热进行数值研究,分析偏置距离与椭圆管短轴半径之比Δx/b和雷诺数(Re)对翅片管空气侧换热和流动的影响。结果表明:在计算的Re数范围内,随着Re数的增加,努谢尔特数(Nu)逐渐增加,阻力系数(f)变化不明显;Δx/b=0.5的翅片管(Nu)数比Δx/b=-0.5的翅片管增大约4.83%~8.7%,Δx/b=0.5的翅片管外空气侧阻力系数比Δx/b=-0.5的翅片管增加约4.3%。就翅片效率而言,Δx/b=-0.5时翅片的效率最高。     

长导程双头螺旋槽管换热与流动阻力性能试验研究

对3种相同基管外径、不同螺旋尺寸的长导程双头螺旋槽管进行试验,并与相同基管外径的光滑管进行比较.计算不同工况下螺旋槽管和光滑管管内和管外的表面传热系数,进行换热综合性能评价,并分析了螺旋尺寸对换热性能和流动阻力性能的影响.结果表明:长导程双头螺旋槽管的管内表面传热系数在旺盛湍流区和过渡流区都优于光滑管,且在过渡流区表面传热系数的增大程度更大,而壳程表面传热系数在旺盛湍流区和过渡流区的增大程度有限,螺旋尺寸主要影响管内换热而对管外换热的影响不大,本试验中1号管的综合性能最好.     

三维双侧水平强化管R245fa凝结换热性能试验

由于制冷剂R11和R123对臭氧层有破坏作用,为完成环保新工质R245fa对R11和R123的替代工作,对R245fa在内螺纹外斜翅片的三维双侧强化管外的凝结换热性能进行试验。数据处理过程中,采用Wilson图解法获得管内水侧对流换热系数及其计算关联式,再利用热阻分离法获得管外凝结换热系数。研究表明:试验中管内对流换热系数高于管外冷凝换热系数,所以管外侧的传热热阻是占据主导地位的传热热阻;相对于光管,R245fa在三维双侧强化管管内换热强化换热倍率为3.58,管外强化换热倍率为2.48;对实验数据进行拟合,得到管外换热系数的变化规律和凝结换热关联式。     

Turbulent heat transfer in a horizontal helically coiled tube

An experiment has been conducted in detail to study the turbulent heat transfer in horizontal helically coiled tubes over a wide range of experimental parameters. We found that the enhancement of heat transfer in the coils results from the effects of turbulent and secondary flows. With Reynolds number increasing to a high level, the contribution of the secondary flow becomes less to enhance heat transfer, and the average heat transfer coefficient of the coil is closer to that in straight tubes under the same conditions. The local heat transfer coefficients are not evenly distributed along both the tube axis and the periphery on the cross section. The local heat transfer coefficients on the outside are three or four times those on the inside, which is half of the average heat transfer. A correlation is proposed to describe the distribution of the heat transfer coefficients at a cross section. The average cross-section heat transfer coefficients are distributed along the tube axis. The average value at the outlet section should not be taken as the average heat transfer coefficient. © 1999 Scripta Technica, Heat Trans Asian Res, 28(5): 395–403, 1999     

低温地热双循环式发电系统横管喷淋降膜蒸发器传热性能实验研究

以R245fa为工质,搭建有机朗肯循环（ORC）发电系统横管喷淋降膜蒸发器传热测试平台,研究有机工质喷淋密度,地热水初温及流率等因素对管外换热系数的影响。实验结果表明：随着有机工质喷淋密度、地热水初温、地热水流率的增大,传热系数均先增大后减小。最后,根据实验结果,对现有横管喷淋降膜蒸发器的管外传热系数经验公式的参数进行修正。     

CFD modeling of heat transfer of CO2 at supercritical pressures flowing vertically in porous tubes

Computational fluid dynamics (CFD) tool has been used for investigation of convective heat transfer of CO₂ in two porous tubes. Effects of some important parameters such as pressure, inlet temperature, mass flow rate, wall heat flux and porosity on temperature distribution and local heat transfer coefficients have been studied numerically. Near the supercritical conditions, these parameters are very effective on temperature gradient and local heat transfer coefficients. For example at p = 9.5 MPa, under the same conditions, the heat transfer coefficient in a tube with particle diameters of 0.1–0.12 mm is about 20–30% higher than when the particle diameter of 0.2–0.28 mm were used. The heat transfer coefficient increases with decreasing pressure and increasing mass flow rate. Also the porosity of the bed has the important role on the heat transfer. The CFD predictions have been compared to the experimental data and showed pretty good agreement.     

椭圆钎焊与双金属轧制翅片管传热与阻力性能的试验对比

对椭圆钎焊翅片管和双金属轧制翅片管换热器的传热及阻力性能进行试验对比研究,试验得到了一系列工况下的传热数据与管外空气流动阻力数据,给出了相应的传热系数、流动阻力曲线。从总传热系数中分离出管外空气侧的对流换热系数,得到了具有一定应用价值的管外换热的计算关联式;拟合得到了管外阻力计算关联式。结果表明:椭圆钎焊翅片管比双金属轧制翅片管的传热系数约高9%,管外换热系数约高17%,且管外空气流动阻力约低11%。     

泡沫铝通道内瞬态流动的平均换热系数

针对泡沫铝金属填充矩形通道内的对流换热开展了瞬态实验研究,分析了泡沫铝孔径(孔隙率)、流体流量(流速)等关键参数的影响。为了有效地处理实验数据,重新定义并推导了平均换热系数的计算公式,得到了泡沫铝通道内流动的平均换热系数,并引入了基于渗透率的雷诺数和达西数,确定了相关换热、流动准则数关系。实验研究表明,流速的增大有利于对流换热的强化:而平均换热系数对泡沫金属孔径较敏感;对于低孔隙率泡沫金属,渗透率成为影响换热强度的主要因素,相同或接近的孔隙率下,孔径越大,渗透率和达西数越大,越有利于换热,且压损减小。     

Convective Heat Transfer and Pressure Drop over Elliptical and Flattened Tube

A numerical study on the effect of the effect of elliptical and flattened tube bundle geometry on the convective heat transfer and pressure drop is presented in this article. The analysis has been carried out to evaluate the performance of these bundle geometries in the design of a compact and effective single phase shell and tube heat exchanger. The temperature, velocity, and pressure drop profiles are obtained from solving the mass, momentum, and energy conservation equations. The comparison is made for inline and staggered bundle with different pitch to diameter ratio and inlet velocity for elliptical and flattened tubes. The pitch to diameter ratio is varied from 1.25 to 2.5 for Reynolds number ranging from 200 to 2000 which is in the laminar flow region. The heat transfer coefficient over the staggered and inline tube bundle decrease with an increase in pitch. The same kind of variation is also observed for the pressure drop in the case of both elliptical and flattened tube bundle. The study shows that the transverse pitch with respect to cross flow affects more than the longitudinal pitch.     

Experiment of flow regime map and local condensing heat transfer coefficients inside three dimensional inner microfin tubes

INTRODUCTIONSince1980s,thestudyofthein-tubecondensingheattransferenhancementhasbecomeimportant.Thetwodimensionalinnerfintubes,twodimensionalinnermicrofintubes(inthefollowingsimplycalled2Dtubes)andinserterswerepaidattentiontoforthein-tubecondensinghea...     

Thermal and flow behavior of ice slurries in a vertical rectangular channel—Part II. Forced convective melting heat transfer

The convective heat transfer characteristics of ice slurries flowing vertically upward in a rectangular channel have been experimentally investigated. At steady state, the local heat transfer coefficients were obtained during convective melting, and the effects of ice fraction, Reynolds number and wall heat flux were determined. To gain more insights into the flow properties, local measurements of the axial mixture velocity, temperature and ice fraction distributions were also made near a heated wall. Four main factors were shown to enhance the convective heat transfer coefficient of ice slurries relative to single-phase flow: (1) the ice fraction, (2) thermally and/or hydrodynamically developing flow conditions, (3) mixed-convection and (4) non-Newtonian effects experienced at Re < 4000. Two new and simple heat transfer correlations are also proposed for the practical design of compact heat exchangers involving phase change ice slurries.     

抽吸式自然对流换热及扰流柱对其影响

用稳态法进行了竖直平行平板间加扰流柱的抽吸式自然对流换热实验研究,全面考虑了包括辐射损失在内的各项修正,得到了精度不低于10％的经验公式。实验发现,对于不加扰流柱的平行平板通道,当温差大于50℃时,换热系数随间距的变化存在极值;小间距加扰流柱时,扰流柱对流动的阻滞起主要作用,换热系数低于无扰流柱时的值。     

Finite element analysis of the flow and heat transfer of solid particles in moving beds

A numerical analysis for the flow and heat transfer of solid particles in moving beds of heat exchangers is presented. The solid particles pass through a bundle of heat source tubes as the result of the gravitational force. Heat energy is transferred through direct contact of particles with the heat source tubes. A viscous-plastic fluid model and a convective heat transfer model are employed in the analysis. The flow field dominantly determines the total heat transfer in the heat exchanger. As the velocities of solid particles around the heat source tubes increase, the heat transfer from the tubes also increases. Examples are presented to show the performance of the numerical model. The effect of flow on heat transfer has also been studied. © 1998 John Wiley & Sons, Ltd.