Effect of wall temperature modulation on the heat transfer characteristics of droplet-train flow inside a rectangular microchannel

The numerical studies of water–oil two-phase slug flow inside a two-dimensional vertical microchannel subjected to modulated wall temperature boundary conditions have been discussed in the present paper. Many researchers have contributed their efforts in exploring the characteristics of Taylor flows inside microchannel under constant wall heat flux or isothermal wall conditions. However, there is no study available in the literature which discusses the impact of modulated thermal wall boundary conditions on the heat transfer behavior of slug flows inside microchannels. Hence, to bridge this gap, an effort has been made to understand the heat transfer characteristics of the flow under sinusoidal wall temperature conditions. Initially, a single phase flow and heat transfer study was performed in microchannels, and the results of the fully developed velocity profile and heat transfer rate were validated with benchmark analytical results. Then an optimal selection of the combination of sinusoidal thermal wall boundary conditions has been made for the two-phase slug flow study. Later, the effects of amplitude(0 b ε b 0.03) and frequency(0 b ω b 750π rad·s~(-1)) of the sinusoidal wall temperature profile on the heat transfer have been studied using the optimal combination of the wall boundary conditions. The results of the numerical study using modulated temperature conditions on channel walls showed a significant improvement in the heat transfer over liquid-only flow by approximately 50% as well as over two-phase flow without wall temperature modulation. The non-dimensional temperature contours obtained for different cases of temperature modulation clearly explain the root cause of such improvement in the heat transfer. Besides,the results based on the hydrodynamics of the flow have also been reported in terms of variation of droplet shapes and film thickness. The influence of Capillary number on the film thickness as well as heat transfer rates has also been discussed. In addition, the measured film thickness has also been compared with that calculated using standard empirical and analytical models available in the literature. The heat transfer rate obtained from the numerical study for the case of unmodulated wall temperature was found to be in a close match with a phenomenological model to evaluate slug flow heat transfer having a mean absolute deviation of 7.56%.     

Enhanced heat transfer in rectangular duct with punched winglets

Thermal performance of a heat exchanger duct with punched winglets(PWs) mounted on the upper duct wall has been examined for Reynolds number(Re) ranging from 4100 to 25,500. In the present experiment, two types of PWs: punched delta-and elliptical-winglets(P-DW and P-EW) with four punched-hole sizes were tested at a fixed attack angle, optimal relative pitch and height. Also, data of solid delta-and elliptical-winglets(DW and EW) were included for comparison. The investigation has shown that the P-DW yields higher thermal-performance enhancement factor(η) than the P-EW. Although the solid DW and EW with no punch have the highest heat transfer and friction loss, the PWs yield better η than the solid ones. For PWs, the P-DW with smaller hole size has the peak heat transfer and friction loss around 5.7 and 40 times over the smooth duct, respectively but the optimum η of 2.17 is seen for the one with a certain hole size. The PWs provide η at about 5%–8% above the solid winglets.     

曲面弓形折流板换热器壳程流体流动与传热

提出一种新型折流板--曲面弓形折流板,并构造曲面弓形折流板换热器,采用数值模拟和实验相结合的方法研究其壳程传热和流动阻力性能。在实验方面,设计了实验用曲面弓形折流板和普通弓形折流板换热器试样,其中换热器管束采用可拆连接形式,以考察不同折流板结构和板间距的影响。通过改变管程及壳程流量和管程流体进口温度,获得了大量对应于不同折流板结构的壳程压力降和传热系数实验数据。在模拟方面,利用Fluent软件建立了曲面弓形折流板换热器和普通弓形折流板换热器流体数值分析模型,得到了壳程流体流场分布及壳程压力降和传热系数。结果发现,在相同结构参数和流动条件下,曲面弓形折流板换热器壳程压力降比普通弓形折流板换热器降低9%～24%,而壳程传热系数比普通弓形折流板换热器提高3%～11%。     

180°矩形弯管流场的实验测量和数值模拟

对180°矩形弯管内的湍流流场用智能型五孔球探针进行了测量，同时采用FLUENT 6.2对其进行数值模拟。流场测量和数值模拟结果表明：180°矩形弯管由于流体旋转产生的离心力导致了弯管内压力分布的变化，使弯管内切向速度发生变化。切向速度在流体开始旋转0°~60°区间内侧的速度增大、压力减小，外侧的速度降低、压力增大；当转过60°截面后，外侧的速度增大、压力减小，内侧的速度降低、压力增大。     

180°矩形弯管流场的实验测量和数值模拟

引言 弯管广泛应用于化工、石油、水利、发电等工业领域,如离心式压缩机的流道、离心泵的流道、流体输送管道的转向等.弯管流道内的流体流动不同于一般直管流道的流体流动.由于流体在弯管流道中受到管道曲率的影响,流动时产生指向弯管外侧的离心力,导致压力分布发生变化,进而引发切向速度分布发生变化.     

Mass and heat transfer from or to a single sphere in simple extensional creeping flow

The first detailed numerical investigation on the mass and heat transfer both outside and inside a solid or liquid sphere immersed in a simple extensional flow for a larger range of Peclet numbers (1–100,000) is presented. By making use of the known Stokes velocity field at small Reynolds numbers, a finite difference method with the control volume formulation is adopted to solve the convection‐diffusion transport equation. Simulation results show that the transport rate, which is represented by Sherwood number, is significantly affected by Peclet number and viscosity ratio. The flow direction, no matter a uniaxial extensional flow or a biaxial extensional flow, has no effect on the total transport rate but affects the concentration distribution a lot. Some comparisons between present simulations and previous studies are made to validate each other and confirm the reliability and applicable scopes of reported correlations. A few new correlations are put forward to predict the transfer rate at finite Peclet numbers for various values of viscosity ratios. © 2011 American Institute of Chemical Engineers AIChE J, 58: 3214–3223, 2012     

强化换热凹槽管内流动与传热数值模拟

以水为工作介质,应用三维常物性不可压缩流体稳态湍流模型,对凹槽管内的流动、阻力与传热性能进行了模拟研究,并与光滑管进行对比。结果表明,凹槽管主要通过扰动、漩涡使层流底层的局部温度梯度变大,从而使换热性能比光滑管强。凹槽结构能显著增强流体的扰动和相互掺混,并产生径向漩涡,减小边界层厚度,加剧流体湍流,促使边界层表面快速更新,从而强化传热,但同时也使其流动阻力增加。最后应用场协同理论,从局部换热角度分析了凹槽管强化换热的机制。强化换热另一重要途径是使换热器内速度场与温度梯度场之间夹角变小,改善速度场与温度场协同程度。     

含油R32管内流动沸腾换热特性测试及关联式开发

R32作为低温室效应制冷剂得到广泛应用,空调器中循环的介质是制冷剂与润滑油的混合物,掌握R32-润滑油混合物的流动沸腾特性是R32空调器优化设计的关键。本文的目的是参照空调器实际运行工况,测试R32-润滑油混合物的管内流动沸腾换热特性,开发传热系数关联式。新搭建了具有防爆功能的R32-润滑油混合物管内换热性能测试台,采用换热管为7 mm铜管,测试的质流密度200~400 kg/(m²·s)、干度0.2~0.7、油浓度0~5%。实验结果表明,R32-润滑油混合物管内流动沸腾传热系数随质流密度的增大而增大;在中低干度下传热系数随油浓度的增大而增大,在高干度下随油浓度增大先增大后减小并于3%油浓度处取得最大值。基于混合物物性与流型开发了传热系数关联式,预测值与85%的实验数据的误差在±20%内。     

内插梯形扰流片的矩形通道内涡流和传热特性

利用Realizable k-ε湍流模型对带缺口的梯形扰流片进行流动和传热特性的数值模拟,研究了梯形扰流片的缺口位置及流动方式对矩形通道内流场以及传热的影响,同时通过对涡量、流线、流速分布、压力变化、湍流强度等的分析,揭示了扰流片强化传热的机理。结果表明,逆流时Nusselt数比顺流时提高了21.7%,同时摩擦因子也提高了25%。顺流时内侧缺口绕流片提高了传热系数的同时也增加了摩擦阻力,而外侧缺口的绕流片降低了传热系数同时也降低了形状阻力。研究发现较低Reynolds数下(10000相似文献   

粗糙表面不同粗糙元间局部流动与传热特性

用规则微小横肋模拟固体表面粗糙度,数值模拟和分析粗糙元间局部流动和换热特性.结果显示,在粗糙段各粗糙元间流动结构会发生变化,大体上可分为入口区、中间区和出口区.入口区流动变化比较快,不同粗糙元间壁面涡结构显示出很大不同;中间区则出现较稳定的单涡结构;与入口区类似,出口区流动结构又发生比较大的改变.阻力曲线从入口处粗糙元间的“W”形,逐渐演化到出口处正弦曲线形;换热曲线基本呈中间高、边沿低的趋势.研究表明,特定粗糙段各粗糙元间流态变化导致局部阻力和换热特性有所不同.     

垂直窄矩形通道内流动沸腾传热研究

在热虹吸条件下，实验研究了缝宽接近于气泡脱离直径型的窄矩形通道内流动沸腾的传热特性。发现其对流蒸发传热中有时处于过渡流状态。首次为对流蒸发传热系数建立了一个通用算式。还将该算式与加和模型相结合，为窄矩形通道形成了第一个完整的流动沸腾传热算法。该算法的预测值与实验数据相比，其平均绝对偏差为１４．９％。     

矩形槽强化膜状冷凝换热数值分析与实验研究

研究了饱和蒸汽在垂直矩形槽表面的层流膜状冷凝换热,对物理模型进行了适当的简化,建立了冷凝液的流体流动方程,并与二维导热方程耦合,用数值方法解方程,通过迭代,得到了传热量与坐标z的关系。为了验证数值分析,建立了饱和蒸汽在矩形槽表面冷凝的实验模型和实验装置,测量了在不同的温差下的热通量,并与计算热通量做了对比。结果表明,热通量数值分析值与实验值吻合较好,最大偏差不超过10%,说明关于矩形槽强化冷凝换热的数值分析具有可行性,可用于矩形槽的参数设计和换热计算。     

竖直窄矩形通道内环状流的流动传热特性

为了研究竖直窄矩形通道内环状流的流动传热特性,建立了窄矩形通道内环状流的数学物理模型,并进行了实验验证。通过数值求解环状流的数学物理模型得到了环状流区域的压降梯度、沸腾传热系数和液膜内的速度分布。结果表明窄矩形通道内的环状流模型能够很好地预测环状流区域的压降梯度和沸腾传热系数,而且环状流液膜内速度在法向的分布是非线性的,在层流边界层区速度梯度较大。热通量和窄矩形通道的尺寸对液膜的流速有很大影响,随热通量的增加和窄矩形通道尺寸的减小液膜的流速逐渐增加,然而质量流速对液膜流速的影响较小,而且随质量流速的增加液膜的速度逐渐减小。     

小型叠板式换热器传热与流阻特性实验

研究了一种小型叠板式换热器,它由多块紫铜薄板叠合而成,薄板表面加工有2个分布腔和微通道阵列,微通道采用精密铣削加工.通过实验研究不同流速下微通道深度、分布腔形状以及换热器放置方式对传热、流阻和综合性能的影响.结果表明,传热体积是决定单位体积传热系数大小的首要因素,在传热体积相等情况下,单位体积换热面积是决定单位体积传热系数大小的主要因素.在3种影响因素中,微通道深度对换热性能影响较大,微通道深度为1 mm的换热器传热性能比深度为2 mm的提高了30%～60%;换热器放置方式对流阻和综合性能影响较大,换热器入口放置在下端时压力损失比放置在上端时下降约26%,综合性能则提高了约30%.     

三种翅管式换热器传热与阻力特性的试验研究

为考察大管径下不同翅片形式对空气侧强化传热的影响,对6排平直,开缝、纵向涡翅片管换热器空气侧的传热及阻力性能进行了试验研究,发现开缝翅片的传热性能高于纵向涡翅片和平直翅片,但相应阻力也增加,在试验的Re数范围内整理出了传热和阻力的关联式,并分别在相同质量流量、相同压降和相同泵功三种准则下对三种换热器进行了传热与阻力的综合比较,结果表明开缝翅片综合性能高于平直翅片和纵向涡翅片.     

卧式螺旋管内流动换热壁温分布特性

螺旋管是一种高效换热管型,在卧式螺旋管内流动时受到重力和离心力的作用,工质流动方向时刻改变,产生二次回流,从而表现出了不同于其他管型的流动与传热特性。以卧式螺旋管为研究对象,选取底部截面、上升段截面、顶部截面三个具有代表性的位置截面,采用逐渐增大热通量法研究了其在单相对流和过冷沸腾条件下壁温的分布特性。为螺旋管换热器的设计与应用提供了参考依据。     

管内插入旋流片的流动和传热的数值模拟

为了获得较好的传热性能,提出一种在缩放管内间隔插入旋流片的复合强化传热方法。采用水为流动介质,在湍流状态下,通过数值模拟,详细研究了缩放管内插入旋流片的速度场与热流场的分布特性及其相互间的协同。对缩放管插入不同结构旋流片的热阻性能与缩放管和光滑管进行了比较,模拟结果表明,在相同条件下,传热系数和阻力系数最大分别提高25%,120%,120%,370%。缩放管与旋流片的复合使用能有效地改善速度场和温度场的协同程度,是提高换热性能的一个有效手段。     

管壳式换热器强化传热的数值模拟分析

阐述了换热器管程强化传热强化管和管内插入物2种结构及其性能特点,并用场协同原理和数值模拟方法分析了其强化传热机理。模拟分析的结果为:光滑管内,速度矢量与温度矢量二者之间的夹角接近90℃,传热效果较差;在管内插入物情况下:两者之间的夹角小于90℃,流场和温度场的协同性更好:从而实现了强化传热。     

小型轴向槽道热管蒸发段的换热系数

通过试验的方法对直径为6mm、长为210mm的小型轴向槽道热管蒸发段换热系数进行研究,并得出计算模型.热管管内工作介质为水,蒸汽饱和温度分别为40℃、50℃和60℃.通过测定,得出在水平工作状况下,热管管内气化换热系数受传输功率.温度差和饱和蒸气压的影响不大.气化形式以蒸发为主.并且通过半理论分析和试验的方法得到在试验条件范围内,热管蒸发段换热系数基本稳定在30 500x(1±25%)W/(m2·℃).     

Numerical study on steady and transient mass/heat transfer involving a liquid sphere in simple shear creeping flow

A numerical method is utilized to examine the steady and transient mass/heat transfer processes that involve a neutrally buoyant liquid sphere suspended in simple shear flow at low Reynolds numbers is described. By making use of the known Stokes velocity field, the convection‐diffusion equations are solved in the three‐dimensional spherical coordinates system. For the mass transfer either outside or inside a liquid sphere, Sherwood number Sh approaches an asymptotic value for a given viscosity ratio at sufficiently high Peclet number Pe. In terms of the numerical results obtained in this work, two new correlations are derived to predict Sh at finite Pe for various viscosity ratios. © 2013 American Institute of Chemical Engineers AIChE J, 60: 343–352, 2014