方形通道内非牛顿流体的强化传热

对非牛顿流体在小尺寸方形通道内的低雷诺数受迫对流传热进行了实验研究。实验用介质为１５００ｗｐｐｍＣａｒｂｏｐｏｌ－９３４中性水溶液。通道顶壁受到等热流加热。结果表明，流体粘弹性与传热的相互作用取决于雷诺数的大小。当表观雷诺数Ｒｅ＞１１．５时，非牛顿流体开始强化对流传热。Ｒｅ数越高，传热强化的程度越大。流体的阻力系数则几乎不受粘弹性的影响。     

周期性通道内非牛顿流体的流动与换热的实验研究

本文用实验方法研究了两种不同浓度的非牛顿流体（ＰＡＭ溶液）在不同温度下的流变曲线及其在周期性通道内的流动与换热规律。实验结果表明,两种ＰＡＭ溶液的稠度系数Ｋ随温度升高而减小,而流变指数ｎ则随温度升高而增加。两种浓度ＰＡＭ溶液在周期性通道内的换热强度和阻力损失均比水高。同时,在Ｒｅ 在１０～５０的范围内,通道内非牛顿流体的流动与换热规律可整理成形如 Ｎｕ＝ Ｃ１Ｒｅ＊ｍ１和 ｆ＝ Ｃ２Ｒｅ＊,ｍ２的形式。     

混合板式换热器板通道的数值模拟研究

本文采用二维非稳态数学模型、周期性充分发展边界条件对混合板式换热器中的板通道内的流动和换热进行数值模拟。在Re=4407～22035工况范围内,计算了半圆型、半椭圆Ⅰ型和半椭圆Ⅱ型三种不同纵截面形状波纹板通道内的流动和换热性能。数值模拟结果分析表明,波纹通道内的流动会因为流体从凸面流向凹面时产生回流而形成涡,即产生扰动,强化了后面的换热,但同时增大了阻力。并得出半圆型通道换热最强,Nu为椭圆Ⅱ型的1.4倍;但其阻力最大,压降为椭圆Ⅱ型的1.9倍。     

正方形小通道内的非牛顿流体-氮气两相流动流型的实验研究

对水力直径为2.5 mm的正方形小通道内的非牛顿流体-氮气的垂直向上两相流动流型进行了可视化实验,工质分别为:浓度0.2%的聚丙烯酰胺(PAM)和0.2%的黄原胶(XG)水溶液,表观气速0.1～100 m/s,表观液速0.01～6 m/s.观察到的典型流型有:弹状流、搅拌流、弹环状流和环状流,其中弹环状流未见于水-空气上升流动.在PAM-氮气实验中发现了一种新流型-泡状-弹状流.通过流型图对比,发现非牛顿流体的搅拌流区域较牛顿流体窄,弹状-搅拌流转变线也明显右移,非牛顿流体的黏性对流型转变的影响较大.     

基于CPV电池散热的射流冲击分形微通道换热器的结构优化

针对聚光光伏(CPV)电池高热流密度散热问题,本文提出了射流冲击与分形微通道散热相结合的解决方案,对其流动和换热进行了模拟.首先对分形微通道的分形级数进行分析,四级相比三级分形微通道换热系数只增加了4.62%,压降却升高了54.37%;接着对管道截面形状进行优化,对圆形截面,方形渐缩截面和扁管截面内流体的流动进行了模拟,结果表明在换热量相近的情况下,扁管拥有最低的压降;随后对比分叉处倒圆角、倒角和Y形三种布置形状,结果表明Y形布置有效地减少了内部流体的涡旋区,能够在牺牲较少的换热面积的条件下,将压降降低85.51%.最后在相同水力直径条件下研究单个喷嘴、均匀喷嘴阵列、非均匀喷嘴阵列射流冲击分形微通道的换热性能,模拟结果表明,非均匀喷嘴阵列分形微通道拥有最佳的换热性能,且压降降低了25.99%.     

变物性参数对微通道内纳米流体强制对流换热的影响

对二维微通道内Al_2O_3-水纳米流体的强制对流换热进行了数值研究。主要研究纳米流体的变热物性参数、纳米粒子体积分数φ和Re数对纳米流体强制对流换热的影响。研究表明:在Re数和纳米颗粒体积分数φ一定时,变热物性参数纳米流体比定热物性参数纳米流体在微通道内的强制对流换热强。在Re数一定时,随着纳米粒子体积分数φ的增加,纳米流体换热性能增强。在纳米粒子体积分数φ一定时,随着Re数的增加,纳米流体的换热能力也随之增加。     

小通道扁管内纳米流体流动与传热特性

建立了测量小通道扁管内纳米流体流动与对流换热性能的实验系统,测量了不同粒子体积份额的水-Cu纳米 流体的管内对流换热系数和摩擦阻力系数,实验结果表明,在相同雷诺数条件下,小通道扁管内纳米流体的对流换热系数 大于纯液体,且随粒子的体积份额的增加而增大,而纳米流体的阻力系数并未明显增大。     

不同纳米流体在微通道内的流动传热特性研究

采用数值模拟的方法研究了不同工质在微通道内流动传热特性的差异。对比了去离子水、纳米流体Al₂O₃/Water、CuO/Water、TiO₂/Water、Cu/Water等工质在微通道内的流动传热特性,并研究了纳米颗粒的浓度对流动换热特性的影响。结果表明：CuO/Water作为冷却工质时的对流换热系数比水增加了9.6%,微通道底面平均温度降低了2.6 K,换热性能明显优于其他几种纳米流体。由于纳米颗粒的加入,纳米流体的粘度比水大,进出口的压降比水大。纳米颗粒的体积分数越大,对流换热系数越大,纳米流体在微通道内的换热性能越好。     

小通道平行流扁管内纳米流体的流动换热特性研究

本文对TiO_2-水,Cu水和SiO_2-水3种纳米流体在一种多孔小通道平行流扁管中的单相流动换热特性进行了实验研究。该扁管矩形通道水力直径1.67 mm,整个实验过程Re变化范围97~6200。实验结果表明:纳米流体起到强化换热效果的同时,也伴随着阻力的增加。此外,纳米流体相对于基液,出现了转捩提前现象。Nu随体积浓度的增加,先增加后减小,使得每种纳米流体都存在一个最优浓度。0.01%的TiO_2-水纳米流体在Re=6200时,Nu增长最大,达到43%。最后,利用性能评价标准对纳米流体的适用性进行了综合评定,0.01%的TiO_2-水纳米流体具有最佳性能。     

离散倾斜肋的传热强化及流动特性

本文以高湿透平叶片中腔内部冷却为应用背景，对两相对壁面具有三维离散斜肋的方形收敛通道内的传热进行了实验研究。本文获得的详尽的局部换热系数图谱、平均换热系数及复杂的近壁流场，可用于强化传热的设计。     

Increasing heat transfer of non-Newtonian nanofluid in rectangular microchannel with triangular ribs

In this study, computational fluid dynamics and the laminar flow of the non-Newtonian fluid have been numerically studied. The cooling fluid includes water and 0.5 wt% Carboxy methyl cellulose (CMC) making the non-Newtonian fluid. In order to make the best of non-Newtonian nanofluid in this simulation, solid nanoparticles of Aluminum Oxide have been added to the non-Newtonian fluid in volume fractions of 0–2% with diameters of 25, 45 and 100 nm. The supposed microchannel is rectangular and two-dimensional in Cartesian coordination. The power law has been used to speculate the dynamic viscosity of the cooling nanofluid. The field of numerical solution is simulated in the Reynolds number range of 5 < Re < 300. A constant heat flux of 10,000 W/m² is exercised on the lower walls of the studied geometry. Further, the effect of triangular ribs with angle of attacks of 30°, 45° and 60° is studied on flow parameters and heat transfer due to the fluid flow. The results show that an increase in the volume fraction of nanoparticles as well as the use for nanoparticles with smaller diameters lead to greater heat transfer. Among all the studied forms, the triangular rib from with an angle of attack 30° has the biggest Nusselt number and the smallest pressure drop along the microchannel. Also, an increase in the angle of attack and as a result of a sudden contact between the fluid and the ribs and also a reduction in the coflowing length (length of the rib) cause a cut in heat transfer by the fluid in farther parts from the solid wall (tip of the rib).     

相变乳状液在蛇形管中的流动和传热特性

用正十四烷、复合表面活化剂和水制成O/W型相变乳状液作为相变蓄冷及传热介质,对其作为非牛顿流体和固液两相流的流动和传热特点进行了分析,通过测试得出不同浓度乳状液在蛇形管道中的对流换热Nυ数和摩擦系数,并给出了换热和阻力的无量纲关联式。指出了对这类特殊相变蓄冷材料作进一步研究的方向。     

The effect of velocity and dimension of solid nanoparticles on heat transfer in non-Newtonian nanofluid

In this investigation, the behavior of non-Newtonian nanofluid hydrodynamic and heat transfer are simulated. In this study, we numerically simulated a laminar forced non-Newtonian nanofluid flow containing a 0.5 wt% carboxy methyl cellulose (CMC) solutionin water as the base fluid with alumina at volume fractions of 0.5 and 1.5 as the solid nanoparticle. Numerical solution was modelled in Cartesian coordinate system in a two-dimensional microchannel in Reynolds number range of 10≤Re≤1000. The analyzed geometrical space here was a rectangular part of whose upper and bottom walls was influenced by a constant temperature. The effect of volume fraction of the nanoparticles, Reynolds number and non-Newtonian nanofluids was studied. In this research, the changes pressure drop, the Nusselt number, dimensionless temperature and heat transfer coefficient, caused by the motion of non-Newtonian nanofluids are described. The results indicated that the increase of the volume fraction of the solid nanoparticles and a reduction in the diameter of the nanoparticles would improve heat transfer which is more significant in Reynolds number. The results of the introduced parameters in the form of graphs drawing and for different parameters are compared.     

非牛顿流体在周期性渐扩渐缩通道内层流流动与换热的实验研究

本文用实验方法研究了浓度为１５００ｗｐｐｍ的聚丙烯酰胺（ＰＡＭ）溶液在周期性渐扩渐缩通道内层流流动与换热问题。研究表明,大约经过８个周期后流动进入充分发展阶段,而换热则要经过大约２０个周期后才能进入充分发展阶段。在充分发展阶段,阻力系数和换热Ｎｕｓｓｅｌｔ数随广义 Ｒｅｙｎｏｌｄｓ数的变化关系式分别为： ｆ＝ １３５ × Ｒｅ＊－０．６３２和 Ｎｕ＝ １０．５ × Ｒｅ＊０．５９８。     

Rapid communication: Transverse spin with coupled plasmons

The steady two-dimensional flow and heat transfer of a non-Newtonian power-law nanofluid over a stretching surface under convective boundary conditions and temperature-dependent fluid viscosity has been numerically investigated. The power-law rheology is adopted to describe non-Newtonian characteristics of the flow. Four different types of nanoparticles, namely copper (Cu), silver (Ag), alumina (Al ₂ O ₃) and titanium oxide (TiO ₂) are considered by using sodium alginate (SA) as the base non-Newtonian fluid. Lie symmetry group transformations are used to convert the boundary layer equations into non-linear ordinary differential equations. The transformed equations are solved numerically by using a shooting method with fourth-order Runge–Kutta integration scheme. The results show that the effect of viscosity on the heat transfer rate is remarkable only for relatively strong convective heating. Moreover, the skin friction coefficient and the rate of heat transfer increase with an increase in Biot number.     

圆截面直流道中微粒黏弹性聚焦机理研究

微粒黏弹性聚焦技术近年来受到了广泛的研究重视, 但影响粒子聚焦特性的关键参数调控机理仍不清楚. 基于此目的, 本文量化研究了圆截面直流道中非牛顿流体诱导微粒黏弹性聚焦的行为, 给出了流速和流道长度对粒子聚焦特性的调控机理. 具体而言: 首先, 对比分析不同黏度牛顿流体(水和22 wt%甘油水溶液)和非牛顿流体(8 wt%聚乙烯吡咯烷酮水溶液)中粒子横向迁移行为, 发现非牛顿流体中粒子将在弹性力主导下聚焦至流道中心区域, 而牛顿流体中粒子则在惯性升力主导下迁移形成Segré-Silberberg圆环. 其次, 量化分析粒子尺寸和驱动流速对黏弹性聚焦效果的影响, 发现随着流速的增加, 粒子聚焦效果逐渐变好并最终趋于稳定, 且大粒子较小粒子具有更好的聚焦效果. 最后, 研究粒子沿流道长度的动态聚焦过程, 推导并验证了粒子聚焦所需安全流道长度的数学模型, 发现大粒子聚焦所需安全流道长度显著短于小粒子. 上述研究结果对于提升粒子黏弹性聚焦机理和过程的理解, 实现微粒聚焦特性的灵活控制具有非常重要的意义.     

Casson fluid flow and heat transfer over a nonlinearly stretching surface

A boundary layer analysis is presented for non-Newtonian fluid flow and heat transfer over a nonlinearly stretching surface. The Casson fluid model is used to characterize the non-Newtonian fluid behavior. By using suitable transformations, the governing partial differential equations corresponding to the momentum and energy equations are converted into non-linear ordinary differential equations. Numerical solutions of these equations are obtained with the shooting method. The effect of increasing Casson parameter is to suppress the velocity field. However the temperature is enhanced with the increasing Casson parameter.     

INTERNAL HEAT TRANSFER TO POWER-LAW FLUID FLOWS THROUGH POROUS MEDIA

Heat transfer and pressure drop were measured for flow of aqueous solutions of Carbopol 934 ® and Carbopol EZ1 ® through a vertical tube filled with porous media. The heated stainless steel test section has an inside diameter of 2.25 cm, and is 200 diameters long. The porosity was varied from 0.32 to 0.68 by using uniform spherical glass beads. Uniform heat flux thermal boundary condition was imposed by passing direct electric current through the tube wall. Over a range of 45 < Re a < 7,000, 21 < Pr a < 58, 0.62 < n < 0.80, 1.6 < D/d < 4.5, and polymer concentration from 250 to 500 ppm, the friction factor data agreed with the Newtonian prediction. Heat transfer to power-law fluids increases with increasing Re a and Pr a , and decreasing porosity. A new correlation was proposed for predicting the heat transfer to power-law fluid flows through confined porous media.