Electro-osmotic flow of a second-grade fluid in a porous microchannel subject to an AC electric field

Studies on electro-osmotic flows of various types of fluids in microchannel are of great importance owing to their multifold applications in the transport of liquids, particularly when the ionized liquid flows with respect to a charged surface in the presence of an external electric field. In the case of viscoelastic fluids, the volumetric flow rate differs significantly from that of Newtonian fluids, even when the flow takes place under the same pressure gradient and the same electric field. With this end in view, this paper is devoted to a study concerning the flow pattern of an electro-osmotic flow in a porous microchannel, which is under the action of an alternating electric field. The influence of various rheological and electro-osmotic parameters, e.g., the Reynolds number, Debye-Huckel parameter, shape factor and fluid viscoelasticity on the kinematics of the fluid, has been investigated for a secondgrade viscoelastic fluid. The problem is first treated by using analytical methods, but the quantitative estimates are obtained numerically with the help of the software MATHEMATICA. The results presented here are applicable to the cases where the channel height is much greater than the thickness of the electrical double layer comprising the Stern and diffuse layers. The study reveals that a larger value of the Debye-Huckel parameter creates sharper profile near the wall and also that the velocity of electro-osmotic flow increases as the permeability of the porous microchannel is enhanced. The study further shows that the electro-osmotic flow dominates at lower values of Reynolds number. The results presented here will be quite useful to validate the observations of experimental investigations on the characteristics of electro-osmotic flows and also the results of complex numerical models that are necessary to deal with more realistic situations, where electro-osmotic flows come into the picture, as in blood flow in the micro-circulatory system subject to an electric field.     

Effects of Navier slip on unsteady flow of a reactive variable viscosity non-Newtonian fluid through a porous saturated medium with asymmetric convective boundary conditions

A study on the effects of Navier slip, in conjunction with other flow parameters, on unsteady flow of reactive variable viscosity third-grade fluid through a porous saturated medium with asymmetric convective boundary conditions is presented. The channel walls are assumed to be subjected to asymmetric convective heat exchange with the ambient, and exothermic chemical reactions take place within the flow system. The heat exchange with the ambient obeys Newton's law of cooling. The coupled equations, arising from the law of conservation of momentum and the first law of thermodynamics, then the derived system are nondimensionalised and solved using a semi-implicit finite difference scheme. The lower wall slip parameter is observed to increase the fluid velocity profiles, whereas the upper wall slip parameter retards them because of backflow at the upper channel wall. Heat production in the fluid is seen to increase with the slip parameters. The wall shear stress increases with the slip parameters while the wall heat transfer rate is largely unaltered by the lower wall slip parameter but marginally increased by the upper wall slip parameter.     

铸件充型过程流动与传热耦合模拟

对铸件充型过程流动与传热藕合模拟技术进行了研究，建立了综合考虑辐射、对传导等多种传热行为的有限差分法计算公式，并在Visual C 6．0平台上，自主开发了可集成于CASTSoft/CAE Ver5.0流场计算模块CASTFlow。新算法的应用提高了软件的计算效率，增加了代码的重用性。以法兰铸件为例，CAST-Flow与MAGMASoft的模拟结果基本一致，说明所提出的计算方法是可取的。     

分形油藏非牛顿幂律流的精确解及动态特征

本文研究了分形油藏无限大地层和有界地层非牛顿幂律流渗流模型 ,利用拉氏变换和分解的方法求得了井底定流量生产无限大地层及有界地层 (包括封闭和定压地层 )五种情况非牛顿幂律流的实空间解析解 ,通过数值积分法和拉氏数值反演分析了分形油藏非牛顿幂律流的压力动态特征以及分形参数和边界对压力动态的影响。     

螺旋流混合器内部流动及换热分析

构建了一种无运动部件的螺旋流混合器三维模型,运用计算流体力学理论,采用湍流、非定常和传热模型,模拟了冷、热水在混合器中螺旋混合的过程,并通过对混合器内流线和温度分布等的分析,探讨了混合器内部流动和能量交换的规律,以及进口流速和混合器容积的变化对其出口温度的影响。计算结果表明:螺旋流混合器可以得到了比较好的混合效果;在一定的阻力下,进口速度越大越易形成旋流,混合效果越好;螺旋流混合器的容积变化对混合效果影响明显,随着容积的增大,调整时间显著增加。     

多孔介质中粘弹性液体广义流动分析

将分数阶导数引入渗流力学建立了多孔介质中具有松驰特性的非牛顿粘弹性液体的含有分数阶导数的不稳定渗流模型，利用离散逆Laplace变换技巧和广义Mittag-Eeffler函数研究了多孔介质中非牛顿松弛粘弹性液分数阶流动特征。对任意的分数阶导数得到了精确解，并先求出了长时和短时渐进解，然后用拉普拉斯数值反演Stehfest方法分析无限大地层粘弹性液的流动。结果表明粘弹性流体对分数导数的阶数具有极强的敏感性。     

非Newton流体在内管做轴向往复运动的偏心环空中非定常流的速度分布

本文建立了双极坐标系下非Newton流体在内管做轴向往复运动的偏心环空中非定常流的控制方程；并以HPAM水溶液为例，用有限差分法对控制方程进行了数值求解，绘制了这种流动的速度分布曲线；分析了环空内外管的偏心距、环空内管的冲程和冲次对流体在偏心环空中速度分布的影响。分析结果表明：随着环空偏心距减小，最宽和最窄间隙处的流体速度分布越来越接近；随着环空内管的冲程和冲次的增加，环空内管的轴向往复运动对流体速度分布的扰动增大。     

城市污水类非牛顿幂律湍流流动特性研究

城市污水的流动特性是其作为冷热源应用的关键问题。基于非牛顿流的切应力本构式、卡门对数分布与尼古拉兹实验,推导了速度分布、阻力系数与平均速度及雷诺准则的关系式,定义了特征雷诺数、特征广义雷诺数及其表达式,分析给出了层流、紊流的临界雷诺数,临界特征广义雷诺数为215。以特征广义雷诺数得到了阻力系数的理论曲线,对比了理论曲线与实验曲线及其与清水阻力曲线。以特征雷诺数表征的阻力系数与牛顿流体一致,阻力系数的理论与实验曲线吻合,为清水的400％以上。     

Natural convective heat transfer and nanofluid flow in a cavity with top wavy wall and corner heater

A numerical analysis of natural convection of nanofluid in a wavy-walled enclosure with an isothermal corner heater has been carried out. The cavity is heated from the left bottom corner and cooled from the top wavy wall while the rest walls are adiabatic. Mathematical model has been formulated using the single-phase nanofluid approach. Main efforts have been focused on the effects of the dimensionless time, Rayleigh number, undulation number, nanoparticle volume fraction and length of corner heaters on the fluid flow and heat transfer inside the cavity. Numerical results have been presented in the form of streamlines, isotherms, velocity and temperature profiles, local and average Nusselt numbers. It has been found that nanoparticle volume fraction essentially affects both fluid flow and heat transfer while undulation number changes significantly only the heat transfer rate.     

Numerical investigation of entropy generation and heat transfer of pulsating flow in a horizontal channel with an open cavity

In this study, the entropy generation and the heat transfer of pulsating air flow in a horizontal channel with an open cavity heated from below with uniform temperature distribution are numerically investigated. A numerical method based on finite volume method is used to discretize the governing equations. At the inlet of the channel, pulsating velocity is imposed for a range of Strouhal numbers St_p from 0 to 1 and amplitude A_p from 0 to 0.5. The effects of the governing parameters, such as frequency and amplitude of the pulsation, Richardson number, Ri, and aspect ratio of the cavity, L/H, on the flow field, temperature distribution, average Nusselt number and average entropy generation, are numerically analyzed. The results indicate that the heat transfer and entropy generation are strongly affected by the frequency and amplitude of the pulsation and this depends on the Richardson number and aspect ratio of the cavity. The pulsation is more effective with the aspect ratio of the cavity L/H= 1.5 in terms of heat transfer enhancement and entropy generation minimization.     

突扩通道内横掠圆管对流换热的数值模拟

采用非稳态数学模型对突扩通道内横掠圆管流动与换热进行了数值模拟。分别对不同雷诺数Re以及不同通道突扩比Er情况进行数值模拟。结果表明:随着Re的变化,数值解有稳态,周期振荡和混沌。同时随着雷诺数的增加,圆管附近的换热不断增强。对于不同突扩比,当10≤Re≤100和250≤Re≤300时,圆管的换热随着通道突扩比的增加而变弱;当100Re≤200时,圆管换热并不单调减弱。     

Experimental investigation of flow past a circular cylinder with hydrophobic coating

A series of experiments are performed in a gravitational low-speed water tunnel to investigate the influence of the hydrophobic coating on the flow past a circular cylinder. The mean velocity and turbulence intensity profiles behind the cylinders are measured using the hot-film anemometer while the separation angles are obtained with the flow visualization technology. For the Reynolds number lower than 3 800, the hydrophobic coatings are in the Cassie state, the velocity deficit and the turbulence intensity in the wake of the hydrophobic cylinders are lower than those of the smooth cylinders which implies the drag reduction effect of the hydrophobic coatings. When the Reynolds number becomes higher than 6 600, the hydrophobic coatings turn into the Wenzel state. Through decomposing the velocity data in the turbulent wake into different scales based on the orthogonal wavelet transform, it is found that the total turbulence intensity in the wake of the hydrophobic cylinders becomes almost the same as in the wake of the smooth cylinders while the intensity of the large scales of vortex components in the wake of the hydrophobic cylinders is still lower. Furthermore, the separation angles show the same trend as a function of the Reynolds number but always take smaller values for the hydrophobic cylinders.     

微通道流体物性对流动与传热影响的数值模拟和实验研究

针对微矩形通道内单相层流流动和传热特性进行实验研究和三维数值模拟研究.微矩形通道宽为50μ m,高为200μ m,工质采用去离子水.讨论流体热物性随温度变化对流动传热特性的影响.并把实验数据、模拟结果和文献参考值比较.结果显示,恒热流边界条件下,由于热量加入导致流体温度沿流动方向升高,摩擦系数沿流动方向有递减趋鼢势.进口处的Nu数有最大值,在L*=0.2处降到最小值,之后又攀升到充分状态时的Nu数值.在充分发展段,Nu数保持不变.黏性耗散对热起始段的作用不能忽略.     

水位变化对粗砂土内流动与传热的影响

该文以粗砂土为研究对象,对粗砂土在不同水位条件下流动与传热情况进行研究。建立粗砂土三维模型,采用有限体积法进行数值计算,并利用恒温实验平台进行验证。结果表明:在正常条件下粗砂土内以对流换热为主,其流线呈顺时针方向流动;粗砂土内水气交界面处存在温度场和流态的突变,出现了半涡形结构和高于壁面温度的现象;瑞利数对粗砂土对流换热影响显著,与Nu数呈指数关系;在实验中温度呈非线性变化,各测点温度值分布为4层2区;模拟值与实验值基本一致,三维模型具有更高的准确度。     

基于非结构网格求解换热器流场的并行计算

该文利用基于非结构化网格的有限容积方法,对两种典型结构换热器内的冷态流场进行了数值模拟,分别给出了换热器内管组对齐排列和交错排列时的流体速度场、压力场和湍动能分布,并对这两种不同管组布置结构的流场进行了比较和分析。其中,采用Delaunay三角化方法在多连通区域内生成非结构化网格,并采用SIMPLE算法离散流动问题的控制微分方程,由此形成的线性代数方程组中的稀疏矩阵则通过并行化的GMRES算法在PC机群上进行多机并行求解,且并行编程环境为基于PC网络分布式内存模式的MPI。实际应用表明：基于网络并行的GMRES算法能获得较高的加速比和效率。     

表面活性剂减阻流体传热与阻力关系试验研究

针对在水中添加表面活性剂可以出现阻力系数减少而同时传热性能发生变化的现象,采用测压差装置和热电偶测温系统分别对二维流道内不同的减阻流体的减阻性能、传热性能进行了试验,研究了浓度、配比、雷诺数和热流密度对减阻流体传热与阻力系数之比值的影响。结果表明,表面活性剂减阻流体的传热系数考尔朋（Colburn）因子与阻力系数之比和水的情况有较大的差异;当减阻流体的减阻率稳定时,随着雷诺数的增大,考尔朋恩因子与阻力系数比值缓慢下降;达到临界雷诺数后,考尔朋因子与阻力系数之比值波动较大,并且比值迅速增大到接近没有添加减阻剂的水平。     

Numerical investigation of flow and heat transfer performances of horizontal spiral-coil pipes

The flow and heat transfer performances of horizontal spiral-coil pipes of circular and elliptical cross-sections are studied.The numerical results are compared with the experimental data,to verify the numerical method.The effects of the inlet water mass flow rate,the structural parameters,the helical pitch and the radius ratio on the heat transfer performances are investigated.Performances of the secondary fluid flow with different radius ratios are also investigated.Numerical results demonstrate that the heat transfer coefficient and the Nusselt number increase with the increase of the water mass flow rate or the helical pitch.The maximum heat transfer coefficient and the maximum Nusselt number are obtained when the radius ratio is equal to 1.00.In addition,the fluid particle moves spirally along the pipe and the velocity changes periodically.The particle flow intensity and the spiral movement frequency decrease significantly with the increase of the radius ratio.Besides,the secondary flow profile in the horizontal spiral-coil pipe contains two oppositely rotating eddies,and the eddy intensity decreases significantly along the pipe owing to the change of curvature.The decreasing tendency of the eddy intensity along the pipe increases with the increase of the radius ratio.     

Theoretical analysis of slip flow on a rotating cone with viscous dissipation effects

This paper is concerned with the mutual effects of viscous dissipation and slip effects on a rotating vertical cone in a viscous fluid. Similarity solutions for rotating cone with wall temperature boundary conditions provides a system of nonlinear ordinary differential equations which have been treated by optimal homotopy analysis method(OHAM). The obtained analytical results in comparison with the numerical ones show a noteworthy accuracy for a special case. Effects for the velocities and temperature are revealed graphically and the tabulated values of the surface shear stresses and the heat transfer rate are entered in tables. From the study it is seen that the slip parameter ? enhances the primary velocity while the secondary velocity reduces. Further it is observed that the heat transfer rate 1/2xNu Re? increases with Eckert number Ec and Prandtl number Pr.     

同轴套管间涡旋流动及强化传热数值模拟

相对旋转两同轴套管间的涡旋流动,能够带来二次流强化传热传质作用,在航空、水处理、生态保护、生物工程和膜分离等领域都具有广泛的应用价值。本文使用Fluent软件,对同轴套管间涡旋流动及传热特性进行数值模拟,考察了内管转速、内外管壁面温差等操作参数变化对同轴套管间流体传热性能的影响,分析了涡旋流动与传热效率之间的关联关系。模拟结果表明:内管转速增加在流场中形成泰勒涡,涡流扰动增大了高温壁面与流体间的热流密度,增强了流体传热效率。增大内外管壁面温差,也可加强流体传热性能,但其强化作用不及内管转速的强化作用显著。受流场中泰勒涡影响,流体速度、温度及热流密度沿轴向的分布都呈正弦状周期性波动,在相邻两涡交界面处,流体传热性能最好,在涡中心处的传热性能最差。     

Couple stress fluid flow in a rotating channel with peristalsis

This article describes a new model for obtaining closed-form semi-analytical solutions of peristaltic flow induced by sinusoidal wave trains propagating with constant speed on the walls of a two-dimensional rotating infinite channel. The channel rotates with a constant angular speed about the z-axis and is filled with couple stress fluid. The governing equations of the channel deformation and the flow rate inside the channel are derived using the lubrication theory approach. The resulting equations are solved, using the homotopy perturbation method(HPM), for exact solutions to the longitudinal velocity distribution, pressure gradient, flow rate due to secondary velocity, and pressure rise per wavelength. The effect of various values of physical parameters, such as, Taylor's number and couple stress parameter, together with some interesting features of peristaltic flow are discussed through graphs. The trapping phenomenon is investigated for different values of parameters under consideration. It is shown that Taylor's number and the couple stress parameter have an increasing effect on the longitudinal velocity distribution till half of the channel, on the flow rate due to secondary velocity, and on the number of closed streamlines circulating the bolus.