低Reynolds数下内置三棱柱通道的流动与传热特性

以放置在二维水平通道内阻塞比为1/5、1/4、1/3的等边三棱柱为对象,数值研究了空气绕流柱体时的流动及传热特性,分析了在不同阻塞比下顶角迎风和顶角背风放置时阻力系数、升力系数、Nusselt数及Strouhal数随Reynolds数的变化关系。研究表明,三棱柱顶角背风布置的阻力系数和升力系数均显著大于顶角迎风布置,升力系数变化速率随阻塞比增加而增加。当阻塞比为1/3时,尾部两列旋涡出现交错影响,阻力系数和Strouhal数显著增大。受通道和柱体共同影响,随着Reynolds数增加,Strouhal数基本趋于定值。阻塞比增加对于Nusselt数影响较小,且顶角迎风时远大于顶角背风,最大差值为24.5%。最后,给出了内置三棱柱通道绕流的Nusselt数准则关联式。     

板式换热器相变流动的传热及压降特性

建立了三维单流道模型,模拟了波纹倾角和波纹节距对相变流动传热和压降特性的影响。结果表明,沸腾传热和压降特性同时受到流动形式和触点影响,"曲折型"流动有利于液膜蒸发增强传热,触点对气相的扰动作用小于液相,因此在气相体积分数较高的区域的触点处气液相变转化现象不明显。传热系数随波纹倾角的增加而增大,随波纹节距的增加而减小,但在相同入口Reynolds数下波纹节距λ=16 mm比14 mm的传热系数大,主要是因为流动形式的改变在某种程度上减弱了触点数目减少对传热的影响。β=75°和λ=10 mm时传热性能最好。沸腾流动的压降随波纹倾角的增大先升高后降低,在β=65°左右达到峰值,随着波纹节距的增大而降低。     

磁场对熔盐射流冲击传热的影响

以永磁铁构建定磁场,进行外加磁场作用下熔盐射流冲击传热的实验研究,并得到Nusselt数Nu驻点关联式和径向分布。结果表明,在驻点区范围内,Nusselt数较无磁场作用时增大,传热得到比较明显的增强,而在壁面射流区,这种强化传热效果逐渐减弱。此外,当Reynolds数Re一定时,熔盐Nusselt数随着磁场强度的增加而增大,且驻点处强化传热效果最为显著。在Reynolds数Re=6400与磁场强度B=2800 Gs条件下,熔盐驻点Nusselt数Nu₀提高约6％,可见磁场作用对熔盐射流冲击传热具有一定的强化效果。     

凹槽表面的流动与传热特性

采用大涡模拟对底面为凹槽的矩形通道内湍流流动与传热特性进行了研究,为验证所采用数值方法的准确性与可靠性,将平直矩形通道内模拟结果与文献中的直接数值模拟结果进行了比较,换热面Nusselt数与采用Dittus-Boelter公式计算所得Nu进行了比较,计算误差小于5%。以凹槽表面为底面的矩形通道的数值模拟结果表明:通道底面的凹槽结构改变了凹槽表面处流动结构,不同的凹槽高度和长度对流动阻力和换热效果的影响不同,在特定的几何参数下,与平直矩形通道相比,凹槽表面时均Nusselt数提高了近50.5%,时均摩擦阻力系数减小了近35.17%,综合系数增加了73.89%。通道内的流动结构显示:凹槽表面附近存在流体垂直流向壁面区域,在垂直流动区域内流体出现前、后分流,分流位置处Nusselt数增加明显,摩擦阻力系数没有明显增加,其综合流动特性最好。     

并联双通道内超临界水的脉动传热特性

在压力26~30 MPa,时均质量流速420 kg·m^-2·s^-1、热通量0~130 kW·m^-2工况范围内,对垂直上升并联双通道内超临界水的脉动传热特性进行了实验研究。根据实验数据分析了流量脉动对壁温的影响,脉动振幅率(A_f)和脉动频率数（Wo）对时均Nusselt数的影响,并将脉动时均Nu与超临界传热关联式进行了比较。结果表明：壁温随流量脉动而波动,脉动周期相同,相位相反,流量脉动振幅越大,壁温波动也越大;低频脉动时均Nu随脉动振幅率的增大逐渐减小,随脉动频率数的增大先减小后基本不变;高频脉动时均Nu随脉动振幅率的增大而缓慢增大,随脉动频率数的增大而减小;关联式预测的Nu较实验值普遍偏高。     

多孔介质内复合对流传热传质的数值分析

采用数值方法分析了具有非均匀内热源的竖直套管中复合对流传热传质,考查内热源分布系数M和热质二浮力比N对速度、温度、浓度分布以及Nusselt数和Sherwood数的影响。结果表明：当N〉1时速度V为正,其值随N的增加而增大;当N〈-1.5时,V则先负后正。随着M增大,等温线和等浓度线分布更加密集,传热和传质过程更加明显。     

竖直套管内带热源的多孔介质中的自然对流

为研究内热源存在时多孔介质中流动和传热的特性,采用数值方法对上下底绝热、内外壁存在温差时,竖直套管中带均匀容积热源的多孔介质内的自然对流进行了分析。考查了内、外Rayleigh数之比Rai／Ra、高宽比A及半径比k对流场、温度场以及局部和平均Nusselt数的影响。研究结果表明,Rai／Ra和A较大时,内、外壁面附近分别形成两个逆向环流,温度场出现θ>1的高温区和θ<1的低温区,高温区的范围随Rai／Ra和A的增大而扩大。内壁面局部Nusselt数沿管长先增大后下降,分界点位置随k的减小和A的增大下移;外壁面局部Nusselt数在管上部区域迅速增大。内壁面平均Nusselt数Nui与Rai／Ra的增加呈现负的线性关系。     

三维槽道的周期性层流流动与传热

应用数值模拟方法分析了以周期性方式布置不同节距比(PR=L/H)、倾角为45°挡板槽道的流动特性与传热特性。挡板在槽道上下壁面对齐布置高度比(BR=b/H)为0.2,以便在槽道内部形成一对沿流向的反向旋涡。数值模拟计算Reynolds数范围为100~1000,流体介质为空气。计算结果表明:布置斜置挡板后,在槽道内诱导产生了沿流向的旋涡流,且在测试段中旋涡流冲击槽道上下壁面和槽道一侧壁面,结果使得传热效率提高;分析了范围内平均Nusselt数比Nu/Nu₀、摩擦系数比f/f₀及传热增强系数η随节距比PR和Reynolds数的变化关系,并建立了相应的准则关系式;当挡板间节距比为0.5时,传热增强系数可达最大值3.0。     

插入圆柱状丝网管对单相对流传热的强化

提出在传热管内插入圆柱状丝网管调控流场进而强化传热的方法。通过在管内同心插入圆柱状丝网管,将流通截面分成中心区和环隙区,流体流经丝网入口处时受到阻力较大,大部分流体流向环隙区,使得速度场受到调控。环隙区与中心区内冷热流体掺混,强化传热。为验证这一思想搭建强制对流换热实验台,以去离子水为工质,Reynolds数为2392～20175,热通量为50.18～282.88 kW·m^-2。通过对局部、平均Nusselt数及摩擦压降数据的研究,结果表明：同光管传热相比,插入丝网管后平均Nusselt数提高,传热强化系数为1.21～1.84,且最大强化系数发生在过渡流内。入口段强化效果明显,局部传热强化系数最高可达到2.64。而强化传热的同时摩擦压降增大6.1～10.6倍。同时对该结构的传热强化机理进行分析：流场受到丝网管的调控作用,进而强化传热。     

三角槽道低 Reynolds 数脉动流与柔性壁耦合特性研究

以水为工质对三角槽道低 Reynolds 数脉动流与柔性壁耦合特性进行了实验研究。通过传热与流动实验,分析了脉动频率(W)、脉动振幅(A)、柔性壁特性对脉动流传热及流动的影响。同时,通过可视化实验,研究了柔性壁与脉动流之间的响应特性,解析了柔性壁形变与振频对脉动流传热及流动的作用机制及分离贡献。研究结果表明,柔性流道脉动流可以实现强化传热与流动减阻双重效果,但强化传热效果相对较弱(传热效率提升0~50%),适用于以减阻为主要目的的换热场合;柔性壁减阻与削弱强化传热效率,源于柔性壁形变造成时均流通截面积增大(流体流速下降)、W与A的增大减弱脉动能量;W的增加将使得柔性壁振动对脉动流强化传热效率的削减逐步趋于主导地位,而A的增加将使得柔性壁变形对脉动流强化传热效率的削减逐步趋于主导地位;脉动流阻力的削减主要来自于柔性壁的变形(D₁>70%),而柔性壁振频对于脉动流能量耗散的抑制作用较为次要。     

Numerical Analyses on the Adhesive Contact between a Sphere and a Longitudinal Wavy Surface

The adhesive contact between a sphere and a longitudinal wavy surface is simulated numerically. A modified simulation method is proposed using the Newton BI-CGSTAB method in a rectangular coordinate. The effective Tabor parameter is proposed. It is found that when the amplitude of the wavy surface is larger, the contact area is smaller and the pull-off force is smaller. Jump-in from noncontact phenomena occurs when the Tabor parameter is large. Jumping from one ridge to the next ridge occurs when the effect of the Tabor parameter is large and the amplitude of the wavy surface is not too small. Jumping from noncontact to full contact is affected by the amplitude and the wave number of the wavy surface and is also affected by the Tabor parameter.     

A combined active/passive scheme for enhancing the mixing efficiency of microfluidic devices

The straight microchannels used in conventional microfluidic devices yield a poor mixing performance because the fluid flow is restricted to the low Reynolds number regime, and hence mixing takes place primarily as a result of diffusion. In an attempt to improve the mixing efficiency of pressure-driven microfluidic flows, the current study applies periodic velocity perturbations to the species flows at the microchannel inlet and incorporates a wavy-wall section within the mixing channel. Numerical simulations are performed to analyze the respective effects on the mixing efficiency of the geometric amplitude of the wavy surface, the length of the wavy-wall section, and the Strouhal number of the periodic velocity perturbations. Overall, the results reveal that the mixing performance is improved by increasing the geometric wave amplitude or length of the wavy-wall section and by applying a Strouhal number in the range 0.33-0.67.     

The Effects of Surface Waviness and Length on Electrokinetic Transport in Wavy Capillary

An electrokinetic model for a wavy capillary has been developed. Poisson‐Nernst‐Planck and Navier‐Stokes equations constitute the model that governs fluid and ionic fluxes and electric potential distribution inside the capillary. In the present paper, a finite wavy cylindrical capillary with a large reservoir at both capillary ends is analyzed using finite element method. The model is used primarily to examine the influence of capillary surface waviness on the electrokinetic transport behaviours. Different frequencies and amplitudes of the wavy surface are considered to investigate the influence of surface waviness on electrokinetic transport. Fluctuations in potential and ionic concentration distribution increase with the increase in either amplitude or frequency of the capillary surface waviness. However, for higher frequencies the fluctuation diminishes for all surface waviness amplitudes. It is observed that for any irregularity in the capillary surface results in higher salt rejection. Salt rejection is found to be dependent on capillary axial length as well as flow velocity. A critical Peclet number, beyond which salt rejection attains a constant steady value, dictates maximum salt rejection.     

Pulsating Jet Impingement Heat Transfer Enhancement

This article presents results from a numerical study of pulsating jet impingement heat transfer. The motivation is to seek conditions offering a significant enhancement compared to steady flow impingement drying. The CFD software package FLUENT was used for simulating slot-type pulsating jet impingement flows with confinement. The parameter study included velocity amplitude ratio, mean jet velocity, and pulsation frequency. The distance from nozzle exit to surface was three times the hydraulic diameter of the nozzle. The Reynolds number based on the nozzle hydraulic diameter and jet temperature was 2,460 with a mean jet velocity of 30 m/s, which is the base case of the numerical experiments. Results showed that time-averaged surface heat transfer increased with increasing velocity amplitude for the same mean jet velocity. Large velocity amplitudes helped enhance heat transfer by two mechanisms: high jet velocity during the positive cycle and strong recirculating flows during the negative cycle. For the cases with different mean jet velocities but the same maximum velocity, time-averaged surface heat flux decreased with decreasing mean jet velocity. As for the effects of pulsation frequency, with high-velocity amplitude ratio, time-averaged surface heat fluxes were at the same level regardless of frequency. However, at low-velocity amplitude ratio, high frequency caused stronger recirculating flows resulting in greater heat transfer compared to the cases with a lower frequency.     

Pulsating Jet Impingement Heat Transfer Enhancement

This article presents results from a numerical study of pulsating jet impingement heat transfer. The motivation is to seek conditions offering a significant enhancement compared to steady flow impingement drying. The CFD software package FLUENT was used for simulating slot-type pulsating jet impingement flows with confinement. The parameter study included velocity amplitude ratio, mean jet velocity, and pulsation frequency. The distance from nozzle exit to surface was three times the hydraulic diameter of the nozzle. The Reynolds number based on the nozzle hydraulic diameter and jet temperature was 2,460 with a mean jet velocity of 30 m/s, which is the base case of the numerical experiments. Results showed that time-averaged surface heat transfer increased with increasing velocity amplitude for the same mean jet velocity. Large velocity amplitudes helped enhance heat transfer by two mechanisms: high jet velocity during the positive cycle and strong recirculating flows during the negative cycle. For the cases with different mean jet velocities but the same maximum velocity, time-averaged surface heat flux decreased with decreasing mean jet velocity. As for the effects of pulsation frequency, with high-velocity amplitude ratio, time-averaged surface heat fluxes were at the same level regardless of frequency. However, at low-velocity amplitude ratio, high frequency caused stronger recirculating flows resulting in greater heat transfer compared to the cases with a lower frequency.     

Hydrothermal performance analysis of various surface roughness configurations in trapezoidal microchannels at slip flow regime

The effects of various surface roughness geometrical properties including roughness height (5%, 10%, 15%), number (3, 6), and shape (rectangular and triangular) on the flow and heat transfer of slip-flow in trapezoidal microchannels were investigated. The effects of mentioned parameters on the heat transfer coefficient through the microchannel, average Nusselt number and pressure drop for Reynolds number of 5, 10, 15 and 20 were examined. The obtained results showed that increasing the roughness height and number increases the pressure drop due to higher stagnation effects before and after roughness elements and decreases the Nusselt number due to higher recirculation zones effects than obstruction effects. The most reduction in Nusselt number and the most increment in pressure drop occur at the roughness height of 15%, roughness number of 6 and Reynolds number of 20 by about 10.6% and 52.8% than the smooth microchannel respectively.     

Influence of Tortuous Geometry on the Hydrodynamic Characteristics of Laminar Flow in Microchannels

Flow visualizations are performed to study the hydrodynamic characteristics of laminar flow in tortuous microchannels for a wide range of Reynolds numbers. The detailed flow patterns in wavy channels are identified and found to be greatly affected by geometrical parameters. In wavy channels, steady flow exists at low Reynolds numbers, but above a critical number unsteady flow develops. The critical Reynolds number is found to depend on the characteristics of the channel path. Recirculation zones form immediately after the inner corners of the sharp bends, with their size and magnitude of the recirculation velocity increasing with higher Reynolds numbers. Large fluctuations in recirculation zone locations highlight the importance of these flow features in the development of transient flow. The flow behaviors play very important roles in determining the pressure drop in wavy channels relative to straight channels.     

边界条件对颗粒-流体对流传热的影响

对单个球形颗粒与周围流体的对流传热进行数值模拟,考察了等温边界、等热流量边界和流固耦合边界条件的影响。结果表明,流固耦合边界和等温边界所得时均面积加权努塞尔数与经验公式计算结果基本一致,等热流量边界模拟结果大于其它两种边界条件结果。时间平均局部面积加权努塞尔数的分布表明,当流动稳定且不发生分离时,努塞尔数从前滞点到后滞点逐渐减小;当出现非稳态涡旋时,努塞尔数从前滞点到分离角附近逐渐减小并出现最小值,后逐渐增大直至后滞点。