扩缩型微通道内流体流动及传热特性数值分析

微通道内扩缩结构的布置,有利于诱发二次涡流,提高传热性能。基于单侧矩形肋微通道(MC-OSRR),提出了单侧矩形肋和空腔(MC-RR.RC)、单侧矩形肋和扰流柱(MC-RR.RPF)两种组合结构微通道优化模型。在验证数值方法有效性的基础上,以去离子水为流动介质,运用Fluent软件模拟研究了不同体积流量、壁面温度、进口温度条件下扩缩型微通道的流动与传热特性。结果表明,空腔和扰流柱结构的引入,能进一步诱发二次流,促进流动混合,低体积流量下有利于增强单侧矩形肋微通道整体性能;相比空腔结构,布置于通道中央的扰流柱结构对主流体的扰动更为剧烈,高体积流量下无法在控制阻力增加幅度的基础上强化单侧矩形肋微通道传热。     

燃机叶片平行肋扰流内冷通道传热特性研究Part2:耦合传热特性

平行肋扰流通道是高温透平叶片常用的冷却结构。本文通过批量(约1000次)的气热耦合数值计算对不同几何结构平行肋扰流冷却通道的流动与耦合传热特性进行研究.对于固定压差通道,存在使固体外壁温度最低的肋片布置形式及对应的最优冷气量,针对某重型燃机动叶设计了不同肋片角度的平行肋扰流冷却通道并进行了气热耦合数值计算,表明60°肋能在最小的冷气量下取得最强的换热,验证了机理研究的结论.     

基于LBM的部分热活跃边界下多孔腔体内自然对流换热

采用Boltzmann方法模拟部分热活跃边界下的多孔腔体内自然对流,探讨不同热边界布置方案、孔隙度、Da数及Ra数对其流动传热的影响.数值计算表明：Da=10^-4时,腔体内中央出现一个循环流模式,只在Ra数很大时孔隙度才对传热有影响; Da=10^-2时,腔体内出现两个循环流,在Ra数很小时孔隙度对传热产生强烈的的影响.热活跃边界位置影响腔体内流体对流传热的强度,加热边界布置在底部、而冷却边界布置在顶部（Bottom-Top布置方式）,对多孔腔体内对流传热最有利,优于全热边界布置方式的传热效果.     

涡轮叶片内冷通道高性能肋流动与传热

涡轮叶片内部冷却通道内壁面布置扰流肋是一种高效的强化传热措施.本文在8500~60000雷诺数范围内,对直肋、斜肋、V肋和W肋开展实验研究,得到了四种肋的换热和流阻特性.实验结果表明:1)W肋具有最高的平均努塞尔数,是光滑通道充分发展流动传热性能的2.2~2.6倍,其次是V肋和直肋,而斜肋的传热性能最低,约是光滑通道的1.7倍;2)W肋表现出最大的流阻性能,是光滑通道的2.5~3.7倍,其次是V肋和直肋,而斜肋流阻最小,约为光滑通道的1.8~2.5倍。W肋具有最优的综合热性能,而直肋的综合热性能最低。另一方面,本研究还通过瞬态液晶热像技术获得了W肋表面详细局部传热分布,实验结果表明W肋中间的顶点迎风区域是强换热区,该区域与气流相互作用,热边界层较薄,有效强化了换热能力。     

微酒窝通道与圆柱面凹槽通道对流传热强化分析

结合对流传热场协同原理分析了微酒窝通道、圆柱面凹槽通道及低肋通道强化传热特点,研究发现酒窝与圆柱面凹槽强化传热主要原因为:1)增加近壁区流体扰动,促进酒窝或凹槽内部流体与主流之间的传热;2)酒窝与凹槽均可扩展传热面积,进而提高总传热量。与低肋通道相比,酒窝与圆柱面凹槽仅对其附近流体的流动产生影响,而对主流流体的流动影响较小,进而阻力增加较少。提出传热量单元性能参数PEC_A作为评价指标,酒窝通道综合性能参数略高于圆柱面凹槽通道,而远高于低肋通道。     

管外流诱导管束振动强化传热试验研究

本文提出了利用管外流体诱导振动实现强化传热的新方法.采用了一种新的传热元件-弹性管束,它对管外流体流动具有良好的振动响应特性.提出了正置三角柱脉动流发生装置,该装置可诱导起弹性管束一定频率的周期性振动,并对强化传热具有显著的促进作用.在恒热流条件下,对管外水流诱导振动强化对流换热规律进行了试验研究,得到了管外对流换热的准则方程式.     

极限重力下冲压发动机再生冷却中燃油流动及传热特性研究

为了了解重力水平对超燃冲压发动机再生冷却中航空燃油流动及传热特性的影响,本文采用有限容积法对不同重力水平下水平圆管内RP-3航空煤油的流动和传热进行三维数值模拟。水平圆管内径为1.8 mm,壁厚为0.2 mm,长为300 mm,管内工质为RP-3航空煤油,管壁受到q=400 kW/m~2的均匀热流密度加热。研究结果表明,根据重力水平的不同可将二次流流型沿管长的变化归纳为3种流型演变规律。0g时截面流型由入口段的汇流发展为源流,最终再次变为汇流;当重力水平增加至0.1g时,在管道轴线上方出现涡状流动,涡胞中心沿管长方向逐渐向下移动,最终流型转变为汇流;当重力水平增加至0.3g时,涡状流动涡胞中心位于管道轴线下方,沿着管长方向快速上移至轴线上方,最终发展至管道轴线下方。随着重力水平的增大,水平圆管轴线上方流体的湍动能逐渐增大,而轴线下方流体的湍动能却逐渐减小。流体湍动能受浮升力效应诱导二次流影响呈非对称分布。重力水平的增大能强化下管壁附近的对流换热,而抑制上管壁附近的对流换热,在上壁面出现了传热恶化现象。管壁平均对流换热系数会随重力水平的增大而增大。     

强化对流传热场协同唯象机制及其控制

根据对流传热强度与流体中存在的各种内场及其外场的关系,从唯象上阐明了降低温度边界层厚度、增加流体扰动和增加近壁面速度梯度的强化对流传热方法的物理机制,其本质是控制流体中内场及其外场之间的相互协同。给出了当流体中存在内场和外场时强化对流传热场协同控制方法,以此可指导发展强化对流传热单元的新方法。     

冲击冷却对燃气轮机尾缘双流程内冷通道传热和压损的影响

燃机内部冷却的目标是采用更小的流量和压力损失实现通道对流传热系数的提高,并利用均匀的冷却减小热应力。本文采用数值模拟手段,研究了局部冲击冷却对带侧向出流的尾缘双流程通道传热和压损的影响。研究表明:动叶尾缘通道由于侧向出流,在页顶出现一个显著的大回流涡,当地传热系数低,易造成叶片局部温度和热应力偏高。通过在U型通道的隔板布置冲击子孔,可以有效地减小通道中分离涡的影响,提高通道末端的传热,并在对通道平均传热系数影响不大的前提下,显著降低通道阻力。使射流与靶面尽量垂直有助于达到最好的冷却效果。     

水平旋转圆管外混合对流的数值分析

一、前言 工程应用中常常涉及到旋转体的传热问题,如透平机械等。在强化传热中采用的螺纹管、肋片管等,由于螺纹糟或肋片的导向作用,往往会使流体的流动除了主流外,还有绕体的旋转流,因此研究旋转流对传热的影响是很有意义的。有关这方面的研究已作了不少工作。     

An experimental study on convective heat transfer performance of steam and air flow in V-shaped rib roughened channels

An experimental study on heat transfer characteristics of steam and air flows in a V-shaped ribbed channels was conducted. The effects of Reynolds numbers and rib angles on heat transfer of steam and air were obtained. The area-averaged Nusselt numbers of steam flow at a Reynolds number of 12,000 were 13.9%, 20.6%, 27.1%, and 27.9% higher than those of air flow for rib angles of 90°, 75°, 60°, and 45°, respectively. The correlations for Nusselt number in terms of Reynolds number and rib angle for steam and air in V-shaped ribbed channels were developed.     

内螺旋肋管流动与传热特性的实验研究

对六种内螺旋肋管进行了流动与传热的实验研究,实验管内径为16．25-16．69 mm,内螺旋肋高为0．28-0．44 mm,螺旋肋牙数为40-45,螺旋角为43°-45°．研究表明,内螺旋肋管可以有效地强化传热,本文所研究的管型的传热强化倍率为1．67-2．99．比较了两种评价内螺旋肋管性能的方法．用Webb模型及Ravigururajan模型对内螺旋肋管进行了性能预测并与实验值进行了比较．两个模型的预测值与本试验结果有较大偏差,相对而言,传热模型稍优．     

Analysis of heat transfer and nanofluid fluid flow in microchannels with trapezoidal,rectangular and triangular shaped ribs

In the present study, the effect of triangular, rectangular and trapezoidal ribs on the laminar heat transfer of water-Ag nanofluid in a ribbed triangular channel under a constant heat flux was numerically studied using finite volume method. Height and width of ribs have been assumed to be fixed in order to study the effect of different rib forms. Modeling were performed for laminar flow (Re=1, 50 and 100) and nanofluid volume fractions of 0, 2% and 4%. The results indicated that an increase in volume fraction of solid nanoparticle leads to convectional heat transfer coefficient enhancement of the cooling fluid, whereas increasing the Nusselt number results in a loss of friction coefficient and pressure. Also, along with the fluid velocity increment, there will be an optimal proportion between heat and hydrodynamic transfer behavior which optimizes performance evaluation criteria (PEC) behavior. Among all of the investigated rib forms, the rectangular one made the most changes in the streamlines and the triangular form has the best thermal performance evaluation criteria values. For all studied Reynold numbers, heat transfer values are least for rectangular rib from. Therefore, trapezoidal ribs are recommended in high Reynold numbers.     

Experimental Investigation of Local Heat Transfer in a Square Duct With Continuous and Truncated Ribs

In the present study, the turbulent heat transfer and fiction in a square duct roughened by continuous and truncated ribs on one wall has been investigated experimentally. The ribs are oriented transversely to the main stream in a periodic arrangement. For both cases, the rib height-to-hydraulic diameter ratio is 0.15, the rib pitch-to-height ratio is fixed at 12, and the Reynolds number varies from 8,000–20,000. Liquid crystal thermography is applied to demonstrate detailed distribution of heat transfer coefficient between a pair of ribs. The results show that the horseshoe vortices produced by truncated ribs are quite different from the flow structures altered by continuous ribs. It is noted that continuous ribs give higher heat transfer augmentation and pressure drop than truncated ribs. Moreover, the truncated ribs cannot be employed to eliminate hot spots which occur in the corresponding continuous types.     

Liquid crystal heat transfer measurements in a rectangular channel with solid and slit rib

An experimental investigation on the heat transfer effectiveness of solid and slit ribs mounted on the bottom surface of a rectangular channel has been carried out at Reynolds numbers of 13400, 22600, 32100 and 40800. The rib height to hydraulic diameter ratio (e/D _h)set during experiment is equal to 0.0624. The surface Nusselt number results from transient liquid crystal thermography are presented. The heat transfer enhancement performance analysis has been carried out using entropy generation principle. The slit rib is superior to solid rib from both heat transfer augmentation and pressure penalty point of view. The performance of the slit rib is a function of the open area ratio (β) and the location of the slit (b) from the bottom test surface. The optimum open area ratio is 20% and the slit located symmetrically from the top and bottom surface of the rib is the optimum location of the slit. The heat transfer augmentation of the slit rib (β=20%) is 61% in comparison to 40% for the solid rib at Re=32100 and the pressure penalty for the slit rib is 7% lower than the solid rib. The entropy generation for the slit rib is 33% less than that of the solid rib.     

Influence of T-semi attached rib on turbulent flow and heat transfer parameters of a silver-water nanofluid with different volume fractions in a three-dimensional trapezoidal microchannel

This study aimed at exploring influence of T-semi attached rib on the turbulent flow and heat transfer parameters of a silver-water nanofluid with different volume fractions in a three-dimensional trapezoidal microchannel. For this purpose, convection heat transfer of the silver-water nanofluid in a ribbed microchannel was numerically studied under a constant heat flux on upper and lower walls as well as isolated side walls. Calculations were done for a range of Reynolds numbers between 10,000 and 16,000, and in four different sorts of serrations with proportion of rib width to hole of serration width (R/W). The results of this research are presented as the coefficient of friction, Nusselt number, heat transfer coefficient and thermal efficiency, four different R/W microchannels. The results of numerical modeling showed that the fluid's convection heat transfer coefficient is increased as the Reynolds number and volume fraction of solid nanoparticle are increased. For R/W=0.5, it was also maximum for all the volume fractions of nanoparticle and different Reynolds numbers in comparison to other similar R/W situations. That's while friction coefficient, pressure drop and pumping power is maximum for serration with R/W=0 compared to other serration ratios which lead to decreased fluid-heat transfer performance.     

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).     

Heat transfer and flow characteristics of turbulent slot jet impingement on plane and ribbed surfaces

A computational study is carried out to assess the suitability of various RANS based turbulence models for slot jet impingement on flat and ribbed surfaces with various values of Reynolds number and jet to plate spacing. The com-puted results are compared with the reported experimental data. It was observed that none of the turbulence models considered predicted the heat transfer data accurately. However, some models predicted the experimental data with good trends, e.g., secondary peak and several spikes in Nusselt number for ribbed surface, with a precise computation of the stagnation point Nusselt number. Further, the effects of slot width, rib pitch and jet to ribbed surface spacing were investigated for jet impingement on a ribbed surface. It was observed that the local Nusselt number increased with slot width and rib to plate spacing. It was also observed that increasing Reynolds number had a positive effect on the local heat transfer. With increasing rib pitch the local Nusselt number increased near the stagnation zone but de-creased downstream. The observed flow pattern was different for jet impingement on a ribbed surface than that on a flat surface.     

Obstacle influence on the flow structure and mass transfer in a boundary layer with ethanol combustion on horizontal surface

Experiments with ethanol combustion on horizontal surfaces revealed the most general properties of a boundary layer with chemical and phase transformations. The list of flow features includes development of large-scale structures and manifestation of volumetric forces, which impact the flow stability and heat and mass transfer. It was demonstrated that the range of velocities ensuring flame existence is wider for flow past a rib than for flow past a backward-facing step. The nature of mass transfer in a reactive flow past an obstacle is transient and remains of that kind until the flame blow-off. For a flow above a horizontal wall at Reynolds numbers Re < 5·10⁴, the intensity of mass transfer is twice higher than for combustion below the wall. When the combustion occurs below the wall, the surface temperature gradients are higher.     

Heat transfer at a laminar free convection and separated flow past a rib in a vertical channel with isothermal walls

The results of numerical computations of a free laminar convection and heat transfer between two parallel isothermal plates in the presence of a single rib on the channel surface are presented. The investigations have been conducted for a channel with the aspect ratio AR = L/w = 10, where L is the channel height, and w is the distance between the plates. An infinitely thin adiabatic rib was located on one of the channel walls in the middle of its height. The relative rib height l/w was varied in the range 0÷0.8. The wall temperature was higher than the ambient temperature, and the Rayleigh number was varied in the range Ra = 10²÷10⁵. The main attention has been paid to the study of the influence of the rib height and the Rayleigh number on local and integral heat transfer and the Reynolds number in the channel (the convective thrust). A fundamental difference in the heat transfer over the channel height has been shown on the ribbed wall and on a smooth surface. The computational results have been compared with the case of a symmetric distribution of the ribs on the both walls with the integral height equal to a single rib.