高温下板翅式回热器翅片性能的试验研究

本文针对微型燃气轮机的板翅式回热器进行高温试验研究。采用等雷诺数法对回热器在600℃高温下进行了传热与阻力性能试验。通过测量冷热两侧的工质流量、进出口温度及进出口压力,得到各工况下回热度、压损及雷诺数(Re)等热力参数。得到该型式翅片表面的强迫对流换热准则关联式,并拟合了传热因子j、阻力因子f和性能综合评价指标PEC与Re的准则关联式,研究发现换热系数随Re的增大而增大,j因子受Re的影响较小,f因子受工质压力的影响较为明显,拟合关联式时应考虑这种影响。同时发现,随着Re的增大,冷热两侧性能综合评价指标均增大。     

梭形百叶窗结构内传热强化数值分析

提出一种新型百叶窗结构—梭形百叶窗,它与一般的矩形百叶窗结构相比,流道从翅片中间截面到管连接处逐渐变宽,有较多流体冲刷管壁表面,增加管壁附近流体温度梯度,从而使传热增强。同时降低了流体流动阻力。具有较好的流动和传热性能。应用FLUENT软件对两种百叶窗结构下空气的流场、温度场和压力场进行了CFD研究,分析不同Re数对换热和流动性能的影响。     

倾角不同波纹板片组合板式换热器流动换热分析

通过数值模拟,分析了倾角不同板片组合的板式换热器单流道模型换热和阻力的情况,并结合场协同理论进行分析。模拟结果表明:传热因子j、阻力因子f及面积质量因子j/f均随Re的增大而降低,其值均偏向于倾角较小的板片组合。协同角θ随着Re的增大先增大后基本不变,场协同数Fc随着Re的增大而下降,倾角较大组合的协同性要优于倾角较小的组合。j、f均随着上壁面倾角β_t的增大而增大,在下壁面倾角β_b≤60时变化趋势较大,βb60时,变化趋势降低。倾角不同的板片组合换热器的性能介于构成"热混合"的常规板片性能之间。     

锂电池液冷结构设计与仿真分析

锂离子电池近年来被广泛应用于电动汽车之中,其对温度具有很高的灵敏度,温度过高会使得电池组电化学性能严重下降。采用双进双出的微通道液冷板换热器对电池进行热管理,为了优化液冷板的冷却性能分析了入口Re数、冷却液温度、流道宽度和流道进出口位置对电池组热性能的影响。结果表明,在单体电池以3C电流倍率放电时该结构能够有效地将电池组温差控制在5℃以内。增大入口Re数、流道宽度对电池组温度变化影响较小;改变流道宽度与进出口位置能够有效改善电池组的温度均匀性。     

黏弹性流体基铜纳米流体流动与传热实验研究

成功建立了流体流动阻力和换热性能测试实验台,在45℃的流体温度下,对不同铜粒子体积分数和基液浓度的纳米流体在湍流状态下的对流换热特性和流动阻力进行了实验测量。实验结果表明:黏弹性流体基液中添加纳米粒子后,在降低对应基液减阻率的同时能明显增强传热性能.例如,将1.0%体积分数的铜纳米粒子添加到质量分数为6×10~(-4)的基液中所形成的黏弹性流体基纳米流体其综合性能指数K=0.47,表现了很好的传热强化和减阻性能.     

螺旋波纹结构对盘管换热器传热特性的影响

为增强盘管换热器的传热性能,提出一种螺旋波纹状的盘管结构,通过Fluent分别对螺旋波纹盘管和普通盘管的对流换热过程进行数值模拟研究,分析入口流速对管道传热特性的影响。结果表明：两种盘管压降随入口流速的增大而升高,不同入口流速下的螺旋波纹盘管综合评价指标PEC值大于1,当入口流速为0.524 m/s时,其努塞尔数比普通盘管高出31.3%。螺旋波纹结构增强了盘管内的二次流效应,缓解了传热不均匀的现象。     

人字形波纹板相变流动及换热特性研究

针对人字形波纹板流道内蒸发换热过程进行了三维建模和数值模拟,研究了不同波纹倾角下人字形波纹板蒸发换热时流动及换热特性,并与单相流换热特性进行了对比。结果表明,蒸发相变对人字形波纹板内流体的流动形态影响不大,波纹倾角对人字形波纹板表面气相体积分数分布影响较大;相同波纹参数下,蒸发换热换热系数比单相流换热换热系数提高20%~100%;随着波纹倾角的增大,蒸发换热增强,波纹倾角为75°时换热性能最优。     

扭曲流道内混沌对流及传热特性研究

采用数值方法对两种扭曲流道(平面流道和曲面流道)进行了研究,分析了扭曲流道和平直流道在雷诺数处于20～1 000范围内时的混沌对流、传热和流阻特性,对比了流道的综合传热性能。结果表明：两种扭曲流道具有混沌特征,雷诺数越大,混沌对流强度越高;流道的传热性能随着雷诺数增大而提高,平面流道传热略优于曲面流道,显著优于平直流道;流道的摩擦系数与雷诺数近似成反比,两种扭曲流道摩擦系数均高于平直流道,曲面流道略低于平面流道。不同流道的综合传热性能在不同的雷诺数范围下具有优势,当雷诺数取1 000时,曲面流道强化传热评价指标值约为1.55,比平面流道高约4.3%,曲面流道倾向于在较高的雷诺数下取得更好的综合性能。     

脉动参数对波纹通道内传热强化的影响

采用曲线坐标系下压力与速度耦合的SIMPLER算法,数值研究了波纹通道内脉动流动与换热情况,流动Re数的范围为5~500,Pr数为0.7.计算考察了脉动参数如脉动频率和振幅对通道内强化传热和压力损失的影响.研究结果表明,流动阻力特性呈周期性余弦规律变化,换热Nu数呈正弦规律变化;频率、振幅的增大,使得阻力脉动幅度增大.受入口脉动流的影响,通道内的旋涡发生周期性的脱落、增长和迁移,从而增强了流体之间的扰动和掺混,强化了传热;传热的强化效果随着振幅的增大而增强,但在特定入口脉动流下,相同振幅不同频率下的强化效果几乎一致.     

准逆流平板膜接触器流道内流体流动与传热模拟

研究了具有准逆流特征的六边形平板膜接触器流道内流体流动与传热特性。选择平板膜接触器内两块膜之间的流道和流道内的流体作为研究对象,建立相应的物理模型和表征流体流动与传热的控制方程,再通过计算流体软件fluent对控制方程进行离散和求解,得到计算单元内流体流动的f Re数、平均努塞尔数Nu_m,分析在不同的流道内不同流体的流动和传热特征。研究结果对用于液体空气加湿或除湿的准逆流平板膜接触器的性能评估和结构设计与优化提供参考。     

扁平管交错蛇形短翅片强化传热的研究

针对火电空冷凝汽器采用的扁平管蛇形翅片长度较大,空气在翅片间流动对强化传热的效果受到边界层发展抑制的缺陷,根据锯齿翅片通过破坏边界层发展强化传热的思想,提出一种扁平管交错蛇形短翅片结构。实验结果表明,扁平管交错蛇形短翅片的传热性能优于原有结构,在不同雷诺数Re范围,努塞尔数Nu增加了1.4%～16%;同时空气侧流动阻力也明显增加,摩擦系数f增加了18%～45%。由综合评价指标PEC也可以得到,扁平管交错蛇形翅片有效地强化了空气侧的换热。     

周期性波纹通道内换热与流动的数值研究

采用曲线坐标系下压力与速度耦合的SIMPLER算法,数值研究了一种紧凑换热器中波纹通道内周期性充分发展的层流流动与换热情况,流动Re数的范围为100~1100,Pr数为0.7.计算考察了不同波纹高度、波纹间距对流动与换热的影响,并对模型参数进行了性能评价.计算结果表明,整体Nu数及fRe数随着流动Re数的增加而增加.随着波纹高度的增加或波纹间距的减小,换热增强,特别是在高Re数下波纹高度的增加更加强化换热.最佳波纹高度和间距分别为1.15 mm和13 mm.     

FRICTION AND CONVECTION STUDIES OF R-134a IN MICROCHANNELS WITHIN THE TRANSITION AND TURBULENT FLOW REGIMES

Fluid flow and heat transfer characteristics of single-phase flows in microchannels for refrigerant R-134a were experimentally investigated. Experiments were conducted using rectangular channels micromilled in aluminum with hydraulic diameters ranging from approximately 112 to 210 w m and aspect ratios that varied from 1.0 to 1.5. Using overall temperature, flow rate, and pressure drop measurements, friction factors and convective heat transfer coefficients were experimentally determined for steady flow conditions. Effects of Reynolds number, relative roughness, and channel aspect ratio are examined in predicting friction factor and Nusselt number for the experiments. Experiment results indicated that transition from laminar to turbulent flow occurred between a Reynolds number of 2,000 and 4,000. Friction factor results were consistently lower than values predicted by macroscale correlations but exhibited the same trends with Reynolds numbers of macroscale correlations. Nusselt number results also exhibited a similar pattern of lower values obtained in the experiments than those predicted by commonly used macroscale correlations. Nusselt number results also indicated that channel size may suppress turbulent convective heat transfer and surface roughness may affect heat transfer characteristics in the turbulent regime.     

Experimental Study of Flow Through a Blade Trailing Edge with a Wavy Partition

Abstract

Flow through a blade trailing edge passage has been experimentally investigated in this article. This rectangular passage is divided into two channels by either a straight or a 120° wavy partition. Fluid in the first channel was injected into the second channel via 14 openings (holes) distributed uniformly and in the crests and troughs for the wavy configuration. Particle image velocimetry and a charge coupled device camera were used in experiment, with Reynolds number varying from 1,800 to 6,700. Experimental results show that the wavy configuration produces an oscillation flow in the second channel. The cause for the oscillation may due to the 120° wavy partition causing shear-layer instabilities and the formation of spanwise eddies by the wavy board. Pictures from the charge coupled device camera revealed strong mixing in the trailing edge, and this mixing could enhance heat transfer, which was confirmed in previous research. The amplitude of oscillation depends on Reynolds number and hole positions. It was observed that the injection flow was not symmetric to the openings, and it was skewed to the direction of the first channel main flow. The ratio of flow rate through the outlet and the bleeding holes was from 1:1 to 1:3.     

The numerical investigation of heat transfer and pressure drop of turbulent flow in a triangular microchannel

In this presentation, the flow and heat transfer inside a microchannel with a triangular section, have been numerically simulated. In this three-dimensional simulation, the flow has been considered turbulent. In order to increase the heat transfer of the channel walls, the semi-truncated and semi-attached ribs have been placed inside the channel and the effect of forms and numbers of ribs has been studied. In this research, the base fluid is Water and the effect of volume fraction of Al₂O₃ nanoparticles on the amount of heat transfer and physics of flow have been investigated. The presented results are including of the distribution of Nusselt number in the channel, friction coefficient and Performance Evaluation Criterion of each different arrangement. The results indicate that, the ribs affect the physics of flow and their influence is absolutely related to Reynolds number of flow. Also, the investigation of the used semi-truncated and semi-attached ribs in Reynolds number indicates that, although heat transfer increases, but more pressure drop arises. Therefore, in this method, in order to improve the heat transfer from the walls of microchannel on the constant heat flux, using the pump is demanded.     

Heat transfer and friction factor characteristic of spherical and inclined teardrop dimple channel subjected to forced convection

An experimental and numerical investigation is performed in order to determine the outcome of dimple geometries on the heat transfer and friction factor in a dimple cooling channel subjected to turbulent flow. Two geometries taken into consideration are spherical and inclined teardrop. In order to have a better comparison between the two different dimple channel, the dimple depth, total wetted area of dimple, and dimple pitch have been kept constant. In case of spherical and inclined teardrop dimple channels, heat transfer augmentation, friction losses, and flow pattern have been obtained for a Reynolds Number range from 14,000 to 65,000. The investigation shows that the dimple geometry has a significant contribution to increasing the heat transfer augmentation and determining the flow pattern. The inclined teardrop dimple arrangement shows the maximum heat transfer that is 17% higher than the spherical dimple channel, whereas inclined teardrop dimple results in the rise of friction factor of about 5.93–16.14% times as compared to the spherical dimple within the specified Reynolds number. The inclined teardrop and spherical dimple channel show the heat transfer enhancement of 2.74 to 3.20 times and 2.38 to 2.68 times than that of smooth channels provided thermal boundary conditions and flow conditions are kept same. The numerical study has been performed, which provided a detailed insight into the flow structures and vortex formations in spherical and inclined teardrop dimple channel.     

Numerical simulation of heat transfer and fluid flow of Water-CuO Nanofluid in a sinusoidal channel with a porous medium

This study has compared the convection heat transfer of Water-based fluid flow with that of Water-Copper oxide (CuO) nanofluid in a sinusoidal channel with a porous medium. The heat flux in the lower and upper walls has been assumed constant, and the flow has been assumed to be two-dimensional, steady, laminar, and incompressible. The governing equations include equations of continuity, momentum, and energy. The assumption of thermal equilibrium has been considered between the porous medium and the fluid. The effects of the parameters, Reynolds number and Darcy number on the thermal performance of the channel, have been investigated. The results of this study show that the presence of a porous medium in a channel, as well as adding nanoparticles to the base fluid, increases the Nusselt number and the convection heat transfer coefficient. Also the results show that As the Reynolds number increases, the temperature gradient increases. In addition, changes in this parameter are greater in the throat of the flow than in convex regions due to changes in the channel geometry. In addition, porous regions reduce the temperature difference, which in turn increases the convective heat transfer coefficient.     

涡轮叶片前缘阵列冲击冷却流动及传热特性数值研究

本文采用SST湍流模型模拟了类前缘通道内蒸汽射流阵列冲击冷却的流动与传热特性,分析了雷诺数(Re=10000~50000)、孔径比(d/H=0.5~0.9)和孔间距比(S/H=2~6)对流动及传热性能的影响规律,得到了相应的传热和摩擦关联式。结果表明:在不同雷诺数下,d/H从0.5到0.9变化时,通道压力损失系数降低了76%~79%,靶面平均努塞尔数降低了45%~49%;S/H从2增至6时,通道压力损失系数增加了1.64~1.92倍,靶面平均努塞尔数增加了54%~64%;增大d/H、减小S/H可有效提高类前缘通道蒸汽冲击冷却的综合热力系数。本文研究结果可为未来先进燃气轮机高温涡轮叶片蒸汽冷却结构的设计提供参考和借鉴。     

U型并联多通道中流量分配特性

高增益包层并联多通道流量分配特性直接影响其传热效率,在初始设计中,缺少简洁有效的设计方法。采用Fluent程序对U型布置并联多通道的流量分配特性进行数值模拟,通过敏感性分析,确定了影响流量分配特性的支管长度、支管横截面积、支管个数以及总管横截面积等重要参数,提出了预估流量分配特性的方法。分析表明,无量纲参数Kfd越低,流量分配越均匀；当Kfd低于0.03时,流量丧失比低于5%。