斜截半椭圆柱面涡流发生器强化换热和压降特性的试验研究

在矩形通道内布置一对斜截半椭圆柱面涡流发生器(高宽比h/b=1/2),通过改变斜边倾角α、来流攻角β、前沿间距S、布置方式以及顺列、错列两排涡流发生器研究不同工况下的换热和压降特性.试验雷诺数范围为Re=700～26 800.结果表明,在试验条件下α=12°时的换热效果要好于α=20°,Re=1 300时两者的最高局部对流换热系数分别比平直通道高19.7%和10.7%,Re=26 800时分别为23.2%和18.33%.斜截半椭圆柱面涡流发生器(α=12°)的最优攻角为β=60°,最优前沿间距为S= 20 mm.当Re=26 800,β=60°,S=20 mm时的最高局部对流换热系数约比平直通道高23.2%,压力损失约比平直通道高33.7%.雷诺数较小时,由于回流滞止区的影响,布置两排涡流发生器后,换热效果反而弱于单排.随着雷诺数增大到4 000,双排涡流发生器的换热效果要明显优于单排,Re=26 800时,最高局部对流换热系数约比单排涡流发生器高14.4%,比平直通道高33.4%.通过试验得出斜截半椭圆柱面涡流发生器在β=60°,α=12°时的涡旋作用距离大约在250～300 mm.     

纵向涡发生器在空气预热器中的强化传热数值模拟研究

针对空气预热器中传热性能低下的问题,将纵向涡器运用于空气预热器热管内,以烟气为介质,运用计算软件FLUENT进行数值模拟,研究在不同Re数下,涡发生器对管内烟气的传热及流动阻力的影响,比较了不同攻角及翼高与管内半径之比的直角三角翼涡发生器强化换热效果,并与光管的换热系数和阻力系数进行了对比。分析表明,纵向涡发生器能明显提高换热性能,在所研究的纵向涡发生器中,攻角为45°时,涡发生器强化传热效果较好。随着Re数的改变,具有最佳传热效果的涡发生器结构也会有所不同。     

半椭圆涡流发生器强化换热机理

对单列及双列方式排列的一种斜截半椭圆柱体涡流发生器,以稳态的气、水逆流换热方式为模型,用数值方法模拟了迎流攻角为0°～90°,雷诺数6 000～48 000范围内的传热特性。结果表明:涡流发生器可以诱导纵向涡与流向涡,起到强化传热的作用,纵向涡在强化传热中起主要作用。     

斜截椭圆柱式涡流发生器强化传热的大涡模拟

对流体在放置斜截椭圆柱式涡流发生器矩形槽道内的流动与传热特性进行大涡模拟,得出流场中速度、温度与压力参数的瞬态变化特性,再现温度场、压力场及诱导旋涡的变化过程,并对流动结构及涡流发生器强化传热的机理进行分析。为验证大涡模拟计算结果的准确性,在相同条件下对未布置涡流发生器的空槽道分别采用湍流模型和大涡模拟进行对比计算,两者的计算结果符合较好。计算结果表明：流场中布置的涡流发生器可以诱导漩涡,而由其所诱导的流向涡对强化传热起主要作用。与相同条件下未布置涡流发生器的情况相比,局部对流换热系数可提高64%～105%,平均对流换热系数则可提高17%～36%;涡流发生器附近位置的对流换热系数提高幅度最大,传热面附近流体的流动状况及流动结构与传热密切相关。     

螺旋曲面通道内组合涡发生器的强化传热及优化研究

对螺旋曲面通道壁面上设置三角翼和椭圆柱两种组合涡发生器进行了研究,利用计算机流体软件Fluent进行数值模拟,在雷诺数Re为4000～7000范围内,研究组合间距s、三角翼攻角α、椭圆柱攻角β对换热的影响。由正交实验对涡发生器的结构进行整体优化,得出影响换热因素的主次顺序及优化组合结构,并通过实验比较优化结构与常规结构的综合强化换热效果。结果表明当s＝90 mm、α＝45°、β＝45°时,其综合换热效果最佳。     

强化矩形通道内滴形柱涡发生器传热与流动特性分析

设计了一种滴形柱涡发生器。在雷诺数4 000~20 000范围内,采用数值模拟的方法对布置在矩形通道内的单排单列不同斜截角,以及多排不同攻角、高度、横向与纵向间距的滴形柱涡发生器传热机理与流动特性进行分析,得到各参数的经验关联式,为工业应用提供参考。     

组合涡发生器在螺旋板式换热器上的强化传热研究

在螺旋板式换热器螺旋通道内设置三角翼和椭圆柱组合涡发生器,利用流体计算软件Fluent进行三维数值模拟。研究了Re为4000～7000内组合涡发生器对通道平均Nu和平均阻力系数f的影响,并应用场协同原理进行了分析。与只加装椭圆柱涡发生器的螺旋板式换热器进行对比,结果表明,纵向涡发生器产生的二次流能改善螺旋通道内的速度场与温度场的协同性,起到强化换热作用。在正三角形排列方式下,组合涡发生器通道的平均Nu比椭圆柱涡发生器的平均Nu增大8.7%,阻力因子f减小23.7%,强化换热的效果较好。     

任意斜截椭圆柱面壳体的展开计算

任意斜截椭圆柱面壳体的展开计算湖北拖车厂曹中生１椭圆柱面壳体的应用与分类公路上行驶的油罐车、洒水车的油罐、水箱是图１应用与分类正截椭圆柱面壳体（图１ａ）。为充分地减小流体阻力和提高抗弯强度，两对称相交（或平行）的等直径输送圆管间的连接部分可设计成两斜...     

高粘度流体中转子旋流长度的数值分析

研究了介质为60%甘油时换热管内螺旋两叶片转子的旋流特性。通过实验验证,得到了介质为高粘度流体时强化传热的数值模拟方法。使用该模拟方法,通过对比纵向涡量云图、轴向速度云图和湍动能云图对单个螺旋两叶片转子的旋流长度进行了分析研究,得到螺旋两叶片转子的旋流长度为80mm。同时对内置不同间隔距离转子的换热管的传热及阻力性能进行了模拟研究,并将模拟结果进行了对比以得到转子排列的最佳间隔距离,即转子的旋流长度,从而验证了之前的分析结果。     

小尺度圆柱涡流发生器的传热与流动特性

应用大涡模拟对设置小尺度圆柱涡流发生器矩形槽道底面的流动与传热特性进行研究,小尺度圆柱涡流发生器置于湍流边界层内。分析不同间隙比对槽道流动结构、槽道底面Nusselt数、摩擦因数以及综合性能系数的影响。此外,采用大涡模拟所得槽道计算结果与前人直接数值模拟结果一致性很好,验证所采用数值方法的准确性与可靠性。研究结果表明,与未设置小尺度圆柱涡流发生器的矩形槽道相比,设置小尺度圆柱涡流发生器槽道底面的换热性能可以得到显著提高,同时其流动阻力的增加亦得到有效抑制。当间隙比为2.0时,槽道底面换热性能最佳,其Nusselt数可提高18.76%;而当间隙比为0.5时,槽道底面减阻效果最佳,摩擦因数可减小3.77%。     

Flow topology,heat transfer characteristic and thermal performance in a circular tube heat exchanger inserted with punched delta winglet vortex generators

To improve the heat transfer rate and thermal performance, the punched delta winglet vortex generators, DWVGs, were inserted in the middle of the circular tube heat exchanger. The effects of the flow attack angles and the flow directions were investigated numerically for the Reynolds number Re = 100–2000. The finite volume method and the SIMPLE algorithm were used to study. The results are reported in terms of the flow structure, heat transfer behavior and thermal performance evaluation and also compared with the smooth tube with no vortex generators. As the numerical results, the use of the DWVGs in the tube can improve the heat transfer rate and thermal performance by creating the vortex flow through the tested section. The rise of the flow attack angle results in the increasing strength of the vortex flows. The flow attack angle of 25° performs the highest heat transfer rate and thermal performance, while the flow attack angle of 0o gives the reversed results. The computational results reveal that the optimum thermal enhancement factor is around 2.80 at Re = 2000, α = 25°, with the winglet tip pointing downstream. The correlations on both the Nusselt number ratio and friction factor ratio for the DWVG in the tube heat exchanger are presented.     

Heat transfer enhancement for fin-tube heat exchanger using vortex generators

Vortex generators are fabricated on the fin surface of a fin-tube heat exchanger to augment the convective heat transfer. In addition to horseshoe vortices formed naturally around the tube of the fin-tube heat exchanger, longitudinal vortices are artificially created on the fin surface by vortex generators. The purpose of this study is to investigate the local heat transfer phenomena in the fin-tube heat exchangers with and without vortex generators, and to evaluate the effect of vortices on the heat transfer enhancement. Naphthalene sublimation technique is employed to measure local mass transfer coefficients, then analogy equation between heat and mass transfer is used to calculate heat transfer coefficients. Experiments are performed for the model of fin-circular tube heat exchangers with and without vortex generators, and of fin-flat tube heat exchangers with and without vortex generators. Average heat transfer coefficients of fin-flat tube heat exchanger without vortex generator are much lower than those of fin-circular tube heat exchanger. On the other hand, fin-flat tube heat exchanger with vortex generators has much higher heat transfer value than conventional fin-circular tube heat exchanger. At the same time, pressure losses for four types of heat exchanger is measured and compared.     

CFD analysis of fin tube heat exchanger with a pair of delta winglet vortex generators

Among tubular heat exchangers, fin-tube types are the most widely used in refrigeration and air-conditioning equipment. Efforts to enhance the performance of these heat exchangers included variations in the fin shape from a plain fin to a slit and louver type. In the context of heat transfer augmentation, the performance of vortex generators has also been investigated. Delta winglet vortex generators have recently attracted research interest, partly due to experimental data showing that their addition to fin-tube heat exchangers considerably reduces pressure loss at heat transfer capacity of nearly the same level. The efficiency of the delta winglet vortex generators widely varies depending on their size and shape, as well as the locations where they are implemented. In this paper, the flow field around delta winglet vortex generators in a common flow up arrangement was analyzed in terms of flow characteristics and heat transfer using computational fluid dynamics methods. Flow mixing due to vortices and delayed separation due to acceleration influence the overall fin performance. The fin with delta winglet vortex generators exhibited a pressure loss lower than that of a plain fin, and the heat transfer performance was enhanced at high air velocity or Reynolds number.     

Determination of optimum winglet height of longitudinal vortex generators for the best thermo-hydraulic performance of compact heat exchangers

The effect of winglet height on the thermal-hydraulic performance of finned tube heat exchanger (FTHE) is investigated numerically. The rectangular winglet pairs (RWPs) having 5° attack angle are placed in common flow up (CFU) manner adjacent to the tubes for analysis. The air-side performance evaluation has been done based on the area goodness factor (j/f). The working fluid (air) is considered as incompressible fluid. Additionally, MOORA (multi objective optimization on the basis of ratio analysis) method is employed to get the best performance order of various configurations by taking Nusselt number (Nu) and area goodness factor (j/f) as beneficial attributes and friction factor (f) as a non-beneficial attribute having equal significances. The present study reveals that the rectangular vortex generators having 60 % of the channel height provides the better thermal hydraulic performance compared to the other considered cases.

     

A study on the improvement of heat transfer performance in low temperature closed Thermosyphon

The study focuses on the heat transfer performance of two-phase closed thermosyphons with plain copper tube and tubes having 50, 60, 70, 80, 90 internal grooves. Three different working fluids (distilled water, methanol, ethanol) are used with various volumetric liquid fill charge ratio from 10 to 40%. Additional experimental parameters such as operating temperature and inclination angle of zero to 90 degrees are used for the comparison of heat transfer performance of the thermosyphon. Condensation and boiling heat transfer coefficients, heat flux are obtained using experimental data for each case of specific parameter. The experimental results are assessed and compared with existing correlations. The results show that working fluids, liquid fill charge ratio, number of grooves and inclination angle are very important factors for the operation of thermosyphons. The relatively high rate of heat transfer is achieved when the thermosyphon with internal grooves is used compared to that with plain tube. The optimum liquid fill charge ratio for the best heat transfer performance lies between 25% and 30%. The range of the optimum inclination angle for this study is 20°-30° from the horizontal position.     

基于正交试验的等弦波形管热工性能分析

基于模拟正交试验法并通过Fluent软件分析,开发出新型等弦波形强化换热管（D＝10 mm, L＝45 mm,R＝50 mm）,分析得出了较大的法向分速度冲刷壁面是影响其对流换热的主要因素;利用场协同理论分析了等弦波形管的强化换热原理,计算出协同角,通过与直管对比发现,协同角有明显减小,局部达10°以上;最后从泵功耗指标出发,计算得出与直管相比,换热效率提高47．6％,验证了新型等弦波形强化换热管具有较优越的热工性能。     

卧式半圆柱型涡流发生器的传热与阻力特性及场协同理论分析

为研究卧式半圆柱型涡流发生器的特性,通过数值模拟方法对安装有卧式半圆柱型涡流发生器的矩形通道进行了传热和流阻特性的研究。结果表明:在相同雷诺数Re下,安装有卧式半圆柱型涡流发生器的矩形通道的换热效果明显优于矩形光滑通道,但阻力系数也均大于矩形光滑通道。对比综合性能指标表明,6/8H楞长处的综合特性PEC值最大。通过对场协同理论分析得出,矩形光滑通道和四种不同楞长的卧式半圆柱型涡流发生器的协同数Fc均随着雷诺数Re的增大而减小,而且其变化趋势也越来越平缓,最终逐渐趋近于定值。相同雷诺数Re下,楞长为6/8H时的协同数Fc最大。