【作者】 冯丽丽；

【导师】 杨勇平； 杜小泽；

【作者基本信息】 华北电力大学 ， 热能工程， 2012， 博士

【摘要】 空冷技术是解决富煤缺水地区火力发电的重要选择,近年来在我国北方地区得到大力发展。为弥补空气冷却能力低的不足,火电机组汽轮机排汽的空冷凝汽器需采用翅片管束达到强化传热的目的。空气在翅片间流动的阻力损失以及在扩展表面上的传热特性是影响机组空冷系统性能的关键因素,也是火电空冷凝汽器优化设计的主要方向。首先针对适用于低雷诺数条件下,空气在翅片间的流动及传热的物理数学模型进行分析讨论。以此为基础,分析椭圆管矩形翅片空气侧换热的特性,对提出的偏置矩形翅片强化空气侧的换热进行研究。其次分析钢制扁平管钎焊蛇形铝翅片空气侧的换热特性,对纵向涡强化扁平管蛇形翅片空气侧的换热及新提出的交错蛇形短翅片和间断蛇形短翅片强化扁平管空气侧的换热进行深入研究。偏置矩形翅片强化椭圆管空气侧换热的数值结果表明,翅片偏置于空气进口方向,空气侧总的表面传热系数平均增加了4.2%,且偏置距离越大,表面传热系数越大。将四种不同类型的纵向涡发生器分别冲压于扁平管蛇形翅片的表面强化空气侧的换热。对三角形对涡发生器在翅片表面的冲压位置及涡的排数对流动与换热的影响进行分析,数值结果表明,三角形对涡强化了空气侧的换热,同时阻力增大。在研究风速范围内,攻角为25°的三角形对涡强化换热性能的评价指标PEC达到最高,六对三角形对涡强化空气侧表而传热系数是最高的。且当一对三角形对涡冲压在翅片的几何中心时,平均PEC可达到1.182。经PIV(粒子图像测速仪)系统的测试,得到涡发生器空气流动截面上的速度矢量图,并验证了数值结果的可行性。进一步通过数值和实验研究六对三角形对涡强化扁平管蛇形翅片空气侧流动和换热的特性,并拟合得出努谢尔特数和摩擦系数随雷诺数变化的实验关联式。其中,对于对流换热Re=1500-4500,而流动损失则在Re=500-4500。实验结果表明,空气侧的平均努谢尔特数增加了20.5%-59.8%,平均摩擦系数增加了12.8%-82.6%,且PEC都大于1。红外热像仪测试技术作为一种新的测试方法首次被应用,与原始结构相比,当翅片表面冲压有三角形对涡后,翅片表面的温度降低,尤其涡发生器后方的翅片表面温度明显降低。交错蛇形短翅片强化扁平管空气侧换热的数值和实验研究表明,交错蛇形短翅片破坏了边界层发展,揭示了其强化换热的物理机制。较长翅片断开为短翅片后,翅片表面的局部传热系数不再是单调递减,而是阶跃性地下降。空气侧表面传热系数提高了1.4%-16%。通过翅片表面温度分布的比较,在断开短翅片的空气进口处,翅片表面的温度降低。用数值计算方法对间断短翅片的布置方式(顺排、叉排)、短翅片排数,及不同断开间距,对空气流动与传热的影响进行分析。结果表明,在工程实际应用的空气流速范围内,间断短翅片有效地提高了空气侧的传热性能,且间断结构减小了空气与壁面的接触面积,流动压力损失的增加受到抑制。更多还原

【Abstract】 Recently air-cooling technology has been actively developed in north China where water resource is of shortage, to solve water crisis for power plant. To compensate for the low cooling capacity of the air-cooling technology, finned flat tube is employed. As a main direction to improve the design, the resistance loss among the fins and the heat transfer of the extended surface are the main factors.According to the low Renolds number, the physical model regarding flow and heat transfer characteristic between fins were analyzed. Based on it, the air-side heat transfer characteristics of oval tube with rectangular fins, the oval tube with deviating-geometric-center rectangular fins, the wavy finned flat tube, the wavy finned flat tube with longitudinal vortex generators, a new staggered short fins and discontinuous short wavy fins were investigated in detail.The numerical simulations concerning deviating-geometric-center rectangular fins show that when deviating for the air inlet direction, the average air-sided heat transfer coefficient increased by4.2%. The more distance away for the center is, the bigger the heat transfer coefficent is. Four various types of punched longitudinal vortex generators were employed to enhance air-side heat transfer on the fin surface of flat tube used in direct air-cooled condenser. It was found that the delta winglet pair with attack angle25°could reach the greatest performance evaluation criteria (PEC) under the conditions of the inlet air flow velocity varied from2m s-1to5m s-1. The influences of locations on the fin surface and the row number of longitudinal vortex generator were also discussed. One delta winglet pair at the middle of fin surface and minimum row number, n=1, with the average PEC being1.182, has the best heat transfer performance of all conditions, which can be recommended for practical applications. A visualization experimental study on flow field in wind by Particle Image Velocimetry (PIV) was conducted. The numerical simulations verified that the delta winglet pairs can generator obvious longitudinal vortex pairs at the down-sweep zone.A further experimental and numerical investigation was conducted into flat tube wavy finned channel flows with and without punched six delta winglet pairs. The experimental correlations of Nusselt number and friction factor vs Reynolds number in the experimental region were obtained. Through comparisons between experiments and numerical simulations, the numerical feasibility was verified in the air-side heat transfer of wavy finned flat tube with and without punched six delta winglet pairs. Experimental and simulation results showed that a substantial increase in the heat transfer with six delta winglet pairs generators. It was observed that average Nusselt number increase by20.5%-59.8%and the average friction factor increase by12.8%-82.6%in experiment. And, the performance evaluation criteria, PEC, is greater than1. By employing IR thermography, temperature distribution information about wavy fin surface with and without punched six delta winglet pairs were obtained.It has been proved that the serrated fins can enhance heat transfer obviously by breaking the development of boundary layer periodically. In the present study, the original continuous wavy fins were disconnected to create a new structure, namely, discontinuous short wavy finned. The results showed that, compared with the. original continuous structure, the discontinuity short-wavy-fin flat tube effectively improved the air-side heat transfer at the air velocity of engineering field, and because the discontinuous fins structure reduces the contact area of air and wall, the increase in pressure loss is suppressed.更多还原