【作者】 温娟；

【导师】 唐大伟；

【作者基本信息】 中国科学院研究生院（工程热物理研究所） ， 工程热物理， 2014， 博士

【摘要】 空冷凝汽器作为直接空冷系统的核心换热设备,其气侧流动换热性能直接影响着整个机组的效率。而体积庞大的扁平管束间的相互干扰使管束尾部形成复杂的流动结构,凝汽器气测空气热阻大换热量低,翅片管束间空气流量分配不均,空冷风机与A型凝汽器间的耦合效应等,所有影响因素相互耦合使得空冷系统具备复杂的非线性尺度效应。本文用大涡数值模拟(LES)方法对直接空冷凝汽器空气侧流动换热问题进行了研究。首先基于圆柱绕流,将大涡模拟结果与他人实验数据进行对比分析,验证了大涡模拟在绕流问题中是一种行之有效的湍流模型和数值方法。本文把大涡模拟方法运用到了直接空冷凝汽器气侧流动换热问题的研究中,通过与他人雷诺时均结果的比较,表明大涡模拟能够捕获更多凝汽器流动换热的细节及相关湍流统计信息,能有效预测翅片管后复杂的旋涡运动,为进一步研究空冷凝汽器气侧流动结构与流动换热间所隐含的相互作用及热质传递规律提供理论基础。对不带翅片的扁平管固有的三维流动结构及换热特性进行了大涡模拟研究。计算结果表明,与圆管相比,扁平管的回流区长度、阻力系数都远小于圆管,体现了扁平管在空冷凝汽器应用中的优势。当扁平管长宽比L/D减小时,其具有较小的回流区、较快的旋涡脱落频率、良好的换热特性以及较小的阻力。研究了翅片扁平管束的相互干扰对流动换热的影响,结果表明,当考虑相邻翅片管的干扰时,与单翅片管相比,其边界层分离点、回流区范围、旋涡脱落模式、流动结构及换热特性明显不同。对双翅片管而言,两个翅片扁管尾部的旋涡脱落模式存在明显的相位关系,在扁平管间距为H/D=1.95,Re=4006时,每个扁平管尾部保持各自的流动形态,相互干扰影响较小,当雷诺数增大时非线性相互干扰明显增强。对于单翅片扁管因不受干扰,其两侧翅片的局部努赛尔数分布基本相同。当考虑相邻翅片管干扰时,其两侧翅片表面的局部努赛尔数呈现不同的变化趋势。雷诺数越高,不同翅片表面的局部努赛尔数差距越明显,并且在相同雷诺数下,单翅片管翅片表面的局部努赛尔数明显低于双翅片管,说明相邻翅片扁管尾部的相互干扰引发的二次频率对流动换热有增强的效果,所以在空冷凝汽器的设计中应充分考虑管束之间的相互干扰对流动换热的影响。对冲压三角形涡流发生器翅片通道的流动换热特性进行了大涡模拟研究。结果表明,与光滑的翅片通道相比,加装涡流发生器后,在计算雷诺数范围内,气侧平均努赛尔数Nu增加了约49.68%-54.14%,平均摩擦系数增加了约35.63%-37.35%,与他人实验关联式结果基本吻合。同时对纵向涡的强化传热机理进行了分析,近壁湍流的下扫过程利于换热。在翅片通道中分别布置梯形翼及斜截半椭圆柱面涡流发生器,对其流动换热机理进行了研究。计算结果表明,因斜截半椭圆柱面涡流发生器产生的端部涡和马蹄涡的综合作用,比梯形翼在强化传热方面具有优势。对于梯形翼来讲其下扫过程和上扬过程贡献的雷诺应力基本相同,基于x-y平面的象限分析手段,其雷诺应力分布在第Ⅱ象限和第Ⅳ象限；而斜截半椭圆柱面以上扬过程贡献的雷诺应力为主,其雷诺应力主要分布在第Ⅱ象限。当雷诺应力位于第Ⅳ象限,即下扫过程时,壁面往往具有较大的阻力,而斜截半椭圆柱面涡流发生器的雷诺应力大部分出现在第Ⅱ象限,所以其对壁面产生的阻力较小,揭示了其减阻的机理。基于湍流拟序结构理论,斜截半椭圆柱面涡发生器可同时实现强化传热和流动减阻。对不同波幅比上波长a0/λ=0.01、0.02、0.04、0.06的波纹翅片的流动换热进行大涡模拟研究。计算结果表明,当波纹翅片的波幅较小时,在波谷位置没有回流现象。当波幅为α0/λ＝0.04时出现回流区,随着波幅的增大其回流区范围也在增大。当波幅较大时波纹面上的涡结构增加,并且在展向出现较多不等间距的高速和低速条带结构。不论是在波谷还是在波峰处,脉动速度都是随着波幅的增大而增大。在波纹的迎风侧其换热和剪切力较大,并出现斑块。当α0/λ=0.06时,其具有良好的换热性能和较大的阻力,其综合性能最优。实现了空冷风机与A型凝汽器的整机耦合计算,获得空冷单元内部较为真实的流场分布。计算结果表明,其内部流场分布极不均匀,导致凝汽器出口的温度分布也不均匀,同一出口面上,风机叶片迎风的一侧速度大温度较低,而背风的一侧速度小温度高。环境温度在250K～310K的范围变化时,凝汽器平均进口风速的变化范围不超过10%,并且凝汽器左右两侧的速度分布及大小并不完全相同,两侧相差2.9%左右。凝汽器两侧风量的偏差,导致即使在无环境风的条件下,凝汽器蒸汽管道顶端速度和温度不对称,出现偏置现象。更多还原

【Abstract】 Air-cooled condenser is the key element of the direct air cooling system.The performance of direct air-cooled condensers is affected directly by air-side flow and heat transfer characteristics. The complex flow structures were generated among the wakes behind the bodies when the air flow past multiple wavy finned flat tubes, beacuse of the nonlinear interactions among the wakes. High thermal resistance of air side will lead to low heat transfer effieieney. The flow complexities at the exit of the axial flow fan result in the flow mal-distribution on the surface of the finned tube bundles. There exists the strong coupling relationship between the fan and A-shaped frame condenser cell. All of the influence factors will lead to nonlinear scale effect.The characteristics of the flow and heat transfer on air-side of the direct air-cooled condenser have been simulated using large eddy simulation (LES).The large eddy simulation results of flow around a circular cylinder are in well agreement with the experimental data. Using LES theory and method for the direct air-cooled condensers, can captures the complex turbulent motions of air-cooled condenser in detail. And the complex vortex motion in the wake region of the flat finned-tubes are predicted more precisely than RANS approach. It provides theoretical basis to study the mechanism of flow and heat transfer.Three-dimensional flow structure and heat transfer characteristics of the flat tube without of fins has been studied. The results show that the flat tube have lower air-side pressure drop and smaller wake region. When the aspect ratio L/D of flat tube is reduced, the flat tube have lower air-side pressure drop, smaller wake region and better heat transfer coefficients.When the wake interaction of the adjacent finned tube is considered,the separation points of the boundary layers, the range of wake region,vortex shedding pattern, flow structure and heat transfer have a notable deviation from the single tube. For double finned tube,the vortex shedding from one flat tube have a definite phase relationship with the other tube.At the spacing H/D=1.95and Re=4006, the vortices remain distinct and the interaction between the flat tubes is weaker. The degree of nonlinear wake interaction depends on the Reynolds number at the spacing.When the Reynolds number increases, the nonlinear interference is enhanced.The two side of the single flat finned-tube have the same distribution of local Nusselt number since they experience the same undisturbed fluid, whereas for the two side by-side flat finned-tubes the local Nusselt number distribution on both side of the tube is different, due to the mutual interference of the flat finned-tubes.The difference of the local Nusselt number between the two fins are larger when the Reynolds number increased. Furthermore, the local Nusselt number of the same fin in single finned-tube is lower than the two side-by-side finned tubes at same Reynolds number.The secondary frequency induced by the interactions of the flat finned-tubes have a better effect on heat transfer.The design of ACC should also account for wake interaction.The flow and heat transfer characteristics of flat tube wavy finned channel with punched six delta winglet pairs was studied by large eddy simulation. The results show that the average Nusselt number increase by49.68%-54.14%and the average friction factor increase by35.63%-37.35%.The results are in well agreement with the experimental correlations of Nusselt number and friction factor with Reynolds number by others.The head vortex system and horseshoe vortex are induced by oblique-cut semi-elliptic cylinder shells vortex generator.The combination of the two type vortex system plays a vital role in heat transfer enhancement. The contribution of reynolds stress in sweep process and ejection process are basically same for trapezoidal wing, and the Reynolds stress distribution is in the Ⅱ quadrant and IV quadrant.The reynolds stress contribution of oblique-cut semi-elliptic cylinder shells is in the II quadrant, and the contribution of Reynolds stress is mostly in ejection process. The drag is large when the distribution of Reynolds stress in the sweep process.So the semi-elliptic cylinder shells have low flow drag. Based on the theory of turbulent coherent structures, the oblique-cut semi-elliptic cylinder shells vortex generator can achieve the aim that heat transfer enhancement and flow drag reduction.Large eddy simulation has been employed to flow and heat transfer over wavy fin surface for different amplitude:ao/λ=0.01、0.02、0.04、0.06. The smaller wave amplitude of wavy surface in this study does not form the recirculation in the valley.With the almplitude increasing,the range of the recirculation and the quantity of the vortex is significantly increased.The high and low speed streaks are obvious increased. A number of high wall sheer stress and Nusselt number spindle-shaped spots are observed. The bigger wave amplitude surface have better heat transfer performance.A three dimensional coupled air flow field calculation is carried out for the rotational fan and the stock-still A-shaped frame-work, which can provide an accurate description of the detailed flow filed in air-cooled condenser cell. And the air mal-distribution on the finned tube bundles due to the interaction between the fan and air cooled condenser.With the ambient temperature increasing, the average inlet air velocity of the finned tube bundles is decreased, the variation range is less than10%. Furthermore, the inlet air velocity of the left side and right side of the finned tube bundles are not the same,which is related to the biased flow that the non-symmetric velocity distribution on the top of the exhaust steam pipe even under the no-wind condition.更多还原