【作者】 李锋祥；

【导师】 杨卫民；

【作者基本信息】 北京化工大学 ， 机械设计及理论， 2009， 博士

【摘要】 管壳换热器广泛应用于过程工业众多领域,而其自身的结垢问题一直没能得到有效地解决,从而造成能源的巨大浪费。管程组合转子强化技术具有强化传热与在线自动清洗功能,其灵活的结构和材料设计能够满足各种管壳换热器的工况及装配要求,为管壳式换热设备污垢问题的解决提供了方案。因此,对管程组合转子的性能、作用机理和结构改进展开研究,具有重要的工程应用价值。本文借助实验手段,研究管程组合转子的扰流特性、传热及阻力特性,进而分析其作用机理并提出结构改进方案。通过可视化实验、转子转速测试和PIV流场测试,对管程组合转子扰流的冷态流场特性进行了从定性到定量地分析。理论推导出转子转速公式,认为影响转子转速的因素包括几何、质量属性(流体和转子的密度)和粘性因素。提出在管程存在无旋流,并通过PIV测试结果得到了初步验证。进而分析认为,管程的轴向速度分布与转子旋转相结合,决定了流体的流动状态分布,如径向混流、湍流强度、涡量等。将相邻两组转子之间的空隙定为滞流区,并对滞流区的流动特性进行了详细分析。总结冷态实验,可得出如下结论:(1)管程组合转子对流体造成不同程度、甚至不同方向的旋流,从而强化了流体层间剪切效应,促进流体的径向混合和边界层剪切变薄;(2)滞流区的流动状态复杂无序,湍流强度更大;(3)管程局部有纵向涡产生。管程组合转子扰流的流动特性有利于传热强化。通过传热及阻力实验,对特征长度的选取原则进行了讨论和验证。结合各无量纲特征数的物理意义和实验数据,认为对于管程动态扰流技术,应选取换热管内径进行实验数据的处理和计算,得到的结果能更准确地表征其实际传热强化程度及综合性能。基于这一原则,对管程组合转子的传热及阻力特性进行实验研究。以油为工质的实验表明:在Re=500-8000的层流区和转捩区,管程组合转子可不同程度地提高传热性能,同时也不可避免地增大阻力损失;增大转子外径对传热强化的贡献微弱,阻力系数却明显增大;增大转子螺旋角度对传热强化的贡献显著,同时阻力也有较大增长;螺旋角度最大的一种转子实验数据出现断层,分析是转子运动状态突变所致;其它4种规格的转子实验数据没有出现断层,很可能是由于其导程较大,转子一直处于停转状态;性能综合评价表明,导程最小的转子具有最佳综合性能。可见,强化层流对流传热,主要是要增强管程流体的径向混合。以水为工质的实验表明:在Re=10~4-10~5的旺盛湍流区,管程组合转子可不同程度地提高传热性能,同时也不可避免地增加阻力损失;增大转子外径对传热强化的贡献明显,而阻力系数略有增大;增大转子螺旋角度对传热强化的贡献微弱,却造成阻力的较大增长;性能综合评价表明,导程较大、外径较大的转子具有最佳综合性能。可见,强化湍流对流传热,主要是要加强对管程流体流动边界层和热边界层的扰动。因此,应根据流体的流动状态(层流、过渡流或湍流)选择转子的结构参数。对于火力发电厂的凝汽器,其流动多处于旺盛湍流区,在安装条件允许的情况下,应尽可能选择外径较大、螺旋角度较小的转子。针对传统结构的转子在阻垢性能、扰流特性、传热及阻力特性方面存在的一些问题,提出镂空型转子、转片式转子和边界层剪切型转子三种结构改进方案。对前两种方案进行了可视化实验和以水为工质的传热及阻力实验验证。可视化实验表明:镂空型转子的硬质颗粒污垢抵抗能力有很大程度提高,但仍不能满足颗粒浓度大或长时间运行的工况要求;转片式转子独特的结构设计,彻底避免了硬质颗粒污垢堵塞,可满足阻垢性能要求。传热及阻力实验表明:镂空型结构虽然可以一定程度降低阻力,但其传热性能也由于流体向镂空中心的汇聚效应而下降,其综合性能甚至不如传统结构转子;转片式转子在降低阻力的同时,保持传热性能不下降,因此其综合性能得到显著提升。可见,强化湍流对流传热,最重要的仍是扰动和减薄边界层。中心流体的径向混合对传热强化的贡献不大,反而徒增阻力。因此,提出边界层剪切型转子,其结构能够将流体中心区的能量向边缘传递,造成对边界层的剪切扰动。更多还原

【Abstract】 Shell-and-tube heat exchangers are widely used in many process industry fields and pronouncing tremendous waste of energy for remaining unresolved fouling problem.Tube side assembled rotors(TSAR) can achieve heat transfer enhancement and online cleaning,with flexible design of structures and materials to meet technical requirements of various shell-and-tube exchangers, lighting up the hope for conquering scale formation.Therefore,it is of great importance to study the performance,mechanism and structure modification of tube side assembled rotors.In this paper,many experimental methods were employed to investigate the disturbed flow characteristics,heat transfer and friction loss of TSAR to analyze its mechanism.Moreover,modified structures were put forward aiming for better performance.Visualization experiment,rotate speed measurement and PIV test gave an exhibition of disturbed flow field by TSAR from qualitatively to quantitatively. Rotate speed of rotors was theoretically deduced,considering geometrical, mass and viscous factors.A non-rotating flow velocity was believed to exist somewhere in the tube,which was primarily proved by PIV test.Axial flow velocity distribution of tube side combined with rotation of rotors,determined the flow state,such as radial mixing,turbulent intensity and vorticity.The interspace between two adjacent groups of rotors was named stagnant sect, flow characteristics in which were discussed in detail.To sum up the cold test of TSAR,several conclusions are reasonable as follows:(1) TSAR created swirl flow with different extent and even different direction,which intensified radial mixing and shear effect between fluid layers,attenuating the boundary layer.(2) A complicated flow state existed in the stagnant sect,inducing stronger turbulent intensity.(3) Longitudinal vortexes sometimes occurred in tube side.It is exciting that all these features are good for heat transfer enhancement.Principle for selecting characteristic length was advanced and validated based on heat transfer and friction loss experiment.Considering the physical significance of each characteristic number combined with experiment data,it was made concise that the inner diameter of the tube should be used in data deduction for dynamic tube side flow disturbing technique.Experimental results calculated in this way revealed actual heat transfer performance much better.Thus,experimental investigations were carried out to evaluate the heat transfer and friction loss performance of TSAR.The oil experiment indicated: In the laminar and transition region with Re=500-8000,TSAR augmented heat transfer and inevitably increased friction loss;Bigger rotor diameter hardly contributed to heat transfer enhancement but lead to much more friction loss; Greater rotor pitch angle brought about more remarkable heat transfer enhancement and also more friction loss;Experiment data concerned with rotors with biggest pitch angle displayed a dislocation,which was assumed due to kinestate mutation of rotors;Experiment data concerned with other four types of rotors were continuous,which might imply that these rotors were always at stationary state for their smaller pitch angle;Performance evaluation indicated that rotors with biggest pitch angle behaved best.It can be concluded that radial mixing of fluid should be the principal choice for enhancing laminar convective heat transfer.The water experiment indicated:In the turbulent region with Re=10~4-10~5,TSAR augmented heat transfer and inevitably increased friction loss;Bigger rotor diameter obviously contributed to heat transfer enhancement and lead to not much friction loss;Greater rotor pitch angle brought about almost nothing for heat transfer enhancement but gained more friction loss;Performance evaluation indicated that rotors with biggest diameter and smallest pitch angle behaved best.It can be concluded that attenuation of boundary layer should be the principal choice for enhancing turbulent convective heat transfer.Therefore,structure parameters of rotors should be determined according to the flow region of fluid(laminar,transition or turbulent).With regard to condensers of power plants,rotors with bigger diameter and smaller pitch angle are preferred providing installation condition permits,because the tube side flow was under fully developed turbulence.According to some insufficiencies of traditional rotors detected in anti-fouling performance,flow disturbing characteristic,heat transfer and friction loss performance,through-carved rotor,rotary-vane rotor and boundary layer shearing rotor were invented to fix the problem.Visualization experiment together with heat transfer and friction loss experiment with water as working fluid were launched to examine through-carved rotor and rotary-vane rotor.Visualization experiment indicated:Through-carved rotors provided better resistance against hard particle fouling than traditional rotors, but still stepped far behind the requirement of high concentration and long running;rotary-vane rotors avoided blockage of hard particle fouling successfully due to particular structure design.Heat transfer and friction loss experiment indicated:Through-carved structure reduced friction loss to some extent,but also weakened heat transfer performance for the influx of fluid to through-carved void,with an over-all property even inferior to traditional ones; rotary-vane rotors reduced friction loss while maintaining constant heat transfer performance,thus achieving much better over-all property.It can be concluded that attenuation of boundary layer should be the principal choice for enhancing turbulent convective heat transfer.For turbulent convective heat transfer,radial mixing of fluid hardly contributes to heat transfer augmentation but increases friction loss to no purpose.Consequently,boundary layer shearing rotor was promoted to settle the conflict.It was designed with a particular configuration which can transfer the kinetic energy of central fluid to the edge and margin,shearing,disturbing and attenuating boundary layer.更多还原