【作者】 何兆红；

【导师】 邓先和；

【作者基本信息】 华南理工大学 ， 化学工程， 2010， 博士

【摘要】 管壳式换热器具有机械密封性好、承压能力强的特点,已被广泛应用于石油、化工等领域,但由于传热管的管壁较厚(2～3.5mm),金属消耗量大,随着金属原材料价格的上涨,设备的材料成本会越来越高;而板式换热器可采用薄的金属板(0.5～1mm)作为传热元件,金属消耗量较少,但密封性差,易泄漏,使用范围受到限制。结合管壳式换热器和板式换热器的优点,本文提出的矩形管束换热器可以较好地解决这一问题。矩形管束换热器内部结构主要由矩形缩放管和旋流片构成。矩形缩放管管壁较薄(0.8～1mm),旋流片既插入矩形管管内,又支撑矩形管管束,因此对管程和壳程都起到强化传热作用。本文采用实验和数值模拟的方法对矩形缩放管内插入旋流片的局部传热与流阻性能进行了研究,结果表明:随着旋流片下游距离的增大,局部努塞尔( Nu )数和阻力系数逐渐减小;其传热综合性能要优于光滑矩形管和矩形缩放管,具有很好的强化传热性能;以场协同理论为指导,分析了流道内速度场与热流场的分布情况,发现旋流片能够使流体产生螺旋运动,改善了流道内速度场与热流场的协同程度,强化传热效果明显。虽然矩形管束换热器具有良好的传热性能,但实验发现,当管外流体压力增大时,矩形管的板面易发生变形,为增强板面的抗压性,可采用带自支撑结构的矩形传热管,壳程管束间采用旋流片或扭带支撑。通过对自支撑矩形管束换热器进行的换热器整体传热与流阻性能研究,得到了管程和壳程Nu数关联式。采用正交试验和数值模拟的方法对自支撑矩形管内旋流片支撑结构进行了优化,结果表明,增大旋流片旋转角和减小旋流片间距有利于提高强化传热综合性能,管内插入连续扭带的传热综合效果要优于插入旋流片时的效果。比较分析自支撑矩形管内不同插入物的热阻和湍流度分布与传热综合性能的关系,本文提出了一种新的提高强化传热能力的途径。运用热阻分析和湍流度分析方法,对自支撑矩形通道内无插入物、插入扭带和旋流片三种通道内层流粘性底层、过渡区和湍流中心区的热阻分布以及局部湍流度分布进行比较,结果表明与无插入物相比,插入扭带的热阻分布在粘性底层的热阻比例明显下降,并且三个区的热阻比例分配更均匀。流道内插入旋流片的热阻分布与无插入物时基本相同,粘性底层为控制热阻区域。在湍流度分布方面,扭带和旋流片的局部湍流度分布变化都比较平缓,避免了通道内无内插物时局部湍流度出现过大或过小的局面。根据自支撑矩形管道内不同支撑结构的热阻和局部湍流度分布分析结果与传热综合性能的关系可以得到:当流道内热阻分布和局部湍流度分布比较均匀时,对提高换热器的传热综合性能有利,传热温差和流体输送功可以得到比较有效的利用,可促使传热综合性能的提高。此外,以场协同理论为指导,扭带的插入改善了速度场与温度梯度场的协同性,也证明了自支撑矩形通道内插入扭带能起到强化传热的作用。对自支撑矩形管内插入扭带的流阻与传热性能进行了实验研究,比较分析了在不同Re数下不同扭带插入不同矩形管时的性能。结果表明:Nu数和阻力系数都随着扭率的减小而增大;并且对于某一种扭带,其Nu数随着Re数的增大而增大,阻力系数随着Re数的增大而减小。以传热综合性能评价因子作为目标函数,最优结构匹配参数为:扭带扭率y =3.5,槽间距s=190mm。更多还原

【Abstract】 The shell-and-tube heat exchanger is widely used in petrolic and chemical industry. It has good leak-proof quality and can withstand the high pressure. However , the usage of tube metal material is large resulted from the thick heat transfer tube wall (the thick of 2～3.5mm). As a result of raising price of tube material, the whole cost of shell-and tube heat exchanger is getting higher. Compared with shell-and-tube heat exchanger, the plate metal as heat transfer element in plate heat exchanger is thinner (the thick of 0.5～1mm). Unfortunately, the plate heat exchanger has a poor quality in leak-proof. Integration the advantages of shell-and-tube and plate heat exchangers respectively, the rectangle tube bundle heat exchanger is presented in this paper.Rectangle converging-diverging(RCD) tubes and twisted leaf constitute heat transfer elements of the rectangle tube bundle heat exchanger. The thickness of RCD tube is small. The twisted leaf can be inserted into tube, meanwhile, can support tube bundles externally. So the tube side and shell side are enhanced by the twisted leaf. Local heat transfer and flow resistance characteristics of RCD tube inserted with twisted leaf are investigated by experiment and numerical simulation. The local Nusselt number ( Nu ) and friction factor decreases as the distance to twisted leaf increases, respectively. Combining heat transfer with flow resistance characteristics, the RCD tube inserted with twisted leaf is better than the rectangle smooth tube and RCD tube. Under the guidance of field synergy principle, the distribution of velocity and temperature fields is analyzed. Twisted leaf can produce swirl flow, and the heat transfer enhancement with twisted leaf is the improvement of synergy between velocity and temperature fields.Though the rectangle tube bundle heat exchanger has good propriety of heat transfer enhancement, the experimental results show that the plate of rectangle tube creates destabilizing bucking deformation when the pressure of shell side increases. The rectangle tube bundle heat exchanger with self-support of tube bundle is proposed, which is investigated about the heat transfer and flow resistance by experiment. The relational expression of heat transfer coefficient and velocity has been obtained. The pressure of shell side is higher than that of tube side under the same Reynold number ( Re ) because shell side is supported by twisted leaf.The configuration optimization is studied through orthogonal numerical simulation for self-support of tube bundle inserted with twisted leaf. The results show that it is beneficial to improve the performance by increasing twist angle and decreasing twisted leaf pitch. Furthermore, compared with twisted leaf, the heat transfer efficiency of twisted tape is much better.Based on the relation between the distribution of thermal resistance and turbulent intensity and heat transfer efficiency in self-support of tube bundle with different inserts, a new method of enhancing heat transfer is proposed. The distributions of thermal resistance in viscous sub-layer, buffer region and turbulent core region and local turbulent intensity are compared between the three types including self-support of rectangle tube bundle, inserted with twisted tape and twisted leaf. When the twisted tape is inserted ,the thermal resistance of viscous sub-layer decreased significantly and the thermal resistance distribution become uniform, compared to without-any-insert and with- twisted-leaf-insert. The distribution of local turbulent intensity in self-support of rectangle tube bundle with twisted tape and twisted leaf both change gently. It overcomes the phenomenon, the local turbulent intensity too high or too low. Based on the relation between the distributions of thermal resistance and turbulent intensity and heat transfer efficiency, a new method of enhancing heat transfer is proposed to make the distributions of thermal resistance and turbulent intensity uniform to improve energy efficiency. Besides, the synergy of velocity field and temperature gradient field with twisted tape is improved and heat transfer can be enhanced.Flow resistance and heat transfer characteristics of self-support of rectangle tube bundle inserted with twisted tape, which vary in twist ratio, have been investigated experimentally with different Re and tube bundle. The results show that both Nu and friction factor increase when twist ratio decreases. And Nu increases and friction factor decrease as Re increases for the same twisted tape. The self-support of rectangle tube bundle inserted with twisted tape, twist ratio of 3.5 and groove spacing of 190mm, achieves the best performance in the experiment.更多还原