Study On Flow Resistance And Heat Transfer In Rectangle Tube Bundle Heat Exchanger

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.