Experimental And Numerical Study On The Heat Transfer And Flow Performance For The Circular Tube Fitted With Drainage Inserts

Global warming,energy shortages and other issues put forward new requirements on China's energy security.China proposed new energy planning about these major issues during the “Thirteen five period”.Heat exchangers play an irreplaceable role in various fields.The heat transfer enhancement technology can be used in the shell and tube heat exchanger to effectively improve the energy efficiency.It is an effective way to achieve the purpose of energy conservation.In the thesis,the new type of drainage inserts is proposed to enhance the heat transfer under turbulence conditions and to study its heat transfer and flow performance.The insert is studied both by the experimental and numerical way.The experiment is mainly used for preliminary study and provides verification for the numerical simulation method.In this thesis,the influence of the pitch ratio p on the heat transfer and resistance characteristics of the drainage insert is studied by the experimental method.The influence of the slant angle ?,the groove height H,the size of the insert and the shape of the groove on the heat transfer and resistance characteristics are investigated by numerical simulation.And the PEC is used to evaluate the overall performance of enhanced heat transfer tubes.It is deduced that the drainage insert has the following mechanism of heat transfer enhancement: the groove can guide the cold fluid to the area near tube wall so that the water mixes with the hot fluid.And the cold water causes local disturbance to the boundary layer at the same time.Then the part of the fluid expands to the both sides of the grooves,and flows to the core area after mixing with the hot water near the tube wall,leading to a larger speed of the mainstream.The mechanism above-mentioned and the disturbance of drainage inserts can interact to form a vortex structure,which will further improve temperature uniformity of the mainstream while not considerably improve the resistance.Therefore,the drainage and vortex structure are the main mechanism of heat transfer enhancement of the insert proposed in this thesis.The results show that the smaller the pitch ratio p is,or the larger the slant angle ? is,or the smaller the groove height H is,the better the heat transfer performance is.But the model will generate the larger resistance at cost.According to the value of the PEC,the above-mentioned parameters are preferably p = 3.3,? = 45,and H = 2 mm.The numerical results show that the heat transfer performance of the model is 1.83-1.94 times that of the smooth tube,while the resistance performance is about 7 times that of the smooth tube.The PEC varies between 0.96 and 1.04.The smaller size of the insert can effectively decrease the resistance by 23.3%-25.3%,while also decrease the heat transfer performance by 5.7%-9.8%.So the PEC also decreases.It is also found that the heat transfer,resistance and overall performance of the square groove are better than that of semicircular groove.Drainage inserts have good heat transfer performance under turbulence conditions.The appropriate parameters and dimensions can be selected according to the working conditions.