Numerical Study Of Flow And Heat Transfer In Multi-starts Twisted Dimpled Tubes And Dimples

In order to promote energy utilization to a more efficient direction,many researchers continue to explore new enhanced heat transfer means to effectively improve the heat transfer performance of heat exchangers.At present,the common means of strengthening heat transfer are:increasing the heat transfer area,using shaped tubes,changing the roughness of the heat transfer surface,adding the disturbance element,adding solid particles,etc.In pursuit of better heat transfer performance,the combination of various enhanced heat transfer technologies highlights the advantages of greater enhanced heat transfer,further improving heat transfer efficiency and promoting technological progress in the field of enhanced heat transfer.The heat transfer performance of straight tube can be significantly enhanced by processing it into a shaped tube.A summary of existing enhanced heat exchanger tubes reveals that there is little literature to study the combination of dimples and twisted tubes.In order to investigate the heat transfer and flow characteristics of combined twisted tubes with dimples,this paper numerically investigates the effects of different variations of the start number N of twisted tube,dimple pitch P₂,and dimple depth b on heat transfer and flow characteristics based on the finite volume method（FVM）,and analyzes the cross-sectional velocity field,secondary flow intensity,local Nu_localon the surface of the tube wall,spanwised averaged Nu_s,Se_sdistribution of the tube,etc,as well as the distribution of average Nu,firction coefficient f and heat transfer performance evaluation factor JF with Re.The main conclusions are as follows:（1）Dimple can induce more separated flows in twisted tubes,and the formation of longitudinal vortex behind the dimple makes the boundary layer thinner,improving the uneven distribution of Nu_localon the wall surface of the twisted tube.Meanwhile,the secondary flow intensity in the tube increases under the joint action of the dimple and twisted tube,which promotes the mixing of hot and cold fluids and makes the heat transfer performance of the twisted tube with dimples much higher than that of the twisted smooth tube.（2）When changing the starts number of twisted tube,the average Nu and JF are found to be maximum for the three-start twisted tube with dimples.Re in the ranges of 2000～15000,the average Nu of the three-start twisted tube with dimples increased by 14.6%～16.7%compared that with the three-start twisted smooth tube.When Re=2000,the three-start twisted tube with dimples has a JF maximum of 1.47,and Re in the ranges of 2000～15000,the JF of three-start twisted tube with dimples increased by 6.8%～12.8%compared to that of three-start twisted smooth tube.（3）Under the influence of different dimple pitches,it is show that the average Nu of the three-start twisted tube with dimples is the largest when Re in the ranges of 2000～15000 and P₂=17mm,and its Nu increasing by 16.8%～20.6%compared with that of the three-start twisted smooth tube,while the Nu of the 17mm increased by 1.8%～3.3%compared with that of the other pitches.At Re=2000,the maximum JF of the three-start twisted tube with dimples by a pitch of 17 mm is 1.51,which increased by 16.1%compared to that of the three-start twisted smooth tube.（4）Under the influence of different dimple depths,the JF of the three-start twisted tube with dimples at b=8.8mm had a maximum value of 1.65 at Re=2000.Re in the ranges of2000～15000,the JF value of the three-start twisted tube with dimples for b=8.8 mm increased by 7.4%～27.2%compared to that of the three-start twisted smooth tube.（5）The effects of the working conditions such as the start number N,dimple pitch P₂,dimple depth b and Re on the enhanced heat transfer effect are comprehensively analyzed,and the correlation equations of Nu,firction coefficient f and JF are fitted,which can guide the engineering application of twisted dimpled tube.