Study Of Vortex Strength And Its Relationship With Heat Transfer In The Fin Side Channel Of Annular Finned Tube Heat Exchangers

As a kind of high-efficiency heat exchanger,tube and fin heat exchanger is widely used in chemical,petroleum,metallurgical and food industries because of its small size,large heat transfer coefficient and low resistance.Finned tube heat exchanger is a kind of tube and fin heat exchanger.The traditional finned tube heat exchanger has several types such as annular fins,spiral fins,serrated fins,twisted fins and nail fins.Due to the advantages of easy manufacturing,light weight,low cost,high finning ratio,and strong fluid disturbance due to the discontinuous shape of the fins,the finned tube boundary layer is constantly broken,which strengthens heat transfer of heat exchanger,and is widely used in waste heat boilers,power station boilers,industrial waste heat boilers and other equipment.In the fluid flushing fin tube bundles,there will be produce resonant noise,vortex vibration and other phenomena among tube bundles,causing some damage to the heat exchanger.Therefore,the need to study the vortex characteristics of finned tube heat exchanger,and this paper notes little research on the relationship between vortex performance and heat transfer resistance performance.Thus,the purpose of this paper is to investigate different structural parameters in the flow channel of spiral serrated finned tube bundles,analyze its flow resistance and heat transfer performance,and analyze the relationship between heat transfer performance and vortex strength using appropriate vortex identification.And a new type of interrupted annular slot finned tube is designed by stamping interrupted annular slotted surfaces on annular fins,using the advantages of interrupted annular slotted surfaces with flow conduction,enhanced heat transfer and reduced wake area.Meanwhile,its flow resistance performance,heat transfer performance and vortex strength are studied in comparison with annular fins and serrated fins to evaluate the advantages and disadvantages of their performance.In this paper,through the research and analysis of different finned tube models,a spiral serrated finned tube bundles is selected for study,and a new type of interrupted annular groove finned tube heat exchanger is also established for comparative analysis.Physical and mathematical models are established,and the mesh is reasonably divided,and the SST k-co turbulence model is selected for numerical simulation by the selection of different turbulence models.The effects of transverse tube pitch,longitudinal tube pitch and fin spacing on fluid flow performance,heat transfer performance and vortex strength of spiral serrated finned tube bundles are mainly studied,and the fluid flow performance,heat transfer performance and vortex strength of annular fins,spiral serrated fins and interrupted annular slot fins are compared and analyzed.The following conclusions were obtained:(1)The lift coefficient CL)and drag coefficient(CD)generated by the finned tube both show regular periodic changes with time,and the frequency of CD is approximately twice the frequency of CL.when the Reynolds number(Re)is small,only the main frequency can be captured,namely the shedding frequency of the Karman vortex.When Re increases,in addition to the main frequency,there are some secondary frequencies,namely the vibration frequency between the tube bundles.(2)For the spiral serrated finned tubes,when Re is constant,the increase of the time-averaged Nusselt number(Num)is 6.4%-18.1%as the transverse tube pitch decreases,and the time-averaged Euler number(Eum)increases by 42.4%-50.8%as the transverse tube pitch increases.As the longitudinal tube pitch and fin spacing increase,Num increases,increases up to 33.1%and 21%respectively.While Eum decreases with the longitudinal tube pitch and fin spacing increasing.The heat transfer capacity of spiral serrated fins is the best,interrupted annular groove fins is the second best,and annular fins is the worst.The Num of interrupted annular groove fins is up to 14%higher than that of annular fins,and spiral serrated fins is up to 26%higher than that of interrupted annular groove fins.The Eum of interrupted annular groove fins are about 13%-29%higher than that of annular fins,and the Eum of spiral serrated fins are about 7%-29%higher than that of interrupted annular groove fins.(3)When Re is constant,the vortex shedding frequency f of the spiral serrated finned tubes decreases with the increase of the transverse tube pitch,and increasing the transverse tube pitch,f decreases by 45.7%-49.8%.With the increase of the longitudinal tube pitch and fin spacing,f is increased,respectively,up to 35%and 15%.The transverse tube pitch has the greatest effect on the f of the finned tube,while changing the fin structure has little effect on the f of the finned tube.(4)The relationships between the effects of transverse tube pitch,longitudinal tube pitch and fin spacing on the fluid flow performance,heat transfer performance and vortex strength performance of the spiral serrated finned tubes are analyzed,and the fitted relationships between the Nusselt number Nu,Euler number Eu and Strouhal number St with the range of structural parameters studied in this paper and Re were obtained,and the deviation of the fitted relationships from the simulation results was basically within 10%.(5)There are vorticities formed behind the last row of tubes,moving forward and meanwhile developing in the lateral side,fusing with the vorticities formed behind the lateral adjacent row of tubes,and then form the new vorticities,which flow forward until dissipate.In the vicinity of the fins,the fluid on both sides of the fins is mixed with each other,and the secondary flow intensity of the finned tube is high,while the shedding of the Kármán vortex gradually occurs behind the fin tube.The Karman vortex intensity is relatively large,and the secondary flow intensity is relatively small.It means that the flow vortex and the Karman vortex interfere with each other,and the heat transfer in the finned tube flow channel is affected by both the flow vortex and the Kármán vortex,and both have a strengthening effect on the heat transfer in the finned tube flow channel.(6)The local Nusselt number(Nulocal)is larger at the location where the Karman vortex is generated behind the finned tube,indicating that the shedding of the Karman vortex is beneficial to the heat transfer of the finned tube,while the presence of the tail vortex makes the Nulocal of the finned tube smaller and unfavorable to the heat transfer of the finned tube.The Nulocal variation of annular fins and interrupted annular groove fins is similar,and the Nulocal of interrupted annular groove finned tube is higher than that of annular finned tube due to the existence of interrupted annular grooves.It is consistent with the change in the mean Nusselt number.