Study On The Relationship Between The Intensity Of Secondary Flow With Skin Friction Drag And Form Drag In Flow Channel Of Heat Exchanger

The energy utilization is essentially a process of transfer and transformation of energy.In China’s primary energy consumption structure,the consumption of fossil energy such as coal,oil,and natural gas accounts for 86.4%of the whole structure.Most of the consumption of these fossil energy is the transfer and transformation of heat energy.Therefore,heat the heat exchanger with high efficiency and compact structure is vital to improve the utilization of heat energy.The longitudinal vortex generator can influence the development of the boundary layer to guide the fluid to produce secondary flow,and strengthen the momentum and energy exchange between the fluid in the boundary layer and the mainstream,thereby enhancing heat transfer.Moreover,it will not increase the drag loss significantly.Accordingly,the longitudinal vortex enhanced heat transfer technology becomes one of research priorities of the application of enhanced heat transfer technology to heat exchangers.The drag force in the heat exchanger channel with vortex generator is mainly composed of friction drag and form drag.The contribution of friction drag and form drag to the total drag in the heat exchange channel will be affected by the size of the heat exchanger,the size of the vortex generators,and layout of vortex generators.Current research mainly focuses on the influence of the shape,geometric size,attack angle,location arrangement and other factors of the vortex generator on heat transfer enhancement and the drag increment inside heat exchange channel.There studies of form drag caused by longitudinal vortex generator are rare,let alone the studies of corresponding relationship between drag caused by longitudinal vortex generator and secondary flow intensity.In this article,the characteristics of fluid flow and heat transfer of flat tube bank fin heat exchanger with triangular winglet vortex generators are numerically simulated,and the change of secondary flow intensity inside flat tube heat exchanger air channel is quantitatively examined and analyzed.The numerical simulation and analysis find the corresponding relation between secondary flow intensity and parasitic drag caused by vortex generators with different forms and structure parameters of heat exchange channel.The research results could provide theoretical basis and basic data for the parasitic drag reduction of tube-fin heat exchangers.The following main conclusions are obtained through research:(1)For all the cases studied in this thesis,there is no unique correspondence between f and Re,and there is also no unique correspondence between Se and f.It is shown that neither Se nor Re can uniquely determine the resistance coefficient of the heat exchanger.The correlation between Se and f is also obtained in this thesis.(2)There is no corresponding relationship between the skin friction drag in the air channel of the flat tube bank fin heat exchanger and the secondary flow intensity.This mean that the intensity of the secondary flow cannot determine the skin friction drag within the parameters studied in this thesis.(3)The form drag in the air channel accounts for the main part of the total flow resistance of the channel.There is a linear correspondence between the form drag FD and the volume average Jn ABS,V.The intensity of the secondary flow determines the value of form drag,and the correlation between FD and Jn ABS,V is also obtained in this thesis.(4)The form drag of the vortex generator accounts for the main part of the additional resistance caused by the vortex generator.The proportion of form drag and skin friction drag in the flow resistance caused by the vortex generator is affected by the geometry and placement of the vortex generator.The fluid flow resistance characteristics in the air channel are analyzed by the form drag and skin friction drag.(5)When the shape of vortex generators is determined,The Nusselt number Nu and the friction factor f on the air side of the heat exchanger are not only related to Re,but also related to the fin spacing Tp,the vortex generator winglet height H,and the vortex generator attack angle θ.The correlations of Nu and f with Re,Tp,H,and θ are obtained in this thesis,respectively.(6)When the shape of vortex generators is determined,The friction factor f on the air side of the heat exchanger are not only related to Se,but also related to the fin spacing Tp,the vortex generator winglet height H,and the vortex generator attack angle θ.The correlation off with Re,Tp,H,and θ is obtained in this thesis,respectively.(7)For all the cases studied in this thesis,there is a certain corresponding relationship between the volume average mechanical energy dissipation φm in the air side channel of the flat tube bank fin heat exchanger and the volume average secondary flow intensity Jn ABS,V.It means that the longitudinal vortex induced by the longitudinal vortex generator will cause an increase in mechanical energy dissipation,thereby causing an increase in the total resistance in the channel.