Numerical Study On The Characteristics Of Flow And Heat Transfer In Inner Axial Micro Fin Tube Placed With Vortex Generators

With the continuous progress of industrialization in various countries,energy consumption and environmental pollution have to attract people’s attention.In order to solve these problems,various countries around the world have proposed various measures to save energy and resources and protect the ecological environment.With the rapid economic development,China’s energy demand is also rising,making the domestic energy situation very severe.Therefore,it is imperative to save energy and protect the environment.Heat exchangers are widely used in equipment in the fields of energy,chemical engineering,electric power and metallurgy.Improving the heat exchange performance of the heat exchanger can reduce the overall size of the equipment and improve the heat exchange efficiency,which can achieve the purpose of energy conservation.Improving the heat exchange capacity of the heat exchange tube is helpful to improve the overall heat exchange efficiency of the heat exchanger.The combined enhanced heat exchange tube combined with the special-shaped tube and the spoiler element can achieve better enhanced heat exchange effect.The model studied in this paper is a combination of inner axial micro fin tube and vortex generators for enhanced heat exchange.On the one hand,compared with smooth circular heat exchange tube,the use of inner axial micro fin tube expands the heat transfer area in the tube,and on the other hand,inserting a vortex generator inside the tube can enhance the disturbance of the fluid,generate secondary flow of the fluid,promote the mixing of cold and hot fluid in the tube,and thereby improve the heat transfer ability of the heat exchange tube.In this paper,a physical model of in inner axial micro fin tube with vortex generators is established through Solid Works,and a mathematical model of fluid flow and heat transfer in the tube is established.Then,using the ICEM CFD 19.0 software of ANSYS,the grid of the solid region of the inner axial micro fin tube wall,the grid of the fluid region in the inner axial micro fin tube wall,and the grid of the fluid region and solid region of the vortex generators are divided,and the three parts of the grid are combined;Then,using Fluent 19.0 software,the fluid flow and heat transfer characteristics in inner axial micro fin tube with vortex generators were numerically simulated;Based on this,the effects of different vortex generators shapes and structural parameters on it were investigated;The relationship between the average Nusselt number Nu_mand the secondary flow intensity Se in the tube was analyzed,and the relationship between Nu_mand Se was fitted using the least square method.In this paper,the flow and heat transfer characteristics of the fluid in inner axial micro fin tube with vortex generators are numerically studied.Firstly,the physical model and mathematical model of the in inner axial micro fin tube placed with the vortex generators are established,and then the grid of the solid region of the inner axial micro fin tube wall,the grid of the fluid region inside the inner axial micro fin tube,and the grid of the fluid region and the solid region of the vortex generators are divided by using the ICEM CFD 19.0 software of ANSYS.The three parts of the grid are combined to form a structured grid in the computational domain,Then,the fluid flow and heat transfer characteristics of the in inner axial micro fin tube placed with vortex generators are numerically simulated by using Fluent 19.0 software.The effects of different shape and structure of the vortex generators,different twist ratio of the vortex generators,vortex generators spacing and base tape width on the fluid flow and heat transfer in the tube are compared and analyzed.The relationship between the average Nusselt number Nu_mand the secondary flow intensity Se in the tube was analyzed,and the relationship between Nu_mand Se was fitted using the least square method.The main conclusions are as follows:（1）In the range of Re=3000 to 30000,compared with inner axial micro fin tube,smooth circular tube with built-in vortex generators,smooth circular tube with built-in twisted tape,and smooth circular tube,inner axial micro fin tube with vortex generators have better heat transfer enhancement effects,and the advantages of inner axial micro fin tube and vortex generators combine to play a role in enhancing heat transfer.The Nu_m/Nu₀and f/f₀of inner axial micro fin tube with vortex generators are 1.17 to 2.16 and 3.87 to 5.07,respectively,with a maximum JF of 1.3.（2）The shapes of the vortex generators studied in this paper have little effect on improving the heat transfer capacity of the inner axial micro fin tube;The maximum friction coefficient is rectangular vortex generators and parallelogram vortex generators,followed by right-angle trapezoid vortex generators and isosceles trapezoid vortex generators,and the smallest is for hollow vortex generators.（3）Different shapes of vortex generators will affect the streamline distribution and the secondary flow intensity distribution in vicinity of the connective tape,the base tape and the the vortex generators on the cross section of the main flow direction;In addition to the hollow vortex generators,the vortex generators with different shapes placed in the inner axial micro fin tube has little influence on the velocity distribution and temperature distribution in the cross section of the main flow direction.（4）In the range of Re=3000～30000,the Nu_m/Nu₀of the vortex generators with different shapes and different structural parameters in the inner axial micro fin tube are greater than 1.The in inner axial micro fin tube placed with vortex generators can effectively improve the heat transfer effect in the tube;Re=10000～30000,the enhanced heat transfer factor（JF）of the vortex generators with different shapes and different structural parameters placed in the inner axial micro fin tube is greater than 1,in this range,the inner axial micro fin tube placed with vortex generators has good comprehensive heat transfer performance.（5）The average Nusselt number Nu_mof inner axial micro fin tubes with vortex generators of different shapes and structural parameters increases with the increase of Re,and the friction coefficient f decreases with the increase of Re.The Nussel number Nu_xalong the tube and the secondary flow intensity Se_xalong the tube change periodically with the position of the vortex generators.（6）The local Nussel number Nu_localon the surface of the inner axial micro fin tube wall exhibits periodic changes,and the peak value of Nu_localon the surface of the tube wall will appear at the location where the inner axial micro fin exists;After built-in vortex generators with different shapes and structural parameters,the circumferential location of the vortex generators determines the location of the Nu_localpeak.（7）In the range of Reynolds number Re=3000 to 30000,after an isosceles trapezoidal vortex generator is installed in inner axial micro fin tube,the average Nussel number Nu_min the tube increases as the twisted ratio T_rof the vortex generators decreases;At low Re,Nu_mdoes not change significantly with the change of vortex generators spacing S_t/W,while at high Re,Nu_mdecreases with the increase of S_t/W;The base tape width W_b/W has little effect on Nu_m;At the same Re,the friction coefficient f in the tube increases with the decrease of T_r,the decrease of S_t/W,and the increase of W_b/W.（8）The vortex generators with different structural parameters placed in the inner axial micro fin tube will affect the distribution of streamline,velocity,temperature,and secondary flow intensity on the cross-section of the main flow direction.（9）The average Nussel number Nu_mof the inner axial micro fin tube with isosceles trapezoidal vortex generators with different structural parameters is exponentially related to the secondary flow intensity Se,indicating that the secondary flow induced by the vortex generators plays a key role in improving the convective heat transfer efficiency of the heat exchange tube.