Study On The Characteristics Of Flow And Heat Transfer Enhancement In An Internal Helical Finned Tube Placed With Vortex Generators

Heat exchanger as the most common equipment in industrial production,improve its heat transfer efficiency can effectively save energy.In the process of increasing the heat transfer efficiency of heat exchanger,the technology of heat transfer enhancement in tube has been developed rapidly.Internal ribbed tube and twisted tape are the most common in-pipe heat transfer enhancement devices,which are widely used in all kinds of heat exchangers.The principle of heat transfer enhancement combining internal helical finned tube and twisted tape is to achieve stronger heat transfer enhancement effect by taking advantage of the respective heat transfer enhancement advantages of special-shaped tube and spoiler.In order to investigate the influence of the spoiler element on the heat transfer and flow of the fluid in the internal ribbed tube the modified torsion vortex generator was inserted into the internal helical finned tube for research and analysis.In this dissertation,the physical model and mathematical model of the internal helical finned tube placed with vortex generators are firstly established,and the mesh is divided by self-programming based on Fortran language.The mesh generation techniques such as transfinite interpolation method,two-boundary method and differential equation method are mainly used to discretize the local area.The block structured mesh splicing technique is used to splice the local mesh into a complete computing domain mesh.Then,the laminar flow and heat transfer characteristics of the fluid in the internal helical finned tube with different vortex generators were numerically simulated.The influences of the shapes,the twisted rate,the spacing and the width of based tape of the vortex generators on the fluid flow and heat transfer in the tubes were compared and analyzed,and the optimal shape of the vortex generators was determined.The effects of twisted ratio,spacing and based tape width of the vortex generators on the fluid flow and heat transfer in the internal helical finned tube were obtained.The relationship between the average Nussel number and the secondary flow intensity in the internal helical finned tube are analyzed and the fitting relationship between the average Nussel number and the secondary flow intensity is obtained.Finally,the turbulent flow of the fluid in the internal helical finned tube was numerically studied and the enhancement of heat transfer effect in the internal helical finned tube is determined by the helical micro rib and the twisted vortex generators.The main conclusions of this paper are as follows:（1）When only the shape control angle of the vortex generators is changed,vortex generators of different shapes have a certain influence on the flow and heat transfer of the fluid in the internal helical finned tube,and the parallelogram vortex generator has the strongest heat transfer ability.The drag coefficients of vortex generators with different shapes in the the internal helical finned tube are basically the same.（2）When vortex generators with different structural parameters are inserted into the internal helical finned tube,the average Nu decreases and the resistance coefficient f increases with the increase of Re,thus the position of effective heat transfer enhancement is moved back continuously.With the increase ofΔx,the average Nu decreases gradually,the resistance coefficient f decreases gradually,and the enhanced heat transfer factor JF value increases.（3）When the structural parameters of the vortex generator remain unchanged,the average Nu of the internal helical finned tube with a parallelogram vortex generator increases with the increase of Re,and the drag coefficient f decreases.When Re=100～800,the heat transfer enhancement factor JF of the internal helical finned tube is all greater than 1,and the built-in parallelogram vortex generator can effectively enhance the heat transfer in the tube.（4）At the same Reynolds number Re,the average Nu and the drag coefficient f of the internal helical finned tube with the parallelogram vortex generator decrease with the increase of twisted rate and spacing of the vortex generators;the based tape width has little effect on the average Nu of the internal helical finned tube with the vortex generators.The drag coefficient f increases with the increase of the based tape width.（5）When the structural parameters remain unchanged,the average Nu and the secondary flow intensity Se in the internal helical finned tube with the parallelogram vortex generators with different shapes increase with the increase of Reynolds number Re,and the changing trends are basically consistent.When the other structural parameters remain constant,the the secondary flow intensity in the internal helical finned tube increases with the decrease of the twisted rate of the vortex generators.The secondary flow intensity in the internal helical finned tube decreases with the increase of the distance between twisted parallelogram vortex generators.The secondary flow intensity in the internal helical finned tube is random with the decrease of the based tape width of the twisted parallelogram vortex generators.（6）The local Nu_local on the surface of the internal helical finned tube wall varies periodically,and the peak value of the local Nu_local on the surface of the internal helical finned tube is related to the position of the helical rib.The distribution trend of local Nu_local on the surface of the internal helical finned tube with vortex generators is basically the same,and its peak value is related to the circumferential position of vortex generators arranged in the internal helical finned tube.（7）When the parallelogram vortex generators with different structural parameters is installed in the internal helical finned tube,the average Nu also increases with the increase of secondary flow intensity Se,and the two have a power exponential correlation.（8）The vortex generators play an important role in enhancing heat transfer under laminar flow condition after the internal helical finned tube with the vortex generators.In the turbulent state,the spiral ribbed can effectively enhance the heat transfer.