Study On Convective Heat Transfer Enhancement Of Vortex Generators In Tube Heat Exchanger

Making full use of heat transfer enhancement technology to improve energy efficiency was an effective measure to address the increasingly serious energy and environmental issues facing China.As an important heat exchange equipment,the application of effective heat transfer enhancement technology in heat exchangers was of great significance to improve energy utilization.In the use of heat exchangers,convective heat transfer was one of its basic physical phenomena,and the research around convective heat transfer had an important guiding role in promoting enhanced heat transfer technology and improving the efficiency of heat energy utilization.Based on this,this paper presented theoretical analysis and numerical calculation of convective heat transfer process in a circular tube,and verified the results through experiments.Supported by the theory of enhanced heat transfer by longitudinal vortex,we developed and designed enhanced heat transfer components that could generate longitudinal vortex and enhance heat transfer performance of heat exchangers.Based on the theory of longitudinal vortex enhanced heat transfer,this paper firstly considered the use of an insert in the tube to disturb the fluid and induce longitudinal vortex structure.A new type of enhanced heat transfer element was proposed,called combined delta winglet vortex generator,and investigated the heat and flow behaviors of a heat pipe inserted with the combined delta winglet vortex generator numerically and experimentally.The results showed that the vortex generator could induce six pairs of longitudinal vortex structures in the tube,and these vortices could bring the cold fluid in the core-flow region to the near-wall region,and squeezed the hot fluid in the near-wall region to the core-flow region.After the continuous movement of these longitudinal vortices,the fluid in the tube was fully mixed,which uniformed the fluid temperature,and enhanced the heat transfer efficiency.After comparative analysis,the insertion of this vortex generators would increase the heat transfer efficiency of the heat exchanger to 1.19 in the Reynolds number studied in this paper.However,vortex generators would inevitably bring pressure loss while enhancing heat transfer.The recirculation region at the rear of the vortex generator was the main source of pressure loss.Therefore,reducing the area of the recirculation region was the main way to reduce the pressure loss and improve the overall heat transfer efficiency.In this paper,the pressure loss was reduced by punching holes on the surface of the rectangular winglet vortex generator.Three different area sizes rectangular holes were punched on the surface of the rectangular wing vortex generator,and three different arrangements of vortex generators(fully punched vortex generators,fully smooth vortex generators,and cross-arrangement of punched and non-punched vortex generators)were designed,and their thermal performance in a circular tube was investigated by experiments and numerical simulations.The results showed that opening holes on the surface of the vortex generators reduced their overall heat transfer performance,but also reduced the pressure loss caused by them.When the area of the open hole was larger,a stronger jet was formed after the fluid passed through the hole,and the area of the recirculation area on the rear side of the vortex generator,where the heat transfer was poor,was eliminated.However,the high intensity jet produced a weakening main longitudinal vortex behind the vortex generator,which led to a weakening of the turbulence intensity in the downstream region of the vortex generator and a reduction in heat transfer performance.The all-perforated vortex generator arrangement obtained a better overall heat transfer efficiency compared to the all-perforated vortex generator,while the cross-arrangement of the perforated and non-perforated vortex generators was in between.It should be noted that when the fluid flowed through the open-hole vortex generator,the fluid generated a jet and a low-intensity main longitudinal vortex,and the two fluids mixed and continued to advance,and when they passed through the nonpunched vortex generator,a high-intensity main longitudinal vortex was formed,so the fluid in the tube would show a periodic phenomenon of the intensity of the main longitudinal vortex from weak to strong and then to weak.In a comprehensive comparison,a smaller area hole on the surface of the vortex generator and the insertion of a perforated vortex generator in the pipe were most beneficial to improve the efficiency of the heat exchanger,and the highest combined heat enhancement factor could reach1.12.Subsequently,the heat transfer mechanism of rectangular holes with different parameters on the surface of the rectangular wing was investigated by a combination of numerical and experimental,and the effects of the location,size and pitch of the holes on the heat transfer performance were investigated respectively.The results showed that when the hole area was the same,the closer the hole location was to the wall,the better the integrated heat transfer efficiency was.This was because the closer the hole location was to the wall,the weaker the intensity of the jet generated by the hole,the less the jet affected the main longitudinal vortex of the vortex generator,and the open-hole also reduced the pressure loss caused by the vortex generator.In summary,the open hole improved the efficiency of the heat exchanger.When the holes were connected to the wall,the larger the area of the holes,the more fluid flowed through the holes to form highintensity jets,significantly weakening the heat transfer performance of the vortex generator.Although opening a large area of holes on the surface of the vortex generator significantly reduced the pressure loss caused by it,it led to a worse overall heat transfer efficiency due to the drastic reduction in heat transfer performance.The highest combined heat transfer efficiency of 1.25 was achieved with the studied parameters.Finally,to improve the heat transfer efficiency in the recirculation region,a new type of vortex generator,the punched delta winglet vortex generator,was proposed.The vortex generator consisted of a combination of a delta winglet vortex generator with rectangular holes placed at an inclination,and its thermal performance under turbulent flow was studied by a numerical simulation and experimental investigation.The numerical results showed that the heat transfer efficiency in the recirculation region was significantly improved by the combined effect of the vortex generator’s guidance of the fluid in the core flow region and the jet flow generated in the area near the wall.The pressure loss was also reduced as well.When the hole was connected to the wall,the wider the width of the hole and the lower the height,the better the combined heat transfer efficiency.At the studied Reynolds number,the combined heat transfer efficiency could reach up to1.33.In addition,the results of all sections were analyzed using entropy generation theory to further verify the accuracy of the conclusions.