Numerical Simulation Of Convective Heat Transfer Enhancement For Single Phase Flow In Circular Tubes With Rectangular Vortex Generators

Improving energy efficiency is the key to solving the global energy crisis,and it is also the most urgent task for China to do a good job of“carbon emissions peak and carbon neutrality”.The study of enhanced heat transfer spoiler elements to achieve maximum heat transfer performance with less power consumption plays an important role in promoting the development of modern industry and improving the comprehensive performance of heat transfer equipment,as well as in carrying out energy conservation and emission reduction,protecting the environment and developing green economy.Therefore,based on passive enhanced heat transfer technology,the spoiler element is designed and form the flow field structure with the longitudinal swirl flow characteristics in the tube to achieve enhanced heat transfer.The heat transfer tube of shell and tube heat exchanger was taken as the object of study,the heat transfer and flow characteristics of the heat exchange tube with built-in vortex generators were studied by numerical simulation with uniform heat flux boundary.Firstly,the heat transfer and flow characteristics of rectangular vortex generators and rectangular surface-cut vortex generators in heat transfer tubes were compared and analyzed.It was found that although the cutting structure reduced the flow resistance,it also reduced the degree of disturbance to the fluid,and the fluid medium produced four longitudinal swirls flow downstream of the vortex generators,which accelerated the convergence of hot and cold fluids,improved field synergy of velocity and temperature gradients,and increased the heat transfer efficiency of the heat exchanger tube.Subsequently,the effects of the length-height ratio（L/H）,the attack angle（β）and longitudinal spacing（P）of the rectangular vortex generators on the flow and heat transfer characteristics were analyzed,the results showed that with the increase of length height ratio of vortex generators,the increase range of performance evaluation criteria（PEC）decreased gradually.When the length-height ratio was2,the heat exchange tube had a better comprehensive performance.With the increase of the angle of attack of the vortex generators,the performance evaluation criterion increased at first and then decreased.When the angle of attack was 30°,the performance evaluation criteria had the maximum value in most cases,and the heat exchanger tube had the best comprehensive performance.When the vortex generator length-height ratio was 2,the angle of attack was 30°,and the longitudinal spacing of 100 mm,the PEC ranges from 1.30 to 1.44for currently studied Re conditions.The absorber tube of the parabolic trough solar collector was taken as the object of study,and the numerical simulation study of the absorber tube with built-in vortex generators were carried out with non-uniform heat flux boundary.The vortex generator was composed of a ring belt and four ribs.According to the tilt direction of the ribs,four types of vortex generators M1,M2,M3 and M4 were proposed.The flow and heat transfer characteristics of the absorber tube at Reynolds number 10000-35000 and inlet temperature 373.15 K-473.15 K were analyzed,and the thermal-hydraulic performance of the absorber tube was studied.The results showed that the four kinds of vortex generators M1,M2,M3 and M4 could make the heat transfer fluid（HTF）in the absorber tube generate longitudinal swirls flow.The longitudinal swirls flow reduced the thickness of the thermal boundary layer,accelerated the mixing of cold and hot fluids,and improved the heat transfer capacity.Vortex generators M1and M3 arranged in the reverse direction produce two pairs of reverse rotating longitudinal swirls flow,while vortex generators M2 and M4 arranged in the same direction form one pair of longitudinal swirls flow,so the flow resistance in M1 and M3 was slightly larger than M2and M4.However,from the perspective of heat transfer enhancement effect and comprehensive performance,the vortex generators M3 had the best effect.In the case of M3,the Nu/Nu₀ reached the highest value of 1.238,and the PEC in the cases of M3 was greater than 1,with a maximum value of 1.11.To further improve the heat transfer efficiency of the absorber tube,while considering the impact cooling effect of the longitudinal swirls flow and reducing the non-uniformity of the temperature on the wall of the absorber tube.The effects of the longitudinal spacing and circumferential arrangement of the rectangular vortex generator on the temperature uniformity and heat transfer performance of the absorber tube were investigated.It was found that the turbulence intensity of the fluid inside the tube increases as the longitudinal spacing decreases,resulting in a more uniform temperature distribution inside the absorber tube,the local temperature increase on the tube wall surface caused by the convergence of longitudinal swirls flow was suppressed.The number of ribs was the same as the number of vortices formed,and the flow direction of the vortices changes the degree of synergy between the original velocity and temperature gradients fields,and the temperature distribution on the wall surface of the collector tube was related to the flow direction of the vortices.By changing the number and position of the vortexes,the heat transfer efficiency could be increased and the temperature distribution on the wall of the absorber tube could be improved.When the M3 structure vortex generators arrangement space was 100 mm and the Reynolds number was 10000,the PEC obtained a maximum value of 1.279.Compared with the smooth absorber tube,the maximum and minimum temperature difference between the absorber tube wall of the test section reduced by 8.1%,and the average-temperature difference between the upper and lower half of the absorber tube wall reduced by 39.9%.Therefore,the safety and heat transfer performance of the absorber tube were significantly improved.