Numerical Analysis Of Flow And Heat Transfer Characteristics Of Twisted Elliptical Tubes With Inner And Outer Ribs

Global climate change is a major challenge facing humanity in the 21st century.China is the largest carbon emitter,and the effective promotion of carbon peaking and carbon neutral work requires us to promote high-quality development.As an important piece of equipment in the industrial field,the heat exchanger can effectively reduce energy consumption and carbon emissions by improving its heat transfer efficiency.The main structure of a heat exchanger consists of different arrangements of heat exchanger tubes.Therefore,it is important to improve the heat transfer capacity of heat exchanger tubes to alleviat e the current energy situation.The mathematical and geometric models of heat exchanger tubes were established and validated with Nussle number and drag coefficient as evaluation indexes.The maximum errors were less than 11%,which were basically in line with the variation trend of Nussle number and drag coefficient and met the actual engineering requirements.The heat transfer and flow resistance performance of the twisted elliptic tube,inner rib twisted elliptic tube,outer rib twisted elliptic tube,inner and outer rib twisted elliptic tube were numerically calculated by numerical simulation method using the Realizable k-ε model with different parameters of Reynolds number,torque,aspect ratio,rib shape,rib height,axial rib spacing,rib thickness,etc.The analysis results showed that.(1)When the Reynolds number is in the range of 25000～45000,the Nusselt number and drag coefficient of the twisted elliptical tube increase with the decrease of the torque and the aspect ratio.The comprehensive heat transfer coefficient j is the largest when the torque is 200 mm,and the aspect ratio is 0.8.(2)Using Plackett-Burman design,the influencing factors affecting the heat transfer capacity of the twisted elliptical tube with inner ribs were screened.Takin g p as the evaluation index,the axial rib spacing,rib height and aspect ratio have a significant impact on the heat transfer capacity of the inner rib twisted elliptical tube.When the Reynolds number is 30000～50000,the rectangular inner rib twisted elliptical tube has the largest Nusselt number when the rib height is 0.9mm,the axial rib spacing is 8mm,and the aspect ratio is 0.8;when the rib height is 0.7mm,the axial rib spacing is 12 mm,and the aspect ratio is 0.8,the resistance coefficient is the smallest and the comprehensive heat transfer coefficient j is the largest under this condition.(3)The distribution of average temperature and field synergy angle at different sections of the twisted elliptical tube with rectangular inner rib is analyzed.The upstream and downstream ends of the rib can increase fluid di sturbance,increase the average temperature,reduce the field synergy angle,enhance the heat transfer capacity of the heat exchange tube,and the heat transfer capacity of th e downstream end is better than that of the upstream end.(4)When the air velocity is 3m/s,4m/s,and 5m/s,the Nusselt number of straight outer rib twisted elliptical pipe and perforated straight outer rib twisted elliptical pipe is the largest when the rib height is 1.5mm,the axial spacing of ribs is 5mm,and the rib thickness is 0.5mm.The Nusselt number of curved outer rib twisted elliptical tubes is the largest when the rib height is 2mm,the axial spacing of ribs is 5mm,and the rib thickness is 0.6mm.Under this parameter,the comprehensive heat transfer coefficient J of the outer rib is the largest,which is higher than that of straight outer rib twisted elliptical tube and perforated straight outer rib twisted elliptical tube.(5)Under certain conditions,the waterside and airside of the inner and outer rib twisted elliptical tube combined with the rectangular inner rib and the curved outer rib have almost the same drag coefficient compared with the waterside and the airside of the rectangular inner rib twisted elliptical tube and the curved outer rib twisted elliptical tube,respectively.Changes,however,the Nusselt number increases larger.Under the same Reynolds number,the comprehensive heat transfer coefficient j of the waterside is increased by about 30% on average compared with the rectangular inner rib twisted elliptical tube.In this paper,the comprehensive performance analysis of twisted elliptical tube,twisted elliptical tube with inner rib,and twisted elliptical tube with outer rib has a certain reference value for the enhanced heat transfer of heat exchange tube and the needs of engineering practice.