Research On Heat Transfer Enhancement Characteristics In The Double Pipe Heat Exchanger With Twisted Oval Tubes

Double pipe heat exchangers are widely used in many fields because of their convenience in production and the free increase and decrease of heat transfer area.With the continuous development of modern industrial production and the gradual severity of energy problems,the demand for the heat transfer performance of double pipe heat exchanger is constantly increasing,and the traditional double pipe heat exchanger has been difficult to meet the industrial demand.In order to obtain higher performance of double pipe heat exchanger,it is the main mean to improve its comprehensive performance to optimize the structure of traditional tube heat exchanger by using enhanced heat transfer technology.The twisted oval tube is a hot topic in the research of heat transfer enhancement technology in recent years.Twisted oval tube is a kind of spiral twisted tube with oval cross section.Its twisted wall surface affects the fluid flow inside and outside the tube,making it generate strong secondary flow on the section perpendicular to the main flow,which can effectively destroy the thermal boundary layer near the wall and facilitate heat transfer.In order to improve the performance of traditional tube heat exchangers,two kinds of double pipe heat exchangers with twisted oval tubes are studied in this paper,double pipe heat exchanger with outer straight pipe and inner twisted tube and double pipe heat exchanger with two twisted oval tubes.Numerical simulation was used to study the enhanced heat transfer characteristics of fluid in the annular channel of two kinds of double pipe heat exchangers with twisted oval tubes with different phase angles,aspect ratio of the inner twisted oval tube tube,torsion and torsion ratio of the inner and outer tubes.The flow field,temperature field,heat transfer and resistance changes in the channel were analyzed compared with ordinary straight oval double pipe heat exchanger.This paper shows the following results through numerical simulation research:Affected by the torsion wall of the twisted oval tube,the fluid near the twisted tube wall flows in a spiral shape,and forms a more obvious secondary flow in the annular channel,which effectively destroys the thermal boundary layer and enhances the heat transfer effect of the heat exchanger.Under the same conditions,the heat transfer effect of the two kinds of double pipe heat exchangers with twisted oval tubes is better than that of ordinary straight oval double pipe heat exchanger.Twisting oval tubes improves the performance of the double pipe heat exchanger with twisted oval tubes in laminar flow conditions better than in turbulent flow conditions.The numerical study on the influence of heat transfer in the double pipe heat exchanger with outer straight pipe and inner twisted tube shows that the fluid flow and heat transfer in the annular channel are obviously affected by the internal twisted oval tube.When the aspect ratio of the inner twisted oval tube increases,the average Nussel number Nu and the comprehensive performance evaluation factor JF both decrease,which means that the increase of the aspect ratio is not conducive to heat transfer.When the torsion of the inner tube increases,the mean Nusser number of the heat exchange surface and the comprehensive performance evaluation factor JF both decrease,which means that the smaller the torsion of the inner twisted ellipse tube,the better the heat transfer.When the aspect axis of the internal twisted oval tube is 0.4,and the torsion is 240 mm,the comprehensive performance of the double pipe heat exchanger with outer straight pipe and inner twisted tube is optimal.Compared with the conventional heat exchangers,the average Nussel number on the heat exchange surface of the heat exchangers with outer circle and inner oval tube is increased by5.6%～124.9%,the resistance coefficient f in the annular channel is increased by 3.8%～46.7%,and the comprehensive performance evaluation factor JF is 1.04～1.98.The numerical study on the effect of heat transfer in double pipe heat exchanger with two twisted oval tubes shows that the fluid near the inner and outer twisted tubes flows in a spiral shape under the influence of the torsional wall,and an obvious secondary flow is generated on the cross section perpendicular to the flow direction,which effectively destroys the thermal boundary layer and promotes the mixing of fluids in the channel.Different phase Angle,inner tube aspect ratio,torsion and torsion ratio have different effects on heat transfer of tube heat exchanger.When the phase Angle is 60°,the aspect ratio of the inner tube is 0.4,the inner tube torsion is 240 mm and the torsion ratio is 1,the comprehensive performance evaluation factor JF of the double pipe heat exchanger with two twisted oval tubes reaches the maximum value when the Re is 2600.Compared with the ordinary straight double pipe heat exchanger,the average Nussel number on the heat exchange surface of the double pipe heat exchanger with two twisted oval tubes is increased by 3.1%～108%,the resistance coefficient f in the channel is increased by 4.5%～80.3%,and the comprehensive performance evaluation factor JF is 1.02～1.71.