Numerical Study On Heat Transfer Enhancement Of A Tube Fitted With Coaxial Cross Twisted Tapes For Oil

The oil coolers are widely used heat transfer equipment,which usually enhanced heat transfer of the oil in shell side,such as the integral pin-fin tubes.The integral pin-fin tubes have the problems of oil leaking and flowing around,which weaken the enhancement of heat transfer,the enhanced heat transfer of the oil in tube side is applied.When the lubricating oil flows in the tube,the high viscosity of oil leads to the smaller convection heat transfer coefficient,then to the larger volume of the oil cooler.Improving the convection heat transfer coefficient of oil side can reduce the volume of the oil cooler and save the cost,which is of great significance for the ship power plant.The insertion of traditional twisted tapes has been considered as a good enhancement technology for laminar convective heat transfer in tubes.Based on the traditional twisted tape and field synergy theory,a new type of twisted tape,coaxial cross twisted tapes(CCTT)are proposed in this paper.For reducing the resistance,a regularly spaced coaxial cross twisted tape(RS-CCTT)is proposed on the basis of the coaxial cross twisted tape.In order to improve the heat transfer coefficient further,combined with the fourth generation of heat transfer enhancement technology,a compound insert coaxial cross two twisted tape compound vortex generator(CCDTT-VG)is proposed.In this paper,the coaxial cross twisted tape and its derivatives are taken as the research object,and the heat transfer characteristics and flow characteristics of the new twisted tapes are numerically studied by using CFD software STAR-CCM+.At the same time,the influence of CCTT's twist ratio(y=2,3,4,infinity),spacing rate of RS-CCTT(s=0.5,1,2,3),twist ratio(y=2,3,4,infinity),inclination angle(?=0°,30°,60°)and vortex generator structure of CCDTT-VG on heat transfer enhancement effect are discussed in detail.By analysing the velocity field,temperature field,local convection heat transfer coefficient,Nusselt number,drag coefficient and performance evaluation criteria of the coaxial cross twisted tape and its derivatives,the mechanism of heat transfer enhancement is studied.In addition,the enhanced heat transfer capacity of the coaxial cross twisted tape and its derivatives is compared with the plain tube and the traditional twisted tape.The results show that at the Reynolds number range of 40~1050,the coaxial cross twisted tape and its derivatives can significantly enhance the convective heat transfer of the lubricating oil in the tube.The strengthening capacity of the coaxial cross twisted tape and its derivative increases with the decrease of twist ratio,and the strengthening effect is best when the twist ratio is 2.0.When the twist ratio is 2.0,the performance evaluation criteria of the oil in the heat transfer tube intersecting the coaxial cross double twisted tape is 1.15~1.42 times of the traditional twisted tape,and the performance evaluation criterion of the coaxial cross triple twisted tape is 1.14~1.54 times of the traditional twisted tape.Through the analysis of velocity field and temperature field,it is found that the heat transfer mechanism of coaxial cross twisted tape and its derivatives is to increase the speed of lubricating oil in the tube,increase the temperature gradient of the lubricating oil near the wall,and further reduce the thickness of the thermal boundary layer.