The Characteristics Of Secondary Flow And Heat Transfer Enhancement In Circular Tube With Twisted Tape Or Vortex Generators

The development of heat transfer enhancement technology can reduce the cost of heat transfer equipment and further meet the requirements of temperature control and high heat flux involved, more than that it is an absolutely necessary requirement of energy saving and pollutant reduction. The heat transfer enhancement by using of some special devices to induce secondary flow in heat exchange space is a passive method for obviously improving the heat transfer efficiency. The third generation of heat transfer enhancement technology producing secondary flow to enhance heat transfer is widely used presently. For example, the usage of vortex generators mounted onto or punched out on the fin surfaces of the tube bank fin heat exchanger and the twisted tape inserted into a tube are typical representations. For better application of secondary flow to enhance heat transfer, the first task is to find a quantitative parameter with universal applicability that can specify the intensity of secondary flow, then to establish the quantitative relationship between the intensity of secondary flow and the degree of heat transfer enhancement on the basis of the quantitative description, and finally to find the way to use secondary flow efficiently in heat transfer enhancement. By considering that too large contact area between twisted tape and fluid will increase friction drag and local pressure drop in the tube with twisted tape insert, one research focus of heat transfer enhancement is looking for the simple inserted element with excellent heat transfer performance and optimizing its structure parameters.In this paper, the numerical method and the corresponding computational code are firstly developed to solve the convective heat transfer problem by means of the domain decomposition in a curvilinear non-orthogonal coordinate system. The numerical simulation of laminar fluid flow and conjugate heat transfer in a tube with twisted tape insert is performed, the effect of the fin efficiency of twisted tape on convective heat transfer enheancement is quantitatively analyzed and the effect of the structure parameters of twisted tape on the characteristic of convective heat transfer involved in the tube is analyzed in detail. The non-dimensional correspondence of the absolute vorticity flux is proposed and the physical meanings of the absolute vorticity flux parameter and the non-dimensional correspondence are stated for the first time in the literatures. The intensity of secondary flow in the tube with twisted tape insert is specified by using of the absolute vorticity flux parameter and the non-dimensional correspondence. The relationship between the non-dimensional parameter and the intensity of convection heat transfer is analyzed, and the effect of secondary flow on flow boundary layer is investigated. The contribution of the intensity of secondary flow to heat transfer enhancement and to the increase in flow resistance is quantified, and the relationship between the efficiency of heat transfer enhancement by secondary flow and the thermal boundary condition at the tube wall is analyzed. To reduce the contact area of twisted tape and fluid flow, an insert element for enhancing heat transfer like vortex generator is proposed. The laminar fluid flow and conjugate heat transfer involved in a tube with vortex generators inserts are numerically studied, and then the structure parameters of the vortex generator are optimized by using the orthogonal design method. Finally, the effect of structure parameters of the vortex generator on the characteristics of fluid flow and heat transfer is analyzed, and the relationship between the non-dimensional parameter used to specify the intensity of secondary flow and the intensity of convection heat transfer in tube with vortex generator inserts is also investigated in this paper. The research shows that:(1) The accuracy of the numerical result is closely dependent on the treatment of thermal boundary condition of the twisted tape. The physical model and the corresponding numerical solution procedure can be simplified with the oversimplified treatment of thermal boundary condition, but the numerical results are far away from the experimental ones. The numerical results obtained using the conjugate heat transfer model agree well with the experimental data, and the agreements between the numerical and experimental results can be improved by selecting the appropriate equivalent thermal resistance to consider the effect of the contact state of the twisted tape and the tube wall on heat transfer.(2) The absolute vorticity flux has the physical meaning of the angle velocity of fluid flow rotating round the axis directs along the main flow direction. And the non-dimensional parameter specifying the intensity of secondary flow named as Se presents the ratio of inertial force to viscous force induced by secondary flow.(3) For the circular tube with twisted tape inserts, the non-dimensional correspondence of the absolute vorticity flux, Se, has a linear relationship with the swirl parameter that describes the intensity of the tape-twist induced secondary motion named as Sw. The phenomenon shows that Se is consistent with Sw, but the former can be used to describe the intensity of secondary flow occurred in general cases. In addition, the dimensionless parameter Se has local characteristics.(4) For the tube with twisted tape inserts, the intensity of the convection heat transfer increases with increasing the intensity of secondary flow under either uniform wall temperature (UWT) or uniform wall heat flux (UWHF) boundary condition. Both Nusselt number and the heat transfer enhancement parameter expressing as δNum (=Num-Num,ref) have a good power law correlation with Se when the fin efficiency of the twisted tape was not taken into account. In addition, the efficiency of heat transfer enhancement by secondary flow expressing as ηEN(=Num/Num,ref) under UWT is larger than that under UWHF. Furthermore, Se correlates with JF to some extent, which indicates that Se can be used to estimate the performance of heat transfer enhancement produced by secondary flow. (5) For the parameters of the twisted tape inserts studied, the secondary flow does not obviously change the thickness of flow boundary layer, and it shows that the convection heat transfer enhancement by secondary flow in the tube with twisted tape insert is not enforced by changing of the thickness of the velocity boundary layer.(6) For the structure parameters of vortex generator inserts considered, the main factors that influence the design goal are related with Reynolds number if the design goal adopted in orthogonal design experiment is to maximize the heat transfer enhancement criteria JF. When Reynolds number is equal to400, the main factors that influence the design goal are the axial distance between two vortex generators, the main attack angle of vortex generator, and the ratio of width of base tape, W, to tube inner diameter, D. The numerical results indicate that the parallelogram vortex generator is superior to the right-angle trapezium vortex generator, and the cases of WID=0.3have an advantage over those of WID=0.5, especially under UWT condition.(7) For the vortex generator inserts, with the same structure parameters excepting the main attack angle, the intensity of secondary flow increases with increasing the main attack angle of vortex generator, which results in an increase in both Nusselt number and the efficiency of heat transfer enhancement,ηEN.While with the same structure parameters excepting the axial distance between two vortex generators, the intensity of secondary flow increases with decreasing the axial distance between two vortex generators, and then Nusselt number and the efficiency of heat transfer enhancement,ηEN, are also increasing. With the same structure parameter and flow parameter, the efficiency of heat transfer enhancement under UWT condition is higher than that under UWHF condition.