| Single-phase convective heat transfer is a most common phenomenon in heat exchangers. The application of efficient heat transfer tube elements can significantly improve heat transfer efficiency, reduce energy consumption and facilitate compact design for equipment, which makes it rather meaningful for research on improvement of in-tube heat transfer performance. Considering the specific characteristics of convective heat transfer at low Re numbers, this dissertation proposes a method to enhance in-tube convective heat transfer by longitudinal vortex induced by winglets, and carries out a thorough research on its mechanism of flow disturbance and characteristics of heat transfer enhancement by numerical simulation and experiments.Research shows that convective heat transfer resistance exists not only in near-wall region but in the main flow region at low in-tube Re numbers or turbulence intensity. Based on this,3 types of winglet are designed for round tubes, which are respectively rectangular, trapezoid and triangular, and analysis on their disturbance flow field and heat transfer characteristics is performed. The results show that compared to rectangular and triangular winglets, the trapezoid one, which is wide at the bottom and narrow at the top, can both bring effective disturbance to fluid at boundary layer and promote mixture of fluid in main flow region, while inducing merely moderate flow resistance, which makes good comprehensive performance within the range of research conditions.Results of further numerical simulation shows that the trapezoid winglets can effectively introduce longitudinal vortex structure to fluid in the rear region, increasing velocity component perpendicular to the flow direction, the maximum lateral velocity in transverse section reaching as high as 60% of main flow average velocity. The main flow fluid passing across the winglet upper edge in the middle of flow passage is introduced to near-wall region, which realizes direct heat and mass transfer between the near-wall high temperature fluid and the low temperature fluid in the main flow zone; meanwhile, the development of longitudinal vortex along the wall effectively thins down thermal boundary layer and increases temperature gradient in the near-wall region. Under the mutual effect of the two mechanisms, uniformity of in-tube fluid temperature distribution is significantly improved, and local surface Nusselt number dramatically rises, the maximum of which reaches as high as 10 times that of smooth tube. Within the range of 500<Re<2500, the average in-tube Nu number increases by 0.5 to 3 times.In order to obtain the winglet parameters for optimum comprehensive performance under different conditions, optimization research is performed by three dimensional numerical simulations on winglet spacing, the angle between winglet and surface and winglet amount circumferentially placed, and thorough analysis is carried out on their different flow and heat transfer mechanisms. The results show that:1) smaller winglet spacing causes greater rise for Nu number and flow resistance as well. At Re<2000, the longitudinal vortices induced by winglets last a longer distance, and larger winglet spacing brings about higher PEC values; At Re>6000, dissipative effect strengthens for longitudinal vortices, and smaller winglet spacing leads to higher PEC values;2) Both Nu number and resistance coefficient grow with the increase of the angle a between winglet and surface, while improvement for in-tube Nu number is more remarkable for winglets up-flow placed; within the range of 500<Re<7000, PEC values for tubes with up-folw winglets placed are all above 1.2, the maximum reaching 1.6; 3) Higher amount n of circumferentially placed winglets leads to more compact and intensive vortex flow in transverse section, and greater proportion of fluid involved in flow disturbance, which is beneficial to heat transfer enhancement. The in-tube PEC value is proportional to winglet amount with 500<Re<5000; while the winglet amount effects little on improvement of comprehensive heat transfer performance with Re>6000 as a result of greater flow resistance caused by growing number of winglets.A visualized flow field velocity measurement test bench is set up, and PIV measurement is carried out on disturbance flow field in the rear region of trapezoid winglets embedded in a round tube. The experimental results show that the winglets introduce multi longitudinal vortex structure into the fluid downstream, and symmetrical vortex pairs form in transverse section. When winglets are upfiow placed, the inside of vortex pair demonstrates towards-wall flow, extending to a rather wide range in circumferential direction and lasting a rather long distance downstream the winglet. While when winglets are downstream placed, the inside of vortex pair shows off-wall flow, which lasts longer distance in radial direction, making the best flow disturbance effect occur at the rear region close to the winglet. The maximum lateral velocity in transverse section occurs respectively at the towards-wall flow region and off-wall flow region for vortex pairs, while the maximum radial velocity component occurs at the middle position between two adjacent vortices. Within the range of 500<Re<13000, at 10 mm downstream the winglet, the maximum lateral velocity can reach more than 27% of the main flow average velocity, and the radial 20%, which is beneficial for mass exchange between main flow fluid and that in near wall region. Research results also show that at higher Re numbers, longitudial vortices induced by winglets upwind placed demonstrate higher strength and better durability than those by winglets downstream placed.An in-tube water flow and heat transfer test bench is built, and research on in-tube heat transfer and flow resistance characteristics is carried out for upflow/downstream winglet groups at different winglet spacing. The results show that within the studied parameter range, the heat transfer performance respectively increases by 70%-175% and 60%-130% with upflow and downstream winglet embedded, compared to smooth tubes. Smaller winglet group spacing leads to better heat transfer enhancement performance, with the PEC value corresponding to spacing S=2.5 higher than that of spacing S=5 by 10%-18.7%. Comprehensive in-tube heat transfer performance shows the trend of first-rise-and-then-fall with the increase of Re number. The Re number corresponding to the maximum PEC value for upwind winglet group reduces with winglet spacing, which generally spreads between 2000 to 4000; while the maximum PEC value for downstream winglet group at different spacing occurs at Re=2000 or so. Within the range of parameters studied, for upwind/downstream winglet groups, the Re number ranges for PEC value above 1.4 are respectively 1500<Re<8000 and 1500<Re<6000, which shows that in-tube embedded trapzoid winglets introduces good comprehensive heat transfer performance for flowing fluid at medium and low Re numbers.An in-tube air flow and heat transfer test bench is set up, and effects of winglet group spacing, layout type and arrangement type are studied on heat transfer enhancement and flow resistance characteristics of in-tube air. Comparison of experiment results shows that within the parameter range of experiment conditions, upflow winglet group demonstrates impressive improvement effect for in-tube Nu number, with Nu/Nu0 values ranging from 1.42 to 1.78. For larger winglet spacing (S=10 and 7.5), the PEC value first rises and then falls with the growth of Re number, the maximum value reaching close to 1.25; while for smaller winglet spacing(S=2.5 and 5), the PEC value is rather small at low Re numbers, but it gradually grows with the increase of Re number, the growth rate of which is especially great for Re<1500. As for downstream winglet groups, good comprehensive performance occurs at low Re numbers, with the maximum PEC values all emerging within the range 1500<Re<2000; and for Re>2000, the PEC values at different winglet spacing all declines to certain extent, which implies that downstream winglet group is suitable for heat transfer enhancement at low Re numbers. Besides, for spacing S=7.5, comparison is carried out on the 5 arrangement types of in-tube winglet group, downstream in-line, downstream staggered, upwind in-line, upwind staggered and cross arrangement, with upwind in-line arranged winglet group showing optimum comprehensive performance under the experiment conditions. Eventually, experiment results under different conditions are fitted, and criterion formula for in-tube heat transfer and flow resistance characteristics are obtained for upflow/downstream winglet groups under different arrangement modes, which provides reference for calculation of this type of heat transfer enhancement elements. |
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