Numerical Simulations On Heat Transfer Enhancement And Flow Resistance Characteristics Of Oblique-cut Semi-elliptic Cylinder Shell

Vortex generator is a kind of element for passive heat transfer enhancement, which can be used to enhance the convective heat transfer of air side in air-liquid or air-steam heat exchangers, and to improve the air flow structure. In this thesis, numerical simulations were carried out to analyze the characteristics of heat transfer enhancement and flow resistance of a new kind of vortex generator-oblique-cut semi-elliptic cylinder shell on the basis of the known experimental results. These results can provide available references for the engineering applications.Numerical simulations were carried out to contrast the characteristics of air flow and convective heat transfer enhancement by a pair of single-row vortex generators, including oblique-cut semi-elliptic cylinder shells, oblique-cut semi-elliptic cylinder, oblique-cut semi-circular cylinder shells, rectangular winglets, trapezoidal winglets, triangular winglets, which were installed in a rectangular channel, respectively. Then the oblique angle a, angle of attackβ, height h, distance from the entrance S, spacing of two leading edges s and configuration of oblique-cut semi-elliptic cylinder shells were considered as effecting parameters on heat transfer enhancement and flow resistance of vortex generators in a rectangular channel. Numerical model was based on the prototype of known experimental apparatus. In order to ensure the reliability of numerical procedure, the numerical methods and results were verified by the known experimental results and the grid independence solution.As to heat transfer enhancement, there is no more 10% difference among these vortex generators; oblique-cut semi-circular cylinder and rectangular winglet are the better ones for laminar flow, and rectangular winglet is the better one for turbulent flow; for laminar or turbulent flow, oblique-cut semi-elliptic cylinder shell is a little better than oblique-cut semi-circular cylinder shell which is the worst. And as to flow resistance characteristic, the flow loss of oblique-cut semi-elliptic cylinder shell for laminar flow with some higher Reynolds numbers and turbulent flow is evidently lower than the other five types of vortex generators. Particularly, it can be 22.45% lower than that of rectangular winglet at most for turbulent flow. Then in comprehensive characteristic (j/j0)/(f/f0), oblique-cut semi-elliptic cylinder shell is comparable with triangular winglet for laminar flow, a little higher than the other four types of vortex generators; it is the best one for turbulent flow, which can be 21.88% higher than rectangular winglet. From the above, oblique-cut semi-elliptic cylinder shell has the advantage in flow loss and comprehensive characteristic. The parameters of oblique-cut semi-elliptic cylinder shells have no obvious effect on the comprehensive characteristic for laminar flow. And the oblique-cut semi-elliptic cylinder shells with a=20°,β=15°, h=5 mm,S=100 mm,s=30 mm and configuration of the common flow down (with lower leading edges and higher trailing edges, two concave surfaces facing each other) provide the best comprehensive characteristics under the turbulent flow conditions.Furthermore, heat transfer enhancement and drag characteristics of oblique-cut semi-elliptic cylinder shells in an air cooler of air-conditioning were investigated numerically. Usually, the air flow state is laminar in air-conditioning cooler, the air flow in this simulation was assumed to be laminar. Triangular winglets were selected to compare with oblique-cut semi-elliptic cylinder shells. The results showed that these two vortex generators have almost the same heat transfer enhancement performance. The flow resistance of oblique-cut semi-elliptic cylinder shell is lower than triangular winglet. These two vortex generators have the better effect of flow resistance reduction in staggered arrangement air cooler at higher Reynolds numbers, the reduction of oblique-cut semi-elliptic cylinder shell is 6.40% at most, and that of triangular winglet is 4.39%. The comprehensive characteristic of oblique-cut semi-elliptic cylinder shell is better than that of triangular winglet in addition to air flow of in-line arrangement cooler at some lower Reynolds numbers. Air cooler with vortex generators have an evident increase in the comprehensive characteristic at higher Reynolds numbers, the air cooler with vortex generators can be running at these conditions.