Heat Transfer Enhancement by Using Vortex Generators

Kurzfassung

The efficiency of energy converters, cooling units and heat exchangers is the most important aspect for the economical success of energy conversion systems. In order to enhance the heat transfer passive devices like roughness elements, dimples or cavities are used, mostly in micro heat exchangers or other energy converters on micro scales. On bigger scales other devices in use are guiding plates, diverter strakes or delta shaped winglets. The physical mechanism causing the enhancement of heat transfer is either to generate or amplify sufficiently strong longitudinal vortices which are interacting with the thermal boundary layer. The stratification of the thermal boundary layer near the heated walls is disturbed by these vortices. The convection of warmer fluid perpendicular to the heated wall and the mixing with colder fluid is intensified, and, additionally, further external momentum is transported into the inner boundary layer region. Depending on the specific application the used flow control devices differ in their geometry, dimensions and integration. Dupond et al. [1] used compact texture like embossed vortex generators similar to dimples. Fiebig [2], Gentry et al.[3] and Jacobi et al. [4] examined the flow effects of fins and winglets to improve the heat transfer on plates and in tubes or channels experimentally. They also conducted numerical simulations considering flow control devices with varying shape and geometry to find device design criteria for heat transfer enhancement. Since the vortex induced heat transfer enhancement depends strongly on shape and position of vortex generators the subject of ongoing research is to find design strategies for device shape and placement optimization. In particular, the investigation of confined channel flows with delta wing vortex generators has motivated numerous numerical and experimental studies [2]. Until now the physical mechanisms are not satisfactory understood. Though, in order to improve the design of compact heat exchangers a better physical understanding of the generation of these flow structures is mandatory. Therefore, the task of the presented work is to analyse the interaction between vortices and the thermal boundary layer, the impact of the vortical flow structure on the heated wall and the convective transport of heat within the flow domain. In this numerical study a generic rectangular heat transfer channel with a heated upper wall is subject of the examination. Although various types of flow manipulating devices could be used for generating coor counterrotating vortices disturbing the stratified temperature layers this consideration is restricted to delta wing vortex generators. In this case pairs of generic, prismatic delta wings are used to influence the flow whereas a special local arrangement is focal point of interest: the vortex generator pairs are arranged facing one another: one pair on the hot wall, one on the cold opposite channel wall. The upper pair on the hot wall is generating a so called “common flow up” flow structure, the other pair a “common flow down” flow structure. This leads to a transport of fluid from the hot to the cold wall. Additionally the upper vortex pair feeds and the lower vortex pair. The enforced vortex pair is now able to enhance the mixing of cold and warm fluid in the channel. A focal point of this consideration is the analysis of the vortical flow field and their interaction with the associated temperature field. Vortex analysis techniques are applied to the numerical solutions of the laminar flow. Vortices are detected by Lambda2 values, normalited helicity is used to distinguish between co- and counter-rotating vortices. The heat transfer is examined by the temperature distribution shown on slices and by calculation of so called beta-planes, suitable for the visualisation of the convective transport of momentum and heat in regard to the localisation of the origin of mixing fluid. The usage of visualization techniques like streamline integration and advanced line integration convolution techniques yields a better understanding of the vortical structures and the associated heat transfer. A further topological analysis is done to gain an impression of the main vortical flow structures and the resulting temperature distribution. Separatrix integration reveals significant information of the vortical flow structures and their impact on heat transfer. This work presents results of a comparative analysis, especially of the topological structure of vortices generated by vortex generator pairs and their interaction with the temperature field.