Influence of Winglets on the Tip Vortex of Low Pressure Axial Fans

Low-pressure axial fans serve a wide range of applications and combine the requirements for high flow rates and low-pressure rises. Due to the low Reynolds numbers and low workload of the fluid, the design of low-pressure axial fans differs considerably from compressors in gas turbines. Yet, regulations and customer demand for high efficiency and acoustic standards. A primary goal of the ventilation and air conditioning industry and the automotive industry is to improve the fans' acoustic. Therefore, a lower overall sound power level and tonal components are necessary. One of the sources for a high sound power level is the interaction of the tip vortex with the main flow through the fan or other fan blades. The operating point essentially defines the tip vortex trajectory. Yet, the sweep and shape of the blade tip geometry can influence the bursting of the vortex and its size. The aim of this paper is the investigation of three different blade tip geometries or winglets. By using basic designs, the fundamental influence on the behavior of the tip vortex and the acoustics can be shown. Aerodynamic (including characteristic and wall pressure measurements) and acoustic studies were carried out. The results of the different blade tip geometries are compared to a well-research baseline fan. A qualitative correlation between the local design of the blade tip and the effects on the tip vortex on the one hand and the acoustics, on the other hand, could be shown. A three-dimensional design using winglets leads to a more stable and shifted vortex. A reduction of the overall sound power level in overload and at the design point is shown. Yet, in partial load, the vortex is much more diffuse, which leads to increased broadband noise