Analysis of the Flow Field in Optimized Axial Fans

Three design strategies for low pressure axial fans are compared. Benchmark fans are designed with the blade element momentum (BEM) method. All common validity limits are respected. Optimized fans are designed according to design guidelines obtained in earlier studies by the authors of this work. Here, “optimal” always means maximum possible total-to-static efficiency at exact fulfillment of a specific design point. One typical difference between the benchmark and optimized fans is that the optimization yields considerably smaller hub-to-tip ratios. The two design methodologies are combined for a third series of fans. These fans are also designed with the BEM method, but with the same hub size and blade number as obtained from the optimization. As a consequence of the smaller hub size, the aforementioned validity limits are violated. All three design strategies are applied at three distinct design points which are supposed to outbid the bounds of axial fans according to Cordier’s diagram. The nine resulting fans are simulated by means of steady CFD (Reynolds-averaged Navier Stokes method, RANS). Quality assurance is considered by a grid independence study and a comparison with transient simulations (SAS method). The aerodynamic comparison reveals the weaknesses of the BEM designs which suffer from high exit losses (if designed with common validity limits) or from high hydraulic losses (if designed with small hub). Additionally, BEM designs with small hubs still have unnecessary exit losses which originate from an uneven distribution of flow velocity downstream of the fan. In contrast, the optimized designs enable a small hub without increased losses resulting in a total-to-static efficiency improvement by 2–14 percentage points depending on the design point. The flow fields of all fans are analyzed in detail to find reasons for the superiority of the optimized designs. It is found that optimized fans benefit from an evenly distributed meridional velocity profile downstream of the fan. Reasons are given why such a flow field is hardly achievable by BEM designs. Further advantages of optimized fans are found in the blade shape near the hub which strictly avoids flow separations which in standard designs often compromise the efficiency in the complete blade channel.