**Maximal Achievable Efficiency of Fans**

Cordier Guided Design Strategies

Due to increasing energy cost and the challenges in the context of climate change there is a permanent demand to enhance the energy efficiency of fans. This paper discusses the theoretical aerodynamic efficiency limit that cannot be exceeded regardless of the effort made to optimize the fan. It is distinguished between two efficiency definitions (total-to-total and total-to-static) and four fan types (axial rotor-only, axial with guide vanes, radial rotor-only and radial with volute). For each fan type, the inevitable aero-dynamic losses are estimated as a function of the design point and the Reynolds num-ber. Inevitable losses are e.g. friction losses, shock losses and exit losses. Aiming at the insuperable efficiency limit, the models to estimate the friction losses are based on a set of idealizing assumptions and the exit losses are minimized by an optimal spanwise load distribution. Since the focus is on the aerodynamic efficiency limit, losses in the motor and the drive drain are neglected.

The resulting efficiencies are depicted assuming an exemplary Reynolds number of one million. It is found that the impact of the design point is very strong, especially with re-gard to the exit losses which increase with decreasing specific fan speed and diameter. Friction losses become relevant at design points with high pressure coefficients. At such design points, the width of radial impellers becomes very small and axial fans feature large hub-to-tip ratios wherefore the wall effects from hub and shroud increase. The ef-ficiency of radial fans is further impaired by increased friction between the bottom disc and shroud with the surrounding air.