Experimental Investigations of Mixed Flow Fans with Various Downstream Angles

3.6 Case Studies: Aerodynamics

Axial and centrifugal fans are represented in a wide variety of different shapes on the market. If the design point requires large flow rates with a sufficiently large increase in pressure, the recommended design point is within the range between the axial and radial fan type, in the area of mixed flow fans. Mixed flow fans are still a rarity. Only a very few recommendations can be found in the literature for the design of mixed flow fans and therefore also no empirical coefficients for the diagonal design are available.
A special field of application for mixed flow fans is the cooling of motors and electrical components. Motor cooling fans are installed directly on the motor shaft inside the housing and ensure the removal of the heat, yielded by engine components, into the passing air flow. Major demands for the design point of these fans are large flow rates with the associated high flow velocities for sufficient cooling capacity. The airflow through the engines, especially through the narrow gap between the stator and rotor, causes high pressure losses. Thus, the requirements for the cooling fans are high flow rates with comparatively high pressure increases.
Similar to centrifugal fans, there is also a flow deflection from axial to radial direction in mixed flow fans. The risk of flow separation at the shroud is increased because of the relatively wide blade channels. In order to improve the flow deflection and to prevent the shroud separation, different shroud contours with an increased radius of $R130$ and $R180$ were investigated.
The investigations revealed that a too sharp flow deflection with wide blade channels, such as in R80, cause flow separation at the shroud, while a smooth radius of R180 results in increased friction losses and deteriorated efficiencies. Furthermore, the impact of various downstream angles chi = 30° (almost axially) to chi = 90° (centrifugal) was evaluated for the volume flow and the pressure increase. Changing the downstream angle to smaller chi decreased the flow rates with smaller pressure rises.
Compared to axial fans, the mixed flow fan type already achieves higher pressure rises in a single stage and while operating without a stalling dip. Mixed flow fans can be easily integrated in fluid systems with axially parallel inlet and outlet flow, as well as axial fans and therefore represent a serious alternative in the application field between axial and radial fans.