Impact of Blade Design on Static Pressure Recovery of Rotor-Only Low-Pressure Axial Fans

In a broad range of applications, low-pressure axial fans are installed without outlet guide vanes and are discharging a swirling flow directly into a free atmosphere. A static pressure recovery can occur between a position immediately downstream of the fan trailing edge and a point in the free atmosphere far downstream of the fan. The total-static efficiency calculated with the ambient pressure is directly affected by this pressure recovery in the fan discharge. In this paper, several fans with varying linear work distributions are designed and numerically investigated to quantify the impact of the pressure recovery on the total-static efficiency and its sensitivity with respect to the vortex design. For this, a delayed detached eddy simulation approach based on the Menter shear stress transport eddy viscosity model and the $γ$ transition model was used. The numerical investigation has shown that the static pressure recovery amounts to up to 18% of the total-static fan pressure and can be influenced by the vortex design. Taking the pressure recovery into account, the optimal vortex design in terms of total-static efficiency shifts away from the low dynamic pressure vortex designs, which are usually aspired to minimize the exit loss. Furthermore, the streamline curvature at the fan exit was found to have an overall adverse effect on the static pressure recovery.