Abstract
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 environment. Considering the radial momentum balance, a static pressure recovery between a measurement plane immediately downstream of the fan trailing edge and a point in the free atmosphere far downstream of the fan can take place. When using the total-static efficiency computed with the ambient pressure as a metric, it is directly impacted by this pressure recovery in the fan discharge. In this paper, several fans with varying linear work distribution are designed and numerically investigated to quantify the impact of the pressure recovery on the total-static efficiency. Since in the discharge of such fans, free shear layers are the key aerodynamic feature for which standard RANS-eddy-viscosity simulations are known to be lacking accuracy, DDES simulations are performed. A non-zonal RANS/LES hybrid approach based upon the k-ω-SST eddy viscosity model and the γ transition model was used. The numerical investigation has shown that the pressure recovery has a significant impact on the total-static efficiency 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.
