Design for High Efficiency of Low-Pressure Axial Fans With Small Hub-to-Tip Diameter Ratio by the Vortex Distribution Method

Abstract

Axial fans with a small hub-to-tip diameter ratio (HTR) are widely used in industry, especially for cooling and ventilation purposes. The optimization of their aerodynamic performance is important, for which the vortex distribution method is well-established for axial fans with medium to high HTR. However, only a few studies have focused on small HTR fans. For such fans, downstream backflow regions are often present near the hub. The vortex distribution (polynomial in spanwise coordinate) and the HTR have been determined by maximizing the total-to-static efficiency of a baseline axial fan with a small HTR. For free vortex designs, analytical expressions for the maximum total-to-static efficiency and the optimal HTR have been formulated. By combining the vortex distributions thus obtained with a suitable choice for the spanwise lift coefficient distribution, fan blade designs have been established. The effects of different vortex distributions on the aerodynamic performance have been investigated, employing a computational fluids dynamics (CFD) simulation strategy that has been validated for the baseline axial fan. The current CFD results show that the free and the polynomial vortex distribution designs satisfy the desired pressure rise, with significantly improved total-to-static and total-to-total efficiency (maximum improvement by 3.9% and 4.6% points, respectively). For the free vortex design with larger HTR, neither flow separation nor backflow is observed. For the other designs at the design flow rate, only flow separation near the hub is found. Backflow is observed only for the designs with smaller HTR.