Off-Design Sound Emission of Low-Pressure Axial Fans Under Distorted Inflow Conditions: An Experimental Study

In many applications, low-pressure axial fans are used in off-design operating conditions with a distorted flow field at the fan inlet. To assess the impact of such boundary conditions on the aerodynamic and acoustic fan characteristics, the sound emission of skewed and unskewed fans is compared under different inflow conditions and at matching operating points, including the design point and the off-design range. The results show that the forward-skewed fan has the highest efficiency and the least sound emission in the typical operating range under all considered inflow conditions. However, in the stall range and in the moderately loaded operating range, the backward-skewed fan has a lower sound emission. The unskewed fan has the least favorable characteristics under all operating conditions. For each fan, operating range and inflow condition, specific features in the acoustic spectrum are identified that lead to these findings.