Skewed Blades in Low Pressure Fans: A Survey of Noise Reduction Mechanisms

Skewed blades in low-speed, low-pressure axial fans are becoming more and more popular. They seem to offer a noise reduction in such applications as heat exchange units etc. Many authors have described acoustical models which in some cases address entirely different noise generation mechanisms. First this paper gives a detailed survey of the mechanisms, models and results found in the literature. It becomes evident that no single mechanism explains the noise reduction in a rotor due to blade skew. This might cause the present lack of profound prediction methods for the noise reduction potential. The second part of the paper focuses on our own recent investigations of low-pressure fans with skewed blades. Some observations are reported which are based on numerically computed flow fields in rotors with skewed and unskewed blades and measurements. Compared to conventional or even backward skewed blades forward (i.e. in direction of rotation and/or against the flow direction) skew of the blades increases the flow in the hub region of the rotor which is usually endangered by flow separation. This seems to stabilize the flow field in the rotor because stall is shifted to remarkably low flow rates. Simultanously the radiated noise stays low in a large range of operation. At very high and low flow rates the noise is the same of both skewed and unskewed blades, i.e. it is independent of the blade's geometry.