Optimum Design of Compact, Quiet, and Efficient Ventilation Fans

Axial ventilation fans are used to improve the air quality, remove contaminants, and to control the temperature and humidity in occupied areas. Ventilation fans are one of the most harmful sources of noise due to their close proximity to occupied areas and widespread use. The prolonged exposure to hazardous noise levels can lead to noise-induced hearing loss. Consequently, there is a critical need to reduce noise levels from ventilation fans. Since fan noise scales with the 4-6th power of the fan tip speed, minimizing the fan tip speed and optimizing the duct geometry are effective methods to reduce fan noise. However, there is a tradeoff between reducing fan speed, noise and aerodynamic efficiency. To this end, a new innovative comprehensive optimum design methodology considering both aerodynamic efficiency and noise was formulated and implemented using a multi-objective genetic algorithm. The methodology incorporates a control vortex design approach that results in a spanwise chord and twist distribution of the blades that maximize the volumetric flow rate contribution of the outer radii, i.e. the axial flow velocity increases from the fan hub to the tip. The resulting blade geometry generates a given volumetric flow rate at the minimum fan tip speed. The fan design is complemented by the design of the optimum inlet duct geometry to maximize volumetric flow rate and minimize BL thickness for low noise generation. Good agreement with experimental results validates the design process. The present study also incorporates multi-element airfoils to further increase the aerodynamic characteristics of the fan blades and enable lower fan speeds and noise. Good agreement between experiments and predictions indicate that traditional blade element momentum methods can be implemented in conjunction with multi-element airfoil aerodynamic characteristics with good accuracy. A direct comparison of fans designed with single and multi-element airfoils has shown that fans designed with multi-element airfoils aerodynamically outperform single element fans.