Design of a Low Speed Rim-Supported Fan for Minimum Noise
2.3 Design for Low-noise Fans
Axial flow fans are critical in maintaining a safe work environment by effectively circulating air in occupied areas. However, commercial ventilation fans are often loud and cause noise induced hearing loss from prolonged exposure. Consequently, there is an increasing need for quiet ventilation fans.
The need exists because many commercial ventilation fans are poorly designed aerodynamically and acoustically. Some of the major fan noise contributors are the rotor broadband self-noise, rotor-stator tonal interaction, tip gap noise, and blade steady loading and thickness. Rotor-stator and tip gap noise can be eliminated with a rim supported fan design. However, fan noise due to rotor self-noise, blade steady loading and thickness will not be eliminated for a rim supported fan. These noise sources are proportional to the 4-6th power of the fan tip speed, depending on the noise source. To this end, the present paper presents a control vortex design (CVD) methodology to design the fan blade profile to minimize fan tip speed and noise while preserving aerodynamic performance.
The CVD fan blades are characterized by a span wise changing circulation that ensures a higher flow rate contribution of the blade outer sections, i.e. axial flow increases from the blade hub to the tip. However, a non-uniform span wise circulation is susceptible to radially outward flow that increases near tip losses if the flow is not in radial equilibrium. Consequently, in this study the effect of radial flow is incorporated into the design procedure. To that end, the velocity profile was designed to maintain radial equilibrium and to maximizing the volumetric flow rate. The fan blade sections giving the desired velocity profile constitute the final fan blade design. The fan design has been integrated with an inlet section designed to maximize the fan aerodynamic performance. The fan and inlet section have been 3D printed. The fan is supported at the rim using an array of high speed miniature bearings.
Testing of the fan sound power level, mechanical power, flow rate and velocity profile have shown good agreement with predictions. It also shows better acoustic and aerodynamic performance than commercial fans. Consequently, a low speed rim supported CVD fan is a promising solution to the need of quiet ventilation fans.