Noise attenuation and performance study of a small-sized contra-rotating fan with microperforated casing treatments

Abstract

This study investigates the benefits of noise control for a small-sized contra-rotating (CR) fan using microperforated-panel (MPP) casing treatments over the rotor. Several configurations of this type of casing treatment were proposed and then simulated with the finite element method (FEM) to determine their acoustic performance. In general, the FEM simulations indicated that the configurations of placing the treatment over the front rotor are more acoustically effective than those over the rear rotor. Experimentally, it was found that each axial row of holes of the MPP casing behaves like a stator vane, interacting with the fan rotor and thus generating a noisy interaction tone. However, such a tone can be sufficiently suppressed by optimizing the row numbers at the design stage of the MPP casing to increase the interaction mode and frequency. Experimental results further showed that for the treatment over the front rotor, the fan performance was not significantly influenced, especially in the flow rate region above stall point, while for the treatment over the rear rotor, a pressure loss of no more than 10 Pa was perceived, particularly in the flow rate region around the fan design point. Acoustically, the former was able to reduce overall noise throughout the flow rate region, with a maximum noise reduction of 3.5 dBA around the design point, whereas the latter increased overall noise slightly. Moreover, an enlarged volume lowering cavity stiffness for the treatment over the front rotor was found to improve the reduction of overall noise by 0.7 dBA (from 2.8 dBA to 3.5 dBA) as well as to attenuate rotor-alone tones effectively, of which the front treatment with the regular cavity failed in noise reduction. In contrast, no substantial enhancements of noise reduction were noticed for the rear treatment with an enlarged cavity. It is anticipated that the current investigation can provide a reference for controlling CR fan noise with MPP casing treatments.