Acoustic analysis of small contra-rotating fan noise and its control with microperforated casing treatments

Abstract

Small axial flow fans working in the low-speed regime are frequently used as air coolers or ventilators. Currently, there is a growing interest in adopting contra-rotation due to its inherent aerodynamic advantages. However, acoustically, contra-rotating (CR) fans radiate additional interaction tones, which decreases the sound quality and may cause extra annoyance. In this work, the aeroacoustics of low-speed contra-rotation are investigated using a small CR fan as a test case, and the strategy of reducing its noise radiation with microperforated-panel casing treatments is explored.

First, a sample CR fan is designed using simple radial equilibrium theory supplemented by three-dimensional steady-flow simulations. Its performance is compared with that of the series operation of two fans. At the same operating point, the CR fan can be run at lower rotational speeds, thus quieter, than the series operation, particularly in the range with relatively higher flow rates. Aerodynamic analysis of the unsteady lift force resulting from the impingement of upstream wakes on the rear rotor is performed. Sound radiation is then predicted using a full model of unsteady pressure on blades and a simplified model. The two predictions are compared with each other and with experimental data. A good agreement is reached between the computed and measured interaction tones for the dominating modal indices |m| = 1. However, discrepancies are found for higher-order interaction tones, probably due to their reflection and scattering at the fan inlet or the additional interaction source such as the scattering of rotor-alone tones. The full model novelly characterizes acoustic sources of interaction tones, showing that sources on the rear rotor dominate over those on the front rotor. The former are concentrated in an outboard leading edge region on the blade suction side, while the latter become intensive in an outboard trailing edge region on the blade pressure side.

Acoustic benefits of microperforated-panel casing treatments in noise reduction of the CR fan are evaluated. It is found that a loud tone is generated by casing-rotor interaction, which behaves like a stator-rotor interaction source, and can be suppressed via increasing the interaction modal index and frequency. The front rotor treatments are acoustically superior to those over the rear rotor without imposing significant aerodynamic penalties on fan performance, especially around the design point. An enlarged cavity is beneficial to the former. Thus, a 3.5 dBA reduction in overall noise can be achieved, with interaction tones and rotor-alone tones above 35 dBA all attenuated. The flow effects of the treatments over the front rotor on rotor-rotor interaction are numerically studied. The front rotor tip leakage flow is reshaped by the treatments, suppressing the unsteady characteristics of the static pressure and lift force on the rear rotor. The enlarged back cavity can enhance such suppression. However, the unsteadiness on the front rotor is not significantly influenced by the treatments.

The findings are useful for developing a quiet and efficient CR fan, identifying its significant interaction source locations, and controlling its noise radiation using an appropriate casing treatment configuration to suppress interaction sources.