Analysis and control of fan noise within a converging duct section of limited length

Abstract

The objective of this study is twofold. One is to identify the noise sources of hairdryer system and investigate the corresponding noise generation mechanisms, and the other one is to use the panel with low mass to strength ratio and with micro-perforation to reduce the sound radiation from the fan which is installed in the duct with low aspect ratio. The proposed device will be adopted into the hairdryer to improve the sound quality of the product.Hairdryer is a typical example of the domestic product of simple ducted-fan system with low aspect ratio. A series of acoustical measurements and sound source identification approaches are conducted to search for the dominant sound sources and their characteristics in the spectrum of the hairdryer provided by Kenford Industrial Co., Ltd. The noise is found mainly in the low-to-medium frequency range. In addition, the aerodynamics pattern and aeroacoutics features can be found by computational fluid dynamics (CFD) study. It is found that the maximum vortex occurs at the tip clearance regime and there is unsteady inlet flow due to the irregular structure at the inlet. Different level of the roughness is introduced at the inlet surface of hairdryer and hence the broadband noise at the low frequency regime can be reduced. In order to understand the physics behind, the aerodynamic pattern is also investigated by using particle image velocimetry (PIV) tests which can show the flow field variation at the inlet of the hairdryer. It is found that the rough surface can provide a more uniform inlet flow. However, there is a trade-off enhancement of the noise at the first and second blade passage frequencies. A passive noise control strategy is proposed with a micro-perforated panel (MPP) housing device. It shares the feature of drum-like silencer with MPP covering on the two side-branch cavities but the working principles are different and the source is placed at the centre of the device. The mechanism to control noise by MPP housing is to undergo sound cancellation supplemented with sound absorption instead of sound reflection by drum silencer or plate silencer. To realize it into the real application, the proposed device is to be designed in compact configuration compared with the traditional silencer and muffler in the market. To investigate the performance of the proposed device, a two-dimensional theoretical model for MPP device housing a dipole source is established. The vibro-acoustic interaction is studied in detail to understand the coupling between the panel vibration induced by the dipole source and the sound fields of a duct and backed cavities. With the aid of finite element method (FEM), the analytical model is validated and the device attenuation performance is studied. To optimize the design, parameters such as structural properties of the panel, perforations property of MPP and configuration of the cavities are to be investigated. To conduct the experimental validation, a small speaker is used to simulate the axial fan with dipole nature. It is found that there is good agreement among experiment, analytical model and finite element method. For the real application in the hairdryer, the MPP housing device with a compact geometry can successfully reduce noise at the blade passage frequency. The sound quality of the product will be improved by the utilization of MPP housing device as the loudness has been reduced in the dominant noise frequency region.To further improve the performance of the proposed MPP housing device, a non-uniform section hollow tube with a Herschel-Quincke nature (MPPHQ) is introduced to tackle the drawback of passband in low frequency to acquire a wider stopband. The optimization of the shape, configuration and location of the hollow tube is processed. The optimal performance can be achieved with a compact housing geometry. The hybrid MPPHQ housing maintains the performance of proposed MPP housing device and at the same time, the passband regime which is originally contributed by MPP housing device is released. In addition, the sound attenuation performances of MPP housing device with partitioned cavity to control different nature of sound sources have also been examined, numerically and experimentally. It is found that when the partitioned cavity is properly designed, the performance of MPP housing device can be significantly improved without any enlargement of geometry size. It can be beneficial for the design of silencer in the future.