Physics-based design and optimization of domestic air purifier fan aeroacoustics

Abstract

This thesis investigates the aeroacoustics of centrifugal fan with forward curved blades that is commonly installed in air purifier. Optimization of the noise radiation for this type of application is necessary as end users require high flow rate yet low noise which are contradictory to each other. Identification of the noise sources is important for design improvement. Therefore, a numerical approach is proposed in present study. The calculation is based on incompressible flow assumption and Large Eddy Simulation (LES) approach is adopted to account for the turbulence effects. The proposed numerical approach is validated by comparing the results with published data. The flow field calculation results on a centrifugal fan design show that the strongest flow unsteadiness occurs around the vicinity of fan volute tongue where wake flow interactions, that generated by the blade motions when the blades are moving towards the volute tongue, prevail. Therefore, the influence of the blade tip to volute tongue distance from fan housing on the aerodynamic and aeroacoustic performance is studied in details. The far field noise is calculated by using the Ffowcs Williams and Hawkings (FW-H) equation with information of blade aerodynamic forces only. The calculated spectra of sound pressure level (SPL) are compared with those obtained from experiments which show good agreement particularly at lower frequencies. However, the first harmonic is not calculated and there is about a difference of 3 dB at blade pass frequency (BPF) between calculated and measured results. The acoustic directivity does also not match with the measurement results well. These deficiencies are shown due to the ignorance of the acoustic scattering from the fan housing in the calculation as the solution of the FW-H equation always assumes free-space Green's function. In order to study the scattering effect from fan housing, an alternative approach is proposed using Boundary Element Method (BEM) approach. This is achieved by using the flow wall pressure to define appropriate boundary conditions of an equivalent acoustic boundary value problem to the interior acoustic domain. This alternative approach gives calculated SPL spectra and directivity patterns that are matching much better with the experimental results. The deviation of noise level at BPF is narrowed down to within 2 dB and its first harmonic is correctly captured. Having confirmed the accuracy of fan aeroacoustics captured by the proposed numerical approach, optimization of centrifugal fan impeller blade design for low fan noise is carried out. The blade curvature is optimized with Response Surface Methodology (RSM). Quadratic response surface model is developed for searching the optimal combination of the blade inlet and outlet angle. The final optimized blade design was formed to give an increase in flow rate and reduction of noise level. In the final part, an novel two-outlet centrifugal fan design is proposed. This design is constructed by mirroring a single outlet fan design from its central vertical axis. The noise of this two-outlet design is found much quieter than that of original single outlet fan. The BPF tonal noise level is about 40% (~18 dB) lower than when it is a single outlet. The flow rate also increases by about 20%. All these benefits were confirmed by experiments. This achievement also highlights the outstanding capability of physics based design and optimization in new low-noise fan design for such quietness critical applications as domestic air purifier. It should be noted that the current proposed methodology, which combines the flow and noise calculations with existing design optimization techniques, could help to optimize fan with any geometrical parameters according to the performance requirement. This is the ultimate fan noise engineering design methodology that the industry has been looking for over years.