| The distinctive bladeless fan has the advantages of stable blowing, large flow rate and low noise. Therefore many people have paid close attention to it. Even more, its derivatives have been added to the function of heating, purification, sterilization and humidification. Bladeless fan has very potential market prospect. Currently, the public research about bladeless fan in our country and foreign is deficient, we’re lack of the product design theory. It’s not only helps to explore the way to optimize the structures of bladeless fan, but also to conduct further research on the formation mechanism of aerodynamic noise and provide theoretical support for the noise reduction measures, to simulate the flow field and predict the aerodynamic noise of bladeless fan.Numerical method was used to study the flow field and aeroacoustics of bladeless fan through three steps. The first step is steady RANS, the second is transient DES and LES, the last is CAA by generalized Lighthill’s acoustic analogy.The part of steady CFD:(i) The steady field of whole computational domain was simulated by RNG k-ε turbulent model, and the results were consistent with the Dyson’s. Then broadband acoustic source was localized by Lilley’s source term, the localization analysis shows acoustic source was mainly concentrated in the rotation zone, slit zone and Coanda zone, especially in the slit and Coanda zone.(ii) After the sharp slit corner was rounded, the acoustic source core downstream vanished. And with the increase of the slit corner radius, the scale of another acoustic source core at the downstream of the slit exit was lessening slightly, at the same time, the flow rate reduced. The radius of slit corner was suggested to be 2-3mm.(iii) The effect of flaring surface’s camber on the flow characteristics of bladeless fan was studied by two dimensional numerical method. Both the camber inward and camber outward can increase the flow rate and improve the velocity distribution, and the effect of the inward camber was better than the outward camber. The inward camber between CUR1 and CUR2 was suggested.The part of transient CFD:The steady flow information in the first step was mapped onto the boundary of the local computational domain determined by acoustic source localization. The velocity fluctuation of inlet before the slit zone was reconstructed by spectral synthesizer method with the steady flow information, such as u, k, s, et al. Then the transient field was simulated by SA DES, Smagorinsky Lilly LES and Dynamic Kinetic LES separately, the analysis of the negative pressure region, the pressure fluctuation and the Q criteria indicated the most turbulent flow centered in the slit zone, Coanda zone and wake zone of the jet, and the most operative turbulent model was Dynamic Kinetic LES without time limitation, otherwise the suggested selection was SADES.The part of CAA:The transient flow information was transformed into acoustic source by generalized Lighthill’s analogy, then, the acoustic propagation was computed. The time-domain and frequency-domain acoustic source characteristics, the SPL at the position 1 m away from the slit exit with the 45° direction off the centerline and its 1/6 octave, and the acoustic directivity showed the low frequency discrete noise constituted main parts of aerodynamic noise of the bladeless fan, high frequency aerodynamic noise was mainly the turbulent noise. The acoustic predictions based on SA DES and Dynamic Kinetic LES were in good agreement, in terms of. |
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