| Partly-ducted axial flow fans with bellmouth are widely used in civil and commercial air conditioners, which are different from full-ducted and open axial flow fans in structure. In order to improve aerodynamic performance and reduce noise, it is important to study aeroacoustics and flow characteristics of partly-ducted axial flow fans, which has already been one of major problems of research in outdoor units.In this paper partly-ducted axial flow fan system with bellmouth, which are included in split-type outdoor units of air conditioner, are investigated. With help of advanced experiment and CFD simulation, aerodynamic and aeroacoustic performances of partly-ducted axial flow fan with uniform inflow and complex structure of single and collateral axial flow fan inside outdoor units are first studied. Then, complex three-dimensional turbulent flow field inside flow passage of partly-ducted axial flow fan are analyzed, characteristics of tip vortex with its formation, development, stretched and broken down by the interaction with bellmouth are discussed. Next, the methods used to identify noise source of partly-ducted axial flow fan are introduced, which is based on vortex sound theory of low Mach number and homentropic flow together with CFD simulation. Furthermore, vortex shedding noise prediction model based on CFD simulation is developed. In the end, a hybrid method coupling near-field CFD calculation plus the application of an integral nonhomogeneous FW-H function for the far-field is adopted to predict the discrete frequency noise and spectrum of outdoor units in time and frequency domain. At the same time prediction models in time and frequency-domain are discussed, respectively.Main research contents and results of this paper are as follows:1. Set up experimental equipment suitable for aerodynamic-aeroacoustic measurement of axial flow fan. Noise source of outdoor units is obtained by sound intensity method and characteristic of noise spectrum of outdoor units is investigated by spectrum analysis. Flow field of partly-duct axial flow fan system is measured in detail by Particle Imaging Velocimetry (PIV) and Hotwire Anemometer (HWA). With help of experiment and CFD simulations, character of tip vortex and trailing edge vortex of partly-ducted axial flow fan is studied, with flow change in outdoor units also analyzed.2. Major noise source radiated from outlet of axial flow fan in outdoor units is aerodynamic noise, which is acted as dipole noise. Noise spectrum of outdoor units is composed of broadband and discrete frequency noise. Effect of broadband noise is crucial and discrete frequency noise is located at Blade Passing Frequency (BPF) and its harmonics. To collateral axial flow fan inside outdoor unit discrete frequency noise is located at BPF and its harmonics of upside and downside rotor, respectively. Interaction of flow field generated by upside and downside rotor has little effect on total sound pressure level of outdoor units.3. In partly-ducted axial flow fan system with bellmouth, blade tip vortex formed at blade leading edge is generated by pressure gradient between suction side and pressure side, it develops downstream and into flow passages as blades rotate. When blade tip vortex passes by bellmouth, it is extruded by bellmouth, and its structure is stretched and broken down, and noise is radiated. When axial flow fan is included in outdoor units, blade tip vortex further develops mid span at meridional plane. Uneven inflow in outdoor units will induce asymmetry flow field at inlet and outlet, which will increase fluctuation at one blade rotating period. According to distribution of pressure of blade surface at different time of outdoor units, it is shown that interaction between tip vortex and bellmouth is the major factor of unsteady fluctuation of blade surface and has large effect on discrete frequency noise of outdoor units. To collateral axial flow fan system inside outdoor units, the interaction of flow field between upside and downside rotor is weak.4. Vortex line analysis based on trail of tip vortex core of rotor with different setting angles is used to illustrate the interaction between tip vortex and bellmouth and set up the relationship between vortex movement and noise radiation. Based on vortex-sound theory of low Mach number and homentropic flow together with CFD simulation, it is shown that trailing edge vortex is the major noise source of partly-ducted axial flow fan with uniform inflow. To outdoor units, besides effect of trailing edge vortex, inlet turbulence generated by heat exchanger has a little effect on inlet turbulent noise of axial flow fan. On the other hand it is shown by HWA measurement that turbulence generated by heat exchanger will be weakened by mixture of main flow, and turbulence intensity will be little at blade leading edge, and then inlet turbulent noise of outdoor units will be little and has weak effect on total sound pressure of outdoor units.5. Broadband noise and total sound pressure level of outdoor units is predicted by vortex shedding noise prediction model based on three-dimensional steady CFD simulation. It is shown that wake parameter at 5mm downstream of blade is accurate. Error of total sound pressure level of single and collateral axial flow fan system inside outdoor units is 2 and 3 dBA, respectively. That is to say this prediction method is suitable for engineering demand. Discrete frequency noise of outdoor units is predicted in frequency and time domains based on a hybrid method coupling FW-H equation and three dimensional unsteady CFD simulations. With careful CFD simulation, time consumption of time-domain noise prediction is reduced. By numerical analysis of Bessel function in frequency-domain noise prediction model, it is shown that only Bessel function below second order has effect on discrete frequency noise of axial flow fan system with low pressure and low Mach number, which can be used to simplify frequency-domain noise prediction model. By contrast of noise prediction based on compact noise source assumption and stripe theory, it is justified that prediction model based on rotating point source is accurate. Results of time and frequency-domain noise prediction are both consistent with experimental one at BPF and its low order harmonics, which attests that this noise prediction model is accurate in discrete frequency noise prediction of outdoor units.
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