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Numerical analysis of noise characteristics of a contra-rotating axial fan

Numerical analysis of the flow-field in a contra-rotating fan has been performed by using the CFD method. The LES technique is applied to simulate the 3D unsteady whole flow-field of the fan. Several monitoring faces are specified in the flow-field between the two impellers. Three monitoring points, which are respectively near the hub, near the casing and in the middle of the flow passage, are specified on each of the other monitoring faces. FW-H model is introduced to simulate aerodynamic noise. The distribution and characteristics of noise show that the broadband noise mainly occurs at the monitoring faces within the impellers, while away from the rotating blades, the broadband noise fades. The dominant frequencies of the discrete noise are near to the blade passing frequency and the harmonic frequencies of the impellers. Approaching the second-stage impeller, the blade passing frequency of the second-stage and the harmonic frequencies are dominant. The contours of vorticity magnitude for each monitoring face show that the intensity of the broadband noise is principally related to the vorticity magnitude. The discrete spectrum noise is affected by the number of blades and the rotating speed of the two stages of impellers.