A combined acoustic-aerodynamic diagnostics methodology is presented herein, involving the phased array microphone technique and an in-house rotating source identifier processing algorithm. The methodology makes possible the industrial onsite investigation of unducted or short-ducted rotor-only axial fans. Its application is illustrated in a case study of a free-inlet, free-exhausting fan, surveyed from the upstream direction. A methodology is outlined for guaranteeing the appropriateness of averaging of the phased array microphone records. A technique is proposed for eliminating the ambiguity of the angular position of the rotor noise sources. A complete set of noise source maps of various frequencies have been evaluated. Noise sources, such as tip leakage flow and suction side blade boundary layer flow, have been identified. Empirical correlations have been pointed out between indicators of aerodynamic noise and aerodynamic loss along the blade span. Such correlations may contribute to redesign guidelines for simultaneous reduction of noise and improvement of efficiency at prescribed fan performance.
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