For the assessment and improvement of noise reduction concepts and the validation of broadband sound power determination in flow ducts, the detailed knowledge of the in-duct acoustic mode spectrum and subsequent broadband noise sources separation is of great interest. A broadband noise mode analysis method was experimentally applied on broadband sound fields at 4 operation points, which were generated by means of a low speed fan test rig. Two axial sensor arrays were mounted wall-flush upstream of the fan. Measurements were made at operating conditions from 40% to 100% rotor design speed. On the whole, broadband mode waves transmitted in the flow direction is 2dB higher than against the flow direction. Mode amplitude results show that the mode decomposition results strongly depend on the chosen reference microphone. Sound power becomes higher as the speed increases. The deviation of results in and against flow directions calculated with different reference microphones is 4dB at all the operation points except for 40% design speed. A deviation of almost 10dB appears in broadband sound power results with different reference microphone in the frequency range, much higher than the results of previous experiment. The method is more accurate if it considers the phenomenon that mode waves in turbo-machinery are partly correlated with each other.

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