Acoustic Emissions into Discharge Pipes II

Kurzfassung

Noise protection regulations for occupational health and safety require manufactures and operators of centrifugal compressors to accurately predict the sound emissions of these turbomachines. The sound emissions are declared in the form of the emitted sound power level. However, measuring the sound power level emitted by a centrifugal compressor into the connected discharge pipe with exciting measurement systems is either too inaccurate or involves too much measurement effort. Therefore, there is a need in the industry for an efficient and robust measurement method. Currently, measurements are often conducted in accordance with the ISO standard 5136 with a very low number of sensors and limitations to the flow profile. Previous research has shown that the ISO method deviates up to 10 dB for tonal sound components when compared to a highly accurate traversable measurement system. This is problematic because the sound emissions into the connected pipes dominate the tonal components of the total sound power of centrifugal compressors. However, more accurate sound power measurement methods with traversable measurement systems are associated with high costs. In order to develop a new measurement technique for accurate estimation of the tonal sound power of centrifugal compressors, which is robust, easy to install and costeffective, the research project “Acoustic Emission into Discharge Pipes" (FVV Nr. 1227) was initiated in 2015. The research object was the largescale, open loop centrifugal compressor test rig at the Institute of Jet Propulsion and Turbomachinery (IST) at RWTH Aachen University. Two measurement techniques were developed and tested on CFD and CAA simulation results, which were used to numerically simulate the sound generation and the pressuresided sound propagation of the Design 603, a highly loaded centrifugal compressor with a vaneless diffuser. In the followup research project “Acoustic Emission into Discharge Pipes II" (FVV No. 1383), the results of which are presented in this final report, both measurement techniques are implemented in the test rig at IST and used to experimentally measure the sound power level in the discharge pipe of the Design 603. The experimental results are compared with the three numerically simulated operating points from the first project phase. The comparison shows that the experimentally measured sound power levels agree well with the numerical simulation results. Thus, the sound power level in the entire map of the centrifugal compressor with a vaneless diffuser was measured. However, both newly developed methods are evaluated as being universally applicable, allowing radial compressor designs with vaned diffusers to be measured in the future. The reference CFD data was also made available within the project “Acoustic Emission into Discharge Pipes II".