Riedel, Jörg ; Czwielong, Felix ; Becker, Stefan (2022)
Investigation of a New Shielding Material for Slotted Tubes Regarding Sound Power Measurements of Ducted Fans.
FAN 2022 – International Conference on Fan Noise, Aerodynamics, Applications and Systems. Senlis, Frankreich (27.06.-29.06.2022)
doi: 10.26083/tuprints-00021696
Konferenzveröffentlichung, Erstveröffentlichung, Verlagsversion

Kurzbeschreibung (Abstract)

Slotted tube probes are used in acoustic measurement technology, for instance, to measure the sound power level of fans and aggregates in duct flows. For this purpose, an approximately 450 mm long metal tube, which is slotted on one side, is screwed onto the actual measuring microphone. The slot can also be covered with a porous medium. The slotted tube acts as a turbulence screen and is supposed to suppress the flow-related hydrodynamic pressure fluctuations on the one hand and to let the acoustic pressure fluctuations, which are due to the fan, pass through to the microphone on the other. Aramid fabrics, sold under the trade Autor Kevlar®, are increasingly used as shielding material in aeroacoustic wind tunnels due to their material properties. In the present work, the question is investigated to what extent Kevlar® is suitable as a porous covering material for slotted tube probes. For this purpose, a standardized slotted tube probe is built, which is then covered with Kevlar®. The performance of the probe is then tested in an axial fan test rig. In addition, the performance of the probe is compared with an uncovered slotted tube probe. The probes are examined on the test rig at different operating points of the fan and turbulent inflow conditions that differ in terms of length scale and turbulence intensity. It is shown that the slotted tube probe covered with the aramide fabric can detect all acoustic characteristics of the axial fan, such as blade passing frequency and higher harmonics, in comparison to the uncovered probe, whereas the uncovered probe is not able to do so, especially at higher flow velocities. When comparing the probe covered with aramide fabric with a probe covered with a metal grid in an aeroacoustic wind tunnel, the turbulent inflow conditions are decisive for which of the two probes achieves a better signal to noise ratio. The results presented should serve to further improve slotted tube probes, which are often optimised in a university context.

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