Noise reduction of a multistage export / reinjection - Dresser-Rand

The second type of discrete frequency noise is the interaction noise, generated by the
aerodynamic interaction between the rotating impeller blades and the stationary vanes (e.g.,
diffuser vanes or low solidity diffuser vanes (LSD)). The discrete frequency noise due to this
interaction can be further classified into wake interaction and potential field interaction. As the
wake from an upstream object (e.g., an impeller blade) impinges on a downstream object (e.g. a
vane), the velocity deficit in the wake causes the downstream object to experience a momentary
change in angle of attack and in velocity because of the variation of the in-flow velocity in the
wake. Consequently, a fluctuating load is imposed on the downstream object and a dipole noise
source is generated.
The potential field interaction is caused by the local periodic interference between the potential
field of the upstream object such as an impeller blade and the potential field of the downstream
object such as a vane. These potential fields interact with each other as the impeller rotates.
That is, the pressure field of an impeller blade disturbs an LSD vane and the pressure field of an
LSD vane disturbs an impeller blade. These disturbances also cause periodic loading variations
that produce tonal noise at blade passing frequency and higher harmonics.
The discrete frequency noise that results from this interaction is commonly associated with
centrifugal compressors using LSD vanes. As a result, compressors with diffuser vanes are
typically noisier than vaneless diffuser designs.
Duct Resonator Array Location and Design Considerations
There are two major reasons why the aeroacoustic noise source is mainly generated in the
region between the impeller exit and diffuser entrance. First, the gas exiting the impeller is of the