Pressure fluctuations in centrifugal compressor diffusers

The unstable flow phenomenon like rotating stall and surge in centrifugal compressors occur at very low flow rates. These would cause unduly large vibrations in the turbo- machinery components and results in mechanical failure. It is important to detect this phenomenon and avoid it by means of early warning systems. An attempt has been made in this paper to study the pressure fluctuations using high frequency miniature transducers in vane and vaneless diffusers of a centrifugal compressor. The unstable nature of the flow is characterized. The observation from the measurements indicate the pressure fluctuations in the vane diffusers are getting amplified and carried to the exit of the diffuser whereas in the case of vaneless diffuser such a phenomena is not observed. The flow near the leading edge of diffuser plays an important role in diffuser performance. The unsteady pressure fluctuations in the diffuser are characterized in terms of instability parameter at various locations in the diffuser. The location where such an instability parameter is sensitive to the flow coefficient is identified as the diffuser throat. Through this instability parameter it is possible to provide early warning just before compressor goes into stall. (Author