Unsteady pressure at the diffuser outlet

The measured pressure spectra at the diffuser outlet are presented in Fig. 7. The results from different operation points and different circumferential an­gles are shown. At the operation point near the surge, the largest amplitude of the pressure variation is seen at the circumferential angle 348 °. However, the largest amplitude of the pressure variation at the diffuser inlet is seen at the circumferential angle 168° (see Fig. 5). There is no large amplitude of pressure variation at the blade passing frequency at the diffuser outlet. On the other hand, pressure variations at the blade passing frequency are visible at the diffuser inlet.

At the design operation point the largest amplitude of the pressure variation is seen at the frequency of every second blade (see Fig. 7). This is also seen at the diffuser inlet, but the amplitude is larger.

At the operation point near the choke the largest amplitude of the pressure variation is seen at the frequency of every second blade and at the circumfer­ential angle 168°. This is also seen at the diffuser inlet.

The calculated pressure spectra are presented in Fig. 8. The results from the design operation point, the operation point near the choke and different circum­
ferential angles are shown. The pressure variation can be seen at the passing frequency of every second blade at the design operation point. The amplitude of the pressure variation decreases when the circumferential angle increases. The magnitude of the pressure variation in the measured data is equal to the magnitude of the pressure variation in the calculated data at the circumferential angles 168° and 348°. Small amplitude of the pressure variation is also seen both in the measured and in the calculated data at the blade passing frequency.

At the operation point near the choke, the behavior of the pressure is similar to the behavior of the pressure at the diffuser inlet. Only the amplitude of the pressure variations is smaller. The calculated data shows large amplitude pres­sure variations at the frequency of every second blade at the circumferential angles 78° and 348°, which are not seen in the measured pressure spectra.

5. Conclusions

An unsteady fbw field in the vaneless diffuser of a centrifugal compres­sor was investigated. Unsteady static pressure was measured at the diffuser inlet and outlet at different circumferential angles. A time-accurate numer-

Design operation point Operation point near the choke Figure 8. Calculated pressure spectra at different circumferential angles at the diffuser outlet

ical simulation was conducted to the tested compressor. A FFT was made to the measured and calculated static pressure. The provided pressure spectra were analyzed, and the measured and calculated data were compared with each other.

It can be concluded that most of the pressure variations lay at the passing frequency of every second blade. Pressure variations did not vanish in the diffuser and were visible at the diffuser outlet. However, the amplitude of the pressure variations decreased in the diffuser. It can be also concluded that the measured pressure variations were largest at the design operation point.

The time-accurate calculations showed quite good agreement with the mea­sured data. Agreement was very good at the design operation point, even though the computational grid was not dense enough in the volute and exit cone. The time-accurate calculation over-predicted the amplitude of the pres­sure variations in the operation point near the choke.

Acknowledgments

The authors wish to thank the National Technology Agency (TEKES), High Speed Tech Oy Ltd and Sundyne Corporation for financing this research. CSC – Scientific Computing Ltd provided the computer resources for the numerical work. The authors also wish to thank the Laboratory of Applied Thermody­namics at Helsinki University of Technology and Finfb Oy Ltd for their kind cooperation.

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