Unsteady pressure at the diffuser inlet

The measured pressure spectra at the diffuser inlet are presented in Fig. 5. The results from different operation points and different circumferential angles are shown. At the operation point near the surge, one can see small variation of the pressure at the blade passing frequency (about 5050 Hz) at every cir­cumferential angle. The amplitude of the pressure variation is about 200 Pa. At the circumferential angle 168° and 348° one can also see the pressure vari­ations at the frequency of every second blade (about 2525 Hz). The amplitude of the pressure variation is larger at the circumferential angle 168 ° than at the circumferential angle 348°.

At the design operation point, the amplitude of the pressure variations are largest at the frequency of every second blade. The amplitude is a little bit larger at the circumferential angle 168° than at the other circumferential angles. No clear pressure variation is seen at the blade passing frequency.

At the operation point near the choke, larger amplitude of the pressure vari­ations is seen at the circumferential angle 168°. The frequencies of the vari­ations are the blade passing frequency and the frequency of the every other blade. The larger pressure variation is also seen at the circumferential an­gle 348°, and at the blade passing frequency. One can also notice that there are smaller pressure flictuations at the multiplex frequencies of the rotational speed at every operation point. These flictuations get larger when the ft>w through the compressor is increased (see Fig. 5).

The calculated pressure spectra are presented in Fig. 6. The results from the design operation point, the operation point near the choke, and the different cir­cumferential angles are shown. At the design operation point, one can clearly see pressure variation at the blade passing frequency and at the frequency of every second blade. The amplitude of the pressure variation is largest at the circumferential angle 168° and at the frequency of every second blade. The magnitude of the amplitude is the same in the measured and in the calculated pressure spectrum at the frequency of every second blade. On the other hand,
pressure variation is not seen in the measured spectrum at the blade passing frequency (see Fig. 5).

There are larger pressure variations at the multiplex frequency of the rota­tion speed at the operation point near the choke. The largest amplitude of the pressure variations is seen at the frequency of every second blade and at the circumferential angles 78° and 348°. These are not seen in the measured pres­sure spectra (see Fig. 5). Larger amplitude of pressure variations is also seen at the blade passing frequency in the calculated spectra. This is also seen in the measured spectra at the circumferential angles 168° and 348°.