Identification of phenomena preceding blower surge by means of pressure spectral maps

In this paper the concept of 'quasi-dynamic' pressure measurements is introduced and used in order to obtain compressor spectral maps describing the process of surge onset in a centrifugal blower. In the experimental rig pressure signals were recorded at several throttling valve positions and subjected to Fourier analysis. The results are shown in the form of contour maps with frequency on the horizontal and control valve opening on the vertical axis. The maps allow the frequencies dominating at consecutive stages of entering surge to be distinguished. Moreover, comparison of the maps, obtained at different pressure tappings allowed the point of instability onset to be determined. Research confirmed that the first disturbances appeared in the inlet recirculation zone much before the surge. The scale of the disturbances was considerable, amplitude increased about 10 times together with an immediate rise of its mean value. On the other hand, flow structure was not regular as no dominating frequency was detected. When the flow rate continued to decrease the instabilities propagated throughout the impeller towards the diffuser and certain frequency peaks appeared suggesting the formation of regular flow structures. At deep surge spectrum is dominated by one peak that is in good agreement with frequency of the corresponding Helmholtz resonator. Moreover, due to the high accuracy of the 'quasi-dynamic' method it was possible to recognise the damped forms of non-stable phenomena were present in stable working conditions