Experiments were conducted to investigate the interactions between blade vibrations and self-excited flow oscillations in a high performance centrifugal compressor system. Unsteady pressure fluctuations and dynamic stress levels were measured during compressor operation near choke, in self-excited oscillating flow conditions and near surge for four different speed lines. The unsteady pressure field for every operating condition was determined from the simultaneous recording of the output of twelve dynamic pressure transducers which were successively positioned in two peripheral and one meridional planes. Corresponding blade vibration data were collected using an eight channel telemetry system transmitting the outputs of semi-conductor strain gages located on different blades. Analysis of the measurements showed that the unsteady pressure field due to self-excited flow oscillations can be characterized by multiples of rotating and non-rotating pressure patterns at different frequencies. The blade vibration signals clearly demonstrated the excitation of the blade by each of the different unsteady pressure patterns and the frequencies of the unsteady flow field demonstrates the complexity of this flow phenomena and the need to understand the mechanism of its occurrence in order to avoid blade resonance excitation and failure during compressor operation.
The mechanism of blade excitation during the operation of a high-mass-flow, high-pressure-ratio centrifugal compressor has been investigated. This was carried out in the compressor operating range below 60 percent of design speed and in the zone of unsteady flow occurrence, where considerable blade vibration has been measured but no periodic unsteady pressure pattern such as rotating stall could be identified. Experiments conducted to study the mechanism of interactions between flow and blades were accomplished using several measuring methods simultaneously, such as measurements of blade vibration, flow angle at impeller inlet, unsteady pressure at different meridional and peripheral locations, as well as flow visualization by means of oil pattern. Analysis of the measurements showed typical broad-band characteristics of the unsteady pressure field and also for the blade vibration behavior. Results of flow angle investigations at the impeller inlet together with the analysis of oil pattern show that the broad-band pressure fluctuations and blade excitation can be attributed to a strong reverse flow near the suction side of the radial blade in the shroud zone. This reverse flow has its source downstream of the impeller and is extending back up to a location ahead of the impeller inlet. Similar results were obtained when the compressor was operated with vaneless and vaned diffuser configurations.
Experiments were conducted to investigate the characteristics of self-excited flow oscillations in a high-performance centrifugal compressor system with a straight channel radial vaned diffuser. Fast response dynamic pressure transducers on the shroud wall and blade-mounted strain gages were used to identify the onset of the oscillations and their characteristics in space and time. In addition, flow characteristics near the shroud wall were visualized by an oil injection method, showing the extent of upstream directed reverse flow in the impeller range during significant unsteady flow compressor operation. Rotating nonuniform flow patterns were found in a wide range of operating speeds before the occurrence of surge. The number of lobes in the nonuniform flow patterns was dependent on the operating conditions and varied from two to four. Results of this experimental investigation were compared with those obtained from a previous investigation of the same compressor but with a cambered vane diffuser. Considerable similarity between the two configurations was found in the spatial distribution of the unsteady pressure field and in the frequencies of the fluctuations. The stability margin before the occurrence of surge and the operating regimes in which very intense pressure fluctuations were found were however different. In both cases, flow visualization techniques revealed the occurrence of reversed flow near the shroud wall of the impeller. Reverse flow extent up to the leading edge of the splitter blades systematically correlated with the occurrence of a nonuniform pressure pattern rotating with relatively high speed. Low rotational speed pressure patterns were observed when the extent of the reverse flow was up to the leading edge of the long blade. These different flow characteristics can be related to the occurrence of distinct rotating stall cell numbers. This result could be confirmed by unsteady pressure and blade vibration measurements.
A new structure of rotating stall with unusual large-number stall cells (up to 7) has been detected experimentally in a high performance single stage centrifugal compressor system with backswept impeller and vaned diffusers. The number of stall cells is firstly identified by a phase analysis of pressure signals obtained from fast response dynamic transducers located at different circumferential positions on the shroud wall, and then verified by comparing the resulting frequencies of blade vibration with that measured from blade mounted strain gages. Up to 6 transducers in one radial position have been used for more certainty of the phase analysis. For the case of intermittent stall patterns, the frequency analysis of pressure and blade vibration signals is performed separately for sections with different characters of oscillation to avoid the smearing effect if signals of the whole pattern are averaged. As a result, the large-number stall cells, numbered 4–7, were determined in the speed range of 12000–14000 rpm, and in the higher speeds of 15000–16000 rpm right after the occurrence of normal stall cells of 2, when the flow rate is slightly reduced along the constant speed line. For the large-number stall cell case, the measured blade vibration strain has reached such a high level that is already beyond the tolerance of blade material. Moreover, frequencies of blade excitation are always in the vincinity of resonance. These two features, which are not observed for the normal stall cell case and for the excitation of broadband character, show a particular danger of this phenomenon to the compressor operation.
Blade vibration has to be considered in the design of high pressure ratio / high mass flow centrifugal compressors with increasing rotational speed values due to the reduced blade thickness. Results of a theoretical and experimental investigation concerning this problem are described. FE calculations of the stress distribution on the blade for the lower natural frequencies and various vibration tests at rest were carried out to investigate resonance and damping effects. This preparatory work was aimed at determining blade vibration behavior and acquiring fundamental experience for measurements on compressors in operation. Results of blade vibration measurements on compressors with a vaneless diffuser
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