A Systematic Approach to Estimate the Impact of the Aerodynamic Force Induced by Rotating Stall in a Vaneless Diffuser on the Rotordynamic Behavior of Centrifugal Compressors

Abstract

One of the main challenges of the present industrial research on centrifugal compressors is the need for extending the left margin of the operating range of ihe machines. As a result, interest is being paid to accurately evaluating the amplitude of the pressure fluctuations caused by rotating stall, which usually occurs prior to surge. The related aerodynamic force acting on the rotor can produce subsynchronous vibrations, which can prevent the machine's further operation, in case their amplitude is too high. These vibrations are often contained due to the stiffness of the oil journals. Centrifugal compressor design is, however, going towards alternative journal solutions having lower stiffness levels (e.g., active magnetic bearings or squeeze fllm dampers), which will be more sensitive to this kind of excitation: consequently, a more accurate estimation of the expected forces in the presence of dynamic external forces such as those connected to an aerodynamically unstable condition is needed to predict the vibration level and the compressor operability in similar conditions. Within this scenario, experimental tests were carried out on industrial impellers operating at high peripheral Mach numbers. The dedicated test rig was equipped with several dynamic pressure probes that were inserted in the gas flow path; moreover, the rotor vibrations were constantly monitored with typical vibration probes located near the journal bearings. The pressure fleld induced by the rotating stall in the vaneless diffuser was reconstructed by means of an ensemble average approach, thus deflning the amplitude and frequency of the external force acting on the impeller. The calculated force value was then included in the rotordynamic model of the test rig: the predicted vibrations on the bearings were compared with the measurements, showing satisfactory agreement. Moreover, the procedure was applied to two real multistage compressors, showing notable prediction capabilities in the description of rotating stall effects on the machine rotordynamics. Finally, the prospects of the proposed approach are discussed by investigating the response of a real machine in high-pressure functioning when different choices of journal bearings are made.

A numerical approach to predict the rotating stall in the vaneless diffuser of a centrifugal compressor using eigenvalue method

“…The experiments carried out by Abdelhamid (1983) and Bianchini et al (2013) confirmed that the local reverse flow did not necessarily lead to stall. The dependency of flow instability on the diffuser width ratios and diameter were also verified.…”

Study on Two Types of Stall Patterns in a Centrifugal Compressor with a Wide Vaneless Diffuser

“…The number of stall cells does not change when the radius ratio is 1.5, and the circumferential propagation velocity of stall cells does not develop from low to high. (2). The stall frequency variations in wide vaneless diffusers with different radius ratios are different.…”

“…However, instability has plagued the safe and stable operation of small flow compressors, such as rotating stalls and surges. When a rotating stall occurs, the performance of a centrifugal compressor will drop sharply, and the blades will be damaged due to the alternating stress [1][2][3]. Rotating stall can occur in the diffuser and impeller [4].…”

Stall Mode Transformation in the Wide Vaneless Diffuser of Centrifugal Compressors

“…[1][2][3][4][5]. As power machinery, a wide, high-efficiency operating range is required for compressors, considering not only the efficiency but also the stability and safety [6][7][8]. However, the behaviors of compressor operating under off-design conditions are subject to various unstable phenomena which could have negative effects on their performance and then lead to further increased losses and energy consumption.…”