Numerical Investigation of the Unsteady Interaction Within a Close-Coupled Centrifugal Compressor Used in an Aero Engine

Abstract

The present work forms part of a research project of the Institute of Jet Propulsion and Turbomachinery at the RWTH Aachen University in collaboration with GE Aviation. The subject is the detailed numerical analysis of the unsteady flow field focusing on the interaction between the impeller and the passage diffuser of a close-coupled transsonic centrifugal compressor used in an aero engine. The centrifugal compressor investigated is characterized by a close-coupled impeller and passage diffuser with a radial gap of only 3.6%. The close coupling tends to provide a high aerodynamic efficiency but simultaneously cause a high unsteady interaction between the impeller and the diffuser. These unsteady effects can have a significant impact on the performance of both components [1,2] and present a challenge to state-of-the-art numerical methods. With increasing compressor efficiency, the more important it is to have a understanding of the detailed unsteady flow physics. Experimental data was obtained from a state-of-the art centrifugal compressor test rig located at the Institute of Jet Propulsion [3]. Steady and unsteady pressure measurements within the impeller and diffuser are used to gain detailed information on the temporal, time-averaged and spectral pressure distributions within the stage to validate the CFD. The work presented shows the unsteady phenomena caused by the interaction as well as the location and propagation of these phenomena within the centrifugal stage. Within the impeller, the exducer is in first order excited by the BPF of the diffuser, whereas in the diffuser both the BPF, as well as the PPF, are present up until the end of the pipe-diffuser. Significant effects on the integral component performance could only be identified for the impeller. Special focus is paid to evaluate the diffuser upstream pressure field, since this is the major source of unsteadiness within the impeller. The performance of the rotor decreases due to the unsteady interaction. This effect is traced back to the unsteady tip-clearance flow, in which the time-averaged mass transport decreases, whereas the specific entropy production increases in a nonlinear way. Within the diffuser, local effects counteracting with respect to the integral performance are found. In front of the throat, there is less decay in total pressure, as a result of tangentially expanding pressure waves. Within the passage an decrease in flow uniformity in the unsteady flow is identified as the reason for the lower diffusion up until the throat and higher losses within the downstream diffuser passage.

Numerical analysis of the vibration and noise induced by the unsteady flow in a centrifugal compressor

“…The peak frequency of the static pressure fluctuations is the BPF and the maximum amplitudes of the static pressure fluctuations are mainly located at the radial clearances between the impeller and the stationary components, and this conclusion has been presented in many previous researches. 13,31…”

Investigation of two pipe diffuser configurations for a compact centrifugal compressor

“…The unsteady impeller–diffuser interaction and the transit phenomena in pipe diffuser passages were investigated by Grates et al., 5 Sugimoto et al., 6 and Wilkosz et al. 7 by means of both experimental and numerical methods. Strong agreement was shown between the time-averaged predicted results and the test data.…”

The effect of impeller–diffuser interactions on diffuser performance in a centrifugal compressor

“…Reinhard (2003a, 2003b) and Robinson, Casey, Hutchinson, and Steed (2012) studied the impact of the size of the gap between the impeller and the vaned diffuser on the performance of the vaned diffuser; their findings indicate that a smaller gap results in a higher efficiency and a stronger IDI effect. Wilkosz, Zimmermann, Schwarz, Jeschke, and Smythe (2014) and Anish, Sitaram, and Kim (2014) investigated the impact of the IDI on the centrifugal compressors with both high and low Mach numbers, finding that the IDI strongly affects the aerodynamic performance of the diffuser and that it must be considered in the detailed design process. Anish et al (2014) noted that the main factor influencing the diffuser instability is the circumferential variation of the flow angle at the leading edge of the diffuser vane.…”