Infoscience

Thesis

Experimental Investigation of the Aeroelastic Stability of an Annular Compressor Cascade at Reverse Flow Conditions

Compressor surge events are unsafe operating regimes yielding highly unsteady flow fields in which complex aeroelastic phenomena occur. If the blade flutter and forced response behaviour (i.e. aeroelastic stability) can be predicted reliably for normal flow conditions, its assessment at severe-off design conditions remains a critical task for compressor development programs. During the flow reversal sequence of a surge cycle, combined aerodynamic phenomena occur which make the accurate prediction of the unsteady forces acting on the blades difficult to assess. The main objective of this investigation is to increase the physical understanding of the unsteady phenomena present during the reverse flow sequence of a typical deep surge cycle. The analysis of the blade surface unsteady pressure distribution enables the identification of the main physical mechanisms present during such extreme flow operating conditions, as well as the evaluation of their contribution on the blade global aerodynamic stability. The approach adopted consists in performing aeroelastic investigations on an annular compressor cascade at established reverse flow conditions. The investigations are carried out at EPFL, in the annular test facility for non-rotating cascades. The cascade is forced to vibrate in a torsional travelling wave mode (controlled vibration). With an upstream swirled flow corresponding to real axial turbomachine conditions, a constant flow can be set in the test section. The steady-state operating conditions are measured upstream and downstream of the test section, using 5-hole aerodynamic probes. Several cascade blades are equipped with pressure taps at 50% span in order to acquire the steady-state and unsteady blade surface pressure distributions. Static pressure taps are also inserted in the casing wall of the test section to assess the steady-state flow field characteristics in the blade tip area. The inlet flow operating conditions are varied in order to determine their influence on the blade unsteady aerodynamic forces. This study presents the measurement results and analyses in details the aerodynamic response of a cascade subjected to controlled vibrations at reverse flow conditions. The data analysis is oriented towards both physical and practical approaches. In particular, the following features are addressed: Identification of the main unsteady physical mechanisms influencing the unsteady aerodynamic forces acting on a blade oscillating and subjected to reverse flow conditions. Determination of the influence of the inlet flow condition variations on these unsteady mechanisms. Evaluation of the contribution of each unsteady phenomenon to the blade aerodynamic stability (in terms of stabilizing or destabilizing impact). Assessment of the key parameters to control in order to minimize flutter risks in case that reverse flow conditions should occur. The data analysis reveals that during the reverse flow sequence of the surge cycle, blade interaction mechanisms play a major role in the blade aerodynamic stability. The global aerodynamic damping coefficient highlights this feature and indicates that aerodynamic instability exists for some operating conditions. A second unsteady phenomenon was detected, generated by the uncommon steady-state flow field characteristics present at reverse flow operating conditions. Within this frame, the presence of a large recirculation zone on the blade suction side was identified, influencing the blade aerodynamic stability. From a more general point of view, this study constitutes a step forward to the understanding of the blade loading processes occurring during a typical deep surge sequence. Results highlight the impact of the steady-state and unsteady phenomena on the blade loading level at reverse flow conditions. For one test case, the measured data was compared with numerical results, performed in parallel to the measurements. The results indicate that even though the agreement is reasonable, the correct prediction of the aerodynamic damping curve re- quires the consideration of a complex blade interaction mechanism. Within this frame, since not many experiments exist at reverse flow conditions, these experimental results are also a precious data source for CFD validation. They enable the improvement of the prediction/simulation accuracy of the compressor performance at off-design flow conditions.

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