Influence of Rotor Tip Flow Field Mis-Prediction on Rotating Disturbance Near Surge in a High Speed Multistage Compressor

Abstract

In this work, unsteady numerical results and high-frequency measurements are investigated from nominal to loaded operating points with the objective to contribute to the understanding of pre-stall rotating disturbances. A 3.5 stages high speed axial multistage compressor is investigated on a 2 MW test rig in the laboratory of fluid mechanics (LMFA) at Ecole Centrale de Lyon, France. The compressor has been built by Snecma, and is representative of modern high-pressure rear blocks of a modern aircraft engine. The unsteady numerical results predict a rotating disturbance in the tip flow field of the rotor 2 at the loaded operating point. It causes a frequency which is not a multiple of the periodicity of the compressor, and is rotating at about 72% of the shaft speed. The mesh independency of this disturbance is ensured. The analysis of the circumferential and axial propagation of the disturbance reveals a rotating instability like phenomenon. Most characteristic is the very important periodic oscillation of the tip leakage vortex trajectory, leading to a modulation of the leakage flow in the neighboring tip gap. The influence of the neighboring blade rows is investigated by filtering their unsteady contribution by means of mixing planes up and/or downstream of the rotor 2. In either case, the rotating disturbance is found to be still present. There are no traces of this rotating disturbance in the high-frequency measurements investigated at a near surge operating point. A spike like stall inception and almost instantaneous surge inception is identified. The mis-prediction of the tip region flow field in the rotors 2 and 3 is believed to cause the mis-prediction of the pre-stall disturbance.

Wall-Modeled Large-Eddy Simulations of a Multistage High-Pressure Compressor

“…The radial locations in the computation match the positions of experimental probes. In addition to the uncertainties in measurement chains, Schreiber et al [48] have recently quantified some discrepancies between experimental and existing numerical studies of CREATE. First, the geometry of the computational domain is simplified as it accounts neither for leakages at hub and casing between each row, except below S1, nor for hub and tip clearances of IGVs.…”

“…URANS simulations of CREATE have been assessed against experimental data [38,5] allowing to study a casing treatment for control of tip leakage flows [17]. Crevel et al [6] and Schreiber et al [48] performed URANS simulations at low mass flow rate for instability investigation, even simulating a full surge cycle [7]. However some limitations in the application of URANS to axial compressors could be evidenced.…”

Analysis of a high-pressure multistage axial compressor at off-design conditions with coarse Large Eddy Simulations

“…It was also shown that the plenums located upstream and downstream the compressor need to be accounted for to simulate a full surge cycle. Recently, on a different version of CREATE, the URANS simulation of Schreiber et al (2016) showed 32 rotating instabilities at 72% of shaft speed at a low mass flow rate operating point. All these simulations were performed over an azimuthal sector of the compressor, hence enforcing the amount of instabilities to be a multiple of the circumferential periodicity.…”

Observations on Rotating Instabilities and Spike Type Stall Inception in a High-Speed Multistage Compressor

“…Consequently one temporal period corresponds to 1/16th of a rotor revolution. A profound knowledge-base has been acquired with CREATE for numerical and experimental methods aiming at flow field investigations in the high-speed multistage compressor environment [10][11][12][13]. Standard tip gap sizes are implemented compared to increased tip gaps in rotors 2 and 3 in the previous version of CREATE.…”