Fluid Mechanics and Acoustics Laboratory - UMR 5509

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Home > Teams > Turbomachinery Team > T1 - Instationnarités et Instabilités Aérodynamiques

Study of corner separation in a linear compressor cascade

Theme 1 - Modèles de turbulence pour les écoulements instationnaires en turbomachines
Period 01/10/2013-01/10/2016
Contact(s) OTTAVY Xavier
PhD student ZAMBONINI Gherardo
Partners LU Lipeng, School of Jet Propulsion, Beihang University, Beijing, China

The rapid growth of air traffic associated with more restrictive environmental pollution norms and the reduction of travel costs have nowadays accelerated the research of efficiency limits of aircraft propulsion. The solution generally adopted is to increment the thrust/weight ratio of aircraft engines by increasing the blade loading and reducing the aspect ratio of multistage axial compressors. However, the compressor blade loading is extremely limited by several three-dimensional flow losses, such as end-wall boundary layers, flow separations, leakages, and shocks that degrade overall performance and stability of the entire engine.

The end-wall and corner region is one of the high-loss regions in compressors. The three-dimensional separation in this region, also referred to corner separation, occurs at the junction between the end-wall and the blade suction surface both in rotor and stator of compressor stages. When the aerodynamic load of the blade is higher than a certain value, the size of the separated region suddenly grows and it is commonly described as corner stall (Lei et al., 2008).
Corner stall is caused mainly by: (i) the strong stream-wise pressure gradient, (ii) the presence of secondary flow and (iii) the merging of the wall and the blade boundary layers, but it is beyond the scope of the boundary layer theory and secondary flow theory.
Though deleterious impacts of this phenomenon on the overall performance of the machine are well known (Dong et al., 1987), an effective control has been very difficult to achieve. This is because neither the nature of the corner separation is still not clearly understood, nor the unsteady mechanisms that influence its growth and size are fully quantified.
Computational fluid dynamics (CFD) is a powerful tool to research corner stall, but only overall pattern of corner stall can be captured while unsteady flow details are not well computed. Right now large eddy simulation (LES) is one of the more promising modes of numerical simulation of turbulence, but it still needs to be calibrated when using in turbomachinery.
This study is a part of a more general project AXIOOM devoted to the improvement of the physics understanding and the advanced numerical simulations.

To gain a better knowledge of the mechanisms and to calibrate CFD tools including both RANS and LES, detailed and accurate experiments of three-dimensional flow field through a compressor cascade have been set up at LMFA. The investigation is carried out on a low-speed cascade facility consisting of 13 NACA65-009 blades. The inlet flow velocity is about 38m/s, chord Reynolds number is 3.8E+5.
The extensive and comprehensive data base includes the following measurements:
(1) Oil visualization on the blade and end-wall;
(2) Inlet flow investigations with hot-wire, in freestream flow and boundary layer;
(3) Inlet flow investigations with five-hole probe;
(4) Steady and unsteady pressure on the blade at various span-wise locations and on the end-wall with microphones;
(5) Velocity in the passage with PIV (2D and 3D) and LDA;
(6) Outlet measurements with hot-wire and five-hole probe.

Laser Doppler Anemometry (LDA) measurements carried out on the suction surface of the blade shed a light on the so called bimodal behavior of the tangential velocity. Specific points localized near the mean separation line are characterized by bimodal probability density functions (PDF), in other words two preferred types of flow. Particle Image Velocimetry (PIV) shows the feature of these modes: an attached flow characterized by small separation and a reversed flow due to a large size of the separated region (cf. Fig. 2).

A characterization of the mean and fluctuating component of wall static pressure on the surface of a special developed blade is achieved at first. This fluctuating component is investigated utilizing nineteen high sensitivity condenser microphones plugged into blade cavities which are carefully calibrated. Transfer functions obtained by calibration are exploited to reconstruct the time-dependent pressure signal and finally statistics, cross-correlations, coherence and spectra of fluctuations are analyzed in order to investigate the unsteady characteristics of the corner separation (cf. Fig. 2).

The region affected by corner separation and the instability of incidence near the leading edge are both revealed by RMS of pressure fluctuations. The computation of skewness reveals an unsteady intermittent behavior of the separation line on the suction surface, as well as the tornado-like vortex dropping near the trailing edge. This feature leads to the assumption that the line of separation in front of the detached zone is not steady as described by averaged topologies but it switches intermittently between a position more upstream and a position more downstream on the suction surface, in accord with the bimodal behavior of the velocity.

Figure 1 : Fig.1 - Linear compressor cascade and experimental methods for the characterization of corner separation.

Figure 2 : Fig.2 - Unsteady velocity and pressure measurements with respective results showing the bimodal behavior and the unsteadiness of the separation.

Documents and associated publications

Ma W. (2012). "Experimental investigation of corner stall in a linear compressor cascade". Ph.D. thesis, Ecole Centrale de Lyon.

Ma W., Ottavy X., Lu L., and Leboeuf F. (2013). "Intermittent corner separation in a linear compressor cascade". Experiments in Fluids, 54(6):1–17.

Zambonini G., Ottavy X. (2015). "Unsteady pressure investigations of corner separated flow in a linear compressor cascade". ASME Turbo Expo 2015.

Gao F., Zambonini G., Boudet J., Ottavy X., Lu L., and Shao L., (2015). "Unsteady behavior of corner separation in a compressor cascade: LES and experimental study". European Turbomachinery Conference.