The flow in the vaned diffuser of an aeronautical centrifugal compressor designed by Safran Helicopter Engines is analyzed through steady and unsteady pressure measurements at different rotation speeds. The analysis leads to the identification of different operating zones thanks to a new variable, the alternate rate ?. It allows the characterization of a specific behavior of the vaned diffuser consisting of an alternate stall pattern in two adjacent channels of the diffuser. While it is close to zero at low speed, the alternate rate reaches a maximum value at a higher speed before collapsing with a further increase in the rotation speed. Depending on the value reached by the alternate rate, three distinct regimes of the flow within the diffuser can be distinguished. For low ? values, the regime is the most common one with an equivalent flow pattern in each channel of the diffuser. For moderate ? values, a mild difference of the flow fields which develop in two adjacent channels can be observed but it remains time independent. Finally, for high values of ?, the alternate pattern is amplified and becomes time dependent, pulsating together with the mild surge of the entire compressor.

This paper reports large-eddy simulations (LES) and unsteady Reynolds-averaged Navier-Stokes (URANS) calculations of a turbine rim seal configuration previously investigated experimentally. The configuration does not include any vanes, blades or external flows, but investigates inherent unsteady flow features and limitations of CFD modelling identified in engine representative studies. Compared to RANS and URANS CFD models, a sector LES model showed closer agreement with mean pressure measurements. LES models also showed agreement with measured pressure frequency spectra, but discrepancies were found between the LES and experiment in the speed and the circumferential lobe number of the unsteady flow structures. Sensitivity of predictions to modelling assumptions and differences with experimental data are investigated through CFD calculations considering sector size, interaction between the rim cavity and the inner cavity, outer annulus boundary conditions, and the coolant mass flow. Significant sensitivity to external flow conditions, which could contribute to differences with measurements, is shown, although some discrepancies remain. Further detailed analysis of the CFD solutions is given illustrating the complex flow physics. Possible improvement of a steady RANS model using a priori analysis of LES was investigated, but showed a rather small improvement in mean pressure prediction.

The present study aims at analysing the Boussinesq constitutive relation validity in a corner separation flow of a compressor cascade. The Boussinesq constitutive relation is commonly used in Reynolds-averaged Navier-Stokes (RANS) simulations for turbomachinery design. It assumes an alignment between the Reynolds stress tensor and the zero-trace mean strain-rate tensor. An indicator that measures the alignment between these tensors is used to test the validity of this assumption in a high fidelity large-eddy simulation. Eddy-viscosities are also computed using the LES database and compared. A large-eddy simulation (LES) of a LMFA-NACA65 compressor cascade, in which a corner separation is present, is considered as reference. With LES, both the Reynolds stress tensor and the mean strain-rate tensor are known, which allows the construction of the indicator and the eddy-viscosities. Two constitutive relations are evaluated. The first one is the Boussinesq constitutive relation, while the second one is the quadratic constitutive relation (QCR), expected to render more anisotropy, thus to present a better alignment between the tensors. The Boussinesq constitutive relation is rarely valid, but the QCR tends to improve the alignment. The improvement is mainly present at the inlet, upstream of the corner separation. At the outlet, the correction is milder. The eddy-viscosity built with the LES results are of the same order of magnitude as those built as the ratio of the turbulent kinetic energy k and the turbulence specific dissipation rate ω. They also show that the main impact of the QCR is to rotate the mean strain-rate tensor in order to realign it with the Reynolds stress tensor, without dilating it.

A research centrifugal compressor stage designed and built by Safran Helicopter Engines is tested at 3 IGV (Inlet Guide Vanes) stagger angles. The methodology for calculating the performance is detailed, including the consideration of humidity in order to minimize errors related in particular to operating atmospheric conditions. The shift of the surge line towards lower mass flow rate as the IGV stagger angle increases highly depends on the rotation speed. The surge line shift is very small at low rotation speeds whereas it significantly increases at high rotation speeds. A first-order stability analysis of the impeller and diffuser sub-components shows that the diffuser (resp. impeller) is the first unstable component at low (resp. high) rotation speeds. This situation is unaltered by increasing the IGV stagger angle. At low rotation speeds below a given mass flow rate, rotating instabilities at the impeller inlet are detected at zero IGV stagger angle. Their occurrence is conditioned by the relative flow angle at the tip of the leading edge of the impeller. As the IGV stagger angle increases, the mass flow decreases to maintain a given inlet flow angle. Therefore, the onset of the rotating instabilities is delayed towards lower mass flow rates. At high rotation speeds, the absolute flow angle at the diffuser inlet near surge decreases as the IGV stagger angle increases. As a result, the flow is highly alternate over two adjacent channels of the radial diffuser beyond the surge line at IGV stagger angle of 0°.

The present paper aims at providing an experimental analysis of the path to surge of a centrifugal compressor stage designed and built by Safran Helicopter Engines. Depending on the rotation speed of the compressor, two distinct flow patterns are observed in the radial diffuser at stabilized operating points near the surge, an asymmetric and a symmetric pattern. At medium rotation speed, the alternate pattern consisting of a two-channel pattern in the radial diffuser develops. One passage over two is stalled, the adjacent passage is free and this pattern replicates over the whole circumference while pulsing at a frequency of roughly 12 Hz which is close to the Helmholtz frequency of the test rig. By lowering the rotation speed, the two-channel pattern fades away and gives way to a periodical behavior of the radial diffuser passages called symmetric mode. The flow in each channel is identical presenting a stalled behavior pulsating in phase at a higher frequency of roughly 42 Hz. The two 12 Hz and 42 Hz modes are described and their existences are imputed to a lock-in of the natural frequencies of the instabilities with the acoustic modes of the test rig.