Session: 40-03: Compressor Secondary Flows and Interactions

Paper Number: 154118

Unsteady Flow Analysis of the Full Annulus and Complete Low-Pressure ENOVAL Transonic Compressor 

For the next generation of aircraft engines, the ultra-high bypass ratio configuration is one of the challenges to be met. In this architecture, the fan and low-pressure compressor are separated by a gearbox, which means a high-speed compressor. Compressibility effects are therefore amplified, reinforcing rotor-stator interactions and accentuating losses. Moreover, the presence of transonic flows is a challenge for numerical simulation software, since shocks generally cross the interface separating rotor and stator domains. So, the steady approach based on the Reynolds-Averaged Navier-Stokes equations (RANS) and mixing plane interfaces fails not only to capture rotor-stator interactions, but in addition these shocks impair the behavior of this boundary condition. Unsteady RANS simulations are then quite mandatory to improve the flow and performance prediction of such low-pressure compressors.

Thus, the aim of this study is to carry out unsteady simulations of the complete and full-annulus low-pressure ENOVAL compressor, for two operating points (nominal and near surge). The computational domain consists in an IGV row, three stages and eight struts downstream of the compressor. The structured mesh is composed of 1.35 billion cells (more than 44 million cells for a single blade channel computation). Unsteady RANS simulations are performed with elsA software, developed at ONERA and co-owned by ONERA and SAFRAN. The turbulence is modeled with the k-l model of Smith. The second order upwind scheme of Roe is used for both conservative and turbulent equations. The backward Euler scheme with a time step equal to 1.95e-7 s (31,200 iterations per revolution) is used. The simulations are carried out with 7552 CPU cores on TGCC IRENE cluster.

For the nominal point, the transient phase lasts two revolutions and a half. The simulation is performed for another revolution in the permanent phase for time signal acquisition and time-averaged state computation. Iso-surfaces at different channel heights, as well as slices upstream and downstream of each row, were extracted every 78 time iterations (400 snapshots per revolution). The analysis of unsteady flow focuses not only on the temporal aspect of compressor performance, and comparison with experimental measurements, but also on rotor-stator interactions, in particular interaction frequencies. First results show a small reduction of overall performances by comparison to steady simulation using mixing plane approach. Temporal fluctuations of stage performances increase from the first to the last row, reaching 0.2% in terms of peak-to-peak amplitude. Moreover, the flow analysis reveals the long propagation of shocks upstream, leading to stronger rotor-stator interaction, as potential effects are not quickly mitigated and use of unsteady simulation allows investigating clocking effects, especially for rotors.

Presenting Author: Julien Marty ONERA

Presenting Author Biography: Julien Marty awarded a Ph.D thesis in 2010 at ONERA dealing with impact of turbulence, laminar-turbulent transition and leakage flows on numerical stability limit of HP compressor. He received a Ph.D award of the ISAE-SUPAERO Foundation in December 2011 for this work. He was hired by ONERA in 2009 as a research engineer and his main research interests are the turbulence modeling of separated or vortical flows in compressors and turbines, especially Zonal Detached-Eddy Simulation and transition modelling of unsteady flows in turbine.

Authors:

Julien Marty ONERA
Mohamed Amine Chemak SAFRAN TECH
Sébastien Bourasseau ONERA
Marc Chauvy SAFRAN TECH
Nicolas Habotte SAFRAN AERO BOOSTERS