The aim of this study is to propose a computing method to obtain a detailed simulation of the unsteady flow that develops in multistage turbomachines. The three-dimensional unsteady Reynolds-averaged NavierâStokes equations are solved using a structured multiblock decomposition method. Although this kind of flow solver is very popular in the turbomachine community nowadays, the complex block connectivities used in meshes of industrial configurations can be penalizing for parallel computing. The computing strategy implemented in a modern flow solver is investigated in this paper, with a particular interest in mesh partitioning, communications and load balancing. Advantages and drawbacks of different computing platforms are then discussed, ranging from vector supercomputers to massively scalar platforms. Comparisons are performed regarding criteria such as the elapsed time and the electrical power consumption. The results show that the use of a large number of computing cores (>128) is heavily penalized by communications and load balancing errors, whereas computing performance with a moderate number of computing cores (<128) is mainly driven by the peak power of the architecture. To help users estimate a priori the parallel performance of a task, a tool based on an extension of Amdahlâs law is proposed, showing satisfying results when compared with observations. Finally, an unsteady flow simulation is performed in a complete three-stage compressor at the design operating point. While still far beyond industrial resources, this numerical flow simulation shows that potential breakthroughs in the design of compression systems can be expected.

High-performance Computing to Simulate Large-scale Industrial Flows in Multistage Compressors

This paper presents a throughflow analysis tool developed in the context of the average-passage flow model elaborated by Adamczyk. The Adamczyk’s flow model describes the 3-D timeaveraged flow field within a blade row passage. The set of equations that governs this flow field is obtained by performing a Reynolds averaging, a time averaging and a passage-to-passage averaging on the Navier-Stokes equations. The throughflow level of approximation is obtained by performing an additional circumferential averaging on the 3-D average-passage flow. The resulting set of equations is similar to the 2-D axisymmetric Navier-Stokes equations but additional terms resulting from the averages show up : blade forces, blade blockage factor, Reynolds stresses, deterministic stresses, passage-to-passage stresses and circumferential stresses. This set of equations represents the ultimate throughflow model provided that all stresses and blade forces can be modeled. The relative importance of these additional terms is studied in the present contribution. The stresses and the blade forces are determined from 3-D steady and unsteady databases (a low speed compressor stage and a transonic turbine stage) and incorporated in a throughflow model based on the axisymmetric Navier-Stokes equations. A good agreement between the throughflow solution and the averaged 3-D results is obtained. These results are also compared to those obtained with a more “classical” throughflow approach based on a Navier-Stokes formulation for the endwall losses, correlations for profile losses and a simple radial mixing model assuming turbulent diffusion.

/pdf/on-the-role-of-the-deterministic-and-circumferential-42xhgi3iei.pdf

On the Role of the Deterministic and Circumferential Stresses in Throughflow Calculations

This work was funded by the European Union Framework 6 Project NEWAC:
NEW Aero engine Core concepts (FP6-030876). Additional support from Rolls-
Royce is also gratefully acknowledged.

Tip clearance flows in axial compressors : stall inception and stability enhancement

This study deals with unsteady numerical simulation of casing treatment in a stage of an axial compressor. The stall inception process is first investigated in the compressor with smooth wall. At low mass flow, the simulation shows the development of large reversed flow regions near the casing that leads to rotating stall. Then, a simulation is performed for a compressor stage with casing treatment. Despite a strong impact on the rotor casing flow, the operating range extension is low, meaning that other processes like rotor/stator interactions may play a role in the loss of stability.

/pdf/numerical-simulation-of-an-axial-compressor-with-2mk2km43v7.pdf

Numerical simulation of an axial compressor with nonaxisymmetric casing treatment

Gas turbine compressor performance may encounter deterioration during service for various reasons such as damage by debris from the casing or foreign objects impacting on the blades, typically near the rotor's tip. Moreover, mal-schedule of Variable Stator Vanes (VSVs) during start-up may also result in performance deterioration and reduction in the surge margin. Ability to assess the effect of compressor deterioration using Computational Fluid Dynamics (CFD) is important at both design stage and in service. Compressor blade damage breaks the cyclic symmetry and the VSVs mal-schecule creates mis-match between stages together with geometric variations, thus computations are desirable to be performed using full annulus assemblies. Furthermore, downstream boundary conditions are also unknown during rotating stall or surge and simulations become difficult.

This research presents unsteady time-accurate CFD analyses of compressor performance with tip curl blade damage in a single stage axial flow compressor and VSVs mal-schedule in a 3.5 stage axial flow compressor. Computations were per-
formed near stall boundary to predict rotating stall characteristics. The primary objectives are to characterise the overall compressor performance and analyse the detailed 
flow behaviour. Computations for the nominal blade configurations were also performed for comparison purposes for both compressors. All unsteady simulations were performed at part speeds with a variable nozzle downstream representing an experimental throttle.

For the blade damage study, two different degrees of damage for one blade and multiple damaged blades were investigated and compared with the results from the undamaged case. For the VSVs mal-schedule study, the first two stators were assumed to be variable and were used to create mal-schedule vane settings for the investigation. The effects of blade damage and VSVs mal-schedule on the aerodynamics performance and rotating stall characteristics for both compressor assemblies were investigated respectively and discussed in detail.

Numerical simulations of rotating stall in axial flow compressors

This paper deals with the first instability which occurs in compressors, close to the maximum of pressure rise, called rotating stall. A numerical simulation of these flow phenomena is performed and a comparison with experimental data is made. The configuration used for the simulation is an axial single-stage and low speed compressor (compressor CME2, LEMFI). The whole stage is modeled with a full 3D approach and tip clearance is taken into account. The numerical simulation shows that at least two different mechanisms are involved in the stall inception. The first one leads to a rotating stall with 10 cells and the second one leads to a configuration with only 3 cells. Unsteady signals from the computation are analyzed thanks to a time-frequency spectral analysis. An original model is proposed, in order to predict the spatial and the temporal modes which are the results of the interaction between stall cells and the compressor stage. A comparison with measurements shows that the computed stall inception point corresponds to the experimental limit of stability. The performance of the compressor during rotating stall is also well predicted by the simulation.Copyright © 2006 by ASME

G. J. Michon

Papers

About the Numerical Simulation of Rotating Stall Mechanisms in Axial Compressors