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ANSYS CFX Analysis Improves Performance and Reduces Cost of 15-stage Compressor

One of the difficult tasks facing engineers is the optimization of gas turbine compressor design in orderto improve performance. The axial compressor is amajor module in a gas turbine; the turbine’s overallperformance depends strongly on compressor performance. Siemens AG Power Generation hasused CFD extensively to improve the compressordesign, leading to performance improvement and costreduction. Recently, ANSYS CFX software was usedto analyze the 3-D flow through a 15-stage axial com-pressor (first-family prototype version of the SiemensV84.3A axial compressor), all in one computation. Tothe best of our knowledge, this is the first time in CFDhistory that such work has been performed using acommercial multi-purpose CFD package.

Traditional analysis of axial compressors involvesisolated blade passage analysis, single-stage (rotor-stator) analysis and, sometimes, multi-stageanalysis (two or more stages). As the design demands

and objectives become ever more challenging, so dothe simulation demands, pushing future analysis toinclude more and more of the entire machine in onesimultaneous simulation. The goals of this projectwere to learn how to best approach the analysis of all15 stages of the compressor, considering a variety ofmodeling options and choices.

This work was quite challenging as the simula-tions included tip gaps, mass bleeds and hub leakageflows; they ranged from a single passage to full 360-degree analysis. Within the CFD simulations, various effects were considered: mesh style andrefinement, boundary conditions, steady or transientanalysis and tip clearance, as well as numerical issuesincluding turbulence models and advection models,among others. The total number of nodes used for theentire simultaneous analysis of the flow through theinlet guide vane (IGV) plus 15 stages (31 components,single passage) was approximately 640K and

Thabet Belamri, Andre Braune and Paul Galpin, ANSYS, Inc.Christian Cornelius, Siemens AG Power Generation, Germany

Siemens and ANSYS performed a 210 million DOF transient simulation of 3-D flow through a complex gas turbine compressor.

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CFD model of the 15-stage compressorOverview of a 15-stage compressor

6.2 million nodes for the coarse and fine mesh,respectively. For the 360-degree simulation, a 32- million-nodes mesh was used to compute the entiremiddle section of the compressor (five stages).

Simulations of the compressor were performedunder both design and off-design conditions. A number of steady state and transient simulations ofthe entire compressor were carried out from choke tostall. Convergence of these simulations was based onthe residual mean square (rms) and maximum residuals of momentum, mass and energy equations,

as well as by monitoring important quantities, such asthe predicted total pressure at key locations. Steadystate convergence is obtained in less than 120 itera-tions for a typical simulation, with a second-orderadvection scheme on the 6.2-million-nodes meshstarting from the initial guess to convergence.

In most cases, a series of simulations wasdefined, beginning with establishment of the numericalchoked flow condition and working back toward stallconditions. A variety of comparisons was madebetween experimental and design data. The statorshroud static pressure was compared to the experimental data. The agreement between staticpressure measurements and numerical results is extremely good, even for the coarse mesh. Anexpected disturbance in the predicted static pressureis seen at each mass bleed location on the casing(mb1 to mb5). The largest mass bleed occurs near thetrailing edge of stator 13, corresponding to the largestcasing static pressure disturbance. The agreement todata is similar for the coarse mesh and the fine mesh,with and without leakage. The leakage flow resultagrees most closely with the experimental data.

The local total pressure and temperature werecalculated for three different locations in front of the stator.

The comparison between the predicted resultsand measurements shows a good agreement in bothradial gradients and absolute values. Modeling theleakage flows provides better results near the end wallregions because additional losses occur due to areverse flow near the hub region.

Mesh (32 million nodes) for the 360-degree analysis of themiddle portion of the compressor (five stages)

Mass and momentum convergence history for the 6.2 million-node mesh, single passage. The simulation converged in about120 iterations for this very large problem.

Overview of the mesh for the 15-stage one-passage analysis (6.2 million nodes)

Predicted outlet total pressure convergence history for the 6.2 million-node mesh, single passage

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CFD Update: What’s New in Computational Fluid Dynamics

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Instantaneous static pressure on a mid-spansurface of the 360-degree middle portion ofthe compressor (five stages). This solution is still evolving. ANSYS CFX was able to accurately perform this very large simulationusing parallel capabilities.

Transient simulations can provide more accurateresults than steady state simulations (for example, atoff-design conditions) but at an additional computa-tional cost. The calculations were performed in parallelusing a variety of computing configurations and types(HP Itanium, HP-PARISC, AMD Opteron, networkclusters and MP systems).

Conclusions

In conclusion, a number of simulations were performed to analyze the flow through an entire 15-stage axial compressor. These simulations haveshown:

■ ANSYS CFX provided accurate simulation of flow through the entire axial compressor. Simulations with large meshes up to 32 millionnodes were used.

■ Single passage steady state (stage) and tran-sient CFD analysis of a complete 15-stage

compressor are possible and practical usingthe standard version of ANSYS CFX.

■ Accurate predictions of overall performance,including pressure rise and efficiency, can beobtained.

■ Leakage flows can be modeled in ANSYSCFX. The stator leakage flows introduce additional losses primarily due to boundarylayer disturbance, causing local regions ofreverse flow.

■ A number of best practice recommendationswere determined as a result of this work,which generally should be applicable for suchlarge-scale axial compressor simulations.

For more information on this project, see the proceedings of the 2005 ASME Turbo Expo, June 6 – 9, 2005, papers GT2005-68261 and GT2005-68262. These are available at www.asme.org. �

Total pressure at mid-span using blade-to-blade view Relative total pressure in the meridional view

ANSYS CFX turbopost functions automatically create blade-to-blade and (circumferentially averaged) meridional plots.31

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Comparison between predicted and experi-mental static pressure on the shroud. The axial position 0 corresponds to the inlet of the guidevane, while position 1 is the outlet of the 15thstator. Pref corresponds to the static pressure at the last casing measurement point.

Local profile developments of the total pressurefrom the hub to shroud, plotted at various axiallocations

ANSYS CFX simulation

Reverse flow near the hub

Experiment Relative error in predicted overall compressorefficiency as a function of mass flow rate, comparing simulations from coarse to finemeshes as well as steady state and transientsimulation techniques