Siemens Energy has commissioned an extensive multi-year experimental and numerical (CFD) project to improve its ability to design for and predict compressor stall. The experimental test rig is a half scale six stage axial compressor. The goal of this work is to provide insight into how best to predict the compressor performance map and in particular the stall point by applying state-of-the-art multiple blade row CFD simulation tools. A preliminary CFD analysis quantified numerical, model and systematic error on the first stage of the compressor. Subsequent steady (Mixing Plane) and transient (Time Transformation) CFD simulations of the entire six stage compressor are compared to each other and to experimental data. Both the steady and transient simulations are shown to be computationally efficient and in very good agreement with the experimental data across the full performance map, up to stall inception on multiple speedlines. Physical explanations of the key flow features observed in the experiment, as well as of the differences between the predictions and experimental data, are given.
Modern CFD flow solvers can be readily used to obtain time-averaged results on industrial size turbomachinery flow problem at low computational cost and overall effort. On the other hand, time-accurate computations are still expensive and require substantial resources in CPU and computer memory. However, numerical techniques such as phase shift and time inclining method can be used to reduce overall computational cost and memory requirements. The unsteady effects of moving wakes, tip vortices and upstream propagation of shock waves in the front stages of multi-stage compressors are crucial to determine the stability and efficiency of gas turbines at part-load conditions. Accurate predictions of efficiency and aerodynamic stability of turbomachinery stages with strong blade row interaction based on transient CFD simulations are therefore of increasing importance today. The T106D turbine profile is under investigation as well as the transonic compressor test rig at Purdue. The main objective of this paper is to contribute to the understanding of unsteady flow phenomena that can lead to the next generation design of turbomachinery blading. Transient results obtained from simulations utilizing shape correction (phase shift) and time inclining methods in an implicit pressure-based solver, are compared with those of a full transient model in terms of computational cost and accuracy.
Siemens Energy has commissioned an extensive multiyear experimental and numerical (computational fluid dynamics (CFD)) project to improve its ability to design for and predict compressor stall. The experimental test rig is a half scale six stage axial compressor. The goal of this work is to provide insight into how best to predict the compressor performance map and in particular the stall point by applying state-of-the-art multiple blade row CFD simulation tools. A preliminary CFD analysis quantified numerical, model, and systematic error on the first stage of the compressor. Subsequent steady (mixing plane) and transient (time transformation) CFD simulations of the entire six stage compressor are compared to each other and to experimental data. Both the steady and transient simulations are shown to be computationally efficient and in very good agreement with the experimental data across the full performance map, up to stall inception on multiple speedlines. Physical explanations of the key flow features observed in the experiment, as well as of the differences between the predictions and experimental data, are given.
The effect of upstream tangential blowing on the secondary flows has been studied in a turbine cascade of rotor blades. The aim is to reduce the secondary flows and losses, but in the evaluation an accounting procedure for the energy for blowing is required. The experimental results show that the effect of the increasing blowing is first to thicken the inlet boundary layer, giving greater secondary flow and more loss, and then as re-energisation of the inlet boundary layer takes place together with increasing counter streamwise vorticity, the passage vortex is progressively weakened, with a corresponding reduction in loss. Low rather than high angle blowing is shown to be more effective as the jet is kept closer to the end wall, and strong similarities could be obtained with the flow patterns from previous work with a skewed inlet boundary layer. However when the energy for inlet blowing is included, no net gain is achieved due mainly to the mixing loss of the injected air. Overall gains may be achievable, if combined with such features as injection for film cooling.
The performance of two refinements (non-equilibrium and non-linear) to the Low-Reynolds-Number Launder and Sharma (1974) k-ε–turbulence model in the framework of a turbomachinery Navier-Stokes code is evaluated for subsonic and transonic compressor cascades at design and off-design conditions. The non-equilibrium treatment originally developed by Rodi (1972) for extending the standard high Re-number k-ε–model to account for the imbalance of production and dissipation in far-field wakes has been applied, in combination with the well-established Kato and Launder (1993) modification, to the entire flow field by extending the scaling of the eddy viscosity to the production term of turbulent kinetic energy in its transport equation. Accounting for non-equilibrium (spectral) effects such as non-local strain history in adverse pressure gradients has been found to be essential a) in predicting early separation on the DCA blade of the subsonic compressor cascade at design conditions and b) in obtaining a more accurate position of a swallowed shock in the transonic compressor cascade at off-design. The former improvement has resulted in a better prediction of the flow exit angle and the latter is essential in capturing the physics of the shock/boundary-layer interaction but further refinements are required to improve the overall flow field and hence loss predictions. The potential of the cubic two-equation non-linear stress-strain model extension developed by Suga (1995) has been investigated. This is shown to provide a better baseline for non-equilibrium modeling than the Kato and Launder modified k-ε–model, since it accounts additionally for the misalignment of turbulence and strain tensor principal axes, streamline curvature and partially for non-equilibrium effects at very little additional computational cost. The non-linear model is found to demonstrate the correct trends, though it requires recalibration over a wider range of turbomachinery flows.
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