The Potential of Casing Treatments for Transonic Compressors: Evaluation Based on Axial-Slot and Rotor Blade Optimization

Casing treatments (CTs) have proven their potential to increase the working range of a compressor stage, sometimes even with little or no decrease in efficiency. However, a positive impact on efficiency is only possible if the additional CT-losses are compensated by a reduction of other losses, especially at rotor tip. This appears to be increasingly difficult to achieve for highly efficient modern rotors. In order to analyse how CTs can contribute to improve the overall compressor design, extensive optimization studies are performed, aiming at increasing the stability and efficiency of a transonic compressor stage. Axial-slot CTs and the rotor are optimized separately with a high number of geometric parameters. Selected Pareto-optimal geometries of the two optimizations are combined to study various CTs on different rotors. It is shown that a significant increase in stability can be achieved using axial-slot CTs, exceeding the values that can be reached optimizing the rotor without CTs. However, no combination of optimized rotors and CTs is found that dominates the other geometries in terms of efficiency. Hence, the question whether a CT can contribute to an improved compressor design very much depends on the desired stage design. CTs provide a benefit if a maximum stability range is necessary or if certain design choices lead to a demand for a stability enhancement, that otherwise cannot be achieved. In order to gain a maximum efficiency, a design without CTs appears to be more promising in the first place. Designs with comparably high losses in the rotor tip region, e.g. due to large tip clearances, might also benefit of CTs in terms of efficiency

Circumferential Grooves for a Modern Transonic Compressor: Aerodynamic Effects, Benefits and Limitations

It has been shown in many cases that a notable aerodynamic stability enhancement can be achieved using circumferential grooves on transonic compressors. This advantage, however, often involves degradation in efficiency at design point conditions. In order to analyze the correlations between efficiency, surge margin and other flow quantities on the one hand and the geometric parameters related to circumferential grooves on the other, an automated multi objective geometry optimization of circumferential grooves for a transonic compressor has been performed. For the surge point determination an iterative approach was used to change the static back pressure until the numerical surge limit was determined with a sufficiently small uncertainty. As a result of the optimization two different types of grooves have been identified. The first type is comparatively small and located only little downstream the leading edge of the rotor. It is capable of increasing the surge margin, while only slightly decreasing efficiency. The second groove type is located more towards the trailing edge and significantly bigger in cross sectional size. It can improve the efficiency of the rotor, but at the same time blockage is generated. Combining the two groove types, also the effects combine, resulting in an increased surge margin and increased efficiency. Applying more than one groove of type one further increases the surge margin compared to a single groove, however the gain is limited. Important groove parameters of optimized grooves are further studied, regarding their sensitivity. The working principles and flow phenomena of the grooves increasing the surge margin are analyzed in detail

Simulation of a Multistage Compressor at Low Load Operation with Additional Bleed Air Extraction for Minimum Environmental Load Reduction

The need for more flexible operation of gas-fired power plants has led manufacturers to exploit possibilities to retrofit existing systems and increase turndown capabilities, lowering the minimum environmental load (MEL) - the lowest output at which the unit can operate and still meet environmental emissions limits. A possible measure for the compressor to enable a reduced MEL is to extract significant mass flow rates through the bleed ports in operation with a closed IGV to lower the mass flow entering the combustor, enabling a further load reduction while maintaining emissions. For this measure to be implemented, a stable operation of the compressor has to be ensured at the reduced MEL conditions. It is well known that bleed-air offtake at full speed shifts the loading towards the rear stages of the compressor. At MEL, the rear stages already operate at an increased loading compared to base-load. Therefore, to confirm the viability of bleed offtake as a turndown strategy, the effects on performance and stability have to be quantified for operation at MEL. Of particular interest is how an increase in air extraction through the bleed ports influences the stability of the compressor at MEL, especially when the offtake is from low pressure. Information on the degradation of the stability margin due to the additional bleed-air extraction at MEL is gained through numerical simulations. Full-compressor CFD simulations of an F-class gas turbine at reduced MEL conditions are performed. The influence of several geometrical and numerical modeling details is studied, and the model is validated against a comprehensive set of experimental data. The numerical results show good agreement with the experimental data, even with increased bleed air extraction. It can be concluded that bleed-air extraction is a capable method of reducing the compressor discharge mass flow rate significantly. The extraction rate is limited by the stability of the last compressor stages. To verify the integrity of the whole GT at operation under such conditions, e.g., the thermal state of hot parts, further analysis should be performed

Optimierung von transsonischen Verdichterstufen mit Gehäuseeinbauten

Gehäuseeinbauten (CTs) werden verwendet um den Arbeitsbereich eines Verdichters zu erweitern. In dieser Arbeit wurden Umfangsnuten und Axial-CTs untersucht. Dabei wurde die automatisierte Optimierung eingesetzt, um gezielt Paretooptimale CTs analysieren zu können und so bessere Aussagen über die Zusammenhänge zwischen den Geometrieparametern der CTs und den Auswirkungen auf die Strömung und die Stabilität der Verdichterstufe treffen zu können. Um die Optimierungen durchführen zu können wurde die zur Optimierung eingesetzte Software um einige Module erweitert, die zur Optimierung der CTs nötig sind. Hierzu zählt die parameterbasierte Geometrieerzeugung der CTs, die automatische Vernetzung, Konvergenzkontrolle und Betriebspunktregelung, sowie Pumpgrenzbestimmung von stationären und instationären CFD-Rechnungen. Es wurden zwei umfangreiche Optimierungen, eine unter Einsatz von RANS CFD zur Optimierung von Umfangsnuten und die zweite unter Einsatz von URANS CFD zur Optimierung von Axial-CTs durchgeführt. Durch die Analyse der Optimierungen konnten wichtige Erkenntnisse zur Gestaltung der CTs gewonnen werden und wichtige Gestaltungsmerkmale identifiziert werden. Es wurden Analysen zur Wirkweise der CTs durchgeführt, um die zugrundeliegenden aerodynamischen Mechanismen zu verstehen

Gehäusestrukturierungen für transsonische Verdichter

Mit numerischen Optimierungsstudien wurde untersucht, ob bei der Auslegung eines neuen Verdichters, unter Einhaltung der geforderten Kennfeldbreite, mit Gehäusestrukturierungen (Casing Treatments, CTs) ein höherer Wirkungsgrad erzielt werden kann, als ohne. Axial-CT-, Umfangsnuten-, Rotor- und Gehäusekonturoptimierungen unter Einsatz zeitgenauer CFD und hoher Auflösung der Stabilitätsgrenze wurden für die Frontstufe des DLR-Rig 250 durchgeführt. Prinzipiell lassen sich mit CTs deutliche Erweiterungen des Stabilitätsbereichs erzielen, die darüber hinausgehen, was durch eine Optimierung des Rotors erreichbar ist. Soll primär der Wirkungsgrad einer Stufe verbessert werden, ist eine Rotoroptimierung ohne CT einem Konzept mit CT vorzuziehen. Für die Neuauslegung eines Verdichters kann eine Option mit CT in Betracht gezogen werden, wenn die stabilitätserweiternde Wirkung des CTs eine bestimmte Auslegung erst ermöglicht

Gehäusestrukturierungen für transsonische Verdichter

Mit numerischen Optimierungsstudien wurde untersucht, ob bei der Auslegung eines neuen Verdichters, unter Einhaltung der geforderten Kennfeldbreite, mit Gehäusestrukturierungen (Casing Treatments, CTs) ein höherer Wirkungsgrad erzielt werden kann, als ohne. Axial-CT-, Umfangsnuten-, Rotor- und Gehäusekonturoptimierungen unter Einsatz zeitgenauer CFD und hoher Auflösung der Stabilitätsgrenze wurden für die Frontstufe des DLR-Rig 250 durchgeführt. Prinzipiell lassen sich mit CTs deutliche Erweiterungen des Stabilitätsbereichs erzielen, die darüber hinausgehen, was durch eine Optimierung des Rotors erreichbar ist. Soll primär der Wirkungsgrad einer Stufe verbessert werden, ist eine Rotoroptimierung ohne CT einem Konzept mit CT vorzuziehen. Für die Neuauslegung eines Verdichters kann eine Option mit CT in Betracht gezogen werden, wenn die stabilitätserweiternde Wirkung des CTs eine bestimmte Auslegung erst ermöglicht.In order to analyze whether newly developed compressors can achieve higher efficiencies if casing treatments (CTs) are considered as a stability enhancing measure, a method is developed, to perform automated optimizations and parametric studies with accurate estimation of the numerical stability limit, including the usage of time accurate CFD (URANS). Studies are performed to increase stability and efficiency of DLR-Rig250 Stage 1. Axial-slots, circumferential grooves, the rotor and the casing geometry are optimized. Applying CTs can lead to a significant extension of the stability margin, which goes beyond what is achievable with only a rotor optimization. However, if the efficiency of a stage is to be increased, an optimization of the rotor without CTs should be preferred. For new designs, CTs can be considered, if the stability increasing effect enables a certain design, which otherwise would not be feasible

Optimizing surge margin and efficiency of a transonic compressor

It has been shown in several cases that casing treatment can improve the surge margin of a compressor. The question that is often left unanswered when designing and optimizing a casing treatment for a given compressor stage is whether the casing treatment can still improve the perfor- mance and surge margin when used on an aerodynamically improved rotor. The current work includes the influence of the rotor geometry and casing geometry in the analysis. Several opti- mizations with equal objectives, to improve surge margin and efficiency, were performed. First the different compo- nents were optimized separately. The possible impact of optimizing the rotor, the casing geometry and the casing treatment on surge margin and efficiency are compared in terms of global performance and aerodynamic effects. It is then analyzed how combinations of optimized geometries perform. Of particular interest is how the circumferential grooves perform when used in combination with the opti- mized blade geometry. The optimizations were performed using state of the art CFD and optimization procedures. Constraints on boundary conditions, mass flow rates and stresses were applied in order not to change the behavior of the compressor at design point significantly or to deteriorate the structural mechanics

Automated Optimization of the Non Axisymmetric Casing Endwall of a Fan Rotor

Abstract In this study the end wall casing of a fan rotor is modified in an automated optimization process in order to achieve a higher efficiency and to gain surge margin. The casing end wall is allowed to take non-axisymmetric shapes by means of different parameters describing the contour. The control points defining the contour have only the possibility to move upwards or downwards in the radial direction. Given these non-axisymmetric shapes and that the simulations are conducted in the steady state, this study is a theoretical case not realizable in reality. The main aim is the understanding of the influence of end wall shaping on the flow and what the flow characteristics would be if the losses were minimized. The DLR in-house CFD code TRACE is used. The used optimization tool is AutoOpti, a DLR developed tool, which is based on a genetic algorithm speeded up by surrogate models. During the optimization two operating points are considered: One is calculated at a fixed backpressure (OP1) representing the operating point at maximum stage efficiency for the smooth casing and the other one (OP2) is an operating point near the surge limit calculated at a fixed outlet mass flow regulated by a PID controller. The optimization delivered some improvements that can be seen in the reduced losses in the tip area of the OP1 as well as in the surge margin. It resulted as well in particular surface distortion patterns on the contour responsible for a change in the static pressure distribution in the tip area. The study mainly focuses on the aerodynamic explanation of these improvements analyzed in different members