Physics of Prestall Propagating Disturbances in Axial Compressors and Their Potential as a Stall Warning Indicator
Axial compressors in aero engines are prone to suffering a breakdown of orderly flow when operating at the peak of the pressure rise characteristic. The damaging potential of separated flows is why a safe distance has to be left between every possible operating point and an operating point at which stall occurs. During earlier investigations of stall inception mechanisms, a new type of prestall instability has been found. In this study, it could be demonstrated that the prestall instability characterised by discrete flow disturbances can be clearly assigned to the subject of “Rotating Instabilities”. Propagating disturbances are responsible for the rise in blade passing irregularity. If the mass flow is reduced successively, the level of irregularity increases until the prestall condition devolves into rotating stall. The primary objective of the current work is to highlight the basic physics behind these prestall disturbances by complementary experimental and numerical investigations. Before reaching the peak of the pressure rise characteristic flow, disturbances appear as small vortex tubes with one end attached to the casing and the other attached to the suction surface of the rotor blade. These vortex structures arise when the entire tip region is affected by blockage and at the same time the critical rotor incidence is not exceeded in this flow regime. Furthermore, a new stall indicator was developed by applying statistical methods to the unsteady pressure signal measured over the rotor blade tips, thus granting a better control of the safety margin.
Experimental identification of rotating instability of an axial fan with shrouded rotor
The rotating instability phenomenon is investigated experimentally on a laboratory low-speed axial fan. Unsteady pressure measurements are performed close to the rotor leading edge, in the far-field in the upstream-duct section, and within the rotating frame by using Kulite sensors near the blade tip. As reference, the occurrence of rotating instability is proven for a rotor configuration with large clearance size. Existing explanations assume that rotating instability is induced by the tip clearance vortex. Following these models, the characteristic spectral and modal signatures of rotating instability are generated by the circumferentially coupled unsteady vortex systems within the individual blade passages. However, some early findings on a shrouded rotor setup and a study on an annular cascade without clearance have raised questions about the validity of the existing theories. To answer these questions, a second configuration is investigated using the identical rotor in a shrouded version. Here, a shrouding band covers the blade tips over the whole circumference. By using a comprehensive sensor arrangement in a wide variation of operating conditions, the existence of spectral and modal rotating instability signatures is proven on the shrouded-rotor configuration, as well. In this case, the tip clearance vortex system and, respectively, the leakage flow are suppressed. The present study underlines that novel explanations are required to describe the physical mechanism that generates rotating instability.
Untersuchungen zur aerodynamisch induzierten Schwingungsanregung von Niederdruck-Laufschaufeln bei extremer Teillast
Die heutigen Marktanforderungen fuhren bei Dampfturbosatze zu einem stark variablen Betrieb, der in der Gesamtturbine oder in einzelnen Stufen haufiger eine sehr geringe oder sogar gar keine Durchstromung hervorruft. Dieser Betriebszustand wird Ventilation genannt, er stellt fur den Turbosatz durch die auftretenden thermischen und dynamischen Belastungen eine zusatzliche Beanspruchung dar, die berucksichtigt werden mus. 
An einer dreistufigen Niederdruck-Modelldampfturbine wurden umfangreiche Untersuchungen im Betriebsbereich der extremen Teillast durchgefuhrt, wobei neben der Ermittlung der thermischen Belastung im Bereich der letzten drei Stufen die Bestimmung der dynamischen Schaufelbelastung der letzten Laufschaufelreihe das Ziel war. Die Untersuchungen ergaben maximale Temperaturen von bis zu 300°C zwischen der Vorstufe (L-1) und der Endstufe 
(L-0) bei geschlossenem Frischdampfschieber. Die Gegenuberstellung der gemessenen Ventilationsverluste von ein- und dreistufigem Modellaufbau zeigte, das die Verluste des einstufigen Aufbaus in der gleichen Grosenordnung wie beim dreistufigen Aufbaus lagen. Bei einem Volumenstrom kleiner 23% von der Auslegung stellt sich bei beiden Versuchskonfigurationen fur das letzte Laufrad eine nahezu konstante Leistungsaufnahme ein. 
Pneumatische Stromungsfeldmessungen im Bereich der letzten Stufe zeigen, das sich vor dem Endstufenlaufrad bei abnehmendem Volumenstrom eine starke Zunahme der Geschwindigkeitskomponente in Umfangsrichtung ausbildet; das Fluid wird, beginnend vom Spitzenschnitt, durch die Laufschaufel mitgerissen. 
Im Bereich der extremen Teillast wurde, neben einer moderaten Zunahme der Wechseldeh-nungsbelastung durch eine stochastische Anregung aller Schwingungs-Eigenformen der Endstufenlaufschaufel, in einem schmalbandigen Bereich des Volumenstromes eine dominante Anregung der zweiten und der dritten Eigenform festgestellt. Wahrend bei der dritten Schwingungsform ein nur masiger Anstieg des Amplitudenniveaus gemessen wurde, zeigte sich bei der zweiten Eigenform eine bis zu sechsfache Amplitudenuberhohung gegenuber dem sonst niedrigen Niveau. Die dynamische Belastung lag in allen Betriebsbereichen unter der Dauerschwingfestigkeit der Laufschaufel. 
Bei instationaren Stromungsfeldmessungen wurden im auseren Bereich des Stromungskanals vor und nach der letzten Laufreihe in einem Lastbereich zwischen 11 und 22% des Auslegungsvolumenstromes stark fluktuierende Wechseldruckanteile gemessen. Hierbei handelt es sich um pulsierend umlaufende Druckzellen, die im Relativsystem generiert werden. Der Entstehungsort und der Volumenstrombereich fallen mit der bereits erlauterten starken Zunahme der Stromungsgeschwindigkeitskomponente in Umfangsrichtung zusammen. Die mit abnehmendem Volumenstrom geringere Differenz zwischen Fluidgeschwindigkeit und Umlaufgeschwindigkeit der Laufschaufel fuhrt im Relativ- sowie im Absolutsystem zu einer starken Frequenzanderung dieser Wechseldruckanteile. Weiterhin vergrosert das Auftreten von hoheren Ordnungen der Pulsationsfrequenz die Bandbreite der moglichen Erregerfrequenz. Dadurch stellt dieses Stromungsphanomen eine nicht zu unterschatzende Gefahrdung der Laufbeschaufelung von Endstufen dar, hervorgerufen durch eine zu hohe Schwingbelastung infolge aerodynamisch induzierter Zwangsanregung. 
Today’s market requirements for steam generators calls for a strong increase in the range and severity of operation conditions. As a result of this, steam turbines, or specific rows within the steam turbines are required to operate more frequently under conditions with little or no steam flow. This extreme part load condition, known as windage, results in an increases in the thermal and dynamic load on the blading, which must be considered in the design. 
Extensive investigations on extreme part load conditions were carried out using a model of a three stage low pressure steam turbine. The primary goals were to determine the resulting thermal conditions in the last three stages and to evaluate the dynamic vibration in the last rotating blade row. The investigations showed with closed steam valve temperature up to 300°C between the preliminary stage (L-1) and the last stage (L-0). A comparison of results from a single stage model and the three stage model showed that the windage losses are similar. At volumetric flow below 23% of the rated condition, the power absorption rate was almost constant at the last running blade row. This was the case for both test configurations. 
Pneumatic flow field measurements in the last stage showed a strong increase in circumferential flow for a corresponding decrease in volumetric flow up stream from the last rotating row (the fluid is carried along by the rotating blade). This flow phenomena starts at the tip section of the blade and propagates down the blade with further decrease in flow. 
At part load condition, in addition to the moderate increase of the alternating stress through the stochastic excitation of all natural frequencies of the last running blade in a small range of the volumetric flow, a dominate excitation appeared at the second and third natural frequencies. For the third vibration mode only, a moderate increase in amplitudes occurred, in contrast an enhancement up to the sixth of the normal low level was measured for the second mode. The dynamic stress in the blades, were below the vibration fatigue strength limit under all excitation conditions. 
Measurements of flow field fluctuations in front of and behind the last running blade indicates strong unsteady flow close to the outer flow boundary between 11% up to 22% of the design volumetric flow. It is a matter of rotating pressure cells generated in the relative system. The geometric area as well as the operation range are obviously connected with the strong increase of the circumferential velocity component as aforementioned. The decreasing circumferential velocity difference between fluid and rotating blade at declining volumetric flow leads in the relative- and absolute system to a strong change of frequency for the pressure fluctuation. Additionally the occurrence of higher modes of the fluctuation frequency increases the bandwidth of the excitation frequencies. Thereby this flow phenomena can be a endangerment of the running blade of last stages, arouse by high blade vibration through forced aerodynamic induced excitation.