Investigation on the fluid and structural dynamic behavior of steam turbine inlet valves at part load operation

Die hier vorliegende Arbeit thematisiert das strömungs- und das strukturdynamische Verhalten von Hochdruckventilen, die in Großkraftwerken zur Regelung von Dampfturbinenanlagen genutzt werden. Der Fokus der Arbeit liegt auf der Untersuchung der Strömungszustände und des daraus resultierenden Schwingungsverhaltens des Ventilkegels im Teillastbetrieb der Anlage. Bei Teillastbetrieb herrscht eine hohe Druckdifferenz zwischen dem Ein- und Austritt des Ventils, die dazu führt, dass im Bereich der Drosselstelle die Strömung extrem stark beschleunigt wird und als Strahl in den Ventildiffusor eintritt. Bei der anschließenden Dissipation des Strahls treten verschiedene Strömungsinstabilitäten auf. Diese rufen in bestimmten Betriebspunkten starke Druckfluktuationen hervor, die letztendlich den Ventilkegel zum Schwingen anregen. Durch die aktuell zunehmende Nutzung volatil verfügbarer regenerativer Energieträger ist die Betriebsweise von Dampfkraftwerken signifikanten Änderungen unterworfen. An die Stelle des Volllastbetriebs mit wenigen Anfahrvorgängen tritt eine Betriebsweise mit häufigem Teillastbetrieb, stark variabler Lastabgabe und häufigen Anfahrvorgängen. Da der zuvor wenig beachtete Teillastbetrieb von Dampfventilen aufgrund dieser Änderungen zunehmend an Bedeutung gewinnt, herrscht in diesem Bereich vermehrt Forschungsbedarf. Die aerodynamischen und strukturdynamischen Untersuchungen werden mit numerischen Methoden durchgeführt. Voruntersuchungen an Validierungsfällen ergeben, dass zur Modellierung von typischen Strömungsinstabilitäten zeitabhängige CFD-Berechnungen mit klassischen Zweigleichungsturbulenzmodellen (RANS) nicht ausreichend sind. Stattdessen wird ein hybrides Turbulenzmodell genutzt, die eine Zwischenstellung zwischen den Zweigleichungsmodellen und den Large Eddy Simulationen einnehmen, genutzt. Die CFD Untersuchungen zeigen, dass Wandstrahlablösungen, die bei bestimmten Betriebsbedingungen im Ventildiffusor auftreten, einen maßgeblichen Einfluss auf das Strömungsverhalten haben sowie die Intensität der dynamischen und statischen Lateralkräfte signifikant erhöhen. Aus der Analyse der Berechnungsergebnisse geht hervor, dass das Auftreten der ungünstigen Wandstrahlablösungen direkt mit dem Versagen des Coandă-Effekts in unterexpandierten Wandstrahlen korreliert. Weitere Untersuchungen zeigen, dass bei einem angelegten Strahl ein wesentlicher Teil der dynamischen Axialkraft durch einen akustischen Raummode auf den Ventilkegel aufgeprägt wird. Der Raummode wird durch eine Scherschicht im Ventildiffusor angeregt. Störungen dieser Scherschicht, die durch Einbauten oder Schwingungen des Kegels hervorgerufen werden können, führen zu einer Verminderung der Anregung. Ist der Strahl abgelöst, treten im Ventil durch instationäre Wiederanlegevorgänge des Strahls sowie durch die Oszillation von schrägen Verdichtungsstößen hohe Druckfluktuationen und damit eine hohe dynamische Kraft, die auf den Kegel wirkt, auf. Generell ist unter strömungsmechanischen Gesichtspunkten die angelegte Strömung als günstiger zu bewerten als die abgelöste Strömung. Die strukturdynamischen Analysen, die zum Teil gekoppelt mit den aerodynamischen Untersuchungen durchgeführt werden, zeigen, dass im Fall der abgelösten Strömung intensivere Schwingungen des Ventilkegels auftreten als im Fall der angelegten Strömung.In this thesis the aero dynamic and structural dynamic behavior of high pressure turbine inlet valves used for large power stations is investigated. Emphasis is put on the investigation of the flow field and the flow induced vibrations at part load operation. At this operational range a large pressure difference exists between the inlet and the outlet of the valve causing an extreme acceleration of the flow in the valve gap between the valve plug and the valve seat. The high speed flow being discharged from the gap forms a jet in the valve diffuser. There the kinetic energy of the jet is dissipated by various flow instabilities causing pressure fluctuations. These pressure fluctuations act on the valve plug and hence cause intense vibrations. Due to the increasing use of renewable energy, the operational mode of steam turbine installations is changed from base load operation to intermediate operation with highly variable power output and frequent startup operations. As long time part load operation was not previously considered, there is a demand for research. The study on the aero and structural dynamic phenomena is carried out with numerical models. Pre-studies on validation cases show that classical two-equation turbulence models are not capable to model flow instabilities and the generation of pressure fluctuations properly. A Hybrid turbulence model combining the advantages of the RANS and the SAS-method is used instead. The CFD simulations reveal that wall jet separations existing at certain operation conditions have a significant effect on the flow topology in the valve diffuser and increase the level of the dynamic lateral forces acting on the valve plug. The analysis of the CFD study shows, that the wall jet separation is related to the failure of the Coandă effect in underexpanded wall jets. In case of the flow topology with the attached wall jet CFD simulations predict that a significant amount of the dynamic force is related to an acoustic mode acting on the valve plug. The acoutic mode is excited by a shear layer in the valve diffuser. Pertubations of the shear layer being caused by vortex-generatig devieces or by vibrations of the plug reduce the excitaion of the mode and the level of the dynamic axial forces. If the jet is detached, the oscialtion of oblique shocks and the partial unsteady reattachment of the jet cause intense pressure fluctuations and hence high dynamic forces acting on the valve plug. From an aerodynamic point of view, the flow topology with the attached jet is more favorable than the detached flow topology. Structural dynamic simulations are preformed to analyse the impact of the flow topology on the vibrations of the valve plug. One one way as well as two way coupled calculations are conducted. The structural dynamic analysis predict that the detached flow topology causes higher structural vibrations than the attached flow topology

Critique of accommodating central bank policies and the 'expropriation of the saver' - A review

In parts of the German media, with the support of a number of German economists, the ECB’s low nominal interest rate policy is criticised as unnecessary, ineffective and as expropriating the German saver. This paper provides a review of the relevant arguments. It is recalled that returns on savings are anchored to the real rate of return on capital. Good monetary policy tries to avoid being a source of disturbance in itself, and may be able to smooth the effects of temporary external shocks, but beyond that cannot structurally improve the real rate of return on capital. Against this general background, the paper critically analyses a number of recent arguments as to why low interest rate policies could actually be counterproductive. Finally, the paper reviews what can be done about the medium to long-term real rate of return on capital, which remains in any case the basic issue for the saver, focusing on the specific case of Germany. The key policies identified relate to demographics, education, labour markets, infrastructure and technology. Low growth dynamics in the coming decades and correspondingly low real rates of return on investments are not inevitable

Business Cycle Synchronization in the EMU: Core vs. Periphery. CEPS Working Documents No. 427, 14 November 2016

This paper examines business cycle synchronization in the European Monetary Union with a special focus on the core-periphery pattern in the aftermath of the crisis. Using a quarterly index for business cycle synchronization by Cerqueira (2013), our panel data estimates suggest that it is countries belonging to the core that are faced with increased synchronization among themselves after 2007Q4, whereas peripheral countries decreased synchronization with regards to the core, non-EMU countries and among themselves. Correlation coefficients and nonparametric local polynomial regressions corroborate these findings. The usual focus on co-movements and correlations might be misleading, however, since we also find large differences in the amplitude of national cycles. A strong common cycle can thus lead to large differences in cyclical positions even if national cycles are strongly correlated

Modification of a Steam Valve Diffuser for Enhanced Full Load and Part Load Operation Using Numerical Methods

The flow in a steam turbine inlet valve is investigated and improved by numerical methods. From the fluid dynamic point of view two requirements exist: Low pressure losses are desired at the fully opened valve position and dynamic fluid forces acting on the valve plug should be minimized to reduce valve vibration. Usually these undesired dynamic fluid forces occur when the flow is throttled at part load. It is found that these fluid forces are generated by separated jets in the diffuser. The attachment and the separation of the jet are related to the Coanda effect.By understanding the flow physics a way is found to modify the diffuser design in such a way that the flow separations are reduced. Bell-shaped diffusers are able to reduce the flow losses at full load operation. A diffuser contour that fulfils both requirements is developed

Trade and capital flows - substitutes or complements? An empirical investigation

This paper examines the linkages between the trade of goods and financial assets. Do both flows behave as complements (implying a positive correlation) or as substitutes (negative correlation)? Although a classic topic in international macroeconomics, the empirical evidence has remained relatively scarce so far, in particular for the Euro area where trade and financial imbalance played a prominent role in the build-up of the European sovereign debt crisis. Consequentially, we use a novel dataset, providing estimates for financial flows and its four main categories for 42 countries and covering the period from 2002-2012, to test the so-called trade-finance nexus. Since theoretical models stress that both flows might be influencing each other simultaneously, we introduce a novel time-varying instrumental variable based on capital control restrictions to estimate a causal effect. The results of the gravity regressions support theories that underline the complementarity between exports and capital flows. When testing the trade-finance nexus for different types of capital flows, the estimated coefficient is most pronounced for foreign direct investment, in line with theories stressing informational frictions. Robustness checks in the form of different estimation methods, alternative proxies for capital flows and sample splits confirm the positive relationship. Interestingly, the trade-finance nexus does not differ among countries belonging to the EMU, the European Union or among core and peripheral Euro area countries.In der internationalen Makroökonomik gibt es seit geraumer Zeit eine Diskussion, ob sich Kapital- und Finanzströme entweder als Substitute oder als Komplemente verhalten. Im erstgenannten Fall haben beide Ströme eine positive, im letzteren ein negative Korrelation. Wirtschaftspolitisch ist der Zusammenhang zwischen Kapital- und Finanzströmen u. a. für die Debatten im Kontext von internationalen Handelsdefiziten oder Handelsabkommen relevant, aber auch im Rahmen der wirtschaftlichen Ungleichgewichte innerhalb der EWU. Der Hauptbeitrag des Artikels besteht in einer systematischen Analyse des sogenannten "Trade-Finance-Nexus", basierend auf einem neuen Datensatz, welcher aggregierte Finanzströme und vier Unterkategorien für 42 Sende- und Empfängerländer im Zeitraum von 2002-2012 umfasst. Da theoretische Modelle den simultanen Einfluss von Handels- und Finanzströmen betonen, wird eine neuartige Instrumentvariable - basierend auf einem Kapitalverkehrskontrollen-Index - in die Literatur eingeführt. Die Ergebnisse verschiedener "Gravity"-Regressionen zeigen einen positiven Zusammenhang zwischen bilateralen Exporten und Kapitalflüssen und stützen somit theoretische Modelle, welche die Komplementarität der beiden Ströme betonen. Dieser positive Zusammenhang ist robust über verschiedene Zeiträume, Schätzmodelle (OLS, POLS, Random- und Fixed-Effects-Modelle als auch Instrumentvariablenschätzungen) als auch Datentransformationen hinweg. Separiert man die aggregierten Kapitalströme nach ihren jeweiligen Unterkategorien, ist der geschätzte Koeffizient für ausländische Direktinvestitionen (ADI) am höchsten. Dieses Ergebnis ist konform mit Theorien, welche die ausgeprägten Informationsbedürfnisse von ADI-Strömen betonen. Interessanterweise gibt es keinen signifikanten Unterschied im Zusammenhang zwischen Kapital- und Warenströmen für verschiedene Ländergruppen, so z. B. in Bezug auf Mitgliedsstaaten der EU, EWU und für Kern- als auch Peripherieländer

Trade and capital flows: Substitutes or complements? An empirical investigation

This paper examines the linkages between the trade of goods and financial assets. Do both flows behave as complements (implying a positive correlation) or as substitutes (negative correlation)? Although a classic topic in international macroeconomics, the empirical evidence has remained relatively scarce so far, in particular for the Euro area where trade and financial imbalance played a prominent role in the build-up of the European sovereign debt crisis. Consequentially, we use a novel dataset, providing estimates for financial flows and its four main categories for 42 countries and covering the period from 2002-2012, to test the so-called trade-finance nexus. Since theoretical models stress that both flows might be influencing each other simultaneously, we introduce a novel time-varying instrumental variable based on capital control restrictions to estimate a causal effect. The results of the gravity regressions support theories that underline the complementarity between exports and capital flows. When testing the trade-finance nexus for different types of capital flows, the estimated coefficient is most pronounced for foreign direct investment, in line with theories stressing informational frictions. Robustness checks in the form of different estimation methods, alternative proxies for capital flows and sample splits confirm the positive relationship. Interestingly, the trade-finance nexus does not differ among countries belonging to the EMU, the European Union or among core and peripheral Euro area countries

Trade and capital flows: Substitutes or complements? An empirical investigation

This paper examines the linkages between the trade of goods and financial assets. Do both flows behave as complements (implying a positive correlation) or as substitutes (negative correlation)? Although a classic topic in international macroeconomics, the empirical evidence has remained relatively scarce so far, in particular for the Euro area where trade and financial imbalance played a prominent role in the build-up of the European sovereign debt crisis. Consequentially, we use a novel dataset, providing estimates for financial flows and its four main categories for 42 countries and covering the period from 2002-2012, to test the so-called trade-finance nexus. Since theoretical models stress that both flows might be influencing each other simultaneously, we introduce a novel time-varying instrumental variable based on capital control restrictions to estimate a causal effect. The results of the gravity regressions support theories that underline the complementarity between exports and capital flows. When testing the trade-finance nexus for different types of capital flows, the estimated coefficient is most pronounced for foreign direct investment, in line with theories stressing informational frictions. Robustness checks in the form of different estimation methods, alternative proxies for capital flows and sample splits confirm the positive relationship. Interestingly, the trade-finance nexus does not differ among countries belonging to the EMU, the European Union or among core and peripheral Euro area countries

Trade and capital flows: Substitutes or complements? An empirical investigation

This paper examines the linkages between the trade of goods and financial assets. Do both flows behave as complements (implying a positive correlation) or as substitutes (negative correlation)? Although a classic topic in international macroeconomics, the empirical evidence has remained relatively scarce so far, in particular for the Euro area where trade and financial imbalance played a prominent role in the build-up of the European sovereign debt crisis. Consequentially, we use a novel dataset, providing estimates for financial flows and its four main categories for 42 countries and covering the period from 2002 to 2012, to test the so-called trade-finance nexus. Since theoretical models stress that both flows might be influencing each other simultaneously, we introduce a novel time-varying instrumental variable based on capital control restrictions to estimate a causal effect. The results of the gravity regressions support theories that underline the complementarity between exports and capital flows. When testing the trade-finance nexus for different types of capital flows, the estimated coefficient is most pronounced for foreign direct investment, in line with theories stressing informational frictions. Robustness checks in the form of different estimation methods, alternative proxies for capital flows and sample splits confirm the positive relationship. Interestingly, the trade-finance nexus does not differ among countries belonging to the EMU, the European Union or among core and peripheral Euro area countries

Trade and Capital Flows: Substitutes or Complements? An Empirical Investigation

This paper examines the linkages between the trade of goods and financial assets. Do both flows behave as complements (implying a positive correlation) or as substitutes (negative correlation)? Although a classic topic in international macroeconomics, the empirical evidence has remained relatively scarce so far, in particular for the Euro area where trade and financial imbalance played a prominent role in the build-up of the European sovereign debt crisis. Consequentially, we use a novel dataset, providing estimates for financial flows and its four main categories for 42 countries and covering the period from 2002-2012, to test the so-called trade-finance nexus. Since theoretical models stress that both flows might be influencing each other simultaneously, we introduce a novel time-varying instrumental variable based on capital control restrictions to estimate a causal effect. The results of the gravity regressions support theories that underline the complementarity between exports and capital flows. When testing the trade-finance nexus for different types of capital flows, the estimated coefficient is most pronounced for foreign direct investment, in line with theories stressing informational frictions. Robustness checks in the form of different estimation methods, alternative proxies for capital flows and sample splits confirm the positive relationship. Interestingly, the trade-finance nexus does not differ among countries belonging to the EMU, the European Union or among core and peripheral Euro area countries

Business Cycle Desynchronisation: Amplitude and Beta vs. Co-movement

A high degree of correlation among the business cycles of individual countries is usually seen as a key criterion for an optimum currency area. However, the elasticity with which countries react to the common cycle is equally important. A country with a non-unitary growth elasticity relative to the common area will experience cyclical divergences at the peak and trough of the common cycle. Despite being characterised by highly correlated business cycles, the euro area suffers from widely differing amplitudes