Fehlercharakterisierung zuverlässiger Schaltungen im Selbsttest

Hochintegrierte Schaltungen können immer kleiner, höher getaktet und energieeffizienter hergestellt werden, allerdings können bedingt durch diese technologischen Trends auch vermehrt Schwachstellen im System entstehen. Diese Schwachstellen führen oft während des Produktionstests nicht zu einem Fehlverhalten der Schaltung, während des Betriebs allerdings droht durch die steigende Anfälligkeit gegenüber intrinsischen und äußeren Störeinflüssen sowie Alterungseffekten ein vorzeitiger Ausfall der Schaltung. Solche Frühausfälle werden „Early-Life Fehler“ genannt und können mit einem Standard- Test ohne weitere Anpassungen nicht erkannt werden. Indikatoren für einen Frühausfall können intermittierende Fehler, aber auch kleine Verzögerungsfehler sein. In dieser Arbeit wird ein Selbsttest vorgestellt, der eine Fehlercharakterisierung zur Erkennung von Systemschwachstellen und Vermeidung von Frühausfällen, speziell solche, die sich als intermittierender Fehler oder kleiner Verzögerungsfehler auswirken, mit geringem Hardware- und Zeitaufwand mittels eines Standard-Tests ermöglicht. Hierzu wird im Selbsttest zunächst zwischen permanenten und nicht-permanenten Fehlern unterschieden und eine Klassifikation der nicht- permanenten Fehler mit Hilfe eines voran geschalteten Diagnoseverfahrens und Bayesschen Berechnungen durchgeführt. Hierdurch lässt sich die Produktqualität ohne zusätzliche Ausbeuteverluste erhöhen. Zusätzlich wird ein Test mit erhöhter Betriebsfrequenz vorgestellt, der im Selbsttest kleine Verzögerungsfehler erkennt.As a result of the fact, that todays integrated circuits have smaller features sizes, higher frequencies and are more energy efficient, weak spots can occur in the system. These weak spots can be undetected by the production test, but during system operation they can lead to hard failures, because of increasing susceptibility to intrinsic and external disturbances or aging effects. This early system breakdown is called „early-life failure“ and cannot be detected by a standard test without any adjustments. Indicators of early-life failures could be intermittent faults and also small delay defects. In this thesis a built-in self-test is presented, which characterizes faulty behavior to detect weak spots and avoid early-life failures, especially caused by intermittent faults or small delay defects, with low hardware and time overhead by using a standard test set. In a first step, the test procedure can distinguish between permanent and non-permanent faults. After that, a diagnosis process and Bayesian reasoning implement the classification of the non-permanent faults. With this procedure the product quality can be increased without additional yield loss. Furthermore a Faster-than-at-Speed-Test (FAST) will be introduced, which allows detecting SDDs in a built-in self-test environment without any changes in the ATPG flow.von Dipl.-Wirt.-Ing. Thomas Indlekofer ; Erster Gutachter: Prof. Dr. Sybille Hellebrand, Zweiter Gutachter: Prof. Dr. Ilia PolianTag der Verteidigung: 03.03.2016Fakultät für Elektrotechnik, Informatik und Mathematik der Universität Paderborn, Univ., Dissertation, 201

Assessment of the LES-FGM framework for capturing stable and unstable modes in a hydrogen / methane fuelled premixed combustor

The main objective of this paper is to assess the capability of compressible Large Eddy Simulations (LES) to capture azimuthal combustion instability. The thickened flame model coupled with Flamelet Generated Manifold (FGM) tabulated chemistry is used as the combustion model. LES of an annular combustor is performed for five cases featuring stable and unstable combustion of hydrogen-methane mixtures. The unstable modes feature azimuthal instabilities and this annular combustor is used to test the LES-FGM framework. A consistent methodology is applied across all cases. It is found that LES predicts azimuthal modes for stable cases but these modes are weak and intermittent with pressure fluctuation amplitudes within the order of experimental noise. In addition, the unstable cases capture azimuthal modes that have approximately the same frequency as that of the experiment though the amplitudes of the modes are over-predicted. This suggests that the described LES-FGM framework is able to predict the onset of thermoacoustic instabilities and their qualitative changes with addition of hydrogen. © 2023 The Combustion InstituteAssessment of the LES-FGM framework for capturing stable and unstable modes in a hydrogen / methane fuelled premixed combustoracceptedVersio

The effect of hydrogen on nonlinear flame saturation

We investigate the effect of increasing levels of hydrogen enrichment on the nonlinear response and saturation of premixed bluff-body stabilized methane/hydrogen flames submitted to acoustic forcing. The thermal power is kept approximately constant to preserve the nozzle velocity while increasing the flame speed through hydrogen enrichment. The flame describing function (FDF) is measured for a fixed frequency and three hydrogen–methane blends ranging from 10% to 50% by power, corresponding to 25% to 75% by volume. We show that when the flame is forced at the same frequency at similar power and bulk velocities, increasing levels of hydrogen enrichment increase the saturation amplitude of the flame. To provide insight into the flame dynamics responsible for the change in the global nonlinear response and saturation amplitude, the flames were investigated using high-speed imaging in combination with OH planar laser-induced fluorescence (OH-PLIF) at a range of forcing amplitudes. At lower hydrogen concentrations, the flame is stabilized along the inner shear layer and saturation in the heat release rate (HRR) occurs at lower forcing amplitudes due to large-scale flame–vortex interactions causing flame annihilation as observed in several previous studies. At increased levels of hydrogen enrichment, distinctly different flame dynamics are observed. In these cases, the flame accelerates and propagates across to the outer shear layer, which acts to suppress large-scale flame annihilation during roll-up of both the inner and outer shear layers. This results in a coherent increase in flame surface area with forcing amplitudes significantly increasing the saturation amplitude of the flame. These results show that high levels of hydrogen increase the amplitude response to acoustic forcing leading to higher saturation amplitudes. This suggests that substituting natural gas with hydrogen in gas turbines increases the risk of much higher limit-cycle amplitudes if self-excited instabilities occur

The effect of dynamic operating conditions on the thermoacoustic response of hydrogen rich flames in an annular combustor

Self-excited thermoacoustic instabilities in symmetric annular combustion chambers typically give rise to spinning, standing and mixed azimuthal modes which are time-varying in nature and occur in a highly noisy environment due to turbulent combustion. We investigate the effect of linear ramps of increasing and decreasing equivalence ratio on the operating limits and thermoacoustic dynamics, from near lean blow-off to near flashback, in a laboratory scale annular combustor. The combustor features 12 lean-premixed hydrogen–methane flames at a fuel composition of 70% hydrogen and 30% methane by power. Equivalence ratio ramps were conducted for different thermal powers P=4, 6 and 8 kW per burner and three different ramp times tramp=5, 20, 60 s to simulate dynamic operation. It was found that ramping leads to self-excited instabilities that exhibit repeatable modal dynamics which depend on thermal power, ramp direction and duration. Different types of hysteresis were observed between the upward and downward ramps which affected the amplitudes and the stable operating range. The hysteresis phenomena also showed repeatable behaviour in terms of the nature and orientation angle. In one specific case, the simultaneous existence of two spinning modes was observed before the appearance of mode hopping leading to both an increase in frequency and doubling of the amplitude. High-speed OH* chemiluminescence of the flames showed that the mode hopping was accompanied by a change in the flame shape which becomes more compact and distributed

The effect of hydrogen on non-linear flame saturation

We investigate the effect of increasing levels of hydrogen enrichment on the non-linear saturation of premixed bluff-body stabilised methane/hydrogen flames submitted to acoustic forcing. The thermal power is kept approximately constant to preserve the nozzle velocity whilst increasing the flame speed through hydrogen enrichment. The flame describing function (FDF) is measured for a variety of frequencies and six hydrogen-methane blends ranging from 10% to pure hydrogen by power. We show that when the flame is forced at the same frequency at similar power and bulk velocities, increasing levels of hydrogen enrichment increases the saturation amplitude of the flame. This increase is observed for all cases investigated. To provide insight into the flame dynamics responsible for the change in the saturation amplitude, selected cases were investigated using highspeed imaging in combination with OH-PLIF at different points in the FDF. At lower levels of hydrogen enrichment, the flame is stabilised along the inner shear layer and saturation in the heat release rate occurs at lower forcing amplitudes due to large-scale flame-vortex interactions causing flame annihilation as observed in several previous studies. At high levels of hydrogen enrichment to pure hydrogen, distinctly different flame dynamics are observed. In these cases, the flame accelerates and propagates across to the outer shear layer which acts to suppress large-scale flame annihilation during roll-up of both the inner and outer shear layers. This results in a coherent increase in flame surface area with forcing amplitude significantly increasing the saturation amplitude of the flame. These results show that high levels of hydrogen increases the linear response to acoustic forcing leading to higher saturation amplitudes. This suggests that substituting natural gas with hydrogen in Gas Turbines increases the risk of much higher limit-cycle amplitudes if self-excited instabilities occur

Imperfect symmetry of real annular combustors: beating thermoacoustic modes and heteroclinic orbits

In jet engines and gas turbines, the annular shape of the combustion chamber allows the appearance of self-oscillating azimuthal thermoacoustic modes. We report experimental evidence of a new type of modal dynamics characterised by periodic switching of the spinning direction and develop a theoretical model that fully reproduces this phenomenon and explains the underlying mechanisms. It is shown that tiny asymmetries of the geometry, the mean temperature field, the thermoacoustic response of the flames or the acoustic impedance of the walls, present in any real systems, can induce these heteroclinic orbits. The model also explains experimental observations showing a statistically dominant spinning direction despite the absence of swirling flow, or pairs of preferred nodal line directions.ISSN:0022-1120ISSN:1469-764

Spontaneous and explicit symmetry breaking of thermoacoustic eigenmodes in imperfect annular geometries

This article deals with the symmetry breaking of azimuthal thermoacoustic modes in annular combustors. Using a nominally symmetric annular combustor, we present experimental evidence of a predicted spontaneous reflectional symmetry breaking, and also an unexpected explicit rotational symmetry breaking in the neighbourhood of the Hopf bifurcation which separates linearly stable azimuthal thermoacoustic modes from self-oscillating modes. We derive and solve a multidimensional Fokker-Planck equation to unravel a unified picture of the phase space topology. We demonstrate that symmetric probability density functions of the thermoacoustic state vector are elusive, because the effect of asymmetries, even imperceptible ones, is magnified close to the bifurcation. This conclusion implies that the thermoacoustic oscillations of azimuthal modes in real combustors will systematically exhibit a statistically dominant orientation of the mode in the vicinity of the Hopf bifurcation.ISSN:0022-1120ISSN:1469-764

Experiments and low-order modelling of intermittent transitions between clockwise and anticlockwise spinning thermoacoustic modes in annular combustors

ISSN:1540-7489ISSN:1873-270

Self-excited longitudinal and azimuthal modes in a pressurised annular combustor

A new laboratory scale pressurised annular combustion experiment is introduced and used to generate selfexcited longitudinal and azimuthal instabilities. The experiments are operated at mean pressuresranging from approximately 2 to 3 atmospheres in order to maintain a well defined acoustic boundary at exit. A range of operating conditions is studied parametrically, and it is observed that at high equivalence ratios, the flame stabilisation location propagates upstream, significantly altering the flame structure. The change in flame stabilisation location promotes a transition from a dominant longitudinal to a dominant azimuthal instability. Investigation of the azimuthal instabilities highlights a rich array of frequency content, with significant amplitude pressure and heat release responses observed for not only the fundamental (n = 1), but also higher harmonics (n = 2, 3). These higher harmonics are also shown to exhibit distinct characteristic modal dynamics, shown through probability density functions of the spin ratio. The flame dynamics for three distinct operating states, corresponding to longitudinal modes at two different stabilisation locations, and one corresponding to strong azimuthal modes are studied. These highlight the difference between longitudinal and azimuthal modes, and demonstrate the presence of significant higher harmonic content. The characterisation of both longitudinal and azimuthal modes in a pressurised laboratory scale annular combustor for the first time provides a unique opportunity for understanding the nature of such instabilities in practically relevant configurations