Self-induced Instabilities Research Articles

Forced response of transverse reacting jet in vitiated crossflow to harmonic velocity disturbances

The orthogonally oriented injector configuration typically used in axial fuel staging-capable gas turbine combustors is known to produce complicated thermoacoustic interactions between two distinct reaction zones. Despite the fundamental importance of transfer functions of transverse reacting jets in vitiated crossflow, however, their distinctive properties remain unknown, and associated low-order modeling combined with two different flame transfer functions remains entirely undemonstrated. From detailed measurements of transfer functions of lean-premixed primary and secondary flames in response to harmonic velocity disturbances, here we show that the transfer function of a transverse reacting jet in high-temperature crossflow is described as having relatively larger gain without undulating patterns, near-linear slow decay in magnitude with respect to the forcing frequency, and remarkably shorter duration response time. By leveraging a reduced-order modeling approach to account for the finite time delay between the two distinct transfer functions and by validating the calculation results against velocity and pressure measurement data, we demonstrate that self-induced instabilities in an axial fuel-staged system can be simultaneously driven by the dynamics of primary and secondary flames, and that the triggering of second-stage-induced instability is characteristically connected to higher-order acoustic modes. Such non-axisymmetric intra-combustor interactions generate a cascade of spectral peaks, including the first and third longitudinal modes and their respective higher harmonics, and additional peaks at the sum and difference of two individual frequencies. While the primary flame's dynamics are capable of selectively exciting the first mode, the secondary jet flame exhibits more complicated modal dynamics, manifested as the L3 mode-coupled jet merging-related flame surface annihilation and the L1 mode-coupled lateral movements of the transverse jet column.

High-frequency hydrogen combustion dynamics driven by local flame displacement and multidimensional thermoacoustic interactions

Knowledge of the underlying physical mechanisms responsible for the triggering of high-frequency transverse combustion dynamics is of fundamental importance in the development of heavy-duty gas turbine combustors, aircraft engine afterburners, and bipropellant liquid rocket engines. Detailed information about three-dimensional thermoacoustic interactions and local flame dynamics, however, remains largely unknown and unanticipated, mainly because high-amplitude transverse mode instabilities are challenging to excite and detect in well-controlled sub-scale laboratory environments. To overcome this impasse, here we exploit a spatially tailored rectangular injector assembly consisting of ten equidistant horizontal slit nozzles to eliminate the complications of out-of-plane flame dynamics characterization. A total of 56 datasets of self-induced instabilities were acquired over a wide range of operating conditions to understand spatiotemporal phase dynamics and important mode shapes, in conjunction with 2D Rayleigh angle reconstruction and phase-resolved OH PLIF-based local flame front identification. Experimentally, we show that high-frequency transverse instabilities are excited only under high temperature and high thermal power conditions, manifested as non-evanescent pressure fluctuations at 6.50 kHz strongly coupled to the second-order tangential mode of the rectangular combustion chamber. Two vertically-oriented pressure nodal planes and the characteristic phase transition perpendicular to the horizontal slit injector direction are accurately measured and reconfirmed by Helmholtz simulations in terms of their interpositions and spatial orientation. Remarkably, the periodic formation of co-propagating coherent structures and concomitant local flame displacement/pinch-off are revealed to play an important role in driving the high-frequency hydrogen combustion dynamics.

Spiking ruby revisited: self-induced periodic spiking oscillations leading to chaotic state in a Cr:Al2O3 laser with cw 532-nm pumping

This paper reexamines a 60-year-old mystery of spiking behavior in ruby lasers with a cw 532-nm pump, paying special attention to mode matching between the pump and lasing beam within a ruby crystal placed in a semi-confocal laser cavity. Periodic spiking oscillations were observed in a limited pump power regime, where spikes obeying the generic asymmetric hyperbolic function appeared at a repetition rate around 50 kHz and with a 130-150 ns width and 0.1-0.6 µJ energy depending on the pump power. The physics of the spiking behavior based on Kleinman’s mechanical approach and a plausible interpretation for the periodic spiking oscillation in terms of self-induced mode matching between the pump and laser beams through the self-induced Kerr-lens effect are addressed. The statistical nature inherent to spiking and the associated self-organized critical behavior in the quasi-periodic spiking oscillations as well as chaotic states occurring outside the periodic spiking regime are clarified from a nonlinear dynamics point of view and intriguing statistical properties inherent to chaotic oscillations in usual solid-state lasers subjected to external modulations are shown to be present in the self-induced instabilities of the ruby laser

A Perspective Review of Passive Techniques Applied to Control the Swirling Flow Instabilities From the Conical Diffuser of Hydraulic Turbines

Abstract This paper represents a welcome synthesis of the results obtained by the authors over more than a decade. The reason why such an approach is perfectly justified is found in the novelty of the control techniques of decelerated swirling flows from the conical diffuser of hydraulic turbines. The results presented in this paper refer strictly to the new passive control techniques of the swirling flows instabilities from the conical diffuser of hydraulic turbines. Although the results of these new techniques have been disseminated in various papers, it is difficult to outline an overview from a collection of articles. In addition, a lot of valuable information about modern experimental and numerical investigations is not found in articles that usually distill only the most significant results. Therefore, the present paper achieves a welcome unitary synthesis, useful to specialists in the field of turbomachine hydrodynamics. The reluctance of the turbine manufacturers on active control techniques that use external/additional energy sources led us to the choice of passive control techniques review, especially the ones developed in the last years. The first part of the paper analyzes the specialized literature that includes a variety of passive solutions for mitigating self-induced instabilities of decelerated swirling flow downstream of hydraulic turbines. Such inherent instabilities manifest intensely at far from optimal operating regimes and represent one of the challenges of modern hydraulic turbines. The mitigation of these instabilities is an open problem, so far there are no unanimously accepted technical solutions implemented on prototype turbines. The second part of the paper includes detailed investigations on axial water injection with flow-feedback, but also more recent approaches using adjustable diaphragm in the conical diffuser.

Thermoacoustic coupling regions of premixed-flames in non-adiabatic tubes

Self-induced thermoacoustic instabilities of premixed methane-air flames propagating in horizontal slender tubes are studied both theoretically and experimentally. First, modeling of responsive flames subject to heat-loss corrections, which yield realistic non-isothermal acoustics, is considered. In particular, we compare two coupling mechanisms proposed in the literature, velocity and pressure coupling. Their unstable behavior is controlled by different Flame Transfer Functions (FTF), which define the flame response to acoustic perturbations and are here coupled with a conductive heat-loss scenario in the aforementioned non-adiabatic tubes. Secondly, these mechanisms are characterized by distinct Acoustic Structure Functions (ASF), which are shown to provide the position of constructive coupling of the instabilities and their use as predictors of unstable regions is discussed. It is remarkable that heat losses have a significant impact on the theoretical predictions of the location of these physical mechanisms. Finally, a tube of variable length L and a fixed diameter D=10 mm with wall temperature control is used to exploit nearly viscous laminar flows induced by the propagating flame. The instability regions of each harmonic observed in the experiments are compared to the theoretical predictions. Excellent experimental agreement with the ASF predictions provides further evidence to validate the theoretical non-adiabatic model. These results provide additional evidence for velocity coupling mechanism to be responsible for the initial process that pairs the acoustic modes and the unsteady heat release rate of the flame in a combustion chamber.

Influence of radial fuel staging on combustion instabilities and exhaust emissions from lean-premixed multi-element hydrogen/methane/air flames

The utilization of hydrogen-blended hydrocarbon fuels, or even completely carbon-free fuels like pure hydrogen and ammonia, will be indispensable in the effort to reduce anthropogenic carbon dioxide emissions from heavy-duty land-based gas turbines. In order to make this energy transition, however, low emissions, low dynamics fuel-flexible gas turbine combustion systems will be required. To address some of the key technological obstacles in the development of such systems, we explore a prototype multi-element injector assembly devised to prevent flashback in extremely fast premixed hydrogen flames, paying particular attention to the impact of radial dual-fuel staging on the generation of self-induced instabilities and the formation of nitrogen oxides and carbon monoxide. In conjunction with phase-averaged OH*/CH* chemiluminescence and OH PLIF flame imaging, we carry out extensive measurements over the full range of 0 to 100% H2/CH4 fuel staging conditions, including even/uneven blends of H2/CH4 fuels between inner and outer nozzle groups, partial/complete fuel split cases, and pure H2 or CH4 fuel for all constituent flames, under a constant thermal power condition of 78 kW. Our measurements demonstrate that whereas carbon monoxide concentrations are largely unaffected by the radial fuel staging conditions except under high and pure hydrogen percentage conditions, there exists a strong correlation between total nitrogen oxides emissions and overall adiabatic flame temperature. Integrated analyses of isocontour instability maps reveal that near XH2,g = 0.50 a discontinuous mode transition takes place, where the intermediate-amplitude lower frequency oscillations associated with relatively low hydrogen concentration conditions give way to stronger, higher frequency instabilities under high hydrogen content conditions. While even-blend (non-staging) conditions are characterized by coherent oscillations of the constituent flames, the responses of clustered flames to radial fuel staging conditions are eccentric and complex, manifested as large-scale asynchronous modulations between inner- and outer-stage heterogeneous reaction zones. This observation suggests that in a multi-element injector environment spatiotemporal incoherence driven by inhomogeneous heat release can be used to neutralize self-excited pressure oscillations, by disrupting pressure-heat release coupling processes.

Stability of Hydrogen Turbopump Rotor Shaft Axially Self-Balanced

Abstract Controlling the vibration levels of turbopump rotor shafts is a key feature for the reliability of space engines. Turbopump components are exposed to high static and dynamic stress levels, and therefore are particularly sensitive to high-cycle fatigues. Among the numerous dynamic excitations that affect the turbopump, self-induced instabilities are the most critical ones because of the exponential growth rate of vibration levels. These flutter-like phenomena may account for rotor shaft instabilities of turbopumps designed with an axial balancing system (ABS). Such a system is necessary to avoid heavy static loads on bearings and is commonly used in high power turbopumps in the space industry. It consists of a fluid cavity located in the back of a centrifugal compressor. Instabilities induced by the ABS have been studied within the framework of a research and technology program using a reduced scale hydrogen turbopump demonstrator called TPtech. This paper focuses on experimental and numerical analysis of rotor instabilities induced by the ABS. A coupled dynamic model of the rotor shaft and the ABS cavity is presented. It shows instabilities of the rigid rotor axial mode, but also of an axisymmetric rotor mode. This result is consistent with the data acquired during TPtech test campaign. The instability mechanism is complex as it involves the rotor modes, the flow in the ABS cavity, and its acoustic modes. Therefore, TPtech tests performed in representative conditions are valuable. They have permitted tool validation and have provided design rules to prevent occurrence of such phenomena.

Computational Assessment of Transonic Airfoil-Gust Aeroelastic Response

This study assesses the aeroelastic response that could arise from a periodic vertical gust of different frequencies encountering an airfoil which consists of a trailing-edge control surface (typically used for transonic aileron-buzz studies) and an airfoil undergoing pitching and plunging aeroelastic motion in transonic flow. The flutter phenomena and limit cycle oscillation correspond to two different types of self-induced instabilities, which happen at a Mach number, freestream velocity, and angle of attack combination for which the airfoil or control surface attains a stable oscillation. Gust acts as an external force to an aerostructural system. The airfoil shows a beating pattern under a periodic gust when the gust frequency coincides with the flutter frequency. However, this pattern evident at flutter velocity is absent when the periodic gust encounters an aeroelastic system at limit cycle oscillation velocity. The present work also shows the impact of periodic gust front on the shock/boundary-layer interaction modifying the coefficient of pressure and coefficient of skin friction distribution and leading to a multiple-frequency and multiple-amplitude aeroelastic motion. Fast Fourier transform is applied to the structural response to explore the frequency content corresponding to the structural response and dominant fluid modes for different flow Mach number regimes under the influence of the periodic gust. These frequency characteristics of the structural responses and flowfield in inviscid and viscous flowfield are distinct for different test cases and reveal the underlying physics of several multiple-frequency and multiple-amplitude aeroelastic motions, frequency coalescence, generation of a new frequency, and so on, as shown in this study.

Measurements and analysis of different front-end configurations for monolithic SiGe BiCMOS pixel detectors for HEP applications

This paper presents a small-area monolithic pixel detector ASIC designed in 130 nm SiGe BiCMOS technology for the upgrade of the pre-shower detector of the FASER experiment at CERN. The purpose of this prototype is to study the integration of fast front-end electronics inside the sensitive area of the pixels and to identify the configuration that could satisfy at best the specifications of the experiment. Self-induced noise, instabilities and cross-talk were minimised to cope with the several challenges associated to the integration of pre-amplifiers and discriminators inside the pixels. The methodology used in the characterisation and the design choices will also be described. Two of the variants studied here will be implemented in the pre-production ASIC of the FASER experiment pre-shower for further tests.

Combustion dynamics of multi-element lean-premixed hydrogen-air flame ensemble

The crux of the technical issues facing the development of hydrogen gas turbine engines is the problem of how to enable low NOx, low dynamics combustor operations without detrimental flashback events in extremely fast lean-premixed hydrogen flames. While flashback mechanisms and nitrogen oxides emissions have been investigated extensively for high hydrogen content flames, the self-excited dynamics of lean-premixed pure hydrogen flame ensembles remain unclear. Here we use phase-resolved OH* chemiluminescence and OH PLIF imaging in a series of experiments in a multi-element injector configuration consisting of 293 small-scale injectors with an inner diameter of 3.0 mm. We collect a large experimental dataset to explore a variety of collective phenomena of the lean-premixed hydrogen-air flame ensemble, and perform systematic investigations of self-induced combustion instabilities. Our observations demonstrate that ultra-compact pure hydrogen flames generate high-amplitude pressure perturbations over a broad range of characteristic frequencies between 400 and 1800 Hz, corresponding to the third to tenth order eigenmodes. Low-frequency flame dynamics developed under relatively low equivalence ratio conditions involve a complex balance among several coexisting phenomena, including strong vortex interactions and periodic extinction-reignition processes, giving rise to large-scale asymmetric oscillations of the entire reaction zone. By contrast, the flame surface dynamics at an intermediate frequency of ~600 Hz exhibit prominent symmetric oscillations accompanied by merging and pinch-off of the constituent flames. Unexpectedly, high-frequency instabilities at approximately 1720 Hz (screech tones) are not influenced by such structurally complex flame dynamics, but by exceptionally simple, seemingly linear, flame surface motion without sudden flame area annihilation events like those observed for lower frequency cases. Despite the extremely low level of heat release rate fluctuations, on the order of less than 1%, the clustered premixed hydrogen flames are capable of producing disproportionately large pressure perturbations in excess of 12 kPa, originating from the synchronous phase dynamics of acoustic pressure and clustered flames’ heat release rate oscillations. These findings provide new insight into the driving mechanisms underlying high-frequency combustion dynamics of densely arranged pure hydrogen-air flames.

Stabilisation of the swirl exiting a Francis runner far from the best efficiency point

The decelerated swirling flow in the discharge cone of Francis turbines operated at partial discharge, far from the best efficiency point, develops self-induced instabilities featuring a precessing helical vortex (so-called vortex rope) which hinders the stable and safe turbine operation. This is an intrinsic characteristic of the swirl exiting the Francis runner, which at part load has a relatively large residual flux of moment of momentum as well as an imbalanced specific hydraulic energy with an excess near the band. We address in this paper the question of how one should alter this swirling flow in order to mitigate further instabilities in the discharge cone. In doing so, we consider an actuator disk located in the upstream part of the discharge cone that model a runaway runner which alter the hub-to-shroud angular momentum and specific head distributions without altering the turbine operating point. However, such a runaway runner provides a swirl stabilisation further downstream thereby effectively mitigating the vortex rope.

3D numerical investigations of the swirling flow in a straight diffuser for the variable speed values of the rotor obtained with a magneto-rheological brake

The self-induced instabilities developed in decelerated swirling flows lead to pressure fluctuations. The swirling flows generated under variable speeds of the rotor of a swirl generator using a magneto-rheological brake are numerically and experimentally investigated. Three-dimensional turbulent flow computation is performed for several rotor speeds. Two mean velocity components numerically computed are validated against LDV data on a survey axis located downstream to the rotor for two speed values. The torque on the rotor blade is examined for several speeds. The numerical results are checked against the design values to identify the benefits and limitations of the concepts applied to the design stage. The flux of moment of momentum is modified when the rotor speed is slow down. The circulation, the total pressure and the swirl free velocity are examined on a cross section located downstream to the rotor for all investigated speeds. As a result, the changes of the hydrodynamic flow field are quantified. The unsteady pressure measured on the cone wall at four levels is acquired. The Fourier spectra associated with different self-induced instabilities developed in the cone are examined revealing the distribution of both plunging and rotating components. A low frequency plunging component in a straight diffuser is determined on a speed range of the rotor.

A Novel Passive Method to Control the Swirling Flow with Vortex Rope from the Conical Diffuser of Hydraulic Turbines with Fixed Blades

In this paper, we introduce a novel passive control method to mitigate the unsteadiness effects associated to the swirling flows with self-induced instabilities. The control method involves a progressive throttling cross-section flow at the outlet of the conical diffuser. It adjusts the cross-section area with a diaphragm while maintaining all positions of the circular shape centered on the axis. It improves the pressure recovery on the cone wall while the pressure fluctuations associated with the self-induced instability are mitigated as it adjusts the cross-section area. It can adjust the diaphragm in correlation with the operating conditions of the turbine. We investigated the passive control method on a swirl generator, which provides a similar flow as a hydraulic turbine operated at a partial discharge. The plunging and rotating components are discriminated using the pressure fluctuation on the cone wall to provide a clear view of the effects induced by this passive control method. As a result, the novel proof of concept examined in this paper offers valuable benefits as it fulfils a good balance between the dynamical behavior and the hydraulic losses.

Investigation of self-induced thermoacoustic instabilities in gas turbine combustors

The self-induced thermoacoustic instabilities in partially premixed combustors remain a significant challenge which has received a limited research attention. This numerical work investigates the effect of air-fuel equivalence ratio ∅ on exciting thermoacoustic instabilities and its impact on the characteristics of both the acoustic and combustion fields in a partially premixed swirl combustor. For this purpose, three sets of numerical investigations are studied. The first set is a comparison between the present numerical study and previous experimental work considering the normalized combustor wall temperature. The second set of simulations is performed to predict the behavior of the reacting and non-reacting flows with regard to pressure fluctuations, power spectral density, and the sound pressure levels. The third set of simulations considered the effect of the air-fuel equivalence ratio on the acoustics and combustion characteristics. The results reveal that at an equivalence ratio of 0.5, the sound pressure level for the non-reacting and reacting flows are 98 and 118 dB, respectively. It is concluded that the equivalence ratio plays a crucial rule on exciting combustion instabilities at various amplitudes and frequencies. The sound pressure levels increase with increasing in the air-fuel equivalence ratio due to increase the heat release rate.

Francis turbine with tandem runners: a proof of concept

Hydraulic turbines operating at off-design regimes inherently have a large residual swirl at runner outlet. This swirling flow is further ingested by the draft tube and its deceleration in the discharge cone results in self-induced instabilities (e.g. precessing vortex rope, low frequency pressure pulsations, power swing) that may hinder the turbine operation. Moreover, for low head Francis turbines the draft tube losses sharply increase as the operating point departs from the best efficiency one, practically shaping the efficiency hill chart. We arrive at the conclusion that an effective approach to flatten the hill chart, while mitigating the instabilities in the draft tube cone is to adaptively adjust the swirling flow exiting the runner for variable operating regimes. As a result, a novel concept of tandem runners is introduced, whereas the classical Francis runner is conceptually split into a high-pressure runner with a constant speed and a low-pressure runner with variable speed. We explore this concept using realistic Francis turbine tandem cascades, with constant and variable transport speed, respectively.

Proper Orthogonal Decomposition of Self-Induced Instabilities in Decelerated Swirling Flows and Their Mitigation Through Axial Water Injection

The swirling flow exiting the runner of a hydraulic turbine is further decelerated in the discharge cone of the draft tube to convert the excess of dynamic pressure into static pressure. When the turbine is operated far from the best efficiency regime, particularly at part load, the decelerated swirling flow develops a self-induced instability with a precessing helical vortex and the associated severe pressure fluctuations. This phenomenon is investigated numerically in this paper, for a swirl apparatus configuration. The unsteady three-dimensional (3D) flow field is analyzed using a proper orthogonal decomposition (POD), and within this framework we examine the effectiveness of an axial jet injection for mitigating the flow instability. It is shown that a limited number of modes can be used to reconstruct the flow field. Moreover, POD enables to reveal influence of the jet injection on the individual modes of the flow and illustrates continuous suppression of the modes from higher-order modes to lower-order modes as the jet discharge increases. Application of POD offers new view for the future control effort aimed on vortex rope mitigation because spatiotemporal description of the flow is provided. Thereby, POD enables better focus of the jets or other flow control devices.

A novel scenario of aperiodical impacts appearance in the turbine draft tube

The swirling flow in the discharge cone of hydroturbine is characterized by various self-induced instabilities and associated low frequency phenomena when the turbine is operated far from the best efficiency point. In particular, the precessing vortex rope develops at part-load regimes in the draft tube. This rope can serve a reason of the periodical low- frequency pressure oscillations in the whole hydrodynamical system. During the experimental research of flow structure in the discharge cone in a regime of free runner new interesting phenomenon was discovered. Due to instability some coils of helical vortex close to each other and reconnection appears with generation of a vortex ring. The experiments were fulfilled at the cavitational conditions when a cavity arises in the vortex core. So the phenomenon was registered with help of visualization by the high speed video recording. The vortex ring after the reconnection moves apart from the main vortex rope toward the wall and downstream. When it reaches the area with high pressure the cavity collapses with generation of pressure impact. The mechanism of cavitational vortex rings generation and their further collapse can serve as a prototype of the aperiodical pressure impacts inside the turbine draft tube.

Self-induced flavor instabilities of a dense neutrino stream in a two-dimensional model

We consider a simplifed model for self-induced flavor conversions of a dense neutrino gas in two dimensions, showing new solutions that spontaneously break the spatial symmetries of the initial conditions. As a result of the symmetry breaking induced by the neutrino-neutrino interactions, the coherent behavior of the neutrino gas becomes unstable. This instability produces large spatial variations in the flavor content of the ensemble. Furthermore, it also leads to the creation of domains of different net lepton number flux. The transition of the neutrino gas from a coherent to incoherent behavior shows an intriguing analogy with a streaming flow changing from laminar to turbulent regime. These finding would be relevant for the self-induced conversions of neutrinos streaming-off a supernova core.

A model for precessing helical vortex in the turbine discharge cone

The decelerated swirling flow in the discharge cone of hydraulic turbine develops various self-induced instabilities and associated low frequency phenomena when the turbine is operated far from the best efficiency regime. In particular, the precessing helical vortex ("vortex rope") developed at part-load regimes is notoriously difficult and expensive to be computed using full three-dimensional turbulent unsteady flow models. On the other hand, modern design and optimization techniques require robust, tractable and accurate a-priori assessment of the turbine flow unsteadiness level within a wide operating range before actually knowing the runner geometry details. This paper presents the development and validation of a quasi-analytical model of the vortex rope in the discharge cone. The first stage is the computing of the axisymmetrical swirling flow at runner outlet with input information related only to the operating point and to the blade outlet angle. Then, the swirling flow profile further downstream is computed in successive cross-sections through the discharge cone. The second stage is the reconstruction of the precessing vortex core parameters in successive cross-sections of the discharge cone. The final stage lies in assembling 3D unsteady flow field in the discharge cone. The end result is validated against both experimental and numerical data.

Unsteady pressure measurements of decelerated swirling flow in a discharge cone at lower runner speeds

The decelerated swirling flow in the draft tube cone of hydraulic turbines (especially turbines with fixed blades) is responsible for self-induced instabilities which generates pressure pulsations that hinder the turbine operation. An experimental test rig was developed in order to investigate the flow instabilities. A new method was implemented to slow down the runner using a magneto rheological brake in order to be extended the flow regimes investigated. As a result, the experimental investigations are performed for 7 operating regimes in order to quantify the flow behaviour from part load operation to overload operation. The unsteady pressure measurements are carried out on 4 levels in the cone. The unsteady pressure measurements on the cone wall consist in quantifying of three aspects: i) the pressure recovery coefficient obtained based on mean pressure provides the energetic assessment on the draft tube cone; ii) the unsteady quantities (dominant amplitude and frequency) are determined revealing the dynamic behaviour; iii) the plunging and rotating components of the pressure pulsation. As a result, this new method helps us to investigate in detail the flow instability for different operating regimes and allows investigating various flow control solutions.