Push-pull Modes Research Articles

Influence of rotational asymmetry on thermoacoustic instabilities in a can-annular lean-premixed combustor

Low-frequency combustion instabilities, and ultimately, engine-level dynamics in can-annular gas turbine combustion systems, are driven by acoustic interactions between adjacent combustors through an annular cross-talk section upstream of first stage turbine nozzles. This is an area of substantial technological challenges, because experimental and numerical complexities scale exponentially with the number of combustors, restricting most investigations to a single combustor or a coupled two-combustor configuration. We recently reported that in a can-annular combustion system consisting of four identical lean-premixed combustors pressure oscillations occur in many combinations of characteristic patterns and frequencies, including symmetric (push–push) and asymmetric (push–pull) interactions, when the system is subjected to rotationally symmetric operating conditions. The impact of rotational asymmetry on can-annular thermoacoustics is still a largely open subject; here we use the same experimental setup to perform multiple independent analyses of the effect of broken rotational symmetry on large-scale pattern formations and modal dynamics of multiple eigenmodes. We demonstrate that the presence of rotational asymmetry can lead to fundamentally different dynamic states, including pairwise push-pull modes, spinning azimuthal instabilities, superposition modes, and strong mode localization, which are totally absent under rotationally symmetric conditions. Of particular importance is the experimental observation of spinning azimuthal instabilities in the annular cross-talk section, which are acoustically separate from flame-dynamics-driven standing wave motion in the flame tube sections. This leads to the coexistence of traveling and standing waves in the can-annular system. In conjunction with FEM-based Helmholtz simulations, we determine the origin of the azimuthal modes to be the simultaneous excitation of two different pairwise push-pull interaction modes at the same frequency – a phenomenon known as degenerate eigenmodes.

Influence of asymmetric flame transfer functions on can-to-can acoustic interactions between two lean-premixed combustors

In can-annular gas turbines, low-frequency thermoacoustic instabilities can arise from cross-talk interactions between neighboring combustors upstream of the first-stage turbine nozzles. In this experimental study, we investigate the influence of non-identical flame transfer functions (FTFs) between adjacent combustors on the development of self-excited thermoacoustic oscillations. To create different FTFs, we use five different swirl nozzles, one with high swirl (HS) and four with low swirl (LS), all with different porosities. We find that, compared with the LS FTFs, the HS FTF exhibits a smaller and flatter gain as well as a smaller phase difference. We attribute this behavior to differences in the flame structure and the stabilization mechanisms, namely an inner shear layer-stabilized diverging front in the HS case versus a detached reaction zone in the presence of a central jet with an outer swirl flow in the LS cases. Using two tunable lean-premixed combustors connected via a cross-talk section, we show that (i) symmetric FTF combinations (HS + HS or LS + LS) produce in-phase interactions, leading to push-push modes, but that (ii) asymmetric FTF combinations (HS + LS) produce out-of-phase interactions, leading to push-pull modes. Phase-resolved visualization of the asymmetric cases reveals that the inner shear layer-stabilized HS flame exhibits large angle fluctuations, whereas the aerodynamically stabilized LS flame is characterized by the periodic emergence of a bow-shaped front and an oval structure. For all the conditions tested, we find that asymmetry in the FTFs leads to either (i) a completely stable state with negligible amplitude or (ii) a mildly unstable state with an amplitude lower than that of the equivalent symmetric cases. These findings highlight the potential of using asymmetric FTFs for passive control of cross-talk-driven thermoacoustic instabilities in can-annular combustors.

Cross-talk-interaction-induced combustion instabilities in a can-annular lean-premixed combustor configuration

The prediction of self-excited combustion instabilities in a can-annular gas turbine combustion system is a significant challenge, mainly because the instabilities originate from the acoustic interactions between adjacent combustors via a cross-talk region upstream of the first stage turbine nozzles. Detailed characterization of these instabilities requires a thorough understanding of engine-level dynamics. Until now, a comprehensive experimental examination of such a can-annular configuration had not been conducted. Here we present new experiments using four lean fully-premixed swirl-stabilized combustors connected via a full-annular cross-talk section. We demonstrate that the global fluctuations at limit cycles are either in-phase interactions (push-push modes) or one of two different forms of out-of-phase interactions (push-pull modes), subject to uniform and non-uniform equivalence ratio combinations. Under certain symmetric conditions, the can-annular system undergoes in-phase synchronous modulations (Type I), giving rise to the formation of pressure antinodes at the inlets of the four combustors and in the cross-talk region. By contrast, out-of-phase interactions are sustained in the form of either an alternating pattern in four-coupled combustors (Type II) or a push-pull interaction in two opposite combustors only (Type III). The latter is dictated by strong out-of-phase fluctuations between two of the combustors and a pressure node-like condition – thermoacoustically decoupled from the global fluctuations – over the entire region of the other two combustors, experimentally demonstrating the existence of mode localization in can-annular thermoacoustic instabilities. We show that the mode clustering phenomenon is responsible for the excitation of closely-spaced multiple eigenmodes in the can-annular acoustic environment, and as a consequence the system can feature a mixed state with several distinct types of interaction patterns. By analyzing a large amount of experimental data acquired systematically for coupled two-combustor and four-can-annular configurations, we demonstrate that longitudinal-mode instabilities in a can-annular combustion system will preferentially emerge in the form of out-of-phase interactions.

Mutual synchronization of two flame-driven thermoacoustic oscillators: Dissipative and time-delayed coupling effects.

Low-emissions can-annular gas turbines are prone to develop low-frequency self-excited thermoacoustic oscillations. Such oscillations arise from the coupling between adjacent combustors and can increase wear and thermal stresses. In this experimental study, we explore the mutual synchronization of two thermoacoustic oscillators (i.e., two model combustors) interacting via dissipative and time-delayed coupling, as introduced via a cross-talk section. Unlike most previous studies, our study makes use of a turbulent lean-premixed flame in each combustor, bringing the system configuration closer to that of practical gas turbines. Using stationary and transient measurements, we examine the effect of the cross-talk diameter and length so as to gain insight into the effect of dissipative and time-delayed coupling. We find that strengthening the dissipative coupling promotes mutual synchronization, but that weakening the dissipative coupling leads to weakly coupled or desynchronized oscillations. On operating the two combustors at different conditions, we find a significant reduction in their overall oscillation amplitude for some coupling conditions. On varying the combustor length and examining the transient response, we find elaborate changes in the pressure-heat-release-rate coupling, spontaneous mode transitions between coupled thermoacoustic modes, and the emergence of a rhomboid structure in the phase plane owing to the coexistence of in-phase and out-of-phase synchronization. In the combustion community, these two types of synchronization are known to be associated with push-push modes and push-pull modes. These findings offer new insight into the mutual synchronization of low-frequency, self-excited thermoacoustic oscillations in can-annular gas turbines, paving the way for the development of improved control strategies.

Effect of Noise and Nonlinearities on Thermoacoustics of Can-Annular Combustors

Abstract Can-annular combustors consist of N distinct cans setup symmetrically around the axis of the gas turbine. Each can is connected to the turbine inlet by means of a transition duct. At the turbine inlet, a small gap between the neighboring transition ducts allows acoustic communication between the cans. Thermoacoustic pulsations in the cans are driven by the respective flames, but also the communication between neighboring cans through the gap plays a significant role. In this study, we focus on the effect of the background noise intensity and of the nonlinear flame saturation. We predict how usually clusters of thermoacoustic modes are unstable in the linear regime and compete with each other in the nonlinear regime, each cluster consisting of axial, azimuthal and push-pull modes. Since linear theory cannot predict the nonlinear solution, stochastic simulations are run to study the nonlinear solution in a probabilistic sense. One outcome of these simulations is the various pulsation patterns, which are in principle different from one can to the next. We recover how not only a stronger flame response in one can gives rise to the phenomenon of mode localization, but also how the nonlinearity of the flame saturation and the competition between modes have an effect on the nonlinear mode shape. We finally predict the coherence and phase between cans on the linearized system subject to noise, and compare the predictions with engine measurements, in terms of spectra of amplitude in each can and coherence and phase, observing a good match.

Combustion-acoustic interactions through cross-talk area between adjacent model gas turbine combustors

Longitudinal-mode combustion instabilities in can-annular gas turbine combustion systems are influenced by acoustic interactions between adjacent combustors; these interactions typically occur across an open area between a combustor transition piece and first stage turbine nozzles. Here we show that strongly interacting thermoacoustic modes are excited in two adjacent model gas turbine combustors connected with a cross-talk area aligned normal to the flow direction. The data are analyzed in comparison with the measurement results of a single isolated combustor without the cross-talk area. Our results demonstrate that the dynamics of the multiple combustors are manifested by a push-push, a push-pull, and a bimodal mode, which are induced in response to the difference in equivalence ratios between two combustors, including symmetric and asymmetric conditions. The push-push mode develops due to in-phase interactions between adjacent combustors, leading to the formation of a pressure anti-node in the cross-talk area. By contrast, the push-pull mode is a result of out-of-phase interactions, and forms a pressure node in the cross-talk area. Under marginally asymmetric conditions, the system features bimodal tones with a strong beating oscillation, which results from a combination of closely-spaced push-push and push-pull modes. Mode pairing is the dominant mechanism for the growth of the beating oscillation. The mode selection process is not directly determined by the local flame dynamics, but by the distribution of acoustic energy between the fundamental and higher harmonics in the cross-talk area. When a push-pull mode is initiated in the system, higher harmonic components with a large amplitude begin to emerge, strongly diminishing the fundamental frequency.

The fractography analysis of IN718 alloy after three-point flexure fatigue test

In this study, the high cycle fatigue (HCF) properties of IN718 superalloy with given chemical composition were investigated at three-point flexure fatigue test at room temperature. INCONEL alloy 718 is nickel-chromium-iron hardenable alloy and due to its unique combination of mechanical properties (high-strength; corrosion-resistant and so on) used for production of heat resistant parts of aero jet engine mostly. Mechanical properties of this alloy are strongly dependent on microstructure and on presence of structural features such are principal strengthening phase gamma double prime, gamma prime and due to its morphology less desired delta phases. The mentioned phases precipitate at various temperature ranges and Nb content as well. The three-point flexure fatigue test was performed on ZWICK/ROELL Amsler 150 HFP 5100 test equipment with approximate loading frequency f=150 Hz. The S – N (Stress – Number of cycles) curve was obtained after testing. With the help of scanning electron microscope (SEM), fractography analyses were performed to disclose the fracture features of specimens in different life ranges. The brief comparison of three-point flexure and push-pull fatigue loading modes and its influence on fatigue life is discussed as well.

Upper Extremity Muscular Strength in Push-Pull Tasks: Model Approach Towards Task Design.

Background:Pushing and pulling in workplaces are common actions. Repetitive forceful exertions in long-duration works lead to increased risk of musculoskeletal disorders and injuries.Aim:To investigate the upper extremity strength in generic push–pull modes while using hand tools and forecasting the limits of the workers while frequent or continuous operation.Settings and Design:The study was conducted among men workers in Ahmedabad city, India, and the design was cross-sectional study.Materials and Methods:In all, 100 men were recruited (Group 1: 18–39 years and Group 2: 40–60 years). Upper extremity muscular strength (isometric mode) testing of the preferred hand during push–pull type of manual hand-tool operations was carried out for 60 s. Forecasting of strength to generate predictions for future events (120 s) based on known past events (measured 60 s) was carried out using Holt–Winters time-series model.Statistical Analysis:Descriptive statistics was used for analysis. For prediction model evaluation, WEKA 3.8.2 was used.Results:Anthropometric parameters of both groups were similar, having no effect on generated strength. Largely, pull strength was recorded to be higher than push strength, wherein Group 2 men generated slightly higher strength. Seated strength was also higher than standing exertion. Forecasting reveals steady strength values for Group 1 men, whereas steep decline among Group 2 men with increasing duration of trial.Conclusion:The strength data generated would aid in work schedule design. Strength forecasting model would assist in developing engineering guidelines in the design of tools at workplace.

Transistor configurations in integrated transistor antennas

A uniform method of analysing the six basic integrated antenna structures by taking the relevant device configurations into consideration is presented in this paper. The possibility of height reduction at resonance at low frequency operation is examined for normal and reverse bias operation of the device as well as push-pull and push-push modes of excitation of the antenna structures.Generalized high frequency analysis for the input impedance at feed points of a structure is worked out and results for particular cases, namely f.e.c.l. and f.e.b.l. are included. Possibility of using approximations for resonance at high frequencies is also outlined.

The University of Maryland Isochronous Cyclotron

Design features of the University of Maryland Isochronous Cyclotron are discussed and a description of initial turn-on and operation is given. The four-sector magnetic field was designed with the aid of a one-third scale model magnet utilizing both the experience with the Grenoble Cyclotron and computer studies at the University of Maryland. The original maximum design energy was increased to 140 MeV protons by modification of the spiral angle after full-scale magnet measurements. The dual RF amplifier chain (25 mW to 250 kW per channel) drives two dees independently in both push-pull and push-push modes over a frequency range of 10-22 MHz from 20 to 90 kV.