1,721,014 research outputs found
Large eddy simulations for the flame describing functionof apremixed turbulent swirling flame
No full textComputational prediction of thermoacoustic instabilities arising in gas turbine and aeroengine combustors is an ongoing challenge. Approaches which couple separate treatments for the acoustic waves and the flame all rely on a model for the response of the flame to oncoming acoustic perturbations. In the frequency domain, in the limit of small perturbations, this function is usually given in terms of the so-called Flame Transfer Function (FTF), i.e., a function that relates the heat release rate perturbation at the flame location with longitudinal velocity fluctuations taken at an upstream reference point. With the increase of the amplitude of oscillations, nonlinear combustion process controls the dynamics of the systems. A flame describing function (FDF) is so-defined introducing a dependence of the gain and phase of the FTF on velocity fluctuations amplitude |u'/ū|. The present work uses numerical simulations to obtain the FDF of a turbulent premixed swirling flame. The swirled burner developed at NTNU university is considered in a longitudinal combustor setup. Simulations are performed using Large Eddy Simulations (LES) via the open source Computational Fluid Dynamics (CFD) code, OpenFOAM. The predicted unperturbed flame structure is at first presented and discussed. Subsequently, the nonlinear flame response is characterised submitting the flame to a harmonically varying longitudinal velocity fluctuation, for which the forcing frequency is varied from 300 Hz to 1900 Hz considering two forcing amplitude levels, |u'/ū|=0.1 and |u'/ū|=0.2. The shape of the gain and the phase of the full FDF is discussed and the main characteristics are investigated
A systematic study of nonlinear coupling of thermoacoustic modes in annular combustors
No full textThermoacoustic instabilities in annular gas turbine combustors often involve modes which vary in both the longitudinal and circumferential directions. Recent experimental studies show that during limit cycle oscillations, different thermoacoustic modes may be uncoupled, as is the case in purely longitudinal or circumferential spinning modes. They may also be coupled, for example two counter-rotating circumferential modes combining to give standing or mixed modes, and coupling between circumferential and longitudinal modes giving rise to the slanted mode. Accurately predicting such modal couplings and the resulting spatial pattern of limit cycle oscillations remains an open challenge. This work uses a 2-D low-order network model based on modal expansions, validated against a full 3-D Helmholtz solver, to systematically investigate these couplings. For the first time, low-order network modelling is shown to capture limit cycle oscillations exhibiting both uncoupled and nonlinearly coupled modes, the latter including coupling between counter-rotating circumferential modes and between longitudinal and circumferential modes. It is shown that limit cycle solutions with totally different mode patterns (longitudinal, circumferential spinning, circumferential standing and slanted)can all exist in a given thermoacoustic system, with switches between modal patterns arising from slight changes in parameters such as the flame time delay
Numerical prediction of the Flame Describing Function and thermoacoustic limit cycle for a pressurised gas turbine combustor
No full textThe forced flame responses in a pressurized gas turbine combustor are predicted using numerical reacting flow simulations. Two incompressible1 large eddy simulation solvers are used, applying two combustion models and two reaction schemes (4-step and 15-step) at two operating pressures (3 and 6 bar). Although the combustor flow field is little affected by these factors, the flame length and heat release rate are found to depend on combustion model, reaction scheme, and combustor pressure. The flame responses to an upstream velocity perturbation are used to construct the flame describing functions (FDFs). The FDFs exhibit smaller dependence on the combustion model and reaction chemistry than the flame shape and mean heat release rate. The FDFs are validated by predicting combustor thermoacoustic stability at 3 and 6 bar and, for the unstable 6 bar case, also by predicting the frequency and oscillation amplitude of the resulting limit cycle oscillation. All of these numerical predictions are in very good agreement with experimental measurements
A novel acoustic network model to study the influence of mean flow and axial temperature distribution on spinning limit cycles in annular combustors
No full textThe present article presents a novel acoustic network model accounting for mean flow and axial temperature distribution able to predict spinning limit cycles observed in an annular combustor. This corresponds to the limit cycle observed in the MICCA test facility, developed at EM2C laboratory, CNRS and CentraleSupélec for which experimental data for different azimuthal mode patterns are available. An acoustic network approach is used to model the combustor. At first the acoustic network modelling assumptions have been validated by comparing the predicted frequency of the pure acoustic modes of the system with those obtained by simulating the detailed geometry of the combustor in a Helmholtz solver. This is performed under the zero-Mach-number approximation since in the Helmholtz solver the convective terms are neglected. The analysis is restricted to the resonant modes from 100 to 1000 Hz, the frequency range in which unstable phenomena have been observed during experiments. Acoustic network predictions have been found to be in agreement with the Helmholtz solver results proving the validity of the modelling assumptions. Subsequently, the influence of convective terms and an axial temperature is investigated. In a second step, an experimental global Flame Describing Function (FDF) measured at the operative conditions in which the combustor dynamics were observed is coupled with the acoustic solver. In the frequency domain, the spinning limit cycle is predicted by means of a weakly nonlinear stability analysis. Simulations retrieve a spinning self-sustained mode at a resonant frequency and velocity amplitude level matching the experimental observation and previous numerical simulations proving that the proposed numerical approach is able to correctly model the combustion dynamics and damping level of the analysed combustor
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Get PDF“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Get PDFWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Dispelling the Myths Behind First-author Citation Counts
Get PDFWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
- …
