1,720,961 research outputs found
Large-Eddy Simulations of a Laser-Ignited Subscale Rocket Combustor: Modeling Strategies and Experimental Comparison
No full textTo predict the reliability of laser ignition in a rocket combustor using large-eddy simulations (LESs), it is essential to first ensure that the pre-ignition jet statistics and the dynamics of the hot kernel generated by the energy deposition are accurately captured. In this manuscript, we compare numerical results with experimental data to evaluate the accuracy of the computational approach. First, the jet LES statistics show good qualitative agreement with the particle imaging velocimetry (PIV) data. Quantitative comparisons at several streamwise locations reveal larger differences near the injector, but with local discrepancies of less than 15 m/s in both the mean and fluctuation statistics. Second, we quantify the mean and uncertainties of the hot kernel modeling parameters through a joint analysis of experimental data and direct numerical simulation (DNS) results. This approach accounts for shot-to-shot variability in the simulations, which demonstrate good agreement with the experimental data regarding the ejecta position
Characteristics of Periodic Self-Sustained Detonation Generation in an Rde Analogue
No full textRotating detonation engines (RDEs) are one of the most promising options for improving combustor efficiency through a constant-volume combustion process. RDEs are characterized by continuous detonation propagation in an annular combustion chamber with an implicitly dynamic injection response. An additional benefit is the similarity of these devices to existing engine architectures. However, RDEs have yet to realize their thermodynamic and systemic advantages due the non-ideal physics of detonation in practical devices and the complex interactions between the detonations and the hydrodynamics of the reactants. The design of RDEs is heavily informed by experimental and simulation efforts, but simulations are expensive and often limited by the assumptions of the solver. Experiments have their own challenges; the dynamic reaction zone processes are difficult to examine experimentally in annular combustor geometry. Therefore, an RDE analogue, operating at near-atmospheric conditions with natural gas and oxygen, was developed that emulates the combustor geometry of an RDE in a linear channel that facilitates optical diagnostic capabilities. The experiment permits detailed characterization of the injection, mixing, and ignition processes in an RDE and provides a cross-platform comparison with simulation results, which are often two-dimensional or linear, 3-D domains.A unique phenomenon was discovered in this experiment, wherein a transverse combustion instability developed periodic, kilohertz-rate detonations through a non-linear amplification process. The behavior was highly repeatable and produced dominant cycle frequencies in two distinct regimes: 6 − 8 kHz and 10 − 11 kHz. An investigation of this phenomenon found that these cycle frequencies corresponded to natural dynamics in the oxidizer and fuel manifolds, respectively, and that the transition between regimes was facilitated by the injection pressure ratio between the oxidizer and fuel. This indicated that the injection hydrodynamics were being influenced by the manifold dynamics, and that the hydrodynamics played a key role in the amplification of the instability.The kinetic characteristics of the reactants were examined independently of the injection hydrodynamics as the second key component of the amplification process by altering the reactant chemistry. The combustion morphology was characterized against performance criteria to examine successful behavior. Results showed that cycle frequency and kinetic rates were directly proportional, and that non-linear growth of the flame was possible when the cycle frequency matched the dynamics supplied by the manifolds. When the cycle frequency exceeded the limits of the manifold dynamics, failure of the detonation behavior would occur. A computational analysis of the reactants was used to examine kinetic rate trends with variations in equivalence ratio, oxidizer dilution, and product gas recirculation.Particle image velocimetry (PIV) was performed on the experiment to study the flow structure of the injection process and the interactions with the detonation process. Timeaveraged statistics showed that the detonation induced transverse perturbation to the flow, with varying strength and directionality with respect to the axial location of the shock. A correlation between this behavior and a reactivity gradient, linked to the local product gas residence time, was found. Analysis of the PIV images produced time-resolved measurements of the reactant fill, from which hydrodynamic timescales of the injection process could be obtained. Comparisons between the hydrodynamic and kinetic timescales created an operability map for the test condition which narrows the prediction of the product gas recirculation that occurs in the combustor.The experiments performed in this work has improved understanding of the dynamic injection that occurs during RDE operation. The self-excited generation of detonations through non-linear processes in this experiment brings to light important interactions between the combustor, injector, and manifolds that can improve, or hinder, the performance of RDEs
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
An Experimental Investigation of Self-Excited Combustion Dynamics in a Single Element Lean Direct Injection (LDI) Combustor
Get PDFThe management of combustion dynamics in gas turbine combustors has become more challenging as strict NOx/CO emission standards have led to engine operation in a narrow, lean regime. While premixed or partially premixed combustor configurations such as the Lean Premixed Pre-vaporized (LPP), Rich Quench Lean burn (RQL), and Lean Direct Injection (LDI) have shown a potential for reduced NOx emissions, they promote a coupling between acoustics, hydrodynamics and combustion that can lead to combustion instabilities. These couplings can be quite complex, and their detailed understanding is a pre-requisite to any engine development program and for the development of predictive capability for combustion instabilities through high-fidelity models. The overarching goal of this project is to assess the capability of high-fidelity simulation to predict combustion dynamics in low-emissions gas turbine combustors. A prototypical lean-direct-inject combustor was designed in a modular configuration so that a suitable geometry could be found by test. The combustor comprised a variable length air plenum and combustion chamber, air swirler, and fuel nozzle located inside a subsonic venturi. The venturi cross section and the fuel nozzle were consistent with previous studies. Test pressure was 1 MPa and variables included geometry and acoustic resonance, inlet temperatures, equivalence ratio, and type of liquid fuel. High-frequency pressure measurements in a well-instrumented metal chamber yielded frequencies and mode shapes as a function of inlet air temperature, equivalence ratio, fuel nozzle placement, and combustor acoustic resonances. The parametric survey was a significant effort, with over 105 tests on eight geometric configurations. A good dataset was obtained that could be used for both operating-point-dependent quantitative comparisons, and testing the ability of the simulation to predict more global trends. Results showed a very strong dependence of instability amplitude on the geometric configuration of the combustor, i.e., its acoustic resonance characteristics, with measured pressure fluctuation amplitudes ranged from 5 kPa (0.5% of mean pressure) to 200 kPa (~20% of mean pressure) depending on combustor geometry. The stability behavior also showed a consistent and pronounced dependence on equivalence ratio and inlet air temperature. Instability amplitude increased with higher equivalence ratio and with lower inlet air temperature. A pronounced effect of fuel nozzle location on the combustion dynamics was also observed. Combustion instabilities with the fuel nozzle at the throat of the venturi throat were stronger than in the configuration with fuel nozzle 2.6 mm upstream of the nozzle. A second set of dynamics data was based on high-response-rate laser-based combustion diagnostics using an optically accessible combustor section. High-frequency measurements of OH*-chemiluminescence and OH-PLIF and velocity fields using PIV were obtained at a relatively stable, low equivalence ratio case and a less stable case at higher equivalence ratio. PIV measurements were performed at 5 kHz for non-reacting flow but glare from the cylindrical quartz chamber limited the field of view to a small region in the combustor. Quantitative and qualitative comparisons were made for five different combinations of geometry and operating condition that yielded discriminating stability behavior in the experiment with simulations that were carried out concurrently. Comparisons were made on the basis of trends and pressure mode data as well as with OH-PLIF measurements for the baseline geometry at equivalence ratios of 0.44 and 0.6. Overall, the ability of the simulation to match experimental data and trends was encouraging. Dynamic Mode Decomposition (DMD) analysis was performed on two sets of computations - a global 2-step chemistry mechanism and an 18-step chemistry mechanism - and the OH-PLIF images to allow comparison of dynamic patterns of heat release and OH distribution in the combustion zone. The DMD analysis was able to identify similar dominant unstable modes in the combustor. Recommendations for future work are based on the continued requirement for quantitative and spatio-temporally resolved data for direct comparison with computational efforts to develop predictive capabilities for combustion instabilities at relevant operating conditions. Discriminating instability behavior for the prototypical combustor demonstrated in this study is critical for any robust validation effort Unit physics based scaling of the current effort to multi-element combustors along with improvement in diagnostic techniques and analysis efforts are recommended for advancement in understanding of the complex physics in the multi-phase, three dimensional and turbulent combustion processes in the LDI combustor
An experimental investigation of a hydrogen oxygen flame steam generator
No full textSupercritical steam is used in a wide range of applications such as stationary power generation, locomotive and space propulsion, and chemical processing including coal and biomass gasification. Subject to availability of hydrogen and oxygen, steam generation using direct H2-O2 combustion offers advantages such as zero pollutant emissions, high thermodynamic efficiency and high specific thermal power output. Direct but staged addition of high pressure liquid water to the combustor allows a broad range of process temperatures. This thesis describes a design and an experimental study of a 40 kW steam generator adapted from the aerospace field for application to coal and biomass gasification. The experimental steam generator includes a spark augmented torch igniter and a near stoichiometric H2-O2 combustion chamber capable of operating at design pressure of 1.5 MPa and a maximum flame zone design temperature of 3450 K. Additional steam is generated by direct injection of liquid water into the post-combustion zone to achieve gasification process temperatures in the 1100-1400 K range. The successful design of the present steam generator includes appropriate igniter location and timing and staging and mixing of the H2, O2, and H2O streams. Experimental results of the steam generator are presented at varying equivalence ratios and compared with equilibrium calculations. The characteristic frequencies and magnitudes of unsteadiness are quantified using power spectral density functions of measured pressure fluctuations at select location throughout the steam generator. The development of the design of the steam generator is also tracked through to its present form. Integration of the steam generator to an optically accessible entrained flow coal and biomass gasifier is discussed. Recommendations for future work including computational verification of experimental results and design improvements are discussed
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