1,720,968 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
Self-Excited Instability Regimes of Confined Turbulent Premixed Jet Flames
No full textThe demand for more efficient and cleaner terrestrial gas turbine engines for energy generation has accelerated with stringent emission regulations. Most terrestrial gas turbine engine configurations utilize lean premixed jet stabilized flames for energy extraction. These highly turbulent flames provide the necessary thermal power densities and low NOx emissions but are prone to combustion instabilities. This work studies the stability of a canonical premixed turbulent jet flame to changes in operation condition at elevated pressure. The underlying coupling mechanisms leading to the excitation of different modes are of interest. Different fuel injection schemes are considered, to delineate the influence of system coupled excitation and flow field intrinsic mechanisms that lead to self-excitation of longitudinal and transverse modes in an axisymmetric jet flame.The fundamental longitudinal mode sensitivity to operation conditions was investigated in a technically-premixed configuration, for which the fuel injector is exposed to the system dynamics. Global equivalence ratio fluctuations (φ ) paired with flame-vortex interactions were observed to sustain the 1L mode, for which leaner operating conditions lead to an increase of the limit-cycle amplitude.To solely focus on the shear layer dynamics involved in the feedback loop of the 1L instability a “fully-premixed” configuration (FPC) of the combustor was designed. The elimination of potential φ lead to the excitation of longitudinal as well as transverse, and spinning modes, which can be associated with specific burner operation envelope regions. 1L mode coupled flame dynamics indicate axisymmetric emission patterns in OH∗ emission imaging corresponding to axisymmetric instabilities in the shear layer. Transverse modes correlate with an asymmetric shear layer roll-up process and a flapping motion of the flame. Spinning modes are characterized by high levels of limit-cycle amplitude and a single wave is observed that travels in the annulus of the recirculation zone. From the high speed imaging an azimuthal wave speed of up to 90% of the Chapman-Jouguet velocity for the natural gas - air mixture is computed.The transverse mode is found to be sensitive to changes in chamber pressure and injector velocity. For a baseline injector velocity at which 1T mode excitation occurs, an increase in system pressure lead to an increase of the 1T mode amplitude. The 1T mode excitation is found to be sensitive to the injector velocity, where the highest amplitude are observed for a Strouhal number band of 1.6-1.7 based on the injector diameter.A linear stability analysis (LSA) of the underlying base flow field is performed in order to assess if the underlying shear layer instability modes determine the selection of the instability regime for a given flame condition. Two flow perturbation modes are supported by the flow field, Mode 1 is associated with the recirculation zone domain, where preferred mode frequencies favors coupling with longitudinal acoustics chamber modes. Mode 2 resides in the mixture jet, for which preferred mode frequencies match local 1T acoustic eigenfrequencies of the chamber. It is found that for 1T mode dominated operation cases, the recirculation zone associated mode is stabilized leading to to the excitation of the 1T mode in the injector near field. For operation cases showing predominantly longitudinal combustion instability, the recirculation zone mode (Mode 1) shows elevated growth rates in the injector near field paired with preferred frequencies that are compatible with the longitudinal acoustic eigenmodes
Turbine Inlet Flows Downstream of a Dual Piloted Swirl Flame
No full textAmong the most challenging aspects of modern gas turbine engine design, particularly with aviation applications, is the balance between the extreme conditions necessary to achieve highly efficient combustion that minimizes emissions, and the structural and material limitations of available manufacturing techniques. Due to the difficulty of these problems, computational fluid dynamics and other model-based design tools are often utilized during the development process to better predict how an engine will perform without the high expense of repeated hardware testing. However, these models have limitations in that they often require strong assumptions to reduce the computational expense, and thereby rely on tuning based on empirical data. High-fidelity measurements are needed to not only validate these model outcomes, but also to establish correct boundary conditions. In this work, we report our progress on the development and demonstration of advanced, non-intrusive laser diagnostics in conjunction with traditional instrumentation in a high pressure combustion test rig. Two dimensional flow field velocity measurements have been performed in a region downstream of the combustion zone in a premixed, swirl stabilized combustor. Point velocity measurements were acquired at a series of spatial locations using Laser Doppler Velocimetry (two components). These measurements were succeeded by a series of planer field measurements, encompassing the same spatial locations, using two dimensional Particle Image Velocimetry. The measurements were performed at two different experimental conditions, at combustor inlet temperatures 100°F apart to examine the temperature dependency of the flow field. The following work evaluates the outcomes of these measurements and reveals that a lower combustor inlet temperature produces a central core deficit significantly less drastic than the higher temperature case. Accompanying this reduced-strength deficit are greater amount of velocity fluctuations. An analysis in the frequency domain attempts to identify the driving mechanisms behind the fluctuations
Multi-Scale Flow and Flame Dynamics at Engine-Relevant Conditions
No full textThe continued advancement of gas turbine combustion technology for power generation and propulsion applications requires novel techniques to increase the overall engine cycle efficiency and improved methods for mitigating combustion instabilities. To help address these problems, high-speed optical diagnostics were applied to two different experiments that replicate relevant physics in gas turbine combustors. The focus of the measurements was to elucidate the effect of various operating parameters on combustion dynamics occurring over a wide range of spatio-temporal flow and chemical scales. The first experiment, VIPER–M, enabled the investigation of coupling mechanisms for transverse instabilities in a multielement, premixed combustor that maintains key similarities with gas turbine combustors for land based power generation. The second experiment, COMRAD, facilitated the study of the effect of fuel heating on the combustion performance and dynamics in a liquid-fueled, piloted swirl flame typical of aviation engine combustors.Two different injector lengths were tested in the VIPER–M experiment, and high-speed CH* chemiluminescence imaging and an array of high-frequency pressure transducers were used to characterize the overall combustor dynamics. For all conditions tested, the longer injector length configuration exhibited high-amplitude instabilities, with pressure fluctuations greater than 100% of the mean chamber pressure. This was due to the excitation of the fundamental transverse mode, with a frequency around 1800 Hz, as well as multiple harmonics. Shortening the injector length significantly lowered the instability amplitudes at all conditions and excited an additional mode near 1550 Hz for lower equivalence ratio cases. The delineating feature controlling the growth of the instabilities in the two injector configurations was shown to be the coupling between the transverse modes in the chamber and axial pressure fluctuations in the injectors.Heated fuels were introduced into the COMRAD experiment, and simultaneous 10 kHz stereoscopic particle image velocimetry and OH* chemiluminescence imaging were performed over a range of equivalence ratios and combustor pressures to study the influence of fuel temperature on the flow and flame structure. The main flame was found to move upstream as the fuel was heated, while no changes in the pilot flame location were observed in the field of view at the exit of the injector. The upstream shift of the main flame corresponded to a local increase in the axial velocity, which caused the shear layer between the pilot/main flames and the central recirculation zone to move downstream. Direct comparison of the mean velocity fields relative to the mean flame location showed that heating the fuel caused the velocity normal to the flame front to increase, which is indicative of an increase in flame speed. The changes to the fuel injection and chemical kinetics help explain the local changes to the flow and flame structure, which contribute to an overall increase in combustion efficiency as well as NOx emissions.Lastly, the effect of fuel injection temperature on the presence of an 800 Hz combustion instability in the COMRAD experiment was investigated. High-frequency pressure and highspeed chemiluminescence measurements revealed a decrease in the instability amplitude as the fuel was heated. The coupling between the fuel flow and the unsteady heat release was studied using independent 10 kHz stereoscopic particle image velocimetry and 10 kHz Mie scattering measurements
High-Speed Diagnostics in a Natural Gas-Air Rotating Detonation Engine at Elevated Pressure
No full textGas turbine engines have operated on the Brayton cycle for decades, each decade only gaining approximately one to two percent in thermal efficiency as a result of efforts to improve engine performance. Pressure-gain combustion in place of constantpressure combustion in a Brayton cycle could provide a drastic step-change in the thermal efficiency of these devices, leading to reductions in fuel consumption and emissions production. Rotating Detonation Engines (RDEs) have been widely researched as a viable option for pressure-gain combustion. Due to the extremely high frequencies associated with operation of an RDE, the development and application of high-speed diagnostics techniques for RDEs is necessary to further understand and develop these devicesAn application of high-speed diagnostic techniques in a natural gas-air RDE at conditions relevant to land-based power generation is presented. Diagnostics included high-frequency chamber pressure measurements, chemiluminescence imaging of the annulus, and Particle Image Velocimetry (PIV) measurements at the exit plane of the RDE. Results from a case with two detonation waves rotating clockwise (aft looking forward) in the combustor annulus are presented. Detonation surface plots are created from chemiluminescence images and allow for the extraction of properties such as dominant frequency modes and wave number, speed, and direction. The chamber frequency for the case with two co-rotating waves in the chamber is found to be 3.46 kHz and corresponds to average individual wave speeds of 68% Chapman-Jouguet (CJ) velocity. Dynamic Mode Decomposition (DMD) is applied and indicates the presence of two strong detonation waves rotating clockwise and periodically intersecting with weaker, counter-rotating waves in the annulus at certain times during operation. Singular-Spectrum Analysis (SSA) is used to isolate modes corresponding to the detonation frequency in the signals of velocity components obtained from PIV, maintaining instantaneous phase information. Axial and azimuthal components of velocity are observed to remain nearly 180◦ out of phase. Lastly, approximate angles for the trailing oblique shocks in the combustion chamber are calculated
Characterization of the Secondary Combustion Zone of a Solid Fuel Ramjet
No full textA research-scale solid-fuel ramjet test article has been developed to study the secondary combustion zone of solid fuel ramjets. Tests were performed at a constant core air mass flowrate of 0.77 kg/s with 0%, 15%, and 30% bypass ratios. The propulsive performance analysis results indicate that the 0% bypass case had the highest regression rate and fuel mass flowrate. The regression rate and fuel mass flowrate of fuel without carbon black was the lowest. The specific impulse with air mass flowrate included was highest for the 0% bypass case reaching 130 s and lowest for the 30% bypass case reaching 110 s. For specific impulse with air mass flowrate excluded, the 30% bypass case achieved 2,800 s while the 0% bypass case achieved 1,800 s. The characteristic velocity was greatest for 0% bypass reaching 1,025 m/s and lowest for 30% bypass reaching 900 m/s. The combustion efficiency was highest for the 15% bypass case with carbon black addition approaching 0.82. 50 kHz and 75 kHz CH* chemiluminescence imaging was performed. Analyzing thin slivers of the images over 40,001 frames with frequency-domain techniques showed that most of the high amplitude content occurred below 1-5kHz with small peaks near 20 kHz and 30 kHz. Dynamic mode decomposition (DMD) was performed on sets of 10,001 spatially-calibrated images and their corresponding uncalibrated, uncropped images. Most of the tests exhibited low-frequency axial pumping, transverse modes, and other mode shapes indicative of the secondary injection. The prominence of transverse and other jet-related modes over axial modes appeared to be related to increasing bypass ratio. High-frequency axial modes also appeared in a case thought to have high core-flow momentum that did not appear at these high frequencies for other cases. The DMD modes for 0% bypass were indiscernible due to high soot content. Most of the modes corresponding to the calibrated images also appeared in the uncalibrated images, however, with different mode amplitude rankings. PIV was performed at 5 kHz for one test at 15% bypass. The instantaneous vector fields for these tests displayed local velocities up to 600 m/s. The mean images showed velocities up to 250 m/s. The two-dimensional turbulent kinetic energies reached 200 m2/s2 in several regions throughout the flowfield. The turbulence intensity exceeded 0.20 near the bottom of the flowfield
Heat transfer augmentation in a rectangular duct characterized by an impinging jet inlet : design of experiment
No full textEnergy is one of the most important engineering challenges of this time. Gas turbine engines a,re responsible for nearly twenty-percent of all electricity produced in the United States today. A small increase in the operating efficiency of these engines could lead to massive reduction in the emission of greenhouse gases into the atmosphere as well as the financial burden on the average homeowner paying the monthly energy bill. In order to improve the efficiency of the engine, the Turbine Inlet Temperature of the hot gas coming from the combustor is continually increased. This requires increasingly advanced active cooling methods to maintain component life in the hot stages of the turbo machine.
In this study, a complete experiment is developed for accurate testing of the complex heat transfer and aerodynamic characteristics present in the active cooling design applied to the transition duct of a land-based gas turbine. The transition duct is the component that channels the hot gases from the combustor to the first stage of the turbihe. It is in contact with the hottest mainstream gas flow in the entire machine. The unique cooling design applied to this component is a combination the three main cooling methods. It is characterized by an impinging jet inlet, which splits into two identical channels flowing in exactly opposite directions. The flow travels through these channels, cooling the hot surfaces of the duct through which they are formed. At the flow exit, it is expelled into the hot gas stream flowing from the can-annular combustor to the turbine stage. The channel exit provides a thin film of cool air coverage that protects the metal surface from the harsh temperatures of the hot gas
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
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