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The combined effects of velocity fluctuations (VF) and equivalence ratio fluctuations (ERF) on thermoacoustic flame oscillations are studied experimentally in a gas turbine model combustor. The analysis is based on time-resolved simultaneous measurements of flow field, flame structure and fuel distribution using combined PIV, OH-PLIF and tracer-PLIF at a rate of 10 kHz. In order to separate the effects of VF and ERF, the combustor has been operated in two configurations: a technically (i.e. partially) premixed (TP) configuration where both ERF and VF occur, and a perfectly premixed (PP) configuration where only VF are present. A particular operating condition is selected where the VF are similar for both premixing modes. The corresponding fluctuations of heat release, by contrast, exhibit considerably different spatial patterns for the two modes. In particular, periodic widening and narrowing as well as extinction and re-ignition of the flame base, and a convective motion of the flame zone are observed in the TP flame but not in the PP case. An evaluation of periodic variations of flux rates and heat release shows that the feedback loop of the TP flame includes an additional convective delay of 50°. The time-resolved measurements reveal that this delay leads to a coincidence of fuel-lean unburned gas and high velocity that induces local flame extinction and narrowing of the flame base. When the equivalence ratio later increases, the flame base re-ignites and a widened zone of increased heat release forms that is convected downstream. For the PP case, by contrast, reaction at the flame base is stable and accordingly the flame responds earlier to VF compared to the TP case. The results show that interacting VF and ERF have a strong and complex impact on the thermoacoustic response of turbulent swirl flames, and this impact depends largely on their time-delay. [ABSTRACT FROM AUTHOR]

Unsteady interactions of flow, fuel–air mixing and reaction in a lean partially-premixed turbulent swirl flame are investigated using simultaneous particle image velocimetry and planar laser-induced fluorescence of OH and acetone with a repetition rate of 10 kHz. The flame is operated with methane and air in a gas turbine model combustor at ambient temperature and pressure. Transport and mixing of fuel is visualized by fluorescence of acetone, which is added with 9% by volume to the methane. The dominant unsteady flow structures are a precessing vortex core (PVC) in the shear layer of the inner recirculation zone and the lower stagnation point (LSP) at the flame root. The measurements show that fuel and air are largely separated at the inlet of the combustion chamber, but are then strongly mixed by the PVC. A well-mixed zone of unburned fuel and air is formed around the vortex, and then ignited by recirculating burned gas. At the flame root, the PVC induces periodic changes in the composition of the unburned gas, which varies between pure air and well-mixed fuel and air. Reaction is locally quenched at the LSP when fresh air without fuel is present, and re-ignition takes place when mixed fuel and air arrive at the boundary of recirculating burned gas. Generally, the results show that the enhancement of fuel–air mixing induced by the PVC contributes significantly to the stabilization of the flame, and that the flame dynamics can only be properly understood when an analysis of transient mixing mechanisms is included. [ABSTRACT FROM AUTHOR]

This study was designed to test the feasibility of acquiring simultaneous PIV/OH-PLIF measurements at multi-kHz rates in a turbulent swirl flame at pressures relevant to modern industrial gas turbine combustors. To accomplish this, particle image velocimetry (PIV) and planar laser-induced fluorescence of the hydroxyl radical (OH-PLIF) were applied simultaneously at 3 kHz to study the dynamics of a lean partially-premixed turbulent swirl-stabilized flame of natural gas in an optically accessible, high-pressure combustion test rig at 5 bars. With 0.25 mJ/pulse at 283 nm for the OH-PLIF measurements, an average signal-to-noise ratio (SNR) of 4.1 was achieved over a region measuring 20 × 80 mm. Absorption of the excitation laser proved to be the greatest challenge in this study, resulting in a significant variation in SNR from one side of the OH-PLIF images to the other. A procedure based on modeling the absorption according to the mean OH-distribution was used to semi-quantitatively correct for this effect. A gradient-based edge-detection algorithm was used to identify reaction zone locations in the resulting images. These were used to compute mean distributions of the flame surface density. With 2.5 mJ/pulse at 532 nm for the PIV system, velocity fields measuring 20 × 80 mm were measured at a resolution of 1.25 mm. Consistent with prior measurements in the burner, the flame shows strong thermo-acoustic pulsation, with a peak frequency of 388 Hz. Phase-averages of the PIV and OH ∗ data indicate these pulsations are driven by the same resonant feedback mechanism responsible for thermo-acoustic pulsation in the burner at atmospheric pressure. No evidence of a precessing vortex core, known to dominate the flow-field of the burner at atmospheric-pressure conditions, was observed. [ABSTRACT FROM AUTHOR]

In this investigation the physics of the reacting swirling flow of a commercial industrial gas turbine burner (SGT-100) was researched using combustion Large Eddy Simulation (LES) and experiments. In the experimental studies the flow field, temperature and major species concentrations were measured using Particle Image Velocimetry (PIV), OH Planar Laser-Induced Fluorescence (OH-PLIF), one-dimensional laser Raman scattering and OH chemiluminescence imaging. For the finite-rate chemistry LES, two global and two skeletal reaction mechanisms were utilized to evaluate the accuracy and tradeoffs of global and skeletal reaction mechanisms. This type of assessments has previously been carried out for simple flames but not for industrial flames at laboratory conditions with detailed measurement data. The LES predictions generally show very good agreement with the experimental data for the flow field, temperature and major species. Different reaction mechanisms do not affect the flow field as much as the temperature and species profiles, which show clear imprints of the selected reaction mechanism. The results further indicate that the industrial flame is best captured with the skeletal reaction mechanisms, whilst the global mechanisms predict too compact flames. The results from the skeletal reaction mechanisms are then used in conjunction with the experimental data to assess the flame characteristics which best can be described as interacting flamelets embedded in an environment of distributed reaction zones. [ABSTRACT FROM AUTHOR]

An interview with performance artist Ann Magnuson is presented. When asked about her feeling being typecast as the unliked career woman, she mentions that she always tries to be practical and is very strict to her work. Magnuson believes that all performance environments are variations of a church. She also states that had a positive experience growing up with Christianity and Pentecostal folks in the churches represented on local radio and television.

Abstract: This paper describes an experimental study on the propagation of premixed flames through a flow with well-defined turbulence characteristics. In this study, multi-plane OH planar laser-induced fluorescence (OH-PLIF) and stereoscopic particle image velocimetry (SPIV) were applied simultaneously at 10kHz to measure the local three-dimensional displacement velocity of unstabilized, freely propagating flames passing through a premixed flow of methane and air. The dual-plane OH-PLIF data was used to track both reaction zone location and flame-normal orientation, while SPIV was used to track the three-component velocity field. The vectorial difference of the two produces a direct, time-resolved measurement of the local 3D displacement velocity of the flame. The use of unstabilized, freely propagating flames eliminates spatiotemporal dependence of local 3D displacement velocity associated with burner- or aerodynamically-stabilized test flames. Statistics of local 3D displacement velocities are presented. These show values ranging from slightly negative up to approximately eight times the laminar flame speed. Instantaneous flame-normal orientation with respect to the direction of local fluid velocity is identified as one parameter affecting instantaneous displacement velocities. [Copyright &y& Elsevier]

Abstract: The dynamics of major flow structures were studied in a gas turbine model combustor for perfectly premixed swirl-stabilized flames under a variety of reacting and non-reacting conditions using high-repetition-rate laser diagnostics. The studied combustor is a target case for the International Workshop on Advanced Measurement Techniques and Computational Methods for Premixed and Partially Premixed Combustion. Measurements were taken of the three-component velocity field, OH planar laser induced fluorescence, and OH∗ chemiluminescence at a rate of 10kHz for nine different flow conditions, covering a range of thermal powers (P th =10–35kW) and equivalence ratios (ϕ =0.65–0.8). Under all non-reacting conditions, the dominant flow structure was a helical vortex core (HVC) that rotated around the burner at a frequency represented by a constant Strouhal number St H,NR =0.78. However, igniting the burner significantly altered the flow structures. At most conditions, the strength and frequency of the HVC increased relative to the corresponding non-reacting case. The HVC frequency in such cases was once again represented by a constant Strouhal number of St H,R =0.88, irrespective of the thermal power or equivalence ratio. The HVC frequency was considerably higher than the frequency of the self-excited thermo-acoustic oscillations exhibited by the burner. However, at other conditions, combustion prevented formation of the HVC. In such cases, the dominant flow structure dynamics were periodic shear layer oscillations and shedding of toroidal vortices at the thermo-acoustic frequency. Cases in which combustion prevented formation of the HVC included those at low thermal powers (P th ⩽15kW) and the highest equivalence ratio (ϕ =0.8). A distinct relationship was found between the flow structure geometry and the pressure oscillation amplitude, with cases having an HVC resulting in higher pressure oscillations. [Copyright &y& Elsevier]

Abstract: The mechanisms of the interaction between a precessing vortex core (PVC) and a turbulent swirl flame in a gas turbine model combustor are investigated experimentally using simultaneous PIV and OH-PLIF at repetition rates up to 10kHz. Three operating conditions with different flow rates corresponding to Damköhler numbers of 1, 0.5 and 0.3 are studied. The flames are mainly stabilized in the shear layer of the inner recirculation zone, where also the PVC is located. The measurements show that for all conditions, the PVC interacts strongly with the flame. Two main effects have been observed: On the one hand, the PVC leads to roll-up of the reaction zone at the flame base. This enhances the supply of heat and radicals to the unburned gas, and thus favors subsequent ignition of the unburned gas. On the other hand, the PVC causes considerable aerodynamic stretch of the reaction zones. This can lead locally to disruption, quenching or extinction of the flame. The strength of the two effects changes significantly when the Damköhler number changes. For the case Da=1, the flame roll-up due to the PVC leads to a strong increase of heat release around the vortex. The stretch rates are relatively low and do not strongly affect the reaction progress. In the case Da=0.5, the PVC initially also causes flame roll-up at the flame base. Subsequently, however, roll-up and propagation of the flame are affected by local disruptions and quenching of the reaction zone due to the increased vortex-induced aerodynamic stretch. Consequently, part of the heat release is delayed and shifted to locations further downstream. For Da=0.3, aerodynamic stretch rates further increase and the reaction zones are even more affected by local disruptions and quenching. The zone of heat release is therefore even further elongated in the direction of the flow. [Copyright &y& Elsevier]

Abstract: The structure and stabilization of heated hydrogen jet flames in heated cross-flows was experimentally investigated in a configuration that is analogous to terrestrial gas turbine components. Three flames, with jet velocities ranging from 100 to 200m/s, were investigated using particle image velocimetry and OH planar laser induced fluorescence in a total of 11 x–y and y–z planes. Additionally, laser Raman scattering was performed in the 200m/s jet to characterize the thermo-chemical state. In all cases, the flame along the jet centerline plane consisted of two branches, one stabilized in the jet lee and one lifted above the jet trajectory. The positional stability of the lee-stabilized branch was greater in the higher jet velocity cases due to the larger and stronger recirculation zones created downstream of the injection point. The lifted flame branch was much more dynamic, with measured flame base axial positions ranging from the jet near field to the flame tip. This flame branch instantaneously resided downstream of regions with high extensive principal strain-rate, and the strain-rate significantly affected the thermo-chemical state. The Raman measurements indicated that the base of the lifted flame branch existed in locations where both tribrachial and/or stratified premixed flame behaviors are expected, depending on the instantaneous flame location. Accurately modeling these complex flame structures and flow-flame interactions therefore is necessary to properly simulate jet flames in cross-flows. [Copyright &y& Elsevier]

Abstract: Lean blowout (LBO) of a partially premixed swirl flame is studied using chemiluminescence imaging and simultaneous stereo-PIV and OH-PLIF measurements at repetition rates up to 5kHz. The flame, which is operated with methane and air in a gas turbine model combustor at atmospheric pressure, features a pronounced precessing vortex core (PVC) at the inner shear layer. In the first part of the study, the stabilization mechanism of the flame close to LBO is investigated. The fields of velocity and OH show that near LBO there are essentially two regions where reaction takes place, namely the helical zone along the PVC and the flame root around the lower stagnation point. The zone along the PVC is favorable to the flame due to low strain rates in the vortex center and accelerated mixing of burned and fresh gas. The flame root, which is located close to the nozzle exit, is characterized by an opposed flow of hot burned gas and relatively fuel-rich fresh gas. Due to the presence of high strain rates, the flame root is inherently unstable near LBO, featuring frequent extinction and reignition. The blowout process, discussed in the second part of the study, starts when the extinction of the flame root persists over a critical length of time. Subsequently, the reaction in the helical zone can no longer be sustained and the flame finally blows out. The results highlight the crucial role of the flame root, and suggest that well-aimed modifications of flow field or mixture fraction in this region might shift the LBO limit to leaner conditions. [ABSTRACT FROM AUTHOR]

Abstract: The dynamics of flame-hole reignition were studied experimentally in a turbulent non-premixed jet flame at (flame ‘DLR-B’ from the TNF workshop). Simultaneous measurements of the OH combustion radical and velocity field were performed using planar laser induced fluorescence (PLIF) and particle image velocimetry (PIV) at a sustained rate of 10kHz. The dynamics of the reignition process were tracked through time and two reignition mechanisms were identified. Particular care was taken to reduce the influence of out-of-plane motion on the analyzed events by simultaneously measuring the OH distribution in crossed planes. Flame-holes reignited due to both edge-flame propagation and turbulent transport of burning flame segments. However, the edge-flame propagation mechanism was dominant and accounted for over 90% of the flame-hole reignition rate on average. Furthermore, the presence of large scale turbulent structures adjacent to a flame-hole did not necessarily result in reignition due to turbulent transport. Instead, the edge-flames propagated around the perimeter of such structures, indicating intervening regions of well mixed gas. The range of measured edge-flame propagation speeds agreed well that of highly-preheated premixed flames, with a mode of approximately 4m/s and a mean of approximately 7m/s. [ABSTRACT FROM AUTHOR]

Abstract: 2-D projections of turbulent non-premixed edge flame propagation velocities following spark ignition have been measured in the opposed-jet geometry using simultaneous high-speed OH-PLIF/PIV. The difference in the thickness of the edge between the flame sheet from the OH-PLIF images and the evaporated droplets from the PIV images was about 1–1.5mm, and generally did not effect the calculation of the absolute flame speed, V abs. The local flow velocity in the direction of the flame velocity was calculated and resulted in a measurement of the relative propagation velocity, V r. It was found that, after an initial transient due to the plasma expansion, the mean 〈V abs〉 increased with time and radial distance as the flame travelled outwards and 〈V abs〉 increased with the bulk velocity U b. The fluctuations in V abs also increased with U b. The mean relative propagation velocity, 〈V r〉, for all flames was about 0.75 S L, but increased slightly to 1 S L by the inner burner rim. The RMS of V r was close to 〈V r〉. The PDF of V r was wide with very little content above 3 S L and a non-negligible content at negative relative speeds, i.e. retreating fronts. The measured statistics are in good agreement with 3-D DNS results. Separate analysis of images from successful and failed ignition events shows that the PDF of V r for the failed ignition has relatively lower values than the successful. This PDF peaks at about 0.25 S L, with higher probability of negative V r. The measured value of 〈V r〉/S L is a new result and provides insight on the time taken to fully ignite non-premixed combustion in applications such as the relight of gas turbines. [Copyright &y& Elsevier]

Abstract: Velocity fields and flame structures of a partially premixed swirl flame in a gas turbine model combustor are measured in axial and transverse sections using simultaneous (Stereo-)PIV and OH-PLIF. The flame, operated under atmospheric pressure with air and methane at a thermal power of 10.3kW and a global equivalence ratio of ϕ =0.75, features thermoacoustic oscillations at a frequency f ≈295Hz. The averaged flow field with inner and outer recirculation zones is typical of swirl-stabilized flames, and the instantaneous measurements show the presence of a helical vortex (PVC) located in the inner shear layer. The PVC, which rotates with a different frequency than the thermoacoustic oscillation, leads to an enhanced mixing of burned and unburned gas and thus to stabilization of the flame. Two distinct large-scale structures of velocity and OH are found in the transverse cross-sections. The first type is characterized by a roughly annular region of inflowing unburned gas and distinct inner and outer recirculation zones. In the second type, the region of positive axial velocity forms a spiral, and the recirculation zone consists of an inner region that is connected to the outer parts by a narrow curved zone along the spiral arm. Whereas the first type is a typical scenario of vortex breakdown with a PVC, the transient spiral recirculation zone observed here has, to our knowledge, not yet been reported. A phase-resolved analysis shows that the annular form correlates with low, and the spiral form with high rates of global heat release. [Copyright &y& Elsevier]

Abstract: Time-resolved particle image velocimetry (PIV) and planar laser-induced fluorescence (PLIF), both at 5kHz, were applied simultaneously on extinguishing turbulent opposed jet flames. This repetition rate allowed tracking of transient extinction events in turbulent combustion. The additional information acquired about time history enabled a study of the evolution of vortex-flame interactions leading to extinction from individual events. A newly introduced multidimensional conditioning technique to avoid spatial- and temporal-smearing of important flow field information was developed in order to compare individual extinction events in a meaningful, statistical manner. The conditional statistics show that vortices tend to align around the flame and generate regions of high strain in the region where the flame is about to extinguish. [Copyright &y& Elsevier]

Abstract: Planar laser-induced fluorescence (PLIF) and stereoscopic particle image velocimetry (PIV) were simultaneously applied at 1.5kHz repetition-rate to acquire time-resolved, cinematographic planar measurements of the flamebase stabilization region of a turbulent lifted jet flame. The current work focuses on the development and demonstration of the diagnostic system and presents key examples of the new capabilities it provides. OH-PLIF is used to identify and track the spatial position and shape of the flamefront near the flamebase, while the stereoscopic PIV is used to acquire three-component planar velocity measurements over the same region. Initial results indicate that the significant temporal histories and experimental access to the Vz-velocity component afforded by this system are frequently essential for accurate interpretation of planar measurements in the flow field of a lifted turbulent jet flame. [Copyright &y& Elsevier]

Cryptographic embedded systems are vulnerable to Differential Power Analysis (DPA) attacks. In this paper, Grain-128 stream cipher is implemented using SABL hiding technique. The cipher is designed by BSim 130nm technology and simulated by HSpice software. Simulations show DPA resistivity of SABL implementation of Grain-128 has a major improvement. The paper presents the tradeoffs involved in designing the architecture, the design for performance issues and the possibilities for future development. [ABSTRACT FROM AUTHOR]

SYNOPSIS: The number of total knee arthroplasty (TKA) surgeries performed each year is predicted to steadily increase. Following TKA surgery, self-reported pain and function improve, though individuals are often plagued with quadriceps muscle impairments and functional limitations. Postoperative rehabilitation approaches either are not incorporated or incompletely address the muscular and functional deficits that persist following surgery. While the reason for quadriceps weakness is not well understood in this patient population, it has been suggested that a combination of muscle atrophy and neuromuscular activation deficits contribute to residual strength impairments. Failure to adequately address the chronic muscle impairments has the potential to limit the long-term functional gains that may be possible following TKA. Postoperative rehabilitation addressing quadriceps strength should mitigate these impairments and ultimately result in improved functional outcomes. The purpose of this paper is to describe these quadriceps muscle impairments and discuss how these impairments can contribute to the related functional limitations following TKA. We will also describe the current concepts in TKA rehabilitation and provide recommendations and clinical guidelines based on the current available evidence. LEVEL OF EVIDENCE: Therapy, level 5. [ABSTRACT FROM AUTHOR]

Abstract: A 1D laser Raman system for the simultaneous measurement of the major species concentrations, mixture fraction, and temperature in gas turbine-like flames is presented. The adaptation of the measuring technique to the test rig and the particular challenges of the measurements are described. The gas turbine model combustor was operated with natural gas and preheated air at pressures of 2 and 10 bar. The measurements characterized the mixing and reaction progress and revealed strong effects of turbulence-chemistry interactions with large deviations from chemical equilibrium. The single-shot data obtained along the imaged line of 7mm allowed the investigation of spatial correlations. Together with the results from further measurements with different methods, the Raman data form an experimental data base that is used for the validation of LES simulations. [Copyright &y& Elsevier]

Abstract: The mixing, reaction progress, and flame front structures of partially premixed flames have been investigated in a gas turbine model combustor using different laser techniques comprising laser Doppler velocimetry for the characterization of the flow field, Raman scattering for simultaneous multi-species and temperature measurements, and planar laser-induced fluorescence of CH for the visualization of the reaction zones. Swirling CH4/air flames with Re numbers between 7500 and 60,000 have been studied to identify the influence of the turbulent flow field on the thermochemical state of the flames and the structures of the CH layers. Turbulence intensities and length scales, as well as the classification of these flames in regime diagrams of turbulent combustion, are addressed. The results indicate that the flames exhibit more characteristics of a diffusion flame (with connected flame zones) than of a uniformly premixed flame. [Copyright &y& Elsevier]

Abstract: In this paper, we present a detailed experimental study of turbulence chemistry interactions in the “DLR_B” turbulent jet diffusion flame. The flame operates on mixtures of CH4, H2, and N2 in the fuel stream at Re =22,800 and is a target flame within the TNF workshop. Extinction and re-ignition events can be tracked in real time and related to the underlying flow field phenomena and temperature fields. Time resolved measurements of OH radical concentration fields are performed in combination with temperature and velocity field measurements. For this purpose, we combined high repetition rate (33kHz) PLIF imaging with stereoscopic PIV and double pulse Rayleigh imaging techniques. Comparisons are made with results from multi-scalar Raman/Rayleigh/LIF point measurements that reveal the thermochemical state of the flame. The large deviations from equilibrium observed on resulting OH/temperature joint pdfs could be related to strain rate and Damköhler number variations caused by turbulent flow structures leading to frequent extinctions. The 2D measurement series uniquely reveal the underlying mechanism that can lead to such events. Finally, comparisons are made to strained laminar flame calculations, which are generally found to be in good agreement with the measured data. [Copyright &y& Elsevier]

A swirled CH4/air diffusion flame has been operated in a gas turbine model combustor at the lean blow-off limit. Laser Doppler velocimetry, planar laser-induced fluorescence of OH and CH, and laser Raman scattering were applied in order to measure velocities, flame structures, and the joint probability density functions of major species concentrations, mixture fractions and temperatures. The main goals of the investigations were a better understanding of the complex physical and chemical processes in flames with swirl, e.g., mixing, stabilisation, heat release, and finite-rate chemistry effects, and the generation of an extensive data base for the validation of numerical flame codes. [ABSTRACT FROM AUTHOR]