1,720,961 research outputs found
The effect of oxygen and carbon dioxide concentration on soot formation in non-premixed flames
No full textThe influence of oxygen concentration and carbon dioxide as diluents in the oxidizer side on soot formation was studied by Time Resolved Laser Induced Incandescence (TIRE-LII) and TEM photography in non-premixed co-flowing flames. TIRE-LII method was used to measure the distribution of two-dimensional soot volume fraction and primary particle size. The soot was directly sampled by the thermophoretic method, and its diameter was examined by TEM photography. Two suitable delay times of the TIRE-LII method affecting measurable range and sensitivity were determined by comparing TEM photographs with the TIRE-LII signal. The effects of oxygen concentration and carbon dioxide as diluents in the oxidizer side on soot formation were investigated with these calibrated techniques. An O-2 + (CO2, N-2, and [Ar + CO2]) mixtures in co-flow were used to isolate carbon dioxide effects systematically. The primary particle number concentration and soot volume fraction were abruptly decreased by the addition of carbon dioxide to co-flow. This suppression was resulted from the short residence time in inception region because of the late nucleation and the decrease of surface growth distance by the low flame temperature due to the higher thermal capacity and the chemical change of carbon dioxide. The increase of oxygen concentration in the co-flow caused an enhancement of soot nucleation and thus the residence time increase, but the specific growth rate showed almost the same value regardless of the co-flow mixture in the growth region. This result suggests that the specific growth rate has a weak dependence on the relative change of co-flow conditions in non-premixed co-flowing flames. (C) 2005 Elsevier Ltd. All rights reserved.This research was supported by the Korea Science and Technology Foundation (KOSEF) through the Combustion Engineering Research Center (CERC)
Experiments on the transient effect of evolving jet diffusion flames
No full textThe unsteady behavior of diffusion flames was experimentally studied with a jet diffusion flame under a configuration similar to counterflow. We designed and conducted our experiments in such a manner that the Fuel was ejected downward so that various time histories of strain rate with good reproducibility were obtained. The Fuel jet was ignited by means of a residual flame sustained at the nozzle tip, and the developing process was visualized by both Mie scattering and high-speed shadow imaging. The maximum flame temperature was measured by a compensated thermocouple on the flame tip. ri new method was adopted for the time constants. The results show that the history of the strain rate has a significant role in the unsteady behavior of laminar diffusion flames, especially close to flame extinction. Regarding of quenching histories, the extinction strain rates extend as the slope of the strain rate increases, which has riot previously been explored experimentally Even in the non-quenched flame. an unsteady effect is shown based on the fact that there are two maximum flame temperatures at the same strain rate and the one experiencing the higher strain rate is always lower
Generation of low-frequency alternative flame Behaviors in a lean premixed combustor
No full textCombustion instability causes severe problems, Such as fatal damage to lean premixed gas turbine combustors and shortening the lifetime of the overall system. In this study,, to understand the instability phenomena, an experimental study and a numerical simulation were conducted in a dump combustor with respect to the modulation of the fuel flow, that is, the choked fuel flow and the unchoked fuel flow. The fluctuations of pressure and heat release were measured by a piezoelectric pressure sensor and high-speed intensified charge-coupled device (ICCD) camera, respectively. Various combustion modes occurred in accordance with the equivalence ratio and the fuel Supply conditions. In the low-frequency instability mode, alternative flame behaviors were observed and result from the large equivalence ratio modulation. It is found that, especially in an unchoked fuel flow condition, the modulation of the fuel flow rate affects the characteristics of flame behavior and pressure fluctuations in a lean premixed flame.This work was supported by the Korea
Science and Technology Foundation through the Combustion Engineering Research Center (CERC) at the Korea Advanced Institute of Science and Technology, as well as by Mitsubishi Heavy Industries, Ltd., Japan
Time-dependent measurements of flame temperature and the OH radical in the unsteady extinction of non-premixed flames
No full textThe extinction point, time-dependent flame temperature, relative [OH], and the instantaneous luminosity of a flame during the unsteady extinction process were measured in non-premixed counterflow flames, and characteristics of each parameter near the extinction limit were investigated. We found that the unsteady extinction point is much higher than the steady extinction point and OH radical is a more adequate indicator of extinction than temperature, especially for turbulent and unsteady flames. (c) 2005 The Combustion Institute. Published by Elsevier Inc. All rights reserved
Effect of the unmixedness of unburned gases on the pressure fluctuations in a dump combustor
No full textCombustion instability is a serious obstacle for the lean premixed combustion of gas turbines and can even cause fatal damage to the combustor and the entire system. Thus, enhanced understanding of the mechanisms of combustion instability is necessary for designing and operating gas turbine combustors. In this study, in order to elucidate the instability phenomena, an experimental study was conducted in a rearward-step dump combustor with LPG and air. The fuel supply conditions and the mixing distances (L-fuel) between fuel and air are used as experimental parameters to examine the effects of fuel modulation and unmixedness. The fluctuations of pressure, heat release, and equivalence ratio were measured by a piezoelectric pressure sensor and a high speed intensified charge coupled device (ICCD) camera, respectively. The unmixedness was measured by acetone laser induced fluorescence (LIF) at nonreacting flow because of stratification of the fuel in air. Various combustion modes occurred in accordance with the equivalence ratio and the fuel supply conditions. In the case of the fully premixed condition, the spatial fuel distribution inside the combustion chamber exists in a homogeneous state compared with the partially premixed condition, which leads to instant heat release, with relatively greater intensity for the chemiluminescence of the flame and higher amplitude of pressure fluctuations. On the contrary, in the partially premixed condition, the spatial fuel distribution exists in a heterogeneous state from the combustion chamber entrance, and the flame's burnout or reignition progresses in accordance with the stratified distribution of fuel. The unmixedness of fuel and air leads to a relatively smaller intensity for the chemiluminescence of the flame and reduced amplitude of pressure fluctuations.This work was supported by the Korea
Science and Technology Foundation through the Combustion Engineering Research Center (CERC) at the Korea Advanced Institute of Science and Technology, as well as by Mitsubishi Heavy Industries, Ltd., Japan
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe 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
Extinction limit extension of unsteady counterflow diffusion flames affected by velocity change
No full textThe unsteady extinction limit of (CH4 + N-2)/air diffusion flames was investigated in terms of the time history of the strain rate and initial strain rates. A spatially locked flame in an opposed-jet counterflow burner was perturbed using linear velocity variation, and time-dependent flame luminosity and unsteady extinction limits were measured with a high-speed intensified CCD (ICCD) camera. In addition, the transient maximum flame temperature and hydroxyl (OH) radical were measured as a function of time using Rayleigh scattering and OH laser-induced fluorescence, respectively. In this experiment, unsteady flames survive at strain rates that are much higher than the extinction limit of steady flames and unsteady extinction limits increase as the slope of the strain rate increases or as the initial strain rate decreases. We found that the equivalent strain rate represents well the unsteady behavior in the outer convective-diffusive layer of the flame. By using the equivalent strain rate, we were able to accurately estimate the contribution of the unsteady effect in the Outer convective-diffusive layer to the extinction limit extension, and we also identified the unsteady effect in the inner diffusive-reactive layer of the flame. Consequently, the extension of unsteady extinction limits results from the unsteady effects of both the convective-diffusive layer and the diffusive-reactive layer. The former effect is dominant at the beginning of the velocity change, and the latter effect is dominant near the extinction limit. (c) 2005 The Combustion Institute. Published by Elsevier Inc. All rights reserved.This research was supported by the Korea Science and Technology Foundation (KOSEF) through the Combustion Engineering Research Center (CERC) of KAIST
Variations on the Author
Full text link“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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