1,720,964 research outputs found
Extinction of a premixed flame by a large variation in axial velocity
No full textUnsteady flame propagation in a tube is examined by introducing a mean velocity variation larger than the burning velocity to a stabilized flame for a period longer than the reaction time scale. In our previous work, stabilized propane-air flames were classified as either one-dimensional or two-dimensional flames. The eventual extinction during the velocity increase was categorized as either acoustic extinction or boundary layer extinction. In this work, the effects of a nonunity Lewis number were estimated through experiments with a methane-air flame; the eventual extinction during the velocity decrease was investigated in more detail; and the growth of the extinction boundary layer was analyzed with a transient one-dimensional model of the flame stretch. In our experiments, the Lewis number did not affect the existence or characteristics of the critical velocity and the characteristic time for boundary layer extinction. An additional critical velocity was found, however, for acoustic extinction when the Lewis number was smaller than unity. In the transient one-dimensional model, the velocity transition along the flame was calculated with a continuity equation and an axial momentum equation. The spatial gradient of the burning velocity and the extinction criterion were simplified with the experimental results and some theoretical studies. The analysis shows that the unsteady flame stretch at the flame edge during a large axial velocity variation is the prevailing cause of the growth of the extinction boundary layer. These results provide some evidence that flame stretch affects the behavior of the flame edge; they also suggest the cause of the finger flame. The findings help explain the unsteady behavior of premixed flames near a flammability limit. (C) 2003 The Combustion Institute. Published by Elsevier Inc. All rights reserved
Laminar premixed flame propagation using large axial velocity variation
No full textA stabilization condition for the premixed flame in a tube was investigated and then a mean velocity variation larger than the burning velocity was introduced to the stabilized flame for a period longer than the reaction time scale in order to examine the unsteady behavior of flame propagation. The stabilized flame were classified into two regimes, a one-dimensional regime and a two-dimensional regime, by both the shape and the mass consumption rate. The magnitude and period of the mean velocity variation were treated as experimental parameters. When the large velocity variation was introduced in the same direction as die initial mean velocity, the extinction behaviors were observed and systematically classified into two groups: extinction by boundary layer and extinction by acoustic instability We found out that there exists a critical velocity change and a critical time above which the extinction region develops to the center of the tube and extinguishes the flame. With the velocity variation in the opposite direction of the initial mean velocity, the flame was not extinguished near the wall, and the characteristics of the flame propagation were similar to those of earlier studies on flame propagation. The mechanism of die extinction near the wall is explained by the flame stretch theory, which provides a clue to the stretch effects on the finger flame
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
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
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
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe 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
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
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