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Articoli di riviste sul tema "Stretched Flames"

Autore: Grafiati

Pubblicato: 14 settembre 2024

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1

JU, YIGUANG, HONGSHENG GUO, KAORU MARUTA e FENGSHAN LIU. "On the extinction limit and flammability limit of non-adiabatic stretched methane–air premixed flames". Journal of Fluid Mechanics 342 (10 luglio 1997): 315–34. http://dx.doi.org/10.1017/s0022112097005636.

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Extinction limits and the lean flammability limit of non-adiabatic stretched premixed methane–air flames are investigated numerically with detailed chemistry and two different Planck mean absorption coefficient models. Attention is paid to the combined effect of radiative heat loss and stretch at low stretch rate. It is found that for a mixture at an equivalence ratio lower than the standard lean flammability limit, a moderate stretch can strengthen the combustion and allow burning. The flame is extinguished at a high stretch rate due to stretch and is quenched at a low stretch rate due to radiation loss. A O-shaped curve of flame temperature versus stretch rate with two distinct extinction limits, a radiation extinction limit and a stretch extinction limit respectively on the left- and right-hand sides, is obtained. A C-shaped curve showing the flammability limit of the stretched methane–air flame is obtained by plotting these two extinction limits in the mixture strength coordinate. A good agreement is shown on comparing the predicted results with the experimental data. For equivalence ratio larger than a critical value, it is found that the O-shaped temperature curve opens up in the middle of the stable branch, so that the stable branch divides into two stable flame branches; a weak flame branch and a normal flame branch. The weak flame can survive between the radiation extinction limit and the opening point (jump limit) while the normal flame branch can survive from its stretch extinction limit to zero stretch rate. Finally, a G-shaped curve showing both extinction limits and jump limits of stretched methane–air flames is presented. It is found that the critical equivalence ratio for opening up corresponds to the standard flammability limit measured in microgravity. Furthermore, the results show that the flammability limit (inferior limit) of the stretched methane–air flame is lower than the standard flammability limit because flames are strengthened by a moderate stretch at Lewis number less than unity.

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2

Clavin, Paul, e José C. Graña-Otero. "Curved and stretched flames: the two Markstein numbers". Journal of Fluid Mechanics 686 (28 settembre 2011): 187–217. http://dx.doi.org/10.1017/jfm.2011.318.

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AbstractThe analytical result concerning the Markstein number of adiabatic flames was obtained in 1982 with the one-step Arrhenius model in the limit of a large activation energy. This result is not relevant for real flames. The form of the law expressing the flame velocity in terms of the total stretch rate of the flame front through a single Markstein length is not conserved when the location of the front (surface of zero thickness) changes within the flame thickness. It is shown in this paper that two different Markstein numbers ${\mathscr{M}}_{I} \not = {\mathscr{M}}_{II} $ characterize usual wrinkled flames sustained by a multiple-step chemical network, ${\mathscr{M}}_{I} $ for the modification of the flame velocity due to the curvature of the front and ${\mathscr{M}}_{II} $ for the effect of the flow strain rate. In contrast to ${\mathscr{M}}_{I} $, ${\mathscr{M}}_{II} $ depends on the location of the flame surface within the flame thickness, in such a way that the final result for the flame dynamics is not depending on this choice. The first part of the paper is devoted to present a general method of solution, valid for any multiple-step chemical network. The two Markstein numbers for two-step chain-branching models representing rich hydrogen–air flames and lean hydrocarbon–air flames are then computed analytically in the second part.

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3

Ju, Yiguang, Kaoru Maruta e Takashi Niioka. "Combustion Limits". Applied Mechanics Reviews 54, n. 3 (1 maggio 2001): 257–77. http://dx.doi.org/10.1115/1.3097297.

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Combustion limits and related flame behaviors are reviewed, especially with regard to fundamental problems. As for premixed flames, after a brief historical overview of research on the flammability limit, recent trends of research on planar propagating flames, curved propagating flames, flame balls, and stretched premixed flames are discussed, and then all types of flames are summarized. Finally, instability and dynamics near limits is discussed. With regard to combustion limits of counterflow diffusion flames and droplet flames, their instability is demonstrated, then an explanation of lifted flames and edge flames is presented. Suggestions for future work are also discussed in the concluding remarks. There are 166 references cited in this review article.

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4

Law, C. K. "Dynamics of stretched flames". Symposium (International) on Combustion 22, n. 1 (gennaio 1989): 1381–402. http://dx.doi.org/10.1016/s0082-0784(89)80149-3.

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5

Mikolaitis, David W. "Stretched spherical cap flames". Combustion and Flame 63, n. 1-2 (gennaio 1986): 95–111. http://dx.doi.org/10.1016/0010-2180(86)90114-8.

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6

Thiesset, F., F. Halter, C. Bariki, C. Lapeyre, C. Chauveau, I. Gökalp, L. Selle e T. Poinsot. "Isolating strain and curvature effects in premixed flame/vortex interactions". Journal of Fluid Mechanics 831 (13 ottobre 2017): 618–54. http://dx.doi.org/10.1017/jfm.2017.641.

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This study focuses on the response of premixed flames to a transient hydrodynamic perturbation in an intermediate situation between laminar stretched flames and turbulent flames: an axisymmetric vortex interacting with a flame. The reasons motivating this choice are discussed in the framework of turbulent combustion models and flame response to the stretch rate. We experimentally quantify the dependence of the flame kinematic properties (displacement and consumption speeds) to geometrical scalars (stretch rate and curvature) in flames characterized by different effective Lewis numbers. Whilst the displacement speed can be readily measured using particle image velocimetry and tomographic diagnostics, providing a reliable estimate of the consumption speed from experiments remains particularly challenging. In the present work, a method based on a budget of fuel on a well chosen domain is proposed and validated both experimentally and numerically using two-dimensional direct numerical simulations of flame/vortex interactions. It is demonstrated that the Lewis number impact neither the geometrical nor the kinematic features of the flames, these quantities being much more influenced by the vortex intensity. While interacting with the vortex, the flame displacement (at an isotherm close to the leading edge) and consumption speeds are found to increase almost independently of the type of fuel. We show that the total stretch rate is not the only scalar quantity impacting the flame displacement and consumption speeds and that curvature has a significant influence. Experimental data are interpreted in the light of asymptotic theories revealing the existence of two distinct Markstein numbers, one characterizing the dependence of flame speed to curvature, the other to the total stretch rate. This theory appears to be well suited for representing the evolution of the displacement speed with respect to either the total stretch rate, curvature or strain rate. It also explains the limited dependence of the flame displacement speed to Lewis number and the strong correlation with curvature observed in the experiments. An explicit relationship between displacement and consumption speeds is also given, indicating that the fuel consumption rate is likely to be altered by both the total stretch rate and curvature.

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7

Avula, Murali, e Ishwar K. Puri. "Dioxin formation in stretched flames". Chemosphere 24, n. 12 (giugno 1992): 1785–98. http://dx.doi.org/10.1016/0045-6535(92)90233-h.

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8

Mokrin, Sergey, R. V. Fursenko e S. S. Minaev. "Thermal-Diffusive Stability of Counterflow Premixed Flames at Low Lewis Numbers". Advanced Materials Research 1040 (settembre 2014): 608–13. http://dx.doi.org/10.4028/www.scientific.net/amr.1040.608.

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Dynamics of radiative, near-limit, stretched premixed flames is investigated analytically and numerically. Investigation of counterflow premixed flames stability is important for the development of new combustion technologies such as those associated with low-NOx emission, lean burn and material synthesis. Emphasis is paid on the linear stability of multiple flame regimes. The present analysis, for the first time, gives out a dispersion equation describing growth rate of small spatial perturbations of the flame front. The stability diagram is obtained and the region of instability is distinguished.

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9

Yousif, Alaeldeen Altag, e Shaharin Anwar Sulaiman. "Experimental Study on Laminar Flame Speeds and Markstein Length of Methane-Air Mixtures at Atmospheric Conditions". Applied Mechanics and Materials 699 (novembre 2014): 714–19. http://dx.doi.org/10.4028/www.scientific.net/amm.699.714.

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Accurate value of laminar flame speed is an important parameter of combustible mixtures. In this respect, experimental data are very useful for modeling improvement and validating chemical kinetic mechanisms. To achieve this, an experimental characterization on spherically expanding flames propagation of methane-air mixtures were carried out. Tests were conducted in constant volume cylindrical combustion chamber to measure stretched, unstretched laminar flame speed, laminar burning velocity, and flame stretch effect as quantified by the associated Markstein lengths. The mixtures of methane-air were ignited at extensive ranges of lean-to-rich equivalence ratios, under ambient pressure and temperature. This is achieved by high speed schlieren cine-photography for flames observation in the vessel. The results showed that the unstretched laminar burning velocity increased and the peak value of the unstretched laminar burning velocity shifted to the richer mixture side with the increase of equivalence ratio. The flame propagation speed showed different trends at different equivalence ratio for tested mixtures. It was found that the Markstein length was increased with the increase of equivalence ratio.

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10

Ju, Yiguang, e Yuan Xue. "Extinction and flame bifurcations of stretched dimethyl ether premixed flames". Proceedings of the Combustion Institute 30, n. 1 (gennaio 2005): 295–301. http://dx.doi.org/10.1016/j.proci.2004.08.258.

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11

Wu, C. K., e C. K. Law. "On the determination of laminar flame speeds from stretched flames". Symposium (International) on Combustion 20, n. 1 (gennaio 1985): 1941–49. http://dx.doi.org/10.1016/s0082-0784(85)80693-7.

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12

Sinibaldi, Jose O., Charles J. Mueller e James F. Driscoll. "Local flame propagation speeds along wrinkled, unsteady, stretched premixed flames". Symposium (International) on Combustion 27, n. 1 (gennaio 1998): 827–32. http://dx.doi.org/10.1016/s0082-0784(98)80478-5.

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13

Detomaso, Nicola, Jean-Jacques Hok, Omar Dounia, Davide Laera e Thierry Poinsot. "A generalization of the Thickened Flame model for stretched flames". Combustion and Flame 258 (dicembre 2023): 113080. http://dx.doi.org/10.1016/j.combustflame.2023.113080.

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14

Kortsarts, Y., I. Brailovsky e G. I. Sivashinsky. "On Hydrodynamic Instability of Stretched Flames". Combustion Science and Technology 123, n. 1-6 (gennaio 1997): 207–25. http://dx.doi.org/10.1080/00102209708935628.

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15

Kortsarts, Y., I. Brailovsky, S. Gutman e G. I. Sivashinsky. "On the stability of stretched flames". Combustion Theory and Modelling 1, n. 2 (febbraio 1997): 143–56. http://dx.doi.org/10.1088/1364-7830/1/2/001.

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16

Liu, Changran, Ajay V. Singh, Chiara Saggese, Quanxi Tang, Dongping Chen, Kevin Wan, Marianna Vinciguerra et al. "Flame-formed carbon nanoparticles exhibit quantum dot behaviors". Proceedings of the National Academy of Sciences 116, n. 26 (10 giugno 2019): 12692–97. http://dx.doi.org/10.1073/pnas.1900205116.

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We examine the quantum confinement in the photoemission ionization energy in air and optical band gap of carbon nanoparticles (CNPs). Premixed, stretched-stabilized ethylene flames are used to generate the CNPs reproducibly over the range of 4–23 nm in volume median diameter. The results reveal that flame-formed CNPs behave like an indirect band gap material, and that the existence of the optical band gap is attributed to the highest occupied molecular orbital (HOMO)–lowest unoccupied molecular orbital (LUMO) gap in the polycyclic aromatic hydrocarbons comprising the CNPs. Both the ionization energy and optical band gap are found to follow closely the quantum confinement effect. The optical band gaps, measured both in situ and ex situ on the CNPs prepared in several additional flames, are consistent with the theory and the baseline data of CNPs from stretched-stabilized ethylene flames, thus indicating the observed effect to be general and that the particle size is the single most important factor governing the variation of the band gap of the CNPs studied. Cyclic voltammetry measurements and density functional theory calculations provide additional support for the quantum dot behavior observed.

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17

Pinchak, Matthew, Timothy Ombrello, Campbell Carter, Ephraim Gutmark e Viswanath Katta. "The effects of hydrodynamic stretch on the flame propagation enhancement of ethylene by addition of ozone". Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences 373, n. 2048 (13 agosto 2015): 20140339. http://dx.doi.org/10.1098/rsta.2014.0339.

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The effect of O 3 on C 2 H 4 /synthetic-air flame propagation at sub-atmospheric pressure was investigated through detailed experiments and simulations. A Hencken burner provided an ideal platform to interrogate flame speed enhancement, producing a steady, laminar, nearly one-dimensional, minimally curved, weakly stretched, and nearly adiabatic flame that could be accurately compared with simulations. The experimental results showed enhancement of up to 7.5% in flame speed for 11 000 ppm of O 3 at stoichiometric conditions. Significantly, the axial stretch rate was also found to affect enhancement. Comparison of the flames for a given burner exit velocity resulted in the enhancement increasing almost 9% over the range of axial stretch rates that was investigated. Two-dimensional simulations agreed well with the experiments in terms of flame speed, as well as the trends of enhancement. Rate of production analysis showed that the primary pathway for O 3 consumption was through reaction with H, leading to early heat release and increased production of OH. Higher flame stretch rates resulted in increased flux through the H+O 3 reaction to provide increased enhancement, due to the thinning of the flame that accompanies higher stretch, and thus results in decreased distance for the H to diffuse before reacting with O 3 .

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18

Shu, Tao, Yuan Xue, Wenkai Liang e Zhuyin Ren. "Extrapolations of laminar flame speeds from expanding spherical flames based on the finite-structure stretched flames". Combustion and Flame 226 (aprile 2021): 445–54. http://dx.doi.org/10.1016/j.combustflame.2020.12.037.

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19

YAMAMOTO, Kazuhiro, e Satoru ISHIZUKA. "Temperatures of Positively and Negatively Stretched Flames." JSME International Journal Series B 46, n. 1 (2003): 198–205. http://dx.doi.org/10.1299/jsmeb.46.198.

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20

Buckmaster, J. "The effects of radiation on stretched flames". Combustion Theory and Modelling 1, n. 1 (gennaio 1997): 1–11. http://dx.doi.org/10.1080/713665227.

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21

TEN THIJE BOONKKAMP, J. H. M., e L. P. H. DE GOEY. "A flamelet model for premixed stretched flames". Combustion Science and Technology 149, n. 1-6 (dicembre 1999): 183–200. http://dx.doi.org/10.1080/00102209908952105.

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22

Liakos, H. H., M. A. Founti e N. C. Markatos. "Modelling of stretched natural gas diffusion flames". Applied Mathematical Modelling 24, n. 5-6 (maggio 2000): 419–35. http://dx.doi.org/10.1016/s0307-904x(99)00052-9.

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23

Rotman, D. A., e A. K. Oppenheim. "Aerothermodynamic properties of stretched flames in enclosures". Symposium (International) on Combustion 21, n. 1 (gennaio 1988): 1303–12. http://dx.doi.org/10.1016/s0082-0784(88)80361-8.

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24

Law, C. K., D. L. Zhu e G. Yu. "Propagation and extinction of stretched premixed flames". Symposium (International) on Combustion 21, n. 1 (gennaio 1988): 1419–26. http://dx.doi.org/10.1016/s0082-0784(88)80374-6.

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25

Bechtold, J. K., e M. Matalon. "Effects of stoichiometry on stretched premixed flames". Combustion and Flame 119, n. 3 (novembre 1999): 217–32. http://dx.doi.org/10.1016/s0010-2180(99)00053-x.

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26

Huang, Z., J. K. Bechtold e M. Matalon. "Weakly stretched premixed flames in oscillating flows". Combustion Theory and Modelling 2, n. 2 (giugno 1998): 115–33. http://dx.doi.org/10.1088/1364-7830/2/2/001.

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27

Tien, J. H., e M. Matalon. "On the burning velocity of stretched flames". Combustion and Flame 84, n. 3-4 (aprile 1991): 238–48. http://dx.doi.org/10.1016/0010-2180(91)90003-t.

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28

Liang, Wenkai, Fujia Wu e Chung K. Law. "Extrapolation of laminar flame speeds from stretched flames: Role of finite flame thickness". Proceedings of the Combustion Institute 36, n. 1 (2017): 1137–43. http://dx.doi.org/10.1016/j.proci.2016.08.074.

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29

JOMAAS, G., C. K. LAW e J. K. BECHTOLD. "On transition to cellularity in expanding spherical flames". Journal of Fluid Mechanics 583 (4 luglio 2007): 1–26. http://dx.doi.org/10.1017/s0022112007005885.

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The instant of transition to cellularity of centrally ignited, outwardly propagating spherical flames in a reactive environment of fuelx–oxidizer mixture, at atmospheric and elevated pressures, was experimentally determined using high-speed schlieren imaging and subsequently interpreted on the basis of hydrodynamic and diffusional–thermal instabilities. Experimental results show that the transition Péclet number, Pec = RcℓL, assumes an almost constant value for the near-equidiffusive acetylene flames with wide ranges in the mixture stoichiometry, oxygen concentration and pressure, where Rc is the flame radius at transition and ℓL the laminar flame thickness. However, for the non-equidiffusive hydrogen and propane flames, Pec respectively increases and decreases somewhat linearly with the mixture equivalence ratio. Evaluation of Pec using previous theory shows complete qualitative agreement and satisfactory quantitative agreement, demonstrating the insensitivity of Pec to all system parameters for equidiffusive mixtures, and the dominance of the Markstein number, Ze(Le – 1), in destabilization for non-equidiffusive mixtures, where Ze is the Zel'dovich number and Le the Lewis number. The importance of using locally evaluated values of ℓL, Ze and Le, extracted from either computationally simulated one-dimensional flame structure with detailed chemistry and transport, or experimentally determined response of stretched flames, in the evaluation of Pec is emphasized.

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30

Fursenko, Roman, Sergey Minaev, Hisashi Nakamura, Takuya Tezuka, Susumu Hasegawa, Koichi Takase, Xing Li, Masato Katsuta, Masao Kikuchi e Kaoru Maruta. "Cellular and sporadic flame regimes of low-Lewis-number stretched premixed flames". Proceedings of the Combustion Institute 34, n. 1 (gennaio 2013): 981–88. http://dx.doi.org/10.1016/j.proci.2012.08.014.

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31

Chen, Yung-Cheng, Norbert Peters, G. A. Schneemann, N. Wruck, U. Renz e Mohy S. Mansour. "The detailed flame structure of highly stretched turbulent premixed methane-air flames". Combustion and Flame 107, n. 3 (novembre 1996): 223—IN2. http://dx.doi.org/10.1016/s0010-2180(96)00070-3.

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32

Daniele, S., J. Mantzaras, P. Jansohn, A. Denisov e K. Boulouchos. "Flame front/turbulence interaction for syngas fuels in the thin reaction zones regime: turbulent and stretched laminar flame speeds at elevated pressures and temperatures". Journal of Fluid Mechanics 724 (29 aprile 2013): 36–68. http://dx.doi.org/10.1017/jfm.2013.141.

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AbstractExperiments were performed in dump-stabilized axisymmetric flames to assess turbulent flame speeds (${S}_{T} $) and mean flamelets speeds (stretched laminar flame speeds, ${S}_{L, k} $). Fuels with significantly different thermodiffusive properties have been investigated, ranging from pure methane to syngas (${\mathrm{H} }_{2} \text{{\ndash}} \mathrm{CO} $ blends) and pure hydrogen, while the pressure was varied from 0.1 to 1.25 MPa. Flame front corrugation was measured with planar laser-induced fluorescence (PLIF) of the OH radical, while turbulence quantities were determined with particle image velocimetry (PIV). Two different analyses based on mass balance were performed on the acquired flame images. The first method assessed absolute values of turbulent flame speeds and the second method, by means of an improved fractal methodology, provided normalized turbulent flame speeds (${S}_{T} / {S}_{L, k} $). Deduced average Markstein numbers exhibited a strong dependence on pressure and hydrogen content of the reactive mixture. It was shown that preferential-diffusive-thermal (PDT) effects acted primarily on enhancing the stretched laminar flame speeds rather than on increasing the flame front corrugations. Interaction between flame front and turbulent eddies measured by the fractal dimension was shown to correlate with the eddy temporal activity.

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JU, YIGUANG, HONGSHENG GUO, FENGSHAN LIU e KAORU MARUTA. "Effects of the Lewis number and radiative heat loss on the bifurcation and extinction of CH4/O2-N2-He flames". Journal of Fluid Mechanics 379 (25 gennaio 1999): 165–90. http://dx.doi.org/10.1017/s0022112098003231.

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Effects of the Lewis number and radiative heat loss on flame bifurcations and extinction of CH4/O2-N2-He flames are investigated numerically with detailed chemistry. Attention is paid to the interaction between radiation heat loss and the Lewis number effect. The Planck mean absorption coefficients of CO, CO2, and H2O are calculated using the statistical narrow-band model and compared with the data given by Tien. The use of Tien's Planck mean absorption coefficients overpredicts radiative heat loss by nearly 30 % in a counter flow configuration. The new Planck mean absorption coefficients are then used to calculate the extinction limits of the planar propagating flame and the counterflow flame when the Lewis number changes from 0.967 to 1.8. The interaction between radiation heat loss and the Lewis number effect greatly enriches the phenomenon of flame bifurcation. The existence of multiple flames is shown to be a physically intrinsic phenomenon of radiating counterflow flames. Eight kinds of typical patterns of flame bifurcation are identified. The competition between radiation heat loss and the Lewis number effect results in two distinct phenomena, depending on if the Lewis number is greater or less than a critical value. Comparisons between the standard limits of the unstrained flames and the ammability limits of the counterflow flames indicate that the ammability limit of the counterflow flame is lower than the standard limit when the Lewis number is less than the critical value and is equal to the standard limit when the Lewis number is higher than this critical value. Finally, a G-shaped curve and a K-shaped curve which respectively represent the ammable regions of the multiple flames for Lewis numbers lower and higher than the critical value are obtained. The G- and K-shaped curves show a clear relationship between the stretched counterflow flame and the unstrained planar flame. The present results provide a good explanation of the physics revealed experimentally in microgravity.

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MINAEV, S., R. FURSENKO, Y. JU e C. K. LAW. "Stability analysis of near-limit stretched premixed flames". Journal of Fluid Mechanics 488 (10 luglio 2003): 225–44. http://dx.doi.org/10.1017/s0022112003004853.

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35

Sun, C. J., e C. K. Law. "On the nonlinear response of stretched premixed flames". Combustion and Flame 121, n. 1-2 (aprile 2000): 236–48. http://dx.doi.org/10.1016/s0010-2180(99)00132-7.

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36

Christiansen, E. W., e C. K. Law. "Pulsating instability and extinction of stretched premixed flames". Proceedings of the Combustion Institute 29, n. 1 (gennaio 2002): 61–68. http://dx.doi.org/10.1016/s1540-7489(02)80012-8.

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37

Kim, Tae Hyung, Jeong Park, Osamu Fujita, Oh Boong Kwon e Jong Ho Park. "Downstream interaction between stretched premixed syngas–air flames". Fuel 104 (febbraio 2013): 739–48. http://dx.doi.org/10.1016/j.fuel.2012.07.038.

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CHENG, Z., R. PITZ e J. WEHRMEYER. "Lean and ultralean stretched propane–air counterflow flames". Combustion and Flame 145, n. 4 (giugno 2006): 647–62. http://dx.doi.org/10.1016/j.combustflame.2006.02.006.

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SUENAGA, Yosuke, Hideki YANAOKA e Daisuke MOMOTORI. "Influences of stretch and curvature on the temperature of stretched cylindrical diffusion flames". Journal of Thermal Science and Technology 11, n. 2 (2016): JTST0028. http://dx.doi.org/10.1299/jtst.2016jtst0028.

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Sun, C. J., C. J. Sung, L. He e C. K. Law. "Dynamics of weakly stretched flames: quantitative description and extraction of global flame parameters". Combustion and Flame 118, n. 1-2 (luglio 1999): 108–28. http://dx.doi.org/10.1016/s0010-2180(98)00137-0.

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41

Hao, Guancheng, Bowen Pang, Qilin Zhang, Fei Cui, Sijia Sun e Shuo Liu. "Flame synchronization and flow field analysis of double candles". Journal of Physics: Conference Series 2247, n. 1 (1 aprile 2022): 012030. http://dx.doi.org/10.1088/1742-6596/2247/1/012030.

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Abstract Flame instability is an interesting topic in combustion science, and it is also of great practical significance for designing high-performance burners. In recent years, the synchronization phenomenon caused by a pair of coupled candles has aroused widespread attention, among which in-phase and anti-phase are two notable examples. In order to understand the flow structure of the flashing flame and the reasons for the synchronization of the flame oscillator, COMSOL flow field analysis technology and MATLAB grey analysis technology were used to analyze the flow field of candles in three combustion states and the change of candle combustion state respectively. By analyzing the flow structure of flashing flame, the reasons of different burning states of candles are explained. Moreover, the experimental and numerical simulation results show that the in-phase mode is characterized by the symmetrical formation of vortex concerning the centerline of two groups of flames, and the flames are vertically stretched under the vortex action. The characteristic of the anti-phase mode is that the vortex alternately forms asymmetrically concerning the centerline of two groups of flames, and the non-uniformity and asymmetry of the vortex lead to the instability of the flame surface. The characteristic of the incoherent mode is that the vortices generated by the two candle groups no longer act on each other, and the airflow field between the two candle groups remains approximately unchanged.

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42

Cheng, Zhongxian, Joseph A. Wehrmeyer e Robert W. Pitz. "Lean or ultra-lean stretched planar methane/air flames". Proceedings of the Combustion Institute 30, n. 1 (gennaio 2005): 285–93. http://dx.doi.org/10.1016/j.proci.2004.08.257.

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43

LIU, C. C., T. H. LIN e J. H. TIEN. "Extinction Theory of Stretched Premixed Flames by Inert Sprays". Combustion Science and Technology 91, n. 4-6 (giugno 1993): 309–27. http://dx.doi.org/10.1080/00102209308907651.

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44

Ju, Yiguang, Goro Masuya, Fengshan Liu, Yuji Hattori e Dirk Riechelmann. "Asymptotic analysis of radiation extinction of stretched premixed flames". International Journal of Heat and Mass Transfer 43, n. 2 (gennaio 2000): 231–39. http://dx.doi.org/10.1016/s0017-9310(99)00130-1.

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45

Ibarreta, Alfonso F., e James F. Driscoll. "Measured burning velocities of stretched inwardly propagating premixed flames". Proceedings of the Combustion Institute 28, n. 2 (gennaio 2000): 1783–91. http://dx.doi.org/10.1016/s0082-0784(00)80580-9.

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46

Mishra, D. P. "Numerical studies of stretched CH4–air cylindrical premixed flames". Fuel 83, n. 17-18 (dicembre 2004): 2345–50. http://dx.doi.org/10.1016/j.fuel.2004.05.012.

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47

Salusbury, Sean D., e Jeffrey M. Bergthorson. "Maximum stretched flame speeds of laminar premixed counter-flow flames at variable Lewis number". Combustion and Flame 162, n. 9 (settembre 2015): 3324–32. http://dx.doi.org/10.1016/j.combustflame.2015.05.023.

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48

Abdul Gani, Zeenathul, e N. Muthu Saravanan. "Computational analysis on the stability and characteristics of partially premixed butane air open flames in tubular burner". Thermal Science, n. 00 (2021): 320. http://dx.doi.org/10.2298/tsci210712320a.

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Abstract (sommario):

Partially premixed combustion is one of the developing areas of combustion research that has the advantages of both premixed and diffusion mode of combustion. The present work involves a computational study on the stability and characteristics of partially premixed butane-air flames. The effect of operating parameters like fuel-air ratio, primary aeration, and the presence of co-flow and co-swirl on the stability and flame characteristics has been studied. The simulation results show that the height of the flame decreases with an increase in primary aeration and also in the presence of a co-swirl stream. It has also been found that the stability of flames increases with co-swirl air but deteriorates with the presence of the co-flow air. The flame temperature increases with primary aeration and it has been observed that the peak flame temperature shifts away from the burner mouth for lower primary aeration. It has been observed that the flame stability improves with co-swirl air which is attributed to the recirculation zone created due to the swirl motion which acts as a heat source. The poor stability in the presence of co-flow air is attributed to flame stretching and aerodynamic quenching of the stretched flame lets. The lift off velocity and the stable operating range increases with equivalence ratio and also with co-swirl air.

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49

Devathi, Harshini, Carl A. Hall e Robert W. Pitz. "Numerical Study of the Structure and NO Emission Characteristics of N2- and CO2-Diluted Tubular Diffusion Flames". Energies 12, n. 8 (19 aprile 2019): 1490. http://dx.doi.org/10.3390/en12081490.

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Abstract (sommario):

The structure of methane/air tubular diffusion flames with 65 % fuel dilution by either CO2 or N2 is numerically investigated as a function of pressure. As pressure is increased, the reaction zone thickness reduces due to decrease in diffusivities with pressure. The flame with CO2-diluted fuel exhibits much lower nitrogen radicals (N, NH, HCN, NCO) and lower temperature than its N2-diluted counterpart. In addition to flame structure, NO emission characteristics are studied using analysis of reaction rates and quantitative reaction pathway diagrams (QRPDs). Four different routes, namely the thermal route, Fenimore prompt route, N2O route, and NNH route, are examined and it is observed that the Fenimore prompt route is the most dominant for both CO2- and N2-diuted cases at all values of pressure followed by NNH route, thermal route, and N2O route. This is due to low temperatures (below 1900 K) found in these highly diluted, stretched, and curved flames. Further, due to lower availability of N2 and nitrogen bearing radicals for the CO2-diluted cases, the reaction rates are orders of magnitude lower than their N2-diluted counterparts. This results in lower NO production for the CO2-diluted flame cases.

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50

Kitano, Michio, e Takanori Ohsuka. "Extinction Characteristics of Stretched Premixed Flames of Methane-Propane Mixtures." Transactions of the Japan Society of Mechanical Engineers Series B 61, n. 581 (1995): 325–31. http://dx.doi.org/10.1299/kikaib.61.325.

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