Journal articles on the topic 'Flame'
To see the other types of publications on this topic, follow the link: Flame.
Create a spot-on reference in APA, MLA, Chicago, Harvard, and other styles
Consult the top 50 journal articles for your research on the topic 'Flame.'
Next to every source in the list of references, there is an 'Add to bibliography' button. Press on it, and we will generate automatically the bibliographic reference to the chosen work in the citation style you need: APA, MLA, Harvard, Chicago, Vancouver, etc.
You can also download the full text of the academic publication as pdf and read online its abstract whenever available in the metadata.
Browse journal articles on a wide variety of disciplines and organise your bibliography correctly.
1
Hsu, Ching Min, Dickson Bwana Mosiria, and Wei Chih Jhan. "Flow and Temperature Characteristics of a 15° Backward-Inclined Jet Flame in Crossflow." Energies 12, no. 1 (December 31, 2018): 132. http://dx.doi.org/10.3390/en12010132.
Full textAbstract:
The flow and flame characteristics of a 15° backward-inclined jet flame in crossflow were investigated in a wind tunnel. The flow structures, flame behaviors, and temperature fields were measured. The jet-to-crossflow momentum flux ratio was less than 7.0. The flow patterns were investigated using photography and Mie-scattering techniques. Meanwhile, the velocity fields were observed using particle image velocimetry techniques, whereas the flame behaviors were studied using photographic techniques. The flame temperatures were probed using a fine-wire R-type thermocouple. Three flame modes were identified: crossflow dominated flames, which were characterized by a blue flame connected to a down-washed yellow recirculation flame; transitional flames identified by a yellow recirculation flame and an elongated yellow tail flame; and detached jet dominated flames denoted by a blue flame base connected to a yellow tail flame. The effect of the flow characteristics on the combustion performance in different flame regimes is presented and discussed. The upwind shear layer of the bent jet exhibited different coherent structures as the jet-to-crossflow momentum flux ratio increased. The transitional flames and detached jet dominated flames presented a double peak temperature distribution in the symmetry plane at x/d = 60. The time-averaged velocity field of the crossflow dominated flames displayed a standing vortex in the wake region, whereas that of the detached jet dominated flames displayed a jet-wake vortex and a wake region source point.
2
Yang, Tao, Yuan Ma, and Peng Zhang. "Dynamical Behavior of Small-Scale Buoyant Diffusion Flames in Externally Swirling Flows." Symmetry 16, no. 3 (March 2, 2024): 292. http://dx.doi.org/10.3390/sym16030292.
Full textAbstract:
This study computationally investigates small-scale flickering buoyant diffusion flames in externally swirling flows and focuses on identifying and characterizing various distinct dynamical behaviors of the flames. To explore the impact of finite rate chemistry on flame flicker, especially in sufficiently strong swirling flows, a one-step reaction mechanism is utilized for investigation. By adjusting the external swirling flow conditions (the intensity R and the inlet angle α), six flame modes in distinct dynamical behaviors were computationally identified in both physical and phase spaces. These modes, including the flickering flame, oscillating flame, steady flame, lifted flame, spiral flame, and flame with a vortex bubble, were analyzed from the perspective of vortex dynamics. The numerical investigation provides relatively comprehensive information on these flames. Under the weakly swirling condition, the flames retain flickering (the periodic pinch-off of the flame) and are axisymmetric, while the frequency nonlinearly increases with the swirling intensity. A relatively high swirling intensity can cause the disappearance of the flame pinch-off, as the toroidal vortex sheds around either the tip or the downstream of the flame. The flicker vanishes, but the flame retains axisymmetric in a small amplitude oscillation or a steady stay. A sufficiently high swirling intensity causes a small Damköhler number, leading to the lift-off of the flame (the local extinction occurs at the flame base). Under the same swirling intensity but large swirling angles, the asymmetric modes of the spiral and vortex bubble flames were likely to occur. With R and α increasing, these flames exhibit axisymmetric and asymmetric patterns, and their dynamical behaviors become more complex. To feature the vortical flows in flames, the phase portraits are established based on the velocity information of six positions along the axis of the flame, and the dynamical behaviors of various flames are presented and compared in the phase space. Observing the phase portraits and their differences in distinct modes could help identify the dynamical behaviors of flames and understand complex phenomena.
3
Dunstan, T. D., N. Swaminathan, and K. N. C. Bray. "Influence of flame geometry on turbulent premixed flame propagation: a DNS investigation." Journal of Fluid Mechanics 709 (August 21, 2012): 191–222. http://dx.doi.org/10.1017/jfm.2012.328.
Full textAbstract:
AbstractThe sensitivity of the turbulent flame speed to the geometry of the flame is investigated using direct numerical simulations of turbulent premixed flames in three canonical configurations: freely propagating statistically planar flames, planar flames stabilized in stagnating flows, and rod-stabilized V-flames. We consider both the consumption speed, which measures the integrated rate of burning, and the propagation speed, which measures the speed of an isosurface within the flame brush. An algebraic model for the propagation speed of the leading edge of the flame brush, which is blind to flame geometry, is also applied to the data for the purposes of establishing its range of validity and the causes of its failure. The turbulent consumption speed is found to be strongly geometry dependent, primarily due to the continuous growth of the flame brush thickness. Changes in the structure and consumption speed of instantaneous flame fronts are found to be only weakly sensitive to flame geometry. The turbulent propagation speed is analysed in terms of its reactive, diffusive and turbulent flux components. All three terms are shown to be significant, both through the flame brush and along the leading edge. The leading-edge propagation speed is found to be sensitive to flame geometry only in the V-flames under certain conditions. It is suggested that this apparent geometry dependence, which the model cannot capture, results from the relation between the turbulence and mean flow time scales in these particular cases, and is not intrinsic to the flame geometry itself.
4
GHOSAL, SANDIP, and LUC VERVISCH. "Theoretical and numerical study of a symmetrical triple flame using the parabolic flame path approximation." Journal of Fluid Mechanics 415 (July 25, 2000): 227–60. http://dx.doi.org/10.1017/s0022112000008685.
Full textAbstract:
In non-premixed turbulent combustion the reactive zone is localized at the stoichiometric surfaces of the mixture and may be locally approximated by a diffusion flame. Experiments and numerical simulations reveal a characteristic structure at the edge of such a two-dimensional diffusion flame. This ‘triple flame’ or ‘edge flame’ consists of a curved flame front followed by a trailing edge that constitutes the body of the diffusion flame. Triple flames are also observed at the edge of a lifted laminar diffusion flame near the exit of burners. The speed of propagation of the triple flame determines such important properties as the rate of increase of the flame surface in non-premixed combustion and the lift-off distance in lifted flames at burners. This paper presents an approximate theory of triple flames based on an approximation of the flame shape by a parabolic profile, for large activation energy and low but finite heat release. The parabolic flame path approximation is a heuristic approximation motivated by physical considerations and is independent of the large activation energy and low heat release assumptions which are incorporated through asymptotic expansions. Therefore, what is presented here is not a truly asymptotic theory of triple flames, but an asymptotic solution of a model problem in which the flame shape is assumed parabolic. Only the symmetrical flame is considered and Lewis numbers are taken to be unity. The principal results are analytical formulas for the speed and curvature of triple flames as a function of the upstream mixture fraction gradient in the limit of infinitesimal heat release as well as small but finite heat release. For given chemistry, the solution provides a complete description of the triple flame in terms of the upstream mixture fraction gradient. The theory is validated by comparison with numerical simulation of the primitive equations.
5
Sun, Y., N. Liu, and W. Gao. "Experimental Study on Geometrical Characteristics of a Square Turbulent Buoyant Jet Flame." Journal of Physics: Conference Series 2442, no. 1 (February 1, 2023): 012020. http://dx.doi.org/10.1088/1742-6596/2442/1/012020.
Full textAbstract:
Abstract This paper gives the experimental results for the buoyant jet diffusion flame in square burners. The flame height and lift-off were measured and discussed. The results show that the normalized flame height and lift-off height of square flames are lower than that of round flames with the same experimental conditions, which indicates enhanced air entrainment in square flames. The model of flame height based on the flame Froude-number, which integrates a correction factor to account for the enhancement of air entrainment, reasonably collapses the square flame data under different experimental cases. The critical Froude number for the transition from buoyancy to momentum controlled regime of a square jet flame is highly dependent on the fuel type and burner size. The lift-off height of the square jet flames increases linearly when the initial velocity increases, but also depends on the fuel type and burner size. A unified correlation using a dimensionless flow number derived from the mixedness-reactedness flamelet model is established, which reasonably predicts the lift-off distances of square jet flames.
6
Vance, Faizan Habib, Yuriy Shoshin, Philip de Goey, and Jeroen van Oijen. "Flame Stabilization and Blow-Off of Ultra-Lean H2-Air Premixed Flames." Energies 14, no. 7 (April 2, 2021): 1977. http://dx.doi.org/10.3390/en14071977.
Full textAbstract:
The manner in which an ultra-lean hydrogen flame stabilizes and blows off is crucial for the understanding and design of safe and efficient combustion devices. In this study, we use experiments and numerical simulations for pure H2-air flames stabilized behind a cylindrical bluff body to reveal the underlying physics that make such flames stable and eventually blow-off. Results from CFD simulations are used to investigate the role of stretch and preferential diffusion after a qualitative validation with experiments. It is found that the flame displacement speed of flames stabilized beyond the lean flammability limit of a flat stretchless flame (ϕ=0.3) can be scaled with a relevant tubular flame displacement speed. This result is crucial as no scaling reference is available for such flames. We also confirm our previous hypothesis regarding lean limit blow-off for flames with a neck formation that such flames are quenched due to excessive local stretching. After extinction at the flame neck, flames with closed flame fronts are found to be stabilized inside a recirculation zone.
7
Ebieto, Celestine Ebieto, and Oku Nyong. "Flammability and Gravity Effect of Horizontal and Vertical Propagating Flames in Tube." European Journal of Engineering Research and Science 5, no. 1 (January 14, 2020): 20–26. http://dx.doi.org/10.24018/ejers.2020.5.1.1695.
Full textAbstract:
In the current research, experimental work is investigated for vertically and horizontally downward propagating flames in an open-ended tube. The objective was to study and compare the influence of flammability limits, gravity, and the flame speed in the different tube configuration for two different fuels. The experimental facility included a 20 mm inner diameter tube, 1200 mm in length and an optical access quartz tube made centrally of 700 mm in length. Methane-air and propane-air fuel were compared for both vertically and horizontally downward propagating flames. The flame speed at each equivalence ratios for both fuels was lower for the flame that propagates downward compared to the flame that propagates horizontally. For both fuels, the flammability limits tend to rise for the vertically downward flame. The influence of gravity was seen as the flames become leaner and richer in methane-air and propane-air flames that propagate vertically downwards, causing a transformation in the contour of the flame from a steady curved flame to a vibrating corrugated flame.
8
Ebieto, Celestine Ebieto, and Oku Nyong. "Flammability and Gravity Effect of Horizontal and Vertical Propagating Flames in Tube." European Journal of Engineering and Technology Research 5, no. 1 (January 14, 2020): 20–26. http://dx.doi.org/10.24018/ejeng.2020.5.1.1695.
Full textAbstract:
In the current research, experimental work is investigated for vertically and horizontally downward propagating flames in an open-ended tube. The objective was to study and compare the influence of flammability limits, gravity, and the flame speed in the different tube configuration for two different fuels. The experimental facility included a 20 mm inner diameter tube, 1200 mm in length and an optical access quartz tube made centrally of 700 mm in length. Methane-air and propane-air fuel were compared for both vertically and horizontally downward propagating flames. The flame speed at each equivalence ratios for both fuels was lower for the flame that propagates downward compared to the flame that propagates horizontally. For both fuels, the flammability limits tend to rise for the vertically downward flame. The influence of gravity was seen as the flames become leaner and richer in methane-air and propane-air flames that propagate vertically downwards, causing a transformation in the contour of the flame from a steady curved flame to a vibrating corrugated flame.
9
Buckmaster, J., and T. L. Jackson. "Holes in flames, flame isolas, and flame edges." Proceedings of the Combustion Institute 28, no. 2 (January 2000): 1957–64. http://dx.doi.org/10.1016/s0082-0784(00)80601-3.
Full text
10
Xing, Tie-Ling, Jie Liu, Shi-Wei Li, and Guo-Qiang Chen. "Thermal properties of flame retardant cotton fabric grafted by dimethyl methacryloyloxyethyl phosphate." Thermal Science 16, no. 5 (2012): 1472–75. http://dx.doi.org/10.2298/tsci1205472x.
Full textAbstract:
Thermal properties of flame retardant cotton fabric grafted by dimethyl methacryloy-loxyethyl phosphate were investigated by the atom transfer radical polymerization method. Thermal gravimetric analysis was used to explore the thermal decomposition mode of flamed retardant cotton fabric. The weight loss rate of the flamed retardant cotton was bigger than that of the control cotton fabric, and a more final residual char of flamed retardant cotton was also observed. Flammability tests were used to study the flame retardance property of the flame retardant cotton fabric. The results showed that flamed retardant cotton fabric with 16.8% of weight gain could keep good flame retardance. Scanning electron microscope pictures were applied to investigate the morphology of residual char of the flame retardant samples.
11
Hicks, E. P. "Rayleigh–Taylor unstable flames at higher Reynolds number." Monthly Notices of the Royal Astronomical Society 489, no. 1 (July 30, 2019): 36–51. http://dx.doi.org/10.1093/mnras/stz2080.
Full textAbstract:
ABSTRACT Rayleigh–Taylor (RT) unstable flames are a key component of Type Ia and Iax supernovae explosions, but their complex hydrodynamics is still not well understood. These flames are affected not only by the RT instability, but also by the turbulence it generates. Both processes can increase the flame speed by stretching and wrinkling the flame. This makes it hard to choose a subgrid model for the flame speed in full star Type Ia or Iax simulations. Commonly used subgrid models get around this difficulty by assuming that either the RT instability or turbulence is dominant and sets the flame speed. In previous work, we evaluated the physical assumptions and predictive abilities of these two types of models by analysing a large parameter study of 3D direct numerical simulations of RT unstable flames. Surprisingly, we found that the flame dynamics is dominated by the RT instability and that RT unstable flames are very different from turbulent flames. In particular, RT unstable flames are thinner rather than thicker when turbulence is strong. In addition, none of the turbulent flame speed models adequately predicted the flame speed. We also showed that the RT flame speed model failed when the RT instability was strong, suggesting that geometrical burning effects also influence the flame speed. However, these results depended on simulations with Re ≲ 720. In this paper, we extend the parameter study to higher Reynolds number and show that the basic conclusions of our previous study still hold when the RT-generated turbulence is stronger.
12
Ban, H., S. Venkatesh, and K. Saito. "Convection-Diffusion Controlled Laminar Micro Flames." Journal of Heat Transfer 116, no. 4 (November 1, 1994): 954–59. http://dx.doi.org/10.1115/1.2911471.
Full textAbstract:
Small laminar diffusion flames (flame height ≃2–3 mm) established by a fuel jet issuing into a quiescent medium are investigated. It was found that for these flames buoyancy effects disappeared as the flame size decreased (Fr≫1), and diffusive transport of the fuel was comparable to the convective transport of the fuel. The effect of buoyancy on these flames was studied by examining the flame shape for horizontally oriented burners. A phenomenological model was developed (based on experimentally determined flame shapes) to compare diffusion and convection transport effects. Finally, the flame shapes were theoretically determined by solving the conservation equations using similarity methods. It was seen that when the axial diffusion (in momentum and species equations) terms are included in the conservation equations, the calculated flame shape is in better agreement (as compared to without the axial diffusion term) with the experimentally measured flame shape.
13
Gómez-Meyer, Juan-Sebastián, Subramanyam R. Gollahalli, Ramkumar N. Parthasarathy, and Jabid-Eduardo Quiroga. "Laminar flame speed of soy and canola biofuels." CT&F - Ciencia, Tecnología y Futuro 4, no. 5 (May 31, 2012): 75–83. http://dx.doi.org/10.29047/01225383.223.
Full textAbstract:
In this article, the flame speed values determined experimentally for laminar premixed flames of the vapors of two biofuels in air are presented. The laminar flame speed is a fundamental thermochemical property of fuels, and is essential for analyzing the flame propagation in practical devices, even those employing turbulent flames. The fuels obtained from transesterification of soy and canola oils are tested. Also, the diesel flames are studied to serve as a baseline for comparison. The experiments are performed with a tubular burner; pre-vaporized fuel is mixed with hot air and is ignited. The flame speed is determined at fuel-equivalence ratios of 1; 1,1 and 1,2 by recording the geometry of the flame. The experimental results show that the flame speed of biofuels is lower by about 15% than that of diesel. Also, the maximum value of flame speed is obtained at an equivalence ratio of approximately 1,1.
14
Chong, Cheng Tung, and Simone Hochgreb. "Spray Flame Study Using a Model Gas Turbine Swirl Burner." Applied Mechanics and Materials 316-317 (April 2013): 17–22. http://dx.doi.org/10.4028/www.scientific.net/amm.316-317.17.
Full textAbstract:
A model gas turbine burner was employed to investigate spray flames established under globally lean, continuous, swirling conditions. Two types of fuel were used to generate liquid spray flames: palm biodiesel and Jet-A1. The main swirling air flow was preheated to 350 °C prior to mixing with airblast-atomized fuel droplets at atmospheric pressure. The global flame structure of flame and flow field were investigated at the fixed power output of 6 kW. Flame chemiluminescence imaging technique was employed to investigate the flame reaction zones, while particle imaging velocimetry (PIV) was utilized to measure the flow field within the combustor. The flow fields of both flames are almost identical despite some differences in the flame reaction zones.
15
Fletcher, Thomas H., Denver Haycock, Seth Tollefsen, and David O. Lignell. "Merging of Horizontally and Vertically Separated Small-Scale Buoyant Flames." Fire 4, no. 3 (August 25, 2021): 51. http://dx.doi.org/10.3390/fire4030051.
Full textAbstract:
The purpose of this study was to investigate the merging behavior of small-scale buoyant flames that might be representative of flames from a leaf in a shrub. Zirconia felt pads soaked in n-heptane were suspended on thin rods and spaced both horizontally and vertically. Time-dependent video images from flames from two-pad and three-pad configurations were analyzed to determine merging probability, combined flame characteristics (height, area, and width), and changes in burn time. Correlations of these combined flame characteristics were developed based on horizontal and vertical spacing between the pads. Merging probability correlated with an exponential function that was quadratic in horizontal and/or vertical spacing. Flame heights corrected for vertical inter-pad spacing showed a maximum increase of 50% over single flame heights, and were correlated with an exponential decay function. Flame areas increased by a maximum of 34%, but on average were relatively constant. Corrected flame widths for the merged flames increased by as much as 55% in some configurations, but decreased by up to 73% in other configurations. Burn times for upper pads decreased when there was no horizontal spacing. The limited flame growth observed in these non-overlapping configurations in the horizontal dimension imply that overlapping configurations seem to be necessary for significant flame growth.
16
Singh, Aditya Prakash, V. RatnaKishore, S. Minaev, and Sudarshan Kumar. "Numerical investigations of unsteady flame propagation in stepped microtubes." RSC Advances 5, no. 122 (2015): 100879–90. http://dx.doi.org/10.1039/c5ra21704k.
Full textAbstract:
Flame dynamics near the contraction is governed by flame stretching. Change in fuel–air mass-flux entering flame plays a crucial role in accelerating the propagating flames at certain thermal boundary conditions as flame reaches the contraction.
17
Shih, Hsin Yi, and Jou Rong Hsu. "Computed Extinction Limits and Flame Structures of Opposed-Jet Syngas Diffusion Flames." Applied Mechanics and Materials 110-116 (October 2011): 4899–906. http://dx.doi.org/10.4028/www.scientific.net/amm.110-116.4899.
Full textAbstract:
This paper reports a numerical study on the extinction limits and flame structures of opposed-jet syngas diffusion flames. A narrowband radiation model is coupled to the OPPDIF program, which uses detailed chemical kinetics and thermal and transport properties to enable the study of 1-D counterflow syngas diffusion flames over the entire range of flammable strain rates with flame radiation. The effects of syngas composition, strain rate, ambient pressure, and dilution gases on the flame structures and extinction limits of H2/CO synthetic mixture flames were examined. Results indicate the flame structures and flame extinction are impacted by the composition of syngas mixture significantly. From hydrogen-lean syngas to hydrogen-rich syngas fuels, flame temperature increases with increasing hydrogen content and ambient pressure, but the flame thickness is decreased with ambient pressure and strain rates. Besides, the dilution effects from CO2, N2, and H2O, which may be present in the syngas mixtures, were studied. The flame is thinner and flame temperature is lower when CO2 is the diluents instead of N2. The combustible range of strain rates is extended with increasing hydrogen percentage and ambient pressure, but it is decreased the most with CO2 as the dilution gas due to the dilution effects. Complete flammability limits using strain rates, maximum flame temperature as coordinates can provide a fundamental understanding of syngas combustion and applications.
18
Tillman, S. T., S. Kuchibhatla, K. Annamalai, J. A. Caton, and D. Ranjan. "Interactive Combustion in a Linear Array of 2D Laminar Isolated and Triple Burner Jets." Journal of Combustion 2012 (2012): 1–22. http://dx.doi.org/10.1155/2012/716050.
Full textAbstract:
Many practical combustion systems such as residential gas burners contain dense groupings or clusters of jet flames with sufficiently small spacing between them, which causes flame interaction. The interaction effect, due in part to Oxygen deficiency in the interstitial space between the flames, causes the spreading of flames, which may merge together to form larger group flames. This interactive effect is studied analytically by revisiting the laminar isolated flame theory for 2D jets, for which similarity solutions are readily available in compressible form, and symmetrical interaction zones can be observed. Flame characteristics were studied by obtaining analytical expressions for flame specific parameters such as height and width, lift-off height and blow-off velocity, air entrainment and mixing layer growth. The theory for multiple interacting jets describes an approximate criterion for interburner spacing at which flame interaction and group flame formation are first observed. The analytical framework presented in this paper presented in this paper produced results which were compared with experimental measurements. The experimental apparatus allowed the interburner spacing to be varied from 7.87 mm to 50.8 mm, and measurements of flame height, width, lift-off height and group-flame formation under interactive modes. Images of the evolving flow field were taken and Schlieren images of the multiple 2D jets were also recorded using a digital camera.
19
Clavin, Paul, and José C. Graña-Otero. "Curved and stretched flames: the two Markstein numbers." Journal of Fluid Mechanics 686 (September 28, 2011): 187–217. http://dx.doi.org/10.1017/jfm.2011.318.
Full textAbstract:
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.
20
Li, X. "On the Scaling of the Visible Lengths of Jet Diffusion Flames." Journal of Energy Resources Technology 118, no. 2 (June 1, 1996): 128–33. http://dx.doi.org/10.1115/1.2792703.
Full textAbstract:
Length of jet diffusion flames is of direct importance in many industrial processes and is analyzed by applying scaling method directly to the governing partial differential equations. It is shown that for jet-momentum-dominated diffusion flames, when the buoyancy effects are neglected, the flame length normalized by the burner exit diameter increases linearly with the Reynolds number at the burner exit in the laminar burning regime and decreases in inverse proportion to the Reynolds number in the transitional regime. For turbulent diffusion flames, the normalized flame lengths are independent of the burner exit flow conditions. It is further found that for vertical upward flames, the buoyancy effect increases the flame length in the laminar and transitional regime and reduces the length in the turbulent regime; while for vertical downward flames, the buoyancy effect decreases the flame length in the laminar and transitional regime and increases the length in the turbulent regime, provided that jet momentum is dominated, and there is no flame spreading out and then burning upward like a downward-facing pool fire. Hence, for turbulent flames the flame lengths depend on the Froude number, Fr, and increase (or decrease) slightly as Fr increases for upward (or downward) flames. By comparison, it is found that the foregoing theoretical results are in good agreement with the experimental observations reported in literature.
21
Scoletta, Edoardo, and Wolfgang Polifke. "Impact of stretch on the flame dynamics of laminar premixed flames." INTER-NOISE and NOISE-CON Congress and Conference Proceedings 265, no. 1 (February 1, 2023): 6277–85. http://dx.doi.org/10.3397/in_2022_0933.
Full textAbstract:
Flame transfer functions (FTF) of laminar premixed flames acoustically forced are analytically investigated and modelled. The study is based on the linearized G-equation, which is used to kinematically track the flame front. In order to incorporate combustion properties, the laminar consumption speed is considered varying depending on the flame front stretch. Once written in dimensionless form, the G-equation reveals that the FTF depends on 3 dimensionless parameters: a Strouhal number (St2) that accounts for the convective time of the flow perturbation along the flame-front, the flame aspect-ratio (Lf/R) and the dimensionless Markstein length, adimensionalized by the injector radius. It is shown that the latter term is responsible for a flame-flow feedback that acts as damper or amplifier of the flame perturbation, respectively for thermodiffusively stable or unstable flames. A LOM FTF is derived both for Conical and V-flames, highlighting the impact of stretch for each configuration. Ultimately the obtained FTFs are compared to previously proposed analytical FTFs from the literature, underlining the importance of stretch at high frequency.
22
Pratap, Sathiah, Andrei N. Lipatnikov, and Jerzy Chomiak. "Effects of Flame Development and Structure on Thermo-Acoustic Oscillations of Premixed Turbulent Flames(S.I. Engines, Flame Propagation)." Proceedings of the International symposium on diagnostics and modeling of combustion in internal combustion engines 2004.6 (2004): 599–606. http://dx.doi.org/10.1299/jmsesdm.2004.6.599.
Full text
23
Johnston, J. M., M. J. Wooster, and T. J. Lynham. "Experimental confirmation of the MWIR and LWIR grey body assumption for vegetation fire flame emissivity." International Journal of Wildland Fire 23, no. 4 (2014): 463. http://dx.doi.org/10.1071/wf12197.
Full textAbstract:
The temperature and emissivity of forest fire flames play a key role in understanding fire behaviour, modelling fire spread and calculating fire parameters by means of active fire thermal remote sensing. Essential to many of these is the often-made assumption that vegetation fire flames behave as grey bodies in the infrared (IR). Although the emissivity of flames and its relationship to flame depth has been measured experimentally using thermal imagers working in the long-wave IR (LWIR, 8–12µm), no published study has yet demonstrated relationships in the important mid-wave IR (MWIR, 3–5µm) spectral region, nor conclusively demonstrated that assumptions about grey body behaviour across these two important IR atmospheric windows fit well with reality. Our study explores these issues using measurements of boreal forest fuels burned with flame depths ranging from 0.2 to 4.2 m. Observations of two stable black body sources made through the differing flame depths were used to explore flame spectral emissivities and their relationship to flame depth. We found essentially the same relationship between flame emissivity and flame depth for both spectral regions, (extinction coefficient K=0.7 m–1), confirming that the grey body assumption for forest fire flames in the MWIR and LWIR atmospheric windows appears valid for the fire conditions encountered here.
24
Ravikrishna, RV, and AB Sahu. "Advances in understanding combustion phenomena using non-premixed and partially premixed counterflow flames: A review." International Journal of Spray and Combustion Dynamics 10, no. 1 (November 14, 2017): 38–71. http://dx.doi.org/10.1177/1756827717738168.
Full textAbstract:
Counterflow flames provide an ideal platform for understanding the flame structure and as a model to study the effect of physical and chemical perturbations on the flame structure. This article reviews the advances made in the understanding of combustion dynamics and chemistry through experimental and numerical studies in counterflow non-premixed and partially premixed flames. Key contributions on fundamental aspects such as extinction, ignition and effect of perturbations on the stability of diffusion flames are first summarized and analysed. The review then focuses on the progress made in the understanding of the effect of inert particles and flame suppressants on the flame characteristics. A review of detailed studies on edge flames facilitates further understanding of local quenching and re-ignition phenomena in highly turbulent flames. The influence of radiation model and unsteady flow-conditions on the flame kinetics and dynamics along with work on NOx kinetics has been discussed. The review also outlines that specific experiments need to be carried out over a wide range of conditions for further understanding and validation of numerical models.
25
Kalman, Joseph, Nick G. Glumac, and Herman Krier. "Experimental Study of Constant Volume Sulfur Dust Explosions." Journal of Combustion 2015 (2015): 1–11. http://dx.doi.org/10.1155/2015/817259.
Full textAbstract:
Dust flames have been studied for decades because of their importance in industrial safety and accident prevention. Recently, dust flames have become a promising candidate to counter biological warfare. Sulfur in particular is one of the elements that is of interest, but sulfur dust flames are not well understood. Flame temperature and flame speed were measured for sulfur flames with particle concentrations of 280 and 560 g/m3and oxygen concentration between 10% and 42% by volume. The flame temperature increased with oxygen concentration from approximately 900 K for the 10% oxygen cases to temperatures exceeding 2000 K under oxygen enriched conditions. The temperature was also observed to increase slightly with particle concentration. The flame speed was observed to increase from approximately 10 cm/s with 10% oxygen to 57 and 81 cm/s with 42% oxygen for the 280 and 560 g/m3cases, respectively. A scaling analysis determined that flames burning in 21% and 42% oxygen are diffusion limited. Finally, it was determined that pressure-time data may likely be used to measure flame speed in constant volume dust explosions.
26
B., Aravind, Ratna Kishore Velamati, Aditya P. Singh, Y. Yoon, S. Minaev, and Sudarshan Kumar. "Investigations on flame dynamics of premixed H2–air mixtures in microscale tubes." RSC Advances 6, no. 55 (2016): 50358–67. http://dx.doi.org/10.1039/c6ra05539g.
Full textAbstract:
The flame propagation is affected by flame shape angle and wall heat transfer coeff. A minimum flame propagation speed is observed for planar flames with nearly zero flame shape angle for 580 h−2 K−1 wall heat transfer coeff. range.
27
Luo, Jing, Lian Sheng Liu, and Zi Zhong Chen. "A Study on Flame Structures in CH4 Counterflow Partially Premixed Flame." Applied Mechanics and Materials 694 (November 2014): 474–77. http://dx.doi.org/10.4028/www.scientific.net/amm.694.474.
Full textAbstract:
An experimental and simulation work had been conducted to study a one-dimensional partially premixed methane/air counterflow flame in this paper. Flame images are obtained through experiments and computations using GRIMech 3.00 chemistry were performed for the flames studied. The partially premixing effects upon the flame were revealed by comparing the flame structures and emissions with premixed flames at the same equivalence ratio. The results show the premixed flame only has a single flame structure. However, PPF has distinct double flame structures at present equivalence ratio. Temperature is relatively high in the whole combustion zone for premixed flame, while, for PPF, there are two temperature peaks in a rich premixed reaction zone on the fuel side and a nonpremixed reaction zone on the oxidizer side respectively. For PPF, NO concentration in the nonpremixed zone is much higher compared to that in the rich premixed zone because of higher OH concentration in the nonpremixed zone.
28
Almyali, Hussein M., Zaid M. H. Al Dulaimi, and Mohammed A. Al-Fahham. "A Review: Flame Propagation Dynamics in Open Tubes: Factors Influencing Combustion Conditions and Practical Implementations." Salud, Ciencia y Tecnología - Serie de Conferencias 3 (May 31, 2024): 816. http://dx.doi.org/10.56294/sctconf2024816.
Full textAbstract:
This study examines the combustion conditions of fuel flame propagation within a tube of uniform cross-sectional area, where ignition occurs at the sealed end and the flame propagates in the direction of both the open and sealed ends. Both factors exert an influence on the configuration and functioning of combustion systems. To gain a comprehensive understanding of the impact of practical implementations in various combustion systems, it is imperative to comprehend the variations in flame propagation stages as a result of thermodynamic conditions. The operational conditions involving pressures and temperatures are considerably elevated compared to the natural settings. Numerous studies have been conducted on the propagation of flames within tubes. The present review centers on an extensive examination of the methodologies and procedures employed to investigate the phases of flame propagation, along with the impact of operational parameters on other fuels. Various aspects of flame behavior are explored in this article, focusing on flame formation, propagation, and the factors that influence them. The flame speed, which represents the rate of flame propagation, is influenced by factors such as fuel and oxidizer composition, temperature, pressure, and environmental conditions. The paper discusses the distinction between premixed and non-premixed flames and their respective characteristics. Several studies are cited to demonstrate the impact of oxygen concentration, air swirl, and fuel blending ratios on flame properties. These investigations involve experiments with different fuel-air mixtures, examining parameters such as flame luminance, temperature, soot production, and flame distortion. The measurement of laminar flame speed, which provides insights into fuel-air mixtures' diffusivity, reactivity, and exothermicity, is discussed. Various techniques for measuring laminar flame speed are mentioned, including the study of flame stability and spherical flame propagation. The paper also addresses the influence of flame stretch, which refers to the elongation or compression of a flame due to fluid flow or turbulence. Researchers aim to eliminate the effect of flame stretch to achieve accurate observations. Furthermore, the manuscript delves into factors affecting flame propagation, including the influence of aspect ratio on flame dynamics and flame oscillations. It describes experiments conducted in different geometries to observe changes in flame morphology and propagation velocity. The impact of ignition disturbances and equivalent ratio stratification on flame behavior is also explored. Studies examine the effects of ignition disruption and ignition volume on flame spread dynamics. Additionally, investigations analyze the behavior of flames under disturbances in the equivalence ratio, discussing changes in flame speed, heat release, and flame structure. Overall, these studies contribute to our understanding of flame behavior, combustion processes, and their applications in various fields, including energy production, environmental science, and engineering.
29
Sheykhbaglou, Soroush. "Properties of turbulent non-premixed methane/air flames in a miniature-scale swirl burner under different coaxial airflow swirl numbers." Future Energy 2, no. 1 (February 15, 2023): 27–37. http://dx.doi.org/10.55670/fpll.fuen.2.1.5.
Full textAbstract:
This study investigates the dynamics and properties of non-premixed methane/air flames under three swirl numbers by segmenting flame images using the Otsu thresholding technique. Under three operating conditions, the lean blow out (LBO) and flame length, lift-off height, maximum width, flame angle, and flame pulsing displacements in terms of flame center of gravity, length, and width are measured and compared. A high-speed camera is used to record video of flames, and the image processing of frames collected from a high-speed video was accomplished by using the intermittency distribution method to quantitatively compare flame attributes. The findings show that increasing the swirl number from 0.5 to 0.7 generally has an unfavorable effect on the LBO at given fuel flow rates, and the LBO of flames under 35° (0.6 swirl number) and 40° (0.7 swirl number) swirlers has decreased up to about 15% and 40%, respectively when compared with a 30° swirler (0.5 swirl number). Additionally, observations indicate that the flame length (𝐿) and lift-off height (𝐿𝑂) drop as the swirl number rises, although the flame width (𝑊) and angle (𝛼) show an ascending tendency. Besides, flame lift-off reveals an increasing-decreasing trend with an increment in the airflow, and flame length decreases as the airflow rate increases. It was also observed that flame pulsating displacements in terms of center of gravity (𝛿𝐶𝐺), length (𝛿𝐿), and width (𝛿𝑊) increases with an increase in the fuel flow rate, and as the swirl number is increased, 𝛿𝐶𝐺 and 𝛿𝐿 lessens, while 𝛿𝑊 increases.
30
Baum, M., T. J. Poinsot, D. C. Haworth, and N. Darabiha. "Direct numerical simulation of H2/O2/N2 flames with complex chemistry in two-dimensional turbulent flows." Journal of Fluid Mechanics 281 (December 25, 1994): 1–32. http://dx.doi.org/10.1017/s0022112094003010.
Full textAbstract:
Premixed H2/O2/N2 flames propagating in two-dimensional turbulence have been studied using direct numerical simulations (DNS: simulations in which all fluid and thermochemical scales are fully resolved). Simulations include realistic chemical kinetics and molecular transport over a range of equivalence ratios Φ (Φ = 0.35, 0.5, 0.7, 1.0, 1.3). The validity of the flamelet assumption for premixed turbulent flames is checked by comparing DNS data and results obtained for steady strained premixed flames with the same chemistry (flamelet ‘library’). This comparison shows that flamelet libraries overestimate the influence of stretch on flame structure. Results are also compared with earlier zero-chemistry (flame sheet) and one-step chemistry simulations. Consistent with the simpler models, the turbulent flame with realistic chemistry aligns preferentially with extensive strain rates in the tangent plane and flame curvature probability density functions are close to symmetric with near-zero means. For very lean flames it is also found that the local flame structure correlates with curvature as predicted by DNS based on simple chemistry. However, for richer flames, by contrast to simple-chemistry results with non-unity Lewis numbers (ratio of thermal to species diffusivity), local flame structure does not correlate with curvature but rather with tangential strain rate. Turbulent straining results in substantial thinning of the flame relative to the steady unstrained laminar case. Heat-release and H2O2 contours remain thin and connected (‘flamelet-like’) while species including H-atom and OH are more diffuse. Peak OH concentration occurs well behind the peak heat-release zone when the flame temperature is high (of the order of 2800 K). For cooler and leaner flames (about 1600 K and for an equivalence ratio below 0.5) the OH radical is concentrated near the reaction zone and the maximum OH level provides an estimate of the local flamelet speed as assumed by Becker et al. (1990).
31
Jiang, Xiaozhen, Jingxuan Li, and Lijun Yang. "Nonlinear response of laminar premixed flames to dual-input harmonic disturbances." INTER-NOISE and NOISE-CON Congress and Conference Proceedings 265, no. 4 (February 1, 2023): 3408–19. http://dx.doi.org/10.3397/in_2022_0484.
Full textAbstract:
In gas turbines, aero-engines and rocket engines, flames are always disturbed by perturbations of dual or multiple harmonic frequencies, resulting in corresponding combustion instability. This paper analyses the nonlinear response of laminar premixed flames to dual-input harmonic disturbances to further understand those associated combustion instability. Nonlinear results of flame dynamics were derived from analytical and numerical solutions of the G-equation. The spatial front-tracking of premixed flames was obtained, where types of nonlinear behaviors were classified and related mechanisms of that were elucidated. A dual-input flame description function (DIFDF) was proposed to separately determine global nonlinearities of flame responses of fundamental and higher harmonics frequencies under dual-input disturbances. The fundamental frequency response consists of linear and nonlinear components, and the higher harmonic frequency one is purely nonlinear. The DIFDF properties of conical and "V" flames were compared, with particular emphasis on their differences in nonlinear behavior. The spatial and global effects of the second input frequency on the flame kinematics perturbed by the first frequency were also clarified. Furthermore, the roles of perturbation amplitude and flame height in spatial flame dynamics and DIFDF were quantified.
32
Javareshkian, Alireza, Sadegh Tabejamaat, Soroush Sarrafan-Sadeghi, and Mohammadreza Baigmohammadi. "An experimental study on the effects of swirling oxidizer flow and diameter of fuel nozzle on behaviour and light emittance of propane-oxygen non-premixed flame." Thermal Science 21, no. 3 (2017): 1453–62. http://dx.doi.org/10.2298/tsci140706210j.
Full textAbstract:
In this study, the stability and the light emittance of non-premixed propane-oxygen flames have been experimentally evaluated with respect to swirling oxidizer flow and variations in fuel nozzle diameter. Hence, three types of the vanes with the swirl angles of 30?, 45?, and 60? have been chosen for producing the desired swirling flows. The main aims of this study are to determine the flame behaviour, light emittance, and also considering the effect of variation in fuel nozzle diameter on combustion phenomena such as flame length, flame shape, and soot free length parameter. The investigation into the flame phenomenology was comprised of variations of the oxidizer and fuel flow velocities (respective Reynolds numbers) and the fuel nozzle diameter. The results showed that the swirl effect could change the flame luminosity and this way could reduce or increase the maximum value of the flame light emittance in the combustion zone. Therefore, investigation into the flame light emittance can give a good clue for studying the mixing quality of reactants, the flame phenomenology (blue flame or sooty flame, localized extinction), and the combustion intensity in non-premixed flames.
33
Camacho, Jorge R., and Ahsan R. Choudhuri. "Shapes of Elliptic Methane Laminar Jet Diffusion Flames." Journal of Engineering for Gas Turbines and Power 128, no. 1 (October 21, 2004): 1–7. http://dx.doi.org/10.1115/1.2032449.
Full textAbstract:
Buoyant and nonbuoyant shapes of methane flames issued from a 2:1 aspect ratio elliptic tube burner were measured. Nonbuoyant conditions were obtained in the KC-135 microgravity research aircraft operated by NASA’s Johnson Space Center. A mathematical model based on the extended Burke-Schumann flame theory is developed to predict the flame length of an elliptic burner. The model utilizes Roper’s theoretical method for circular burners and extends the analysis for elliptic burners. The predicted flame length using the theoretical model agrees well with experimental measurements. In general for the elliptic burner the nonbuoyant flames are longer than the buoyant flames. However, measured lengths of both buoyant and nonbuoyant flame lengths change proportionally with the volumetric fuel flow rate and support the L vs Q correlation. The maximum flame width measured at buoyant and nonbuoyant conditions also show a proportional relation with the volumetric fuel flow rate. Normalized buoyant and nonbuoyant flame lengths of the elliptic burner correlate (L∕d∝Re) with the jet exit Reynolds number and exhibit a higher slope compared to a circular burner. Normalized flame width data show a power correlation (w∕d=cFrn) with the jet exit Froude number.
34
Shehata, Mohamed S., Mohamed M. ElKotb, and Hindawi Salem. "Combustion Characteristics for Turbulent Prevaporized Premixed Flame Using Commercial Light Diesel and Kerosene Fuels." Journal of Combustion 2014 (2014): 1–17. http://dx.doi.org/10.1155/2014/363465.
Full textAbstract:
Experimental study has been carried out for investigating fuel type, fuel blends, equivalence ratio, Reynolds number, inlet mixture temperature, and holes diameter of perforated plate affecting combustion process for turbulent prevaporized premixed air flames for different operating conditions. CO2, CO, H2, N2, C3H8, C2H6, C2H4, flame temperature, and gas flow velocity are measured along flame axis for different operating conditions. Gas chromatographic (GC) and CO/CO2infrared gas analyzer are used for measuring different species. Temperature is measured using thermocouple technique. Gas flow velocity is measured using pitot tube technique. The effect of kerosene percentage on concentration, flame temperature, and gas flow velocity is not linearly dependent. Correlations for adiabatic flame temperature for diesel and kerosene-air flames are obtained as function of mixture strength, fuel type, and inlet mixture temperature. Effect of equivalence ratio on combustion process for light diesel-air flame is greater than for kerosene-air flame. Flame temperature increases with increased Reynolds number for different operating conditions. Effect of Reynolds number on combustion process for light diesel flame is greater than for kerosene flame and also for rich flame is greater than for lean flame. The present work contributes to design and development of lean prevaporized premixed (LPP) gas turbine combustors.
35
Sandrowitz, A. K., J. M. Cooke, and N. G. Glumac. "Flame Emission Spectroscopy for Equivalence Ratio Monitoring." Applied Spectroscopy 52, no. 5 (May 1998): 658–62. http://dx.doi.org/10.1366/0003702981944319.
Full textAbstract:
The dependence of UV-visible emission characteristics in hydrocarbon flames as a function of flame equivalence ratio and total flow rate is examined for low-pressure acetylene/oxygen flames used for materials synthesis and for atmospheric-pressure methane/air flames typically seen in industrial boilers and heaters. In both flames, the OH and CH emission features show significantly different variations with respect to changes in equivalence ratio, while variations with changes in total flow rate are nearly identical. These results suggest that flame emission spectroscopy can be used as a sensitive, on-line process diagnostic for equivalence ratio monitoring in flame reactors.
36
Xie, Yu, Zhilong Wei, Teng Zhou, Haishen Zhen, Zihao Liu, and Zuohuang Huang. "Combustion Characteristics of Small Laminar Flames in an Upward Decreasing Magnetic Field." Energies 14, no. 7 (April 2, 2021): 1969. http://dx.doi.org/10.3390/en14071969.
Full textAbstract:
The combustion characteristics of laminar biogas premixed and diffusion flames in the presence of upward decreasing magnetic fields have been investigated in this study. The mechanism of magnet–flame interaction in the literature, in which magnetic fields change the behaviors of laminar flames due to the paramagnetic and diamagnetic properties of the constituent gases, is examined and the results are as follows. The magnetic field has no noticeable effect on premixed flames due to low oxygen concentration of the mixed gas at the injection and the relatively high flow momentum. However, due to the diffusion nature of diffusion flames and paramagnetic property of oxygen in ambient air, oxygen distributions are subjected to the gradient of magnetic flux, thus shortening the height of diffusion flames. Results also show that the flame volume is more strongly varied than flame height. Altered oxygen distributions result in improved combustion and higher flame temperature. In the case of current magnet–flame interaction, the magnetic driving force is combined with gravitational force, and a modified gravity g* as well as gravity modification factor G are derived to characterize the paramagnetism theory of oxygen.
37
Smith, Tracy, Chendhil Periasamy, Benjamin Baird, and S. R. Gollahalli. "Trajectory and Characteristics of Buoyancy and Momentum Dominated Horizontal Jet Flames From Circular and Elliptic Burners." Journal of Energy Resources Technology 128, no. 4 (October 21, 2005): 300–310. http://dx.doi.org/10.1115/1.2358145.
Full textAbstract:
Relative effects of buoyancy and momentum on the characteristics of horizontally oriented circular (Circ) and elliptic (E) burner flames in a quiescent environment over a wide range of jet exit velocities are presented. The major axis of the elliptic burner was oriented horizontally and vertically (referred to as Emaj and Emin flames, respectively). Propane was used as fuel and a small amount of hydrogen was piloted to attach flames to the burner. Global flame characteristics such as flame dimensions, centerline trajectory, emission indices (EI) and radiative fraction, and in-flame transverse concentration and temperature profiles were measured. At a jet exit Reynolds number (Rej) of 2000, based on the area-equivalent diameter of the burner, the flame characteristics were affected by the burner geometry and its orientation. Also, the vertical dimension of the burner exit dictated buoyancy effects. At Rej=12,500, the influence of burner geometry or its orientation was negligible. Elliptic burner flames exhibited lower liftoff and blowout velocities than circular burner flames. Furthermore, the flame stability and nitric oxide emissions were not much affected by the orientation of elliptic burner. Although the elliptic burners produced higher EINO at lower jet exit velocities, the variation in EINO among three burners (Circ, Emaj, and Emin) was insignificant at higher velocities. Some effects of buoyancy on EICO were observed at lower jet exit velocities and the EICO was the lowest for the burners with largest buoyancy flux. Elliptic burner flames produced greater peak flame temperature than the corresponding circular burner flames under most conditions.
38
Roh, Joo-Hyung, Se-Hong Min, and Min-suk Kong. "Flame Segmentation Characteristics of YCbCr Color Model Using Object Detection Technique." Fire Science and Engineering 37, no. 6 (December 31, 2023): 54–61. http://dx.doi.org/10.7731/kifse.7c1d5c35.
Full textAbstract:
The existing YCbCr color model for flame segmentation has a low segmentation performance for various colored flames and mis-segmentation for flame-like colored-object regions. An improved YCbCr color model using an object detection technique is proposed in this study to improve the flame segmentation performance of the existing YCbCr color model. YOLOv8, a deep learning model for object detection, was used to form a bounding box for the flame to prevent the segmentation of the flame-like colored-object region, and flame segmentation in the bounding box was performed. In addition, YCbCr rules were proposed to segment red and yellow flames to improve flame segmentation performance. The performance evaluation showed that the proposed model increased the intersection over union value by approximately 15.4% compared to that of the existing YCbCr model. In terms of the fire prediction performance evaluation, the precision, recall, and F1-score of the proposed model increased by approximately 15.9%, 28.2%, and 24.7%, respectively.
39
VEYNANTE, DENIS, and THIERRY POINSOT. "Effects of pressure gradients on turbulent premixed flames." Journal of Fluid Mechanics 353 (December 25, 1997): 83–114. http://dx.doi.org/10.1017/s0022112097007556.
Full textAbstract:
In most practical situations, turbulent premixed flames are ducted and, accordingly, subjected to externally imposed pressure gradients. These pressure gradients may induce strong modifications of the turbulent flame structure because of buoyancy effects between heavy cold fresh and light hot burnt gases. In the present work, the influence of a constant acceleration, inducing large pressure gradients, on a premixed turbulent flame is studied using direct numerical simulations.A favourable pressure gradient, i.e. a pressure decrease from unburnt to burnt gases, is found to decrease the flame wrinkling, the flame brush thickness, and the turbulent flame speed. It also promotes counter-gradient turbulent transport. On the other hand, adverse pressure gradients tend to increase the flame brush thickness and turbulent flame speed, and promote classical gradient turbulent transport. As proposed by Libby (1989), the turbulent flame speed is modified by a buoyancy term linearly dependent on both the imposed pressure gradient and the integral length scale lt.A simple model for the turbulent flux u″c″ is also proposed, validated from simulation data and compared to existing models. It is shown that turbulent premixed flames can exhibit both gradient and counter-gradient transport and a criterion integrating the effects of pressure gradients is derived to differentiate between these regimes. In fact, counter-gradient diffusion may occur in most practical ducted flames.
40
Abdul, Gani. "Experimental investigation on lift off, blowout and drop back in partially premixed LPG open flames in tubular burner." Thermal Science, no. 00 (2022): 31. http://dx.doi.org/10.2298/tsci211126031a.
Full textAbstract:
The higher pollutant level in non premixed combustion and safety issues pertaining to premixed combustion can be counteracted by partially-premixed mode of combustion. The partially premixed flames (PPF) exhibit the benefits of both premixed and non premixed flames. PPF enhances complete combustion leading to reduced soot formation and hence lower emission. However, the equivalence ratio plays an important role in the stability of such flames. This paper reports the experimental investigation on the flame characteristics and stability of partially premixed LPG-air flames in tubular burner. The stability curve obtained for the base case without any secondary flow shows that the velocity at lift-off, drop-back and blowout increases with increasing equivalence ratio. In the presence of secondary co-flow air, the lift-off and blow off velocity decreases compared to base case indicating poor stability due to extensive flame stretch leading to aerodynamic quenching. The experimental results show that the velocity of flow at lift off, blow out and drop back are higher in the presence of secondary swirl air than the base case. Co-swirl air increases the stability due to better mixing at the flame base with increased residence time. Flame stability deteriorates with co-flow air as co-flow strains the flame boundary due to flame stretch.
41
CRETA, F., and M. MATALON. "Propagation of wrinkled turbulent flames in the context of hydrodynamic theory." Journal of Fluid Mechanics 680 (June 1, 2011): 225–64. http://dx.doi.org/10.1017/jfm.2011.157.
Full textAbstract:
We study the propagation of premixed flames in two-dimensional homogeneous isotropic turbulence using a Navier–Stokes/front-capturing methodology within the context of hydrodynamic theory. The flame is treated as a thin layer separating burnt and unburnt gases, of vanishingly small thickness, smaller than the smallest fluid scales. The method is thus suitable to investigate the flame propagation in the wrinkled flamelet regime of turbulent combustion. A flow-control system regulates the mean position of the flame and the incident turbulence intensity. In this context we study the individual effects of turbulence intensity, turbulence scale, thermal expansion, hydrodynamic strain and hydrodynamic instability on the propagation characteristics of the flame. Results are obtained assuming positive Markstein length, corresponding to lean hydrocarbon–air or rich hydrogen–air mixtures. For stable planar flames we find a quadratic dependence of turbulent speed on turbulence intensity. Upon onset of hydrodynamic instability, corrugated structures replace the planar conformation and we observe a greater resilience to turbulence, the quadratic scaling being replaced by scaling exponents less than one. Such resilience is also confirmed by the observation of a threshold turbulence intensity below which the propagation speed of corrugated flames is indistinguishable from the laminar speed. Turbulent speed is found to increase and later plateau with increasing thermal expansion, this affecting the average flame displacement but not the mean flame curvature. In addition, turbulence integral scale is also observed to affect the propagation of the flame with the existence of an intermediate scale maximizing the turbulent speed. This maximizing scale is smaller for corrugated flames than it is for planar flames, implying that small eddies that will be unable to significantly perturb a planar front could be rather effective in perturbing a corrugated flame. Turbulent planar flames, and more so corrugated flames, were observed to experience a positive mean hydrodynamic strain, which was explained in terms of the overwhelming mean contribution of the normal component of strain. The positive straining causes a decrease in the mean laminar propagation speed which in turn can decrease the turbulent speed. The effect of the flame on the incident turbulent field was examined in terms of loss of isotropy and vorticity destruction by thermal expansion. The latter can be mitigated by a baroclinic vorticity generation which is enhanced for corrugated flames.
42
Avila Jimenez, Cristian D. Avila, Santiago Cardona, Mohammed A. Juaied, Mourad Younes, Aqil Jamal, Thibault F. Guiberti, and William L. Roberts. "Influence of the Pilot Flame on the Morphology and Exhaust Emissions of NH3-CH4-Air Swirl Flames Using a Reduced-Scale Burner at Atmospheric Pressure." Energies 16, no. 1 (December 25, 2022): 231. http://dx.doi.org/10.3390/en16010231.
Full textAbstract:
This work presents an experimental study on the influence of the pilot flame characteristics on the flame morphology and exhaust emissions of a turbulent swirling flame. A reduced-scale burner, inspired by that fitted in the AE-T100 micro gas turbine, was employed as the experimental platform to evaluate methane (CH4) and an ammonia-methane fuel blend with an ammonia (NH3) volume fraction of 0.7. The power ratio (PR) between the pilot flame and the main flame and the fuel composition of the pilot flame was investigated. The pilot power ratio was varied from 0 to 20% for both fuel compositions tested. The NH3 volume fraction in the pilot flame ranged from pure CH4 to pure NH3 through various NH3–CH4 blends. Flame images and exhaust emissions, namely CO2, CO, NO, and N2O were recorded. It was found that increasing the pilot power ratio produces more stable flames and influences most of the exhaust emissions measured. The CO2 concentration in the exhaust gases was roughly constant for CH4-air or NH3–CH4–air flames. In addition, a CO2 concentration reduction of about 45% was achieved for XNH3 = 0.70 compared with pure CH4, while still producing stable flames as long as PR ≥ 5%. The pilot power ratio was found to have a higher relative impact on NO emissions for CH4 than for NH3–CH4, with measured exhaust NO percentage increments of about 276% and 11%, respectively. The N2O concentration was constant for all pilot power ratios for CH4 but it decreased when the pilot power ratio increased for NH3–CH4. The pilot fuel composition highly affected the NO and N2O emissions. Pure CH4 pilot flames and higher power ratios produced higher NO emissions. Conversely, the NO concentration was roughly constant for pure NH3 pilot flames, regardless of the pilot power ratio. Qualitative OH-PLIF images were recorded to further investigate these trends. Results showed that the pilot power ratio and the pilot fuel composition modified the flame morphology and the OH concentration, which both influence NO emissions.
43
JU, YIGUANG, HONGSHENG GUO, FENGSHAN LIU, and 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 (January 25, 1999): 165–90. http://dx.doi.org/10.1017/s0022112098003231.
Full textAbstract:
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.
44
DuttaRoy, Rahul, SR Chakravarthy, and Ashis Kumar Sen. "Experimental investigation of flame propagation in a meso-combustor." Proceedings of the Institution of Mechanical Engineers, Part A: Journal of Power and Energy 234, no. 8 (December 31, 2019): 1131–46. http://dx.doi.org/10.1177/0957650919897755.
Full textAbstract:
The present work aims to study the quenching of propagating flames in meso-combustors for which dimensions are of the order of quenching distances of hydrocarbon fuels. Combustion of gaseous fuels and subsequent flame propagation in a meso-scale combustor duct of square cross-section is studied experimentally. Premixed mixtures of methane, propane, and ethylene with air are considered. Two different variants of flame propagation states are found to occur in the meso-combustor, viz., one undergoing flame propagation till the combustor entry and quenching at the step and the other undergoing wall quenching. Regime transitions across these flame states are mapped comprehensively over a wide range of operating conditions. The radius of curvature of the flame and the dead space between the flame and the wall are determined for those conditions with the aid of curve fitting and image processing techniques using Matlab software. The spatial and temporal variation of both these parameters show a drastic increase during quenching in the wall-quenched case, while it remains nearly constant in the step quenched case. With increasing duct Reynolds number, the flame propagates slower, and the heat conduction to the wall leads to a decrease in the dead space and flattening of the flame, particularly at equivalence ratios corresponding to lower flame speeds. This flame-wall interaction is found to be low for methane, resulting in more heat loss and thereby wall-quenched flames compared to propane and ethylene. None of the ethylene flames were found to suffer wall quenching thereby making it a suitable fuel for meso-/micro-combustors among the three fuels used in the present work.
45
Zhang, Lili, Yongzhang Cui, Pengfei Yin, Wenlong Mao, and Pengzhao Zhang. "Numerical Investigation on the Flame Characteristics of Lean Premixed Methane Flame Piloted with Rich Premixed Flame." Energies 17, no. 14 (July 12, 2024): 3430. http://dx.doi.org/10.3390/en17143430.
Full textAbstract:
The introduction of the pilot flame can stabilize the lean premixed flame and promote its industrial application. However, the interaction mechanism between the pilot and main flames is complicated. To reveal the effect of the pilot flame on the main flame, a laminar lean premixed flame adjacent to a rich premixed pilot flame on one side and a similar lean premixed flame on the other side was considered. A two-dimensional numerical model was adopted with detailed chemistry and species transport, also with no artificial flame anchoring boundary conditions. The results show that the pilot flame could promote the main flame stabilized in different locations with various shapes, by adjusting the stretch, heat transfer, and preferential diffusion in a complicated manner. The pilot flame improves the local equivalence ratio and transfer more heat to the main flame. The growth of the pilot flame equivalence ratio and inlet velocity enhances the combustion on the rich side of the main flame and helps it anchor closer to the flame wall. Both the curvature and strain rate show a significant effect on the flame root, which contributes to the main flame bending towards the pilot flame.
46
Palies, Paul, Milos Ilak, and Robert Cheng. "Transient and limit cycle combustion dynamics analysis of turbulent premixed swirling flames." Journal of Fluid Mechanics 830 (October 5, 2017): 681–707. http://dx.doi.org/10.1017/jfm.2017.575.
Full textAbstract:
Premixed low swirling flames (methane–air and hydrogen–methane–air) are experimentally investigated for three different regimes. Stable, local transient to instability and limit cycle regimes corresponding to three distinct equivalence ratios are considered. Dynamic mode decomposition is applied to the hydrogen–air–methane flame to retrieve the modes frequencies, growth rates and spatial distributions for each regime. The results indicate that a vortical wave propagating along the flame front is associated with the transition from stability to instability. In addition, it is shown that a key effect on stability is the location of the non-oscillating (0 Hz) flame component. The phase-averaged unsteady motion of the flames over one cycle of oscillation shows the vortical wave rolling up the flame front. The Rayleigh index maps are formed to identify the region of driving and damping of the self-sustained oscillation, while the flame transfer function phase leads to the propagation mode of the perturbations along the flame front. The second mechanism identified concerns the swirl number fluctuation induced by the mode conversion. By utilizing hypotheses for the flow field and the flame structure, it is pointed out that those mechanisms are at work for both flames (methane–air and hydrogen–methane–air) and their effects on the unsteady heat release are determined. Both unsteady heat release contributions, the vortical wave induces flame surface fluctuations and swirl number oscillation induces unsteady turbulent burning velocity, are in phase opposition and of similar amplitudes.
47
Tewarson, A., and M. M. Khan. "Extinguishment of Diffusion Flames of Polymeric Materials by Halon 1301." Journal of Fire Sciences 11, no. 5 (September 1993): 407–20. http://dx.doi.org/10.1177/073490419301100503.
Full textAbstract:
Halon 1301 flame extinction results are discussed for the com bustion of polymethylmethacrylate (PMMA), eight composite materials, and carbon in the gas phase. Two types of combustion and flame extinction experi ments were performed: (1) in the Factory Mutual Research Corporation (FMRC) flammability apparatus (50 kW scale) for PMMA and composite materials, and (2) in the FMRC electrical arc apparatus for carbon in the gas phase. For char forming composite materials, mass transfer from the surface was low, turbulent diffusion flames were not generated, and flame extinction oc curred between 3 to 4.5% of Halon 1301, close to the value reported for the lam inar diffusion flames of polymers. For non-charring PMMA, mass transfer from the surface was high, flames were turbulent, and flame extinction was found at about 6% of Halon 1301, contrary to the accepted value of about 4% for the lam inar diffusion flames of polymers. With Halon 1301 the conditions for flame in stability and extinction for combustion efficiency less than about 0.40, with sig nificant increase in the amounts of products of incomplete combustion (such as CO and hydrocarbon), were in agreement with flame instability and extinction found for fuel-rich conditions inside well-ventilated laminar and turbulent diffusion flames, in ceiling layers of combustion products, in enclosure fires, in ventilation-controlled buoyant diffusion flames of polymers, and for flame ex tinction of heptane flames by water. Experiments in the FMRC electrical arc apparatus showed that in the gas phase combustion of carbon vapors generated in high energy arc, chemical heat release rate and combustion efficiency decreased with increase in Halon 1301. At about 7.5% of Halon 1301, conditions were close to flame extinction and at 9.0%, oxidative pyrolysis of carbon was indicated. Concentrations of Br- and F- ions, generated from the decomposition of Halon 1301, were also measured. Concentration of Br- ions was higher than the concentration of F- ions, al though there are three F atoms and only one Br atom in Halon 1301. There was brown deposit on the walls of the apparatus with extensive corrosion of rubber gaskets, electrical fan, and other components. The techniques discussed in this article appear to be attractive for the assess ment of flame extinguishability and corrosive characteristics of fire suppres sants to replace ozone layer depleting Halons.
48
Joo, S. H., K. M. Chun, Y. Shin, and K. C. Lee. "An Investigation of Flame Expansion Speed With a Strong Swirl Motion Using High-Speed Visualization." Journal of Engineering for Gas Turbines and Power 125, no. 2 (April 1, 2003): 485–93. http://dx.doi.org/10.1115/1.1564067.
Full textAbstract:
In this study, a simple linear supposition method is proposed to separate the flame expansion speed and swirl motion of a flame propagating in an engine cylinder. Two series of images of flames propagating in the cylinder with/without swirl motion were taken by a high frame rate digital video camera. A small tube (4 mm ID) was installed inside the intake port to deliver the fuel/air mixture with strong swirl motion into the cylinder. An LDV was employed to measure the swirl motion during the compression stroke. Under the assumption that flame propagates spherically from the each point of the flame front, a diameter of small spherical flames can be calculated from the two consecutive images of the flame without swirl motion in the cylinder. Using the normalized swirl motion of the mixture during the compression stroke and the spherical flame diameters, the flame expansion speed and swirl ratio of combustion propagation in the engine cylinder can be obtained. This simple linear superposition method for separating the flame expansion speed and swirl motion can be utilized to understand the flow characteristics, such as swirl and turbulence, during the combustion process.
49
Yang, Yang, Xiao Liu, and Zhihao Zhang. "Analysis of V-Gutter Reacting Flow Dynamics Using Proper Orthogonal and Dynamic Mode Decompositions." Energies 13, no. 18 (September 17, 2020): 4886. http://dx.doi.org/10.3390/en13184886.
Full textAbstract:
The current work is focused on investigating the potential of data-driven post-processing techniques, including proper orthogonal decomposition (POD) and dynamic mode decomposition (DMD) for flame dynamics. Large-eddy simulation (LES) of a V-gutter premixed flame was performed with two Reynolds numbers. The flame transfer function (FTF) was calculated. The POD and DMD were used for the analysis of the flame structures, wake shedding frequency, etc. The results acquired by different methods were also compared. The FTF results indicate that the flames have proportional, inertial, and delay components. The POD method could capture the shedding wake motion and shear layer motion. The excited DMD modes corresponded to the shear layer flames’ swing and convect motions in certain directions. Both POD and DMD could help to identify the wake shedding frequency. However, this large-scale flame oscillation is not presented in the FTF results. The negative growth rates of the decomposed mode confirm that the shear layer stabilized flame was more stable than the flame possessing a wake instability. The corresponding combustor design could be guided by the above results.
50
ZHOU, RUI, and ZI-NIU WU. "Fire whirls due to surrounding flame sources and the influence of the rotation speed on the flame height." Journal of Fluid Mechanics 583 (July 4, 2007): 313–45. http://dx.doi.org/10.1017/s0022112007006337.
Full textAbstract:
In this paper, we use numerical simulation and laboratory experimental observation to show that fire whirls can be generated spontaneously through the interaction between a central flame and surrounding organized or randomly distributed flames. The momentum of the air stream entrained by the main flame decreases as it crosses a surrounding flame, so that the main flame rotates if surrounding flames are arranged in such a way as to block the passage of the air stream directed towards the centre of the main flame and to favour flows in a particular circumferential direction. An analysis is performed to study the role of the rotation speed in the flame height. It is found that the flame height initially decreases to a minimum owing to the inflow boundary layer wind reducing the initial vertical velocity of gas for low rotation speed and to entrainment enhancement reducing the rising time, and then it increases owing to the pressure reduction at the centre of the rotating vortex and entrainment suppression extending the rising time.