| 1. |
- Adebiyi, Abdulafeez, et al.
(författare)
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Computational simulations of nonequidiffusive premixed flames in obstructed pipes
- 2018
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Konferensbidrag (refereegranskat)abstract
- The impact of the Lewis number, Le, on the dynamics and morphology of a premixed flame front, spreading through a toothbrush-like array of obstacles in a semi-open channel, is studied by means of the computational simulations of the reacting flow equations with fully-compressible hydrodynamics and Arrhenius chemical kinetics. The computational approach employs a cell-centered, finite-volume numerical scheme, which is of the 2nd-order accuracy in time, 4th-order in space for the convective terms, and of the 2nd-order in space for the diffusive terms. The channels of blockage ratios 0.33∼0.67 are considered, with the Lewis numbers in the range 0.2≤Le≤2.0 employed. It is shown that the Lewis number influences the flame evolution substantially. Specifically, flame acceleration weakens for Le>1 (inherent to fuel-lean hydrogen or fuel-rich hydrocarbon burning), presumably, due to a thickening of the flame front. In contrast, Le<1 flames (such as that of rich hydrogen or lean hydrocarbon) acquire an extra strong folding of the front and thereby accelerate even much faster. The later effect can be devoted to the onset of the diffusional-Thermal combustion instability. © 2018 Eastern States Section of the Combustion Institute. All rights reserved.
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| 2. |
- Adebiyi, Abdulafeez, et al.
(författare)
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Effect of surface friction on ultrafast flame acceleration in obstructed cylindrical pipes
- 2019
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Ingår i: AIP Advances. - : American Institute of Physics (AIP). - 2158-3226. ; 9:3
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Tidskriftsartikel (refereegranskat)abstract
- The Bychkov model of ultrafast flame acceleration in obstructed tubes [Valiev et al., "Flame Acceleration in Channels with Obstacles in the Deflagration-to-Detonation Transition," Combust. Flame 157, 1012 (2010)] employed a number of simplifying assumptions, including those of free-slip and adiabatic surfaces of the obstacles and of the tube wall. In the present work, the influence of free-slip/non-slip surface conditions on the flame dynamics in a cylindrical tube of radius R, involving an array of parallel, tightly-spaced obstacles of size αR, is scrutinized by means of the computational simulations of the axisymmetric fully-compressible gasdynamics and combustion equations with an Arrhenius chemical kinetics. Specifically, non-slip and free-slip surfaces are compared for the blockage ratio, α, and the spacing between the obstacles, ΔZ, in the ranges 1/3 ≤ α ≤ 2/3 and 0.25 ≤ ΔZ/R ≤ 2.0, respectively. For these parameters, an impact of surface friction on flameacceleration is shown to be minor, only 1-4%, slightly facilitating acceleration in a tube with ΔZ/R = 0.5 and moderating acceleration in thecase of ΔZ/R = 0.25. Given the fact that the physical boundary conditions are non-slip as far as the continuum assumption is valid, the presentwork thereby justifies the Bychkov model, employing the free-slip conditions, and makes its wider applicable to the practical reality. Whilethis result can be anticipated and explained by a fact that flame propagation is mainly driven by its spreading in the unobstructed portion ofan obstructed tube (i.e. far from the tube wall), the situation is, however, qualitatively different from that in the unobstructed tubes, wheresurface friction modifies the flame dynamics conceptually.
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| 3. |
- Akkerman, Vyacheslav, et al.
(författare)
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Effect of gas compression on flame acceleration in obstructed cylindrical tubes
- 2016
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Ingår i: Spring Technical Meeting of the Eastern States Section of the Combustion Institute 2016. - : Combustion Institute; Curran Associates, Inc.. - 9781510822566
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- The role of gas compression on the process of extremely fast flame acceleration in obstructed cylindrical tubes is studied analytically and validated by computational simulations. The acceleration leading to a deflagration-to-detonation transition is associated with a powerful jet-flow produced by delayed combustion in spaces between the obstacles. This acceleration mechanism is Reynolds-independent and conceptually laminar, with turbulence playing only a supplementary role. In this particular work, the incompressible formulation [Combust. Flame 157 (2010) 1012], Ref. 15 is extended to account for small but finite initial Mach number up to the first-order terms. While flames accelerate exponentially during the initial stage of propagation, when the compressibility is negligible, with continuous increase in the flame velocity with respect to the tube wall, the flame-generated compression waves subsequently moderate the acceleration process by affecting the flame shape and velocity, as well as the flow driven by the flame. It is demonstrated that the moderation effect is substantial, and as soon as gas compression is relatively small, the present theory is in good quantitative agreement with the computational simulations. The limitations of the incompressible theory are thereby underlined, and a critical blockage ratio for with this acceleration mechanism can be evaluated.
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| 4. |
- Akkerman, Vyacheslav, et al.
(författare)
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Impacts of the Lewis and Markstein numbers effects on the flame acceleration in channels
- 2016
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- The effects of flame stretch and thermal/molecular diffusion on the flame acceleration in channels are quantified by means of the analytical and computational endeavours. The internal transport flame properties are accounted in the theory by means of the Markstein number, Mk. Being a positive or negative function of the thermal-chemical combustion parameters, such as the thermal expansion ratio and the Lewis and Zeldovich numbers, the Markstein number either moderates or promotes the flame acceleration. While Mk may provide a substantial impact on the flame acceleration rate in narrow channels, this effects diminishes with the increase of the channel width. The analysis is accompanied by extensive computational simulations of the Navier-Stokes combustion equations, which clarify the impact of the Lewis number on the flame acceleration. It is obtained that, for Le below a certain critical value, at the initial stage of flame acceleration, a globally-convex flame front is splits into two or more "fingers", accompanied by a drastic increase in the flame surface area and associated enhancement of the flame acceleration. Overall, the thermal-diffusive effects substantially facilitate the flame acceleration scenario, thereby advancing a potential deflagration-to-detonation transition.
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| 5. |
- Akkerman, V'yacheslav, et al.
(författare)
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Theory of flame acceleration in open/vented obstructed pipes
- 2016
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Ingår i: 2016 Spring Technical Meeting of the Eastern States Section of the Combustion Institute, ESSCI 2016. - : Eastern States Section of the Combustion Institute.
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Konferensbidrag (refereegranskat)abstract
- A shockless, conceptually-laminar formulation on extremely fast flame acceleration in semi-open obstructed pipes [Physical Review Letters 101 (2008) 164501; Combust. Flame 157 (2010) 1012], Refs. [8-9] is extended to pipes with both ends open/vented. The acceleration is devoted to a powerful jet-flow produced by delayed combustion in the pockets between the obstacles, and it leads to a prompt deflagration-to-detonation transition event. Starting with inviscid approximation, the analysis subsequently incorporates the viscous forces (hydraulic resistance). The theory is validated by the recent experiments [http://arxiv.org/abs/1208.6453], Ref. [11]. It is shown that hydraulic resistance is not required to drive the flame acceleration. In contrast, this is a supplementary effect, which actually moderates the acceleration rate. On the other hand, hydraulic resistance plays an important role: it is responsible for the initial delay, before the flame acceleration onset, observed in the experiments. It is demonstrated that flames accelerate strongly in open/vented obstructed pipes, and the acceleration mechanism is qualitatively the same as that in the semi-open ones. However, because of the flame-generated flow distributed upward and downward of the flame front, the acceleration rate in open pipes is noticeably less than that in the semi-open ones.
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| 6. |
- Alkhabbaz, Mohammed, et al.
(författare)
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Impact of the Lewis number on finger flame acceleration at the early stage of burning in channels and tubes
- 2019
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Ingår i: Physics of fluids. - : American Institute of Physics (AIP). - 1070-6631 .- 1089-7666. ; 31:8
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Tidskriftsartikel (refereegranskat)abstract
- For premixed combustion in channels and tubes with one end open, when a flame is ignited at the centerline at the closed end of the pipe and it propagates toward the open one, significant flame acceleration occurs at an early stage of the combustion process due to formation of a finger-shaped flame front. This scenario is tagged "finger flame acceleration" (FFA), involving an initially hemispherical flame kernel, which subsequently acquires a finger shape with increasing surface area of the flame front. Previous analytical and computational studies of FFA employed a conventional assumption of equidiffusivity when the thermal-to-mass-diffusivity ratio (the Lewis number) is unity (Le = 1). However, combustion is oftentimes nonequidiffusive (Le ≠ 1) in practice such that there has been a need to identify the role of Le in FFA. This demand is addressed in the present work. Specifically, the dynamics and morphology of the Le ≠ 1 flames in two-dimensional (2D) channels and cylindrical tubes are scrutinized by means of the computational simulations of the fully compressible reacting flow equations, and the role of Le is identified. Specifically, the Le > 1 flames accelerate slower as compared with the equidiffusive ones. In contrast, the Le < 1 flames acquire stronger distortion of the front, experience the diffusional-thermal combustion instability, and thereby accelerate much faster than the Le = 1 flames. In addition, combustion in a cylindrical configuration shows stronger FFA than that under the same burning conditions in a 2D planar geometry.
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| 7. |
- Bychkov, Vitaly, et al.
(författare)
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Analysis of flame acceleration in open or vented obstructed pipes
- 2017
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Ingår i: PHYSICAL REVIEW E. - 2470-0045 .- 2470-0053. ; 95:1
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Tidskriftsartikel (refereegranskat)abstract
- While flame propagation through obstacles is often associated with turbulence and/or shocks, Bychkov et al. [V. Bychkov et al., Phys. Rev. Lett. 101, 164501 (2008)] have revealed a shockless, conceptually laminar mechanism of extremely fast flame acceleration in semiopen obstructed pipes (one end of a pipe is closed; a flame is ignited at the closed end and propagates towards the open one). The acceleration is devoted to a powerful jet flow produced by delayed combustion in the spaces between the obstacles, with turbulence playing only a supplementary role in this process. In the present work, this formulation is extended to pipes with both ends open in order to describe the recent experiments and modeling by Yanez et al. [J. Yanez et al., arXiv: 1208.6453] as well as the simulations by Middha and Hansen [P. Middha and O. R. Hansen, Process Safety Prog. 27, 192 (2008)]. It is demonstrated that flames accelerate strongly in open or vented obstructed pipes and the acceleration mechanism is similar to that in semiopen ones (shockless and laminar), although acceleration is weaker in open pipes. Starting with an inviscid approximation, we subsequently incorporate hydraulic resistance (viscous forces) into the analysis for the sake of comparing its role to that of a jet flow driving acceleration. It is shown that hydraulic resistance is actually not required to drive flame acceleration. In contrast, this is a supplementary effect, which moderates acceleration. On the other hand, viscous forces are nevertheless an important effect because they are responsible for the initial delay occurring before the flame acceleration onset, which is observed in the experiments and simulations. Accounting for this effect provides good agreement between the experiments, modeling, and the present theory.
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| 8. |
- Demir, Sinan, et al.
(författare)
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Analysis of "finger" flame acceleration as a stage of a methane air-dust fire in a coal mine
- 2016
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Ingår i: ASME 2015 Power Conference. - : The american society of mechanical engineers. - 9780791856604
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Konferensbidrag (refereegranskat)abstract
- To reveal the inner mechanism of gas explosion, the entire scenario of premixed flame front evolution within an accidental fire is prescribed. Specifically, "finger" flame shape, which is one of the key stages of flame evolution, is scrutinized with the situation of a methane-air explosion. A transition from a globally -spherical front to a finger-shaped one occurs when a flame starts approaching the passage walls. While this acceleration is extremely strong, it stops as soon as the flame touches the passage wall. This mechanism is Reynolds-independent; being equally relevant to micro channels and giant tunnels. The flame speed increases by an order of magnitude during this stage. To implement dusty environments, Seshadri formulation for the planar flame [Combustion and Flame 89 (7992) 333] is employed with a non-uniform distribution of inert dust gradients, specifically, linear, parabolic and hyperbolic spatial dust distribution gradients are incorporated into the "finger" flame shape. This study systematically investigates how the noncombustible dust distributions affect fire evolution, the flame shape, and propagation velocity.
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| 9. |
- Demir, Sinan, et al.
(författare)
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Towards a predictive scenario of a burning accident in a mining passage
- 2017
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Ingår i: Combustion theory and modelling. - : TAYLOR & FRANCIS LTD. - 1364-7830 .- 1741-3559. ; 21:6, s. 997-1022
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Tidskriftsartikel (refereegranskat)abstract
- To reveal the inner mechanisms of a combustion accident in a coalmine, the key stages and characteristics of premixed flame front evolution such as the flame shapes, propagation speeds, acceleration rates, run-up distances and flame-generated velocity profiles are scrutinised. The theories of globally spherical, expanding flames and of finger-flame acceleration are combined into a general analytical formulation. Two-dimensional and cylindrical mining passages are studied, with noticeably stronger acceleration found in the cylindrical geometry. The entire acceleration scenario may promote the total burning rate by up to two orders of magnitude, to a near-sonic value. Starting with gaseous combustion, the analysis is subsequently extended to gaseous-dusty environments. Specifically, combustible dust (e.g. coal), inert dust (e.g. sand), and their combination are considered, and the influence of the size and concentration of the dust particles is quantified. In particular, small particles influence flame propagation more than large ones, and flame acceleration increases with the concentration of a combustible dust, until the concentration attains a certain limit.
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| 10. |
- Demirgok, Berk, et al.
(författare)
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Effect of thermal expansion on flame propagation in channels with nonslip walls
- 2015
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Ingår i: Proceedings of the Combustion Institute. - New York : Elsevier. - 0082-0784 .- 1878-027X .- 1540-7489. ; 35:1, s. 929-936
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Tidskriftsartikel (refereegranskat)abstract
- Propagation of premixed flames in narrow channels is investigated by means of extensive numerical simulations of a complete system of combustion and hydrodynamic equations, incorporating transport properties (thermal conduction, diffusion and viscosity) and Arrhenius chemical kinetics. The system includes mass conservation and Navier–Stokes equations as well as those for the energy and species balance. A flame propagates from the closed end of a channel to the open one. An initially planar flame front gets corrugated due to wall friction and thereby accelerates. It is shown that a flame exhibits an exponential state of acceleration only when the thermal expansion coefficient Θ exceeds a certain critical value Θ>Θc. The quantity Θc is tabulated as a function of the Reynolds number related to the flame propagation, Re, being Θc≈6 for Re=5∼20. The major flame characteristics such as the flame propagation speed and acceleration rate are scrutinized. It is demonstrated that the acceleration promotes with Θ but weakens with Re. In this respect, the present computational results support the theoretical prediction of Bychkov et al . Physical Review E 72 (2005) 046307 in a wide range of Θ and Re. While very good quantitative and qualitative agreement between numerical and theoretical results is found for realistically large thermal expansion, Θ>=8, agreement deteriorates with decreasing Θ. Specifically, while the theory and modeling do not quantitatively agree for Θc<Θ<8, they nevertheless demonstrate a qualitative resemblance (the exponential state of acceleration). Finally, no exponential acceleration at Θ<Θc denotes that the theory completely breaks in that case, but this fits other works in the field and thereby allows reconciling various formulations on the flame acceleration.
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