| 1. |
- Abidakun, Olatunde, et al.
(författare)
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Impacts of fuel nonequidiffusivity on premixed flame propagation in channels with open ends
- 2021
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Ingår i: Physics of fluids. - : American Institute of Physics (AIP). - 1070-6631 .- 1089-7666. ; 33
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Tidskriftsartikel (refereegranskat)abstract
- The present study scrutinizes premixed flame dynamics in micro-channels, thereby shedding light on advanced miniature micro-combustion technologies. While equidiffusive burning (when the Lewis number Le = 1) is a conventional approach adopted in numerous theoretical studies, real premixed flames are typically non-equidiffusive (Le ≠ 1), which leads to intriguing effects, such as diffusional-thermal instability. An equidiffusive computational study [V. Akkerman et al., Combust. Flame 145, 675–687 (2006)] reported regular oscillations of premixed flames spreading in channels having nonslip walls and open extremes. Here, this investigation is extended to nonequidiffusive combustion in order to systematically study the impact of the Lewis number on the flame in this geometry. The analysis is performed by means of computational simulations of the reacting flow equations with fully-compressible hydrodynamics and onestep Arrhenius chemical kinetics in channels with adiabatic and isothermal walls. In the adiabatic channels, which are the main case of study, it is found that the flames oscillate at low Lewis numbers, with the oscillation frequency decreasing with Le, while for the Le > 1 flames, a tendency to steady flame propagation is observed. The oscillation parameters also depend on the thermal expansion ratio and the channel width, although the impacts are rather quantitative than qualitative. The analysis is subsequently extended to the isothermal channels. It is shown that the role of heat losses to the walls is important and may potentially dominate over that of the Lewis number. At the same time, the impact of Le on burning in the isothermal channels is qualitatively weaker than that in the adiabatic channels.
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| 2. |
- 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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| 3. |
- 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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| 4. |
- 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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| 5. |
- 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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| 6. |
- Akkerman, V'yacheslav, et al.
(författare)
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Fast flame acceleration and deflagration-to-detonation transition in smooth and obstructed tubes, channels and slits
- 2013
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Ingår i: 8th US National Combustion Meeting 2013. - : Western States Section/Combustion Institute. - 9781627488426 ; , s. 970-978
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Konferensbidrag (refereegranskat)abstract
- This work is devoted to the comprehensive analytical, computational and experimental investigation of various stages of flame acceleration in narrow chambers. We consider mesoscale two-dimensional channels and cylindrical tubes, smooth and obstructed, and sub-millimeter gaps between two parallel plates. The evolution of the flame shape, propagation speed, acceleration rate, and velocity profiles nearby the flamefront are determined for each configuration, with the theories substantiated by the numerical simulations of the hydrodynamics and combustion equations with an Arrhenius reaction, and by the experiments on premixed hydrogen-oxygen and ethylene-oxygen flames. The detailed analyses demonstrate three different mechanisms of flame acceleration: 1) At the early stages of burning at the closed tube end, the flamefront acquires a finger-shape and demonstrates strong acceleration during a short time interval. While this precursor acceleration mechanism is terminated as soon as the flamefornt touches the side wall of the tube, having a little relation to the deflagration-to-detonation transition (DDT) for relatively slow, hydrocarbon flames; for fast (e.g. hydrogen-oxygen) flames, even a short finger-flame acceleration may amplify the flame propagation speed up to sonic values, with an important effect on the subsequent DDT process. 2) On the other hand, the classical mechanism of flame acceleration due to wall friction in smooth tubes is basically unlimited in time, but it depends noticeably on the tube width such that the acceleration rate decreases strongly with the Reynolds number. The entire DDT scenario includes four distinctive stages: (i) initial exponential acceleration at the quasi-incompressible state; (ii) moderation of the process because of gas compression; (iii) eventual saturation to a quasisteady, high-speed flames correlated with the Chapman-Jouguet deflagration; (iv) finally, the heating of the fuel mixture leads to the explosion ahead of the flame front, which develops into a self-supporting detonation. 3) In addition, we have revealed a physical mechanism of extremely fast flame acceleration in channels/tubes with obstacles. Combining the "benefits" of 1) and 2), this new mechanism is based on delayed burning between the obstacles, creating a powerful jet-flow and thereby driving the acceleration, which is extremely strong and independent of the Reynolds number, so the effect can be fruitfully utilized at industrial scales. Understanding of this mechanism provides the guide for optimization of the obstacle shape, while this task required tantalizing cut-and-try methods previously. On the other hand, our formulation opens new technological possibilities of DDT in micro-combustion.
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| 7. |
- 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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| 8. |
- Bychkov, Vitaly, et al.
(författare)
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Gas compression moderates flame acceleration in deflagration-to-detonation transition
- 2012
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Ingår i: Combustion Science and Technology. - : Informa UK Limited. - 0010-2202 .- 1563-521X. ; 184:7-8, s. 1066-1079
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Tidskriftsartikel (refereegranskat)abstract
- The effect of gas compression at the developed stages of flame acceleration in smooth-wall and obstructed channels is studied. We demonstrate analytically that gas compression moderates the acceleration rate, and we perform numerical simulations within the problem of flame transition to detonation. It is shown that flame acceleration undergoes three distinctive stages: (1) initial exponential acceleration in the incompressible regime, (2) moderation of the acceleration process due to gas compression, so that the exponential acceleration state goes over to a much slower one, (3) eventual saturation to a steady (or statistically steady) high-speed deflagration velocity, which may be correlated with the Chapman-Jouguet deflagration speed. The possibility of deflagration-to-detonation transition is demonstrated.
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| 9. |
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| 10. |
- Bychkov, Vitaly, et al.
(författare)
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Physical Mechanism of Ultrafast Flame Acceleration
- 2008
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Ingår i: Physical Review Letters. - 0031-9007 .- 1079-7114. ; 101:16, s. 164501-1-164501-4
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Tidskriftsartikel (refereegranskat)abstract
- We explain the physical mechanism of ultrafast flame acceleration in obstructed channels used in modern experiments on detonation triggering. It is demonstrated that delayed burning between the obstacles creates a powerful jetflow, driving the acceleration. This mechanism is much stronger than the classical Shelkin scenario of flame acceleration due to nonslip at the channel walls. The mechanism under study is independent of the Reynolds number, with turbulence playing only a supplementary role. The flame front accelerates exponentially; the analytical formula for the growth rate is obtained. The theory is validated by extensive direct numerical simulations and comparison to previous experiments.
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| 11. |
- Bychkov, Vitaly, et al.
(författare)
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Role of Compressibility in Moderating Flame Acceleration in Tubes
- 2010
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Ingår i: Physical Review E. Statistical, Nonlinear, and Soft Matter Physics. - 1063-651X .- 1095-3787. ; 81:2, s. 026309-
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Tidskriftsartikel (refereegranskat)abstract
- The effect of gas compression on spontaneous flame acceleration leading to deflagration-to-detonation transition is studied theoretically for small Reynolds number flame propagation from the closed end of a tube. The theory assumes weak compressibility through expansion in small Mach number. Results show that the flame front accelerates exponentially during the initial stage of propagation when the Mach number 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.
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| 12. |
- Bychkov, Vitaly, 1968-, et al.
(författare)
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Speedup of doping fronts in organic semiconductors through plasma instability
- 2011
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Ingår i: Physical Review Letters. - 0031-9007 .- 1079-7114. ; 107:1, s. 016103-016107
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Tidskriftsartikel (refereegranskat)abstract
- The dynamics of doping transformation fronts in organic semiconductor plasma is studied for application in light-emitting electrochemical cells. We show that new fundamental effects of the plasma dynamics can significantly improve the device performance. We obtain an electrodynamic instability, which distorts the doping fronts and increases the transformation rate considerably. We explain the physical mechanism of the instability, develop theory, provide experimental evidence, perform numerical simulations, and demonstrate how the instability strength may be amplified technologically. The electrodynamic plasma instability obtained also shows interesting similarity to the hydrodynamic Darrieus-Landau instability in combustion, laser ablation, and astrophysics.
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| 13. |
- Chen, Canruo, et al.
(författare)
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On the stabilization mechanism of high-speed deflagrations in narrow channels with heat loss
- 2024
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Ingår i: Proceedings of the Combustion Institute. - : Elsevier. - 1540-7489 .- 1873-2704. ; 40:1-4
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Tidskriftsartikel (refereegranskat)abstract
- Statistically steady supersonic deflagrations are numerically investigated in narrow channels with strong thermal expansion and heat loss. Four modes of flame propagation are observed, namely, extinction, low-speed deflagration, high-speed deflagration, and DDT. It is determined that larger thermal expansion facilitates initiation of high-speed deflagrations while the heat loss can suppress the transition to detonation. The high-speed deflagration mode is shown to be the result of the dynamic balance between thermal expansion and wall heat loss. The limits of high-speed deflagration in terms of the thermal expansion and heat loss coefficients are determined. The statistically steady oscillatory high-speed deflagrations propagate at average velocities close to half of the CJ detonation velocity. The dynamics of the flame front and shock waves are visualized using numerical schlieren. Periodic acceleration and deceleration of the leading shock are identified, and the mechanism of DDT suppression is elucidated.
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| 14. |
- Demirgok, Berk, et al.
(författare)
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Analysis of ethylene-oxygen combustion in micro-pipes
- 2013
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Ingår i: Fall Technical Meeting of the Eastern States Section of the Combustion Institute 2013. - : Combustion Institute. - 9781629937199 ; , s. 155-160
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Konferensbidrag (refereegranskat)abstract
- Propagation of premixed stoichiometric ethylene-oxygen flames in cylindrical pipes of sub/near-millimeter radii is investigated-computationally, analytically and experimentally. Namely, various stages of flame evolution such as quasi-isobaric, exponential acceleration; its moderation due to gas compression; and eventual saturation to the Chapmen-Jouget deflagration are consdiered. Specifically, we have determined the dynamics and morphology of the flame front, its propagation velocity and acceleration rate. Due to viscous heating, the entire process can be followed by the detonation initiation ahead of the flame front. The computational component of this research includes numerical solution of the hydrodynamics and combustion equations with chemical kinetics represented by one-step Arrhenius reaction. The theoretical model accounts for small, but finite Mach number; and it assumes a plane-parallel flame-generated flow, zero flame thickness as well as large thermal expansion and flame-related Reynolds number. The overall study bridges the gap between the experiments of Wu et al. [Proc. Combust. Inst. 31 (2007) 2429] and the analytical formulation of Akkerman et al. [Combust. Flame 145 (2006) 206].
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| 15. |
- 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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| 16. |
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| 17. |
- Dion, Claude, et al.
(författare)
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Dynamics of flame extinction in narrow channels with cold walls: Heat loss vs acceleration
- 2021
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Ingår i: Physics of Fluids. - : AIP Publishing. - 1070-6631 .- 1089-7666. ; 33:3
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Tidskriftsartikel (refereegranskat)abstract
- Propagation of a premixed flame from a closed to an open end in micro-channels with smooth non-slip isothermal walls is considered in the context of flame extinction dynamics. Powerful exponential flame acceleration in micro-channels with adiabatic walls has been demonstrated at the initial quasi-isobaric stage of the process [Bychkov et al., Phys. Rev. E 72, 046307 (2005)]. In contrast to the previous studies, here we investigate flame propagation in channels with isothermal walls. The problem is solved by means of high-fidelity laminar numerical simulations of the complete set of the Navier-Stokes combustion equations. For most of the problem parameter sets chosen, we obtain initial flame acceleration after ignition at the closed channel end. This acceleration resembles qualitatively the adiabatic case, but it develops noticeably slower, in an approximately linear regime instead of the exponential one and persists only for a limited time interval. Subsequently, heat loss to the walls reduces the temperature and hence the volume of the burnt gas behind the flame front, which produces a reverse flow in the direction of the closed channel end. When the amount of the burnt gas becomes sufficiently large, the reverse flow stops the acceleration process and drives the flame backwards with modifications of the flame front shape from convex to concave. Eventually, the flame extinguishes. Qualitatively, the process obtained reproduces a possible combustion failure during deflagration-to-detonation transition observed in previous experiments. We investigate the key characteristics of initial flame acceleration such as the acceleration rate and the maximum speed of the flame tip.
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| 18. |
- Dion, Claude, 1970-, et al.
(författare)
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Flames in channels with cold walls : acceleration versus extinction
- 2015
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Ingår i: MCS 9.
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Konferensbidrag (refereegranskat)abstract
- The present work considers the problem of premixed flame front acceleration in microchannelswith smooth cold non-slip walls in the context of the deflagration-to-detonationtransition; the flame accelerates from the closed channel end to the open one. Recently, anumber of theoretical and computational papers have demonstrated the possibility of powerfulflame acceleration for micro-channels with adiabatic walls. In contrast to the previous studies,here we investigate the case of flame propagation in channels with isothermal cold walls. Theproblem is solved by using direct numerical simulations of the complete set of the Navier-Stokes combustion equations. We obtain flame extinction for narrow channels due to heat lossto the walls. However, for sufficiently wide channels, flame acceleration is found even for theconditions of cold walls in spite of the heat loss. Specifically, the flame accelerates in thelinear regime in that case. While this acceleration regime is quite different from theexponential acceleration predicted theoretically and obtained computationally for theadiabatic channels, it is consistent with the previous experimental observations, whichinevitably involve thermal losses to the walls. In this particular work, we focus on the effectof the Reynolds number of the flow on the manner of the flame acceleration.
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| 19. |
- Feng, Ruixue, et al.
(författare)
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Effect of thermal gas expansion on the fractal structure of outwardly propagating flames
- 2019
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- The role of Darrieus-Landau instability in forming the fractal structure of the freely propagating expanding flame front is studied numerically by solving the two-dimensional Navier-Stokes equations with one step irreversible Arrhenius reaction. Numerical simulation of the radially expanding flames with Le=1 demonstrates that the temporal dependence of mean radius after a certain critical time instant is corresponding to a power law, in line with previous experimental, numerical and theoretical studies. Influence of gas expansion ratio on power-law exponent and flame surface fractal dimension is the focus of this research. It is shown that the expansion coefficient has an effect on fractal structure of the outwardly propagating flame, confirming the assumption that fractal dimension is not a universal parameter.
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| 20. |
- Feng, Ruixue, et al.
(författare)
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Influence of gas expansion on the interaction between spatially periodic shear flow and premixed flame
- 2017
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Ingår i: 11th Asia-Pacific Conference on Combustion 2017. - : Curran Associates. - 9781510856462 ; , s. 807-810
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- It has been previously shown that thermal expansion may have a role in a boundary layer flashback of premixed turbulent flames [Gruber et al., J Fluid Mech 2012], enhancing the flame velocity through flame corrugation. The current study focuses on the two-dimensional numerical investigation of the interaction of a periodic shear flow and a laminar premixed flame. The periodic shear is a simplified model for the oncoming prolonged velocity streaks with alternating regions of high and low velocities found in a turbulent channel flow close to the wall. The parametric study focuses on the amplitude and wavelength of the periodic shear flow, and gas expansion ratio. It has been found that with the increase of the amplitude of the periodic shear flow, as well as the gas expansion, the curved flame velocity increases monotonically. The flame velocity dependence on the periodic shear wavelength is non-monotonic, which is consistent with previous theoretical studies of the curved premixed flame velocity.
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| 21. |
- Feng, Ruixue, et al.
(författare)
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Influence of gas expansion on the propagation of a premixed flame in a spatially periodic shear flow
- 2021
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Ingår i: Combustion and Flame. - : Elsevier. - 0010-2180 .- 1556-2921. ; 227, s. 421-427
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Tidskriftsartikel (refereegranskat)abstract
- It has been previously demonstrated that thermal gas expansion might have a role in boundary layer flashback of premixed turbulent flames [Gruber et al., J Fluid Mech 2012], inducing local flow-reversal in the boundary layer's low-velocity streaks on the reactants’ side of the flame and facilitating its upstream propagation. We perform a two-dimensional numerical investigation of the interaction between a periodic shear flow and a laminar premixed flame. The periodic shear is a simplified model for the oncoming prolonged streamwise velocity streaks with alternating regions of high and low velocities found in turbulent boundary layers in the vicinity of the walls. The parametric study focuses on the amplitude and wavelength of the periodic shear flow and on the gas expansion ratio (unburnt-to-burnt density ratio). With the increase of the amplitudes of the periodic shear flow and of the gas expansion, the curved flame velocity increases monotonically. The flame velocity dependence on the periodic shear wavelength is non-monotonic, which is consistent with previous theoretical studies of curved premixed flame velocity. The flame shape that is initially formed by the oncoming periodic shear appears to be metastable. At a later stage of the flame propagation, the flame shape transforms into the stationary one dominated by the Darrieus-Landau instability.
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| 22. |
- Gruber, Andrea, et al.
(författare)
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Direct numerical simulation of premixed flame boundary layer flashback in turbulent channel flow
- 2012
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Ingår i: Journal of Fluid Mechanics. - : Cambridge University Press. - 0022-1120 .- 1469-7645. ; 709, s. 516-542
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Tidskriftsartikel (refereegranskat)abstract
- Direct numerical simulations are performed to investigate the transient upstream propagation (flashback) of premixed hydrogen–air flames in the boundary layer of a fully developed turbulent channel flow. Results show that the well-known near-wall velocity fluctuations pattern found in turbulent boundary layers triggers wrinkling of the initially flat flame sheet as it starts propagating against the main flow direction, and that the structure of the characteristic streaks of the turbulent boundary layer ultimately has an important impact on the resulting flame shape and on its propagation mechanism. It is observed that the leading edges of the upstream-propagating premixed flame are always located in the near-wall region of the channel and assume the shape of several smooth, curved bulges propagating upstream side by side in the spanwise direction and convex towards the reactant side of the flame. These leading-edge flame bulges are separated by thin regions of spiky flame cusps pointing towards the product side at the trailing edges of the flame. Analysis of the instantaneous velocity fields clearly reveals the existence, on the reactant side of the flame sheet, of backflow pockets that extend well above the wall-quenching distance. There is a strong correspondence between each of the backflow pockets and a leading edge convex flame bulge. Likewise, high-speed streaks of fast flowing fluid are found to be always colocated with the spiky flame cusps pointing towards the product side of the flame. It is suggested that the origin of the formation of the backflow pockets, along with the subsequent mutual feedback mechanism, is due to the interaction of the approaching streaky turbulent flow pattern with the Darrieus–Landau hydrodynamic instability and pressure fluctuations triggered by the flame sheet. Moreover, the presence of the backflow pockets, coupled with the associated hydrodynamic instability and pressure–flow field interaction, greatly facilitate flame propagation in turbulent boundary layers and ultimately results in high flashback velocities that increase proportionately with pressure.
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| 23. |
- Gruber, Andrea, et al.
(författare)
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Modeling of mean flame shape during premixed flame flashback in turbulent boundary layers
- 2014
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Ingår i: Proceedings of the Combustion Institute. - : Elsevier BV. - 0082-0784 .- 1878-027X .- 1540-7489.
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Tidskriftsartikel (refereegranskat)abstract
- Direct numerical simulations of freely-propagating premixed flames in the turbulent boundary layer of fully-developed turbulent channel flows are used for a priori validation of a new model that aims to describe the mean shape of the turbulent flame brush during flashback. Comparison with the DNS datasets, for both fuel-lean and fuel-rich mixture conditions and for Damköhler numbers lower and larger than unity, shows that the model is able to capture the main features of the flame shape. Although further a priori and a posteriori validation is required, particularly at higher Reynolds numbers, this new simple model seems promising and can potentially have impact on the design process of industrial combustion equipment.
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| 24. |
- Huang, Kai, et al.
(författare)
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Combined impact of the Lewis number and thermal expansion on laminar flame flashback in tubes
- 2024
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Ingår i: Fluids. - : MDPI. - 2311-5521. ; 9:1
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Tidskriftsartikel (refereegranskat)abstract
- The understanding of the boundary layer flame flashback (BLF) has considerably improved in recent decades, driven by the increasing focus on clean energy and the need to address the operational issues associated with flashback. This study investigates the influence of the Lewis number (Le) on symmetric flame shapes under the critical conditions for a laminar boundary layer flashback in cylindrical tubes. It has been found that the transformation of the flame shape from a mushroom to a tulip happens in a tube of a given radius, as the thermal expansion coefficient and Le are modified. A smaller Lewis number results in a local increase in the burning rate at the flame tip, with the flame being able to propagate closer to the wall, which significantly increases the flashback propensity, in line with previous findings. In cases with a Lewis number smaller than unity, a higher thermal expansion results in a flame propagation happening closer to the wall, thus facing a weaker oncoming flow and, consequently, becoming more prone to flashback. For Le > 1, the effect of the increase in the thermal expansion coefficient on the flashback tendency is much less pronounced.
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| 25. |
- Huang, Kai, et al.
(författare)
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Numerical study of the influence of the thermal gas expansion on the boundary layer flame flashback in channels with different wall thermal conditions
- 2023
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Ingår i: Energies. - : MDPI. - 1996-1073. ; 16:4
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Tidskriftsartikel (refereegranskat)abstract
- In recent years, boundary layer flame flashback (BLF) has re-emerged as a technological and operational issue due to the more widespread use of alternative fuels as a part of a global effort to promote carbon neutrality. While much understanding has been achieved in experiments and simulations of BLF in the past decades, the theoretical modeling of BLF still largely relies on the progress made as early as the 1940s, when the critical gradient model (CGM) for the laminar flame flashback was proposed by Lewis and von Elbe. The CGM does not account for the modification of the upstream flow by the flame, which has been recently shown to play a role in BLF. The aim of the present work is to gain additional insight into the effects of thermal gas expansion and confinement on the flame-flow interaction in laminar BLF. Two-dimensional simulations of the confined laminar BLF in a channel are performed in this work. The parametric study focuses on the channel width, the thermal gas expansion coefficient, and the heat losses to the wall. This study evaluates the influence of these factors on the critical condition for the flame flashback. By varying the channel width, it is demonstrated that at the critical condition, the incoming flow in narrow channels is modified globally by the thermal gas expansion, while in wider channels, the flow modification by the flame tends to be more local. In narrow channels, a non-monotonic dependence of the critical-condition centerline velocity on the channel width has been identified. The variation of the heat loss to the wall confirms that the wall’s thermal conditions can significantly alter the flashback limit, with the flashback propensity being larger when the thermal resistance of the wall is high. To assess the general applicability of the CGM, the flame consumption speed and the flow velocity near the wall are quantified. The results confirm that the assumption of flame having no influence on the upstream flow, employed in the CGM, is not fulfilled under confinement for a realistic thermal gas expansion. This results in a general disagreement between the simulations and the CGM, which implies that the thermal expansion effects should be accounted for when considering the confined boundary layer flashback limits. It is shown that the critical velocity gradient increases with the gas expansion coefficient for the given channel width and wall thermal condition.
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