| 1. |
- 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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| 2. |
- 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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| 3. |
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| 4. |
- 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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| 5. |
- Ugarte, Orlando, et al.
(författare)
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Effect of wall boundary conditions on flame propagation in micro-chambers
- 2016
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Ingår i: PROCEEDINGS OF THE ASME POWER CONFERENCE, 2015. - : The american society of mechanical engineers.
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Konferensbidrag (refereegranskat)abstract
- Flame dynamics in micro-pipes have been observed to be strongly affected by the wall boundary conditions. In this respect, two mechanisms of flame acceleration are related to the momentum transferred in these regions: 1) that associated with flame stretching produced by wall friction forces; and 2) when obstacles are placed at the walls, as a result of the delayed burning occurring between them, a jet-flow is formed, intensively promoting the flame spreading. Wall thermal conditions have usually been neglected, thus restricting the cases to adiabatic wall conditions. In contrast, in the present work, the effect of the boundary conditions on the flame propagation dynamics is investigated, computationally, with the effect of wall heat losses included in the consideration. In addition, the powerful flame acceleration attained in obstructed pipes is studied in relation to the obstacle size, which determines how different this mechanism is from the wall friction. A parametric study of two-dimensional (2D) channels and cylindrical tubes, of various radiuses, with one end open is performed. The walls are subjected to slip and non-slip, adiabatic and constant temperature conditions, with different fuel mixtures described by varying the thermal expansion coefficients. Results demonstrate that higher wall temperatures promote slower propagation as they reduce the thermal expansion rate, as a result of the post-cooling of the burn matter. In turn, smaller obstacle sizes generate weaker flame acceleration, although the mechanism is noticed to be stronger than the wall friction-driven, even for the smaller sizes considered.
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