Dynamics of flame rings in a thermally-conductive narrow channel: a numerical experiment

Abstract

A numerical experiment is conducted to examine the ignition of a near-flammability-limit low-Lewis-number mixture and the evolution processes of the resulting premixed flames in a circular thermally-conductive narrow channel. A diffusive-thermal model is adopted to describe the fuel mass conservation in the gas phase and energy conservation in both the gas and the channel wall. Spalding's ‘one-dimensional idealization’ approximation is introduced to simplify all these conservation equations to a two-dimensional form over the plane parallel to the wall surface of the channel. As a result, the half channel height h constitutes the primary parameter controlling the heat exchange rate between the gas and the wall, which serves as a conductive heat loss mechanism from the perspective of the gas phase. For relatively large h, following ignition at the centre of the channel by a hot ignition kernel, the resulting flame front suffers diffusive-thermal instability and quickly breaks into a number of discrete reaction cells, which propagate forward towards the boundary of the channel. When h becomes sufficiently small, the reaction cells close upon themselves and transform to ring-shaped flamelets, herein termed flame rings, a 2-D analogue of flame balls formed in 3-D unconfined space. The effects of the half channel height h on the formation and dynamics of the flame rings are examined, together with a demonstration of a possible means to manipulate the flame ring dynamics by exploiting the 2-D character of the narrow channel configuration. It is suggested that, as a simple ground-based buoyancy-suppression strategy, the presently considered narrow channel configuration can be conveniently employed to study the properties related to the flammability limit of combustible mixtures.

Keywords:

• near-limit premixed flame
• flame ball
• flame ring
• narrow channel
• conductive heat loss

Disclosure statement

No potential conflict of interest was reported by the author(s).

Notes

1 Since a change in the grid density will inevitably cause a change in the initial perturbation, which in turn will lead to a change in the pattern of the generated flame rings, to facilitate a meaningful comparison the calculation for the dense grid case started from an intermediate result obtained from the calculation for the coarse grid case, specifically at t = 20, by which time the flame rings have been generated.

Additional information

Funding

This work was supported by the National Natural Science Foundation of China, under grant Nos. 51776136 and U1738117.