The differential diffusion effect of the intermediate species on the stability of premixed flames propagating in microchannels

Abstract

The propagation of premixed flames in adiabatic and non-catalytic planar microchannels subject to an assisted or opposed Poiseuille flow is considered. The diffusive-thermal model and the well-known two-step chain-branching kinetics are used in order to investigate the role of the differential diffusion of the intermediate species on the spatial and temporal flame stability. This numerical study successfully compares steady-state and time-dependent computations to the linear stability analysis of the problem. Results show that for fuel Lewis numbers less than unity, Le F < 1, and at sufficiently large values of the opposed Poiseuille flow rate, symmetry-breaking bifurcation arises. It is seen that small values of the radical Lewis number, Le Z , stabilise the flame to symmetric shape solutions, but result in earlier flashback. For very lean flames, the effect of the radical on the flame stabilisation becomes less important due to the small radical concentration typically found in the reaction zone. Cellular flame structures were also identified in this regime. For Le F > 1, flames propagating in adiabatic channels suffer from oscillatory instabilities. The Poiseuille flow stabilises the flame and the effect of Le Z is opposite to that found for Le F < 1. Small values of Le Z further destabilise the flame to oscillating or pulsating instabilities.

Propagation of symmetric and non-symmetric lean hydrogen–air flames in narrow channels: Influence of heat losses

“…For Le > 1, oscillatory symmetric and non-symmetric solutions can appear. This was shown for both adiabatic channels [10,13,14,19] and channels with heat losses through the wall [9,10]. Of course this is an effect of thermo diffusive instabilities that dominate in narrow channels.…”

“…As we are interested in the symmetry breaking effect reported for flames with Le < 1 [9,10,13,14], we have chosen to study lean hydrogen-air flames with equivalence ratio φ = 0.4. Because in this case the limiting reactant is hydrogen this should correspond to an effective Lewis number close to the Lewis number of hydrogen in the mixture, about Le = 0.3.…”

“…After pioneering work in the subject of flames propagating in ducts several years Email address: Corresponding author: carmen.jimenez@ciemat.es (Carmen Jiménez) ago [4,5], the interest in small-scale combustion systems has revived the studies on the effects of the channel flow rate [6,7], heat-losses [8,9], the Lewis number [10][11][12][13][14] and thermal expansion [15][16][17][18][19] on the flame propagation in narrow channels.…”

Micro/mesoscale combustion based portable power generating system

“…Other studies used time-accurate models, e.g. Pizza et al [56], Nair et al [62] and Jiménez et al [74,91] for H 2 /air, Nakamura et al [59], Miyata et al [61] and Kang et al [78] for CH 4 /air, to capture the flame dynamics involving various modes of flame stabilities and instabilities.…”

“…This energy can be harnessed via combustion in the form of heat and converted to useful power. In order to achieve this, combustion in a narrow passage where the characteristic dimension is of the order of the flame thickness has received considerable attention in the past two decades and is termed microscale (dimension <1mm) or mesoscale (dimension close to flame thickness) combustion [5,46,66,67,70,73,91,106,[123][124][125]. The potential applications include portable power systems and propulsion systems for small scale rockets [4].…”