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.

Keywords:

• premixed flames dynamics
• chain-branching kinetic model
• micro-combustion
• symmetry-breaking bifurcation
• oscillatory instability

Acknowledgements

The authors would like to thank Dr Naud for his help in the detailed chemistry calculations of the thermodynamics and transport properties for the planar premixed flames with the use of the code LFLAM developed at CIEMAT.

Supplemental data

The following supplemental data for this article can be accessed at http://dx.doi. org/10.1080/17509653.2014.946970.

Animation 1: Time evolution of the isocontours of Z for the case Le_F = 4, Le_Z = 1, q = 0.9, and m = −1.2. This corresponds to the case marked with the symbol ▴ in Figure 10.

Animation 2: Time evolution of the isocontours of Z for the case Le_F = 4, Le_Z = 1, q = 0.9, and m = 2. This corresponds to the case marked with the symbol ▾ in Figure 10.

Additional information

Funding

This research was supported by the Spanish Ministerio de Ciencia e Innovación (MICINN) [Projects ENE2011-27686-C02-01, ENE2012-33213]; the Comunidad de Madrid [Project S2009/ENE-1597, CONSOLIDER CSD2010-00011].