Numerical analysis of flame instabilities in narrow channels: Laminar premixed methane/air combustion

Abstract

Premixed flames propagating within small channels show complex combustion phenomena that differ from flame propagation at conventional scales. Available experimental and numerical studies have documented stationary, non-stationary, or asymmetric modes that depend on properties of the incoming reactant flow as well as channel geometry and wall temperatures. This work seeks to illuminate mechanisms leading to symmetry breaking and limit cycle behavior that are fundamental to these combustion modes. Specifically, four cases of lean premixed methane/air combustion—two equivalence ratios (0.53 and 0.7) and two channel widths (2 mm and 5 mm)—are investigated in a 2D configuration with constant channel length and bulk inlet velocity, where numerical simulations are performed using detailed chemistry. External wall heating is simulated by imposing a linear temperature gradient as a boundary condition on both walls. In the 2 mm channel, both equivalence ratios produce flames that stabilize with symmetric flame fronts after propagating upstream. In the 5 mm channel, flame fronts start symmetrically, although symmetry is broken almost immediately after ignition. Further, 5 mm channels produce non-stationary combustion modes with dramatically different limit cycles: in the leaner case ( φ = 0.53), the asymmetric flame front flops periodically, whereas in the richer case ( φ = 0.7), flames with repetitive extinctions and ignitions (FREI) are observed. To further understand the flame dynamics, reaction fronts and flame fronts are captured and differentiated. Results show that the loss of flame front symmetry originates in a region close to the flame cusp, where flow and chemical characteristics exhibit large gradients and curvatures. Limit cycle behavior is illuminated by investigating flame edges that are formed along the wall, and accompany local or global ignition and extinction processes. In the flopping mode ( φ = 0.53), local ignition and extinction in regions adjacent to the wall result in oblique fronts that advance and recede along the wall and redirect the flow ahead of the flame. In the FREI mode, asymmetric flames propagate much farther upstream, where they experience global extinction due to heat losses, and re-ignite far downstream with opposite flame front orientation. In both cases, an interaction of flow and chemical effects drives the asymmetric limit cycles. The lack of instabilities and asymmetries for the 2mm cases is attributed to insufficient wall separation, which is of the same order of magnitude as the flame thickness.

Investigating the Ignition and Stability Limits of Premixed Methane/Air Combustion in Micro-Channels

“…The FREI phenomenon has been investigated for a methane/air mixture in a series of works [18,[25][26][27]. Miyata et al [26] studied the unsteady behavior of the oscillating flames and the FREI for methane/air premixed combustion in microchannels.…”

“…Varying these parameters, the flame can become stable or experience repeated extinctions and ignitions [11,21,29,30]. It is important to highlight that the models and the methods employed in this work have been accepted and acknowledged in the literature [24,27]. In the following sections, the simulation methodology is described.…”

Flammability and Propagation Dynamics of Planar Freely Propagating Dimethyl Ether Premixed Flame

“…The dominant chemical and diffusive processes underlying the hot- and cool-flame dynamics will be revealed, which would be of fundamental and practical importance for the design of advanced combustion systems. Furthermore, some recent studies observed the repetitive ignition and extinction (FRIE) event in microchannel flow reactors with an imposed temperature stratification. − The coupling of inlet flow with the propagating flame front that depends on the local temperature gradient was responsible for the FRIE dynamics. However, when considering the effectiveness of the mixture equivalence ratio or stoichiometry stratification, as well as the intertransition between hot- and cool-flame fronts, the FRIE dynamics would become much more complicated.…”

Numerical Investigations of Micro-Scale Diffusion Combustion: A Brief Review

“…Numerical simulations for both premixed and non-premixed (diffusion) combustion at micro scales have been evolving extensively as the topic is becoming more and more interesting to researchers. In premixed combustion, researchers have concluded that combustion characteristics and flame behavior are very sensitive to the dimensional and thermo-physical parameters [3][4][5][6]. Other than premixed combustion at micro scales, non-premixed or diffusion combustion at such scales has also been under great attention due to the fact that diffusion combustion is a more typical combustion regime to use in practical systems [7].…”

“…Numerical study of micro-jet hydrogen/methane diffusion flames by Gao et al[27]. Top: Computed temperature contours and heat release rate (HRR) isopleths for CH4 flames at fuel jet velocity of 0.4 m/s for (a) T b = 1800 K (b) T b = 300 K, and for H 2 flames at 1.0 m/s for (c) Tb = 1800 K (d) T b = 300 K; Bottom: Computed temperature contours and HRR isopleths at the thermal conductive burner condition (the thermal conductivity of burner wall is 16 W/m-K) for (a) H 2 flame at 2.0 m/s, (b) H 2 /CH 4 80%/20% flame at 1.2 m/s, (c) H 2 /CH 4 20%/80% flame at 0.6 m/s, (d) CH 4 flame at 0.5 m/s. Here the flame temperatures range between 2028 K and 2034 K.…”