Elsevier

Combustion and Flame

Volume 159, Issue 4, April 2012, Pages 1631-1643
Combustion and Flame

Bifurcations and negative propagation speeds of methane/air premixed flames with repetitive extinction and ignition in a heated microchannel

https://doi.org/10.1016/j.combustflame.2011.11.004Get rights and content

Abstract

Detailed behaviors of ignition kernel(s) in a uniform stoichiometric methane/air mixture under the temperature gradient were investigated numerically by using a fundamental system of microcombustion. Bifurcation of the heat release rate peak in an ignition phase at the high wall temperature side, which has been observed in previous experimental and theoretical studies, was successfully reproduced by the present computation. The bifurcated heat release rate peak exhibited negative propagation speed relative to the local flow velocity by consuming a separated methane/air mixture in the downstream side of the boundary zone between an incoming fresh mixture and burned gas. CH4 was completely consumed at the main peak, whereas CO remained unreacted in the wide region behind the main peak. In a weak reaction phase at the low wall temperature side, two bifurcations of heat release rate peak were newly captured. By the two bifurcations, three heat release rate peaks, namely, a main and two bifurcated peaks appeared. The two bifurcations were caused by remaining intermediates such as CH3, CO, H, and OH in the downstream side of the boundary zone. The main and one bifurcated peak disappeared, whereas the other bifurcated peak remained and flowed downstream. The main and two bifurcated peaks exhibited negative propagation speeds relative to local flow velocity by consuming the remaining intermediates. CO which formed in the middle of the boundary zone in the weak reaction phase remained unreacted and kept on flowing downstream, but did not flow out since the next cycle of the ignition phase was initiated there.

Introduction

Since hydrocarbon fuels possess 20–50 times higher energy density than that of electrochemical batteries [1], [2], power-generation devices based on microcombustion with hydrocarbon fuels have been extensively anticipated. Since large heat loss due to the large surface area-to-volume ratio in microscale devices significantly affects flame stability, thermal management, for instance, heat recirculation, has widely been investigated to attain stable microcombustion. A heat-recirculating combustor, termed Swissroll combustor, was first proposed by Lloyd and Weinberg [3]. Subsequently, various studies have been conducted in the context of “excess enthalpy combustion” (e.g., Takeno and Sato [4]). Analytical studies on flame stabilization in a countercurrent and U-shaped heat-recirculating combustors have been conducted by Ju and Choi [5] and Ronney [6]. Flame stabilization and emission of the Swissroll combustor have experimentally investigated by Kim et al. [7]. Ju and Xu [8] and Leach et al. [9] have studied thermal coupling between flame and wall. Recent studies on microscale combustion have been reviewed in [10], [11].

To examine fundamental microcombustion with heat recirculation, a heated microchannel was employed in our previous studies [12], [13], [14], [15]. Figure 1 shows a schematic of a heated microchannel. A fine quartz tube having an inner diameter smaller than the ordinary quenching diameter is heated by an external heat source to form a gradual temperature increase in the flow direction. Figure 2 shows a schematic of the flame responses with a methane/air mixture in a heated microchannel [14]. Unstable flames, termed flames with repetitive extinction and ignition (FREI), in a moderate velocity regime as well as stable normal flames in a high velocity regime and stable weak flames in a low velocity regime have been observed experimentally. These three flame responses have also been observed for other fuels such as dimethyl ether (DME) [16] and n-heptane [17] and reported by an analytical study [18]. Ignition of FREI occurs at the downstream side because of the high wall temperature and the ignition front propagates upstream. Ignition front propagation in FREI is one order of magnitude faster than flame speed. The flame is finally quenched in the upstream region due to the large heat loss to the low-temperature wall. Following some time delays after quenching, a fresh mixture flows in and reignition occurs at the ignition position. This cycle is regularly repeated in FREI. This behavior of FREI has also been observed by detailed laser diagnosis [19], [20] and theoretical works [18], [21], [22]. Similar repetitive extinction and ignition have been observed in a straight channel [23] and a curved duct [24], [25].

Such dynamics of flames on the unstable solution branch significantly affect combustion characteristics in a microcombustor. It has been experimentally confirmed that various flame patterns occur in a heated radial microchannel due to the appearance of FREI [26], [27], which leads to an increase of unburned fuel and CO compositions in burned gas [28]. An analytical study [29] has also shown flame pattern formations in a heated radial microchannel.

To investigate the detailed combustion process of FREI, direct numerical simulation with detailed chemical kinetics for a hydrogen/air mixture has been conducted by Pizza et al. [30], [31]. However, there has been no work conducted on unsteady computation with detailed chemical kinetics for a hydrocarbon fuel. While Pizza et al. [30], [31] have reported strong boundary layer profiles, a simpler plug-flow model was adopted in the present study to investigate the controlling kinetics of FREI dynamics for a hydrocarbon fuel. It would be interesting to compare plug flow and 2-D/3-D models in terms of FREI dynamic in the future.

The objective of the present study was to investigate the detailed combustion process of FREI for a methane/air mixture in a heated microchannel. For that purpose, unsteady one-dimensional computation with detailed chemical kinetics was conducted. Computed overall dynamics of FREI were examined, and then variation of flame structures, chemical reaction steps, and propagation speed were examined.

Section snippets

Previous studies on FREI

Before going into detail, our previous studies on FREI are overviewed in this chapter for the readers’ convenience. Figure 3 shows the dynamics of FREI in a heated microchannel. To date, the dynamics of FREI have been elucidated experimentally and theoretically [13], [14], [18], [32], [33]. A boundary zone between unburned gas of an incoming mixture and burned gas of the previous cycle of FREI is formed, as shown in Fig. 3a. The gas-phase temperature increases by heat transfer between the gas

Computational method

The flow field in a heated microchannel was modeled as the one-dimensional plug flow model [13], [14] and unsteady one-dimensional computation with detailed transport properties and chemical kinetics was conducted for a stoichiometric methane/air mixture. The flame code including heat transfer between gas and wall [13], [14] based on PREMIX [34] was modified for unsteady computations by adding the time-derivative terms [35]. Governing equations are described as follows:ρt+m˙x=0,ρYkt+m˙Yk

Results and discussion

To explain the detailed dynamics of FREI step by step, the overall dynamics of FREI are examined in the first section of this chapter. Then, variation of flame structures is examined. Since three bifurcations of the HRR peak were obtained, chemical reaction processes of the bifurcations are discussed. In the last section of this chapter, propagation speeds of each HRR peak are discussed.

Conclusions

The detailed combustion process of FREI in a heated microchannel was investigated using unsteady one-dimensional computation with detailed chemical kinetics. The following results were obtained:

  • 1.

    The present computational model successfully reproduced FREI and the quasi-steady nature of FREI was proved.

  • 2.

    Bifurcation of the heat release rate peak in the ignition phase of FREI was captured. After the bifurcation, two peaks, namely, a main peak “P1” and a bifurcated peak “P2,” were identified. P2

Acknowledgments

The authors would like to thank Prof. Yiguang Ju of Princeton University for his stimulating discussion on transient modeling of FREI. This work was partially supported by Grant-in-Aid for Scientific Research (B) of Japan (No. 18360097). Collaborative research with Prof. Sergey Minaev of RAS was supported by the Collaborative Research Project of the Institute of Fluid Science, Tohoku University and that with Dr. Aiwu Fan of HUST was supported by the Natural Science Foundation of China (No.

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