Volume 7 Issue 3 - January 9, 2009
Structure and Stabilization Mechanism of a Microjet Methane Diffusion Flame Near Extinction
C.-P. Chen1, Y.-C. Chao1,*, T. S. Cheng2, G.-B. Chen3, and C.-Y. Wu1

1Department of Aeronautics and Astronautics, College of Engineering, National Cheng Kung University
2Department of Mechanical Engineering, Chung Hua University
3Department of Computer Science and Information Engineering, Diwan College of Management

Proceedings of the Combustion Institute, 31:3301-3308, 2007
SCI Category: Engineering, Mechanical, Ranking 3 /107 = 2.8 %

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The advances in MEMS fabrication technologies have led to intensive research interest in high-density and long-operation micro-power generation devices as an alternative power source for numerous applications1. With a energy density of about 100 times higher than that of the most advanced batteries, a micro-scale hydrocarbon combustion system has been considered as a viable alternative to batteries. Therefore, an understanding of the physics and chemistry of the laminar microjet flame is of vital importance.

Due to its small size, the amount of heat released from the flame is very small, whereas the heat loss to the burner would be relatively large. This indicates that the flame might be always operated in a severe condition especially near extinction limit. Experimental measurements have shown that the imaged flame shapes of microjet methane diffusion flames are completely different between the direct photograph and laser shadowgraph2. This fact implies that an intensive characteristic hot region may exist just ahead of the flame zone. Several issues3,4 concerning the structure and stabilization mechanism of the microjet methane diffusion flames are still unrevealed especially in the standoff region. Therefore, in the present study, the chemiluminescence measurements as well as the full chemical reaction mechanism numerical simulation are adopted to examine the microjet methane flame structure and stabilization characteristics in the standoff region. The important radicals that sustain the preflame reaction and hence stabilize the standoff flame near extinction are elucidated through detailed analysis.

Methods of analysis
The laminar microjet methane diffusion flame is stabilized on a vertical straight stainless-steel (AISI 304) tube with an inner diameter (d) of 186 μm and the wall thickness of 79 μm.  Methane is introduced into the quiescent atmospheric air at a volumetric flow rate of 3.9 cc/min corresponding to a near extinction-limit bulk velocity of 2.39 m/s, with the Reynolds and Froude numbers of Re = 27 and Fr = 3131, respectively. The CH* chemiluminescence image of the flame is recorded by a  cooled CCD camera (Cooke SensiCam) with a macro lens and a narrowband interference filter centered at λ = 431 nm with a full width at half-maximum of 10±2 nm.

To numerically model the laminar microjet methane diffusion flame, the governing equations of continuity, momentum, energy, and chemical species for a steady reacting flow are solved by CFD numerical code coupled with CHEMKIN package and full chemical kinetic mechanisms from GRI-Mech 3.0. The burner is placed inside the computational domain, and hence the property located inside as well as outside the burner is calculated. This takes into account the back-diffusion of species into the tube and the heat transfer between the tube and flame. Far field boundary conditions are imposed to the open boundaries. Non-slip and non-catalytic reaction conditions are applied on the burner surface. Radiation heat loss is neglected in the calculation because no soot formation is observed for the flame near extinction. A minimum grid size of 0.05d is placed near the exit of the tube and an enlarged grid size is set forth toward the outer boundaries. The grid-independence study suggests that 0.0093 mm grid spacing is sufficient for resolving the flame features.

Results and Discussion
To look into the micro-flame stabilization characteristics, now focus on the d = 186 μm flame with the condition near extinction limit (ue = 2.39 m/s). The computed CH mass fraction isopleths are compared with the measured CH* radical image as shown in Fig. 1. It can be seen that the computed flame height and flame shape (Fig. 1b) are in very good agreement with measurements (Fig. 1a).  Both results indicate that the flame stands off from the burner port. In view of the success of predicting the flame heights and shapes of microjet flames by numerical simulation, further detailed computation of the flame structures and reaction characteristics is performed.
Fig. 1 Comparison of the measured CH* image (a) with computed CH mass fraction isopleths (b).

The computed results of the 2-D temperature, CH, O2, CH4, CO2, H2O, H2, and CO mass fraction contours as compared with mixture fraction contours of lean (ξ = 0.029), stoichiometric (ξ = 0.055), and rich (ξ = 0.089) limits of the methane flame are shown in Fig. 2. It can be seen that a small amount of O2 is entrained into the standoff region from the gap between the burner wall and flame base and a small amount of CH4 has diffused upstream of burner port. This entrainment could results in slightly partial premixing of fuel and oxygen over the standoff distance. The computed temperature contour shows that the maximum flame temperature locates at the jet centerline near the stoichiometric mixture fraction contour and the maximum of the CH isopleths. The unburned mixtures, the burner wall, and the fuel stream are heated to a temperature higher than 700 K. This fact suggests that the standoff and the flame stabilization are strongly related to the characteristic hot zone and heating of the fuel stream and unburned mixture through the tube wall.
Fig. 2 Computed 2-D temperature and species mass fraction contours for the micro-flame near extinction limit.

Fig. 3 Computed HO2 mass fraction contours, heat-release rate isopleths and H, O, and OH mass flux vectors.
From the discussions of the flame phenomena one could conjecture that the stabilization of a standoff microjet flame could be due to a consequence of flame quenching, preheating of partially premixed mixture and sustaining preflame reaction, and finally forming a stable diffusion flame.  With variable wall temperature condition (as in the real situation), the flame is quenched on the tube wall and creates a gap to allow oxidizer entrainment. As the flame is quenched by the tube wall, heat is transferred through the wall to accelerate fuel decomposition, initiate further reaction, and produce intermediate radicals in the vicinity of the exit. As usual, the HO2 radicals are brisk in the chain-terminating reactions when the flame is quenched on the wall. Detailed examination of the intermediates indicates that the HO2 radicals play an important role in connecting with the stabilization of the flame.  The HO2 mass fraction contours, heat-release rate isopleths and H, O, and OH mass flux vectors are illustrated in Fig. 3. It can be seen that the maximum HO2 appears in the gap region and then decreases to connect with heat-release rate isopleths. The HO2 + CH3 <=> OH + CH3O reaction releases significant amount of heat in the gap region which is believed to form the hot zone (T = 900 ~ 1450 K) that connects to the visible flame as observed by the shadowgraph image. On the other hand, the heat-release rate isopleths, which resemble to those of CH (see Fig. 1), show the reaction kernel (peak reactivity spot) at the flame base as that reported by Takahashi and Katta5.  The chain radicals (H, O, and OH) diffuse onto both sides of the flame zone and in the downward direction to initiate other radical reactions. The dominant exothermic reactions, O+CH3<=>H+CH2O (R10), O+CH3<=>H+H2+CO (R284), OH+CO<=>H+CO2 (R99), OH+CH4<=>CH3+H2O (R98), OH+H2<=>H+H2O (R84), OH+CH2O<=>HCO+H2O (R101), O2+CH2<=>2H+CO2 (R190), O2+CH=>O+HCO (R125), O2+HCO,=>HO2+CO (R168), and H+CH2O<=>H2+HCO (R58), contribute to 81.5% of the total heat-release rate at the reaction kernel (1.15×109 W/m3). As a result, the formation of hot zone, chain radical reactions, and exothermic reactions, thus forming the reaction kernel, stabilize a standoff microjet flame near extinction limit.

The structure and stabilization mechanism of a microjet methane diffusion flame near extinction limit are numerically investigated using detailed transport model and full chemical kinetic mechanisms. For the present d = 186μm micro flame with a velocity ue < 3.68 m/s, buoyancy effect becomes minor and the flame is diffusion controlled. Detailed analysis of the flame structure strongly suggests that the fuel burns in a diffusion flame near extinction limit, which contradicts predictions by simple jet flame model. Although partial premixing may occur in the standoff region, its mixing intensity is not strong enough to generate reaction and hence no double flame structure is observed. Further examination indicates that the flame is stabilized by a hot zone that connects to a reaction kernel, through the formation of HO2 radicals and subsequent key radical reactions.

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[2]T. Ida, M. Fuchihata, Y. Mizutani, in: Microscopic diffusion structures with micro flames. Proceedings of the Third International Symposium on Scale Modeling, ISSM3-E7, 2000.
[3]T. S. Cheng, Y.-H. Li, C.-S. Chen, C.-Y. Wu, C.-P. Chen, Y.-C. Chao, in: Structure of Microjet Methane Diffusion Flames. Proceedings of the 20th International Colloquium on the Dynamics of Explosions and Reactive Systems, 2005.
[4]T. S. Cheng, Y.-C. Chao, C.-Y. Wu, Y.-H. Li, Y. Nakamura, K.-Y. Lee, T. Yuan, T. S. Leu, Proc. Combust. Inst. 30 (2005) 2489-2497.
[5]F. Takahashi, V. R. Katta, Proc. Combust. Inst. 29 (2002) 2509-2518.
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