A numerical study is carried out on the self-igniting laminar methane/hydrogen jet flame in a dilute coflow air. 1 2-D axisymmetric domain for the present simulations of self-ignited laminar lifted methane and methane/hydrogen jet flames in a dilute coflow air.
List of Tables
1 Introduction
Studies on self-igniting laminar raised flames have provided basic understanding of self-igniting turbulent raised flames. They also found that the raised laminar self-igniting flame exists even though the fuel Schmidt number, Sc, is less than 1 [26].
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2 Numerical methods
2-D axisymmetric domain for the present simulations of self-igniting laminar lifted methane and methane/hydrogen jet flames in a dilute coflow air. The profiles of axial velocity, temperature and mass fractions of CH2O and OH of the self-igniting laminar lifted methane/hydrogen jet flame in a co-flow air along the stoichiometric mixture fraction, ξst, isoline for three different grid sizes are shown in Figure 2. The profiles for the grid size of 100 µm and 50 µm are almost identical while there is slight difference between that of 200 µm and the other two profiles.
Profiles of (a) axial velocity, (b) temperature, (c) CH2O and (d) OH mass fractions of a self-igniting laminar rising flame of a methane/hydrogen jet in an air flow along the stoichiometric mixture fraction, ξst, isoline for three different grid resolutions for T0 = 940 K and U0 = 16 m/s. In the r-direction, a grid space of 100 µm is uniformly applied to the domain for 0 ≤ r ≤ 3 cm, and stretched grids are distributed over the rest of the domain. Adiabatic and no-slip conditions are used for the wall boundaries, and a symmetric solution is obtained about the z-axis.
The fully developed tube flow condition with the mean flow rate of the fuel stream velocity, U0, and the uniform cross-flow velocity, UCO, are applied to the fuel stream inlet and the air inlet with the common flow. As performed in a previous study [36], simulations for methane and methane/hydrogen flames are performed by varying U0 with fixed value of UCO.
3 Overall flame characteristics
Methane jet flames
From the isocontours of the mass fraction of several species, it can be easily observed that methane is transported along the centerline of the fuel jet, diffuses into the coflow, and is finally consumed near the flame zone. This indicates that the self-igniting laminar lifted flame is stabilized by a different mechanism than the non-autoignited laminar lifted flame, where the flame is stabilized at a point of stoichiometric mixture fraction isoline where the flame propagation speed balances with the local flow velocity [51, 52]. Isocontours of (a) YOH (left half) and T (right half), (b) YCH4 (left half) and YCH2O (right half), and (c) YH2O2 (left half) and YHO2 (right half) for self-igniting laminar lifted methane jet flames for different mole fractions of O2 in the coflow side, XO2, forU0 = 30 m/s.
Therefore, the existence of stationary lifted methane jet flames confirms that the contribution of self-ignition is critical for the flame stabilization. In this study, the flame with MILD combustion for XO2 = 0.10 shows different characteristics from those in a previous numerical study [46]. In [46], the profiles of temperature, some species, and heat release rate of flame with MAGTE combustion showed different characteristics than those of the flame with tribrachial edge, and the isoline lies far away from the flame zone.
Moreover, ξth isoline lies in the flame zone and the maximum temperature and OH occur along ξth isoline as shown in Figure 4. It is suspected that different characteristics are attributed to the fact that the self-igniting flame with MAGE combustion is obtained in different ways from that in the previous study [46] in which the flame with MAGTE combustion was obtained by dilution of the fuel while it is obtained by dilution of air in the coflow inlet in the present study.
Methane/hydrogen jet flames
This result implies that the reaction zone is also displaced, leading to the displacement of the flame base to the side of the co-flow. Therefore, ignition occurs at higher RH with shorter τig as XO2 decreases due to the shift of the ξst isoline toward the coflow side. As a result, the temperature increases faster along ξfb with decreasing XO2 for U0 = 30 m/s while it increases slowly for U0 = 16 m/s, although it appears to increase rapidly in the early stage due to the slight increase in RH.
This result indicates that at a low fuel jet velocity, a relatively large amount of methane species can be transported by diffusion to the location where the reaction zone is shifted to the coflow side by the dilution, while only a limited amount of methane species can be diffused for a relatively long time . -high fuel jet velocity. The effect of the reaction zone shift can be ignored due to the relatively low fuel jet velocity due to the relatively high HL, and the long diffusion time ensures that a sufficient amount of methane is transported by diffusion to the location where the reaction regime is shifted. Therefore, although the ignition delay time decreases due to the increase in RH, the decreasing ignition delay time due to increasing RH is unable to overcome the increasing ignition delay time due to flame intensity.
As T0 decreases, the flame intensity becomes weak and this leads to an increase in HL. In addition, lCH4 is also investigated for T0 = 950 K and 930 K, and it is identified that similar to the cases of T0 = 940 K, lCH4 is much longer than ∆ξfb in low fuel jet velocity regime, while ∆ξfb is much longer than lCH4 in high fuel jet speed regime.
4 Flame stabilization mechanism
In the previous study [36], the self-igniting laminar lifted methane/hydrogen jet flames under relatively low temperature and high hydrogen ratio (LTHH) condition, its flame mode changes from the MILD combustion to the tribrachial edge flame with increasing U0, and the flame stabilization mechanism also changes from inhomogeneous auto -ignition after autoignition-assisted flame propagation. Similarly, the flame mode changes from the tribrachial edge flame to the MILD combustion as XO2 decreases, and the flame stabilization mechanism also changes from autoignition-assisted flame propagation to inhomogeneous autoignition. As the flame mode changes from the tribrachial edge flame to the MAGTE combustion, the flame intensity becomes weak, and the effect of flame propagation on flame stabilization decreases while that of self-ignition increases to stabilize the flame.
This is because the YOH increases relatively smoothly upstream of the flame base due to the increase of the autoignition effect on the flame stabilization with decreasing XO2. The flame thickness also increases more significantly with decreasing XO2, and is greater for U0 = 16 m/s than for 30 m/s. These results indicate that the effect of self-ignition on the flame stabilization for U0 = 16 m/s is greater than for U0 = 30 m/s.
This is because there is a larger amount of hydrogen at the base of the flame for U0 = 30 m/s due to the differential diffusion effect, so the effect of flame propagation on flame stabilization becomes marginal.
5 CEMA(chemical explosive mode analysis)
It is observed from the figure that the overall contribution of hydrogen to the CEM increases with decreasing XO2. On the other hand, the contribution of methane/hydrogen to the CEM near the flame base increases/decreases slightly with the decrease of U0. To further elucidate the chemical properties upstream of the flame base, the contributions of elementary chemical reactions to the CEM upstream of flame base are investigated.
Although not shown here, a general sequence of methane autoignition is observed before the flame base from the PI analysis [70]: the initiation of a chain reaction that is. At U0 = 30 m/s, the contribution of reactions related to hydrogen oxidation to the CEM is dominant regardless of XO2. Furthermore, although not shown here, it is observed from the PI isocontours in front of the flame base that the contribution of methane oxidation reactions to the CEM is significant near the fuel nozzle, while the hydrogen contribution becomes large near the flow side due to the high diffusivity of hydrogen.
Therefore, it can be identified how the contribution of methane oxidation reactions to CEM changes upstream of the flame base depending on the residence and diffusion time scale at U0 and HL. On the other hand, for U0 = 30 m/s, it decreases significantly with XO2 and finally the contribution of the reaction to CEM almost vanishes for XO2 = 0.10.
6 Conclusion
Therefore, the flame stabilization mechanism changes from autoignition assisted flame propagation to autoignition with decreasing XO2. In addition, it was also identified that the effect of flame propagation on the flame stabilization becomes more significant as U0 increases due to high RF for high U0. Through the chemical explosive mode analysis (CEMA), the chemical kinetic process upstream of the flame base was investigated.
CEMA showed that the reactions related to hydrogen dominantly contribute to CEM for relatively high U0. Its contribution decreases with decreasing U0 due to increase in the contribution of reactions related to methane oxidation. This result implies that the effect of hydrogen ratio, RH on flame stabilization becomes significant for high U0.
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