Combustion characteristics of lean hydrogen/air mixture in a planar micro-channel with a bluff body were investigated experimentally and numerically. Effects of the inlet velocity and equivalence ...ratio on the blow-off limit, combustion efficiency and exhaust gas temperature were examined. The results show that the blow-off limit is greatly extended as compared with that of the micro-combustor without a bluff body. Moreover, the blow-off limit increases as the equivalence ratio is increased from 0.4 to 0.6. Furthermore, with the increase of inlet velocity, the flame front is prolonged and becomes narrower, and the high temperature segment of outer wall shifts downstream. In addition, the combustion efficiency and exhaust gas temperature increase first and then decrease with the increase of the inlet velocity. Finally, comparatively high combustion efficiency can be maintained over the whole combustible velocity range at a moderate equivalence ratio.
► A planar micro-combustor with a bluff body was developed in the present work. ► Combustion characteristics were investigated experimentally and numerically. ► Blow-off limit is notably extended and it increases with the equivalence ratio. ► Flue temperature and efficiency vary in non-monotonic ways with inlet velocity.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
We recently developed a micro-combustor with a triangular bluff body, which has a demonstrated 5-time extension in the blow-off limit compared to straight channel. In the present work, the effect of ...bluff body shape on the blow-off limit was investigated with a detailed H2/air reaction mechanism. The results show that the blow-off limits for the triangular and semicircular bluff bodies are 36 and 43 m/s respectively at the same equivalence ratio of 0.5. Analyses reveal that flame blowout occurs due to the stretching effect in the shear layers for both the triangular and semicircular bluff bodies. Moreover, it is found that the triangular bluff body has a smaller blow-off limit because of the stronger flame stretching as compared with the semicircular case. Calculations indicate that the two cases have negligible differences in heat losses because the reaction zones and high temperature regions are located in the combustor centers. Therefore, the heat losses have a negligible effect on the difference in the blow-off limit of the two micro-combustors.
•Effect of bluff body shape on blow-off limit of a micro-combustor was investigated.•Blowout occurs due to flame stretching for triangular and semicircular bluff bodies.•Triangular bluff body has a lower blow-off limit due to stronger flame stretching.•Heat losses have a negligible effect on the difference of blow-off limit.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
Heat transfer processes are of significance for combustion in microscale channel. In the present paper, three different values (i.e., ε = 0.1, 0.5 and 0.92) are selected to numerically investigate ...the effect of external surface emissivity on the flame-splitting limit (the critical velocity when the combustion efficiency drops to 80%) using Fluent 6.3. The results demonstrate that the flame-splitting limit has a negatively monotonic variation trend with the external surface emissivity. This feature is interpreted from viewpoints of heat-loss from the exterior wall and heat recirculation through the upstream wall. The analyses show that the heat-loss ratio decreases with the reduction of external surface emissivity, which gives rise to a more intense reaction in the cavity and a higher wall temperature level. Therefore, more heat is recirculated to upstream wall and a better preheating of the fresh mixture can be achieved. Consequently, the chain reactions of H2/air mixture start earlier and the reaction rates are much faster for a smaller surface emissivity. The combined actions of heat loss and heat recirculation lead to a faster consumption of the hydrogen fuel at the flame tip, and a larger flame-splitting limit can be obtained for a smaller surface emissivity.
•Effect of emissivity on flame-splitting limit of micro cavity-combustors was studied.•Flame-splitting limit increases as the external surface emissivity is reduced.•Heat-loss ratio drops sharply with the decrease of external surface emissivity.•The recirculated heat amount rises with the decrease of external surface emissivity.•The fuel consumption rate at the flame tip is faster for smaller surface emissivity.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
•Recent progress in flame stabilization methods for micro-combustors was reviewed.•Flame anchoring approaches are divided into single, combined, and other methods.•Single methods include flame ...holders, heat recirculation, catalyst, and additives.•The combined methods are combinations of two or more of these single methods.•Future directions towards the development of micro-combustors were presented.
The microscale combustion-based energy and power generation systems are potential alternatives to traditional batteries owing to the much higher energy density of hydrogen and hydrocarbon fuels. However, the surface-area-to-volume ratio of the micro-combustors is very large, which makes the flame difficult to sustain stable owing to the large heat-loss. Therefore, it is very necessary to develop effective flame stabilization technologies for combustion-based micro energy and power systems. Great progress has been made in the past decade, which was reviewed in this article. The flame stabilization technologies for micro- and meso-scale combustors are divided into three categories, namely, single flame stabilization technologies, combined flame stabilization technologies, and other flame stabilization technologies that could not be included in the former two. The single flame stabilization technologies include flame holders (e.g., bluff body, wall cavity), heat recirculation strategy (e.g., porous media, thermally orthotropic wall), catalyst, and additives in the fuel. The combined flame stabilization technologies consist of two or more of these single flame stabilization methods, such as Swiss-roll combustor with a bluff-body, micro catalytic combustor with a bluff-body, and so on. The underlying mechanisms of these flame anchoring strategies were also discussed. The guidelines to develop micro-combustors with excellent flame stabilization ability were summarized at the end of this review article.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
We recently developed a micro-combustor with a bluff body, which has a demonstrated 3- to 5-time extension in the blow-off limit. In the present work, the dimension effect of the bluff body on the ...blow-off limit (indicated by the blockage ratio, ζ) was investigated with a detailed H2/O2 reaction mechanism. The results indicate that the blow-off limits for ζ = 0.3, 0.4 and 0.5 are 20, 31 and 36 m/s, respectively. Analyses reveal that for ζ = 0.3, flame blowout occurs due to insufficient recirculation zone size. However, flame blowout occurs due to the stretching effect in the shear layers when ζ = 0.4 and 0.5. Calculations indicate that the three cases have negligible differences in heat loss because the high temperature zones are located in the combustor centers; therefore, their effects on the combustor walls are mitigated.
•The effect of bluff body size on the blow-off limit of a micro-combustor was studied.•The blow-off limit correlates positively with the blockage ratio.•A blockage ratio of 0.3 can't create a large recirculation zone to anchor the flame.•Blockage ratios of 0.4 and 0.5 induce flame blowout due to the stretching effect.•Differences in heat-loss ratios are negligible for various blockage ratios.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
In a previous study, we fabricated a well-designed mesoscale combustor with a flame holder and preheating channels to improve the flame stability under extreme combustion conditions. This combustor ...can also act as a model for studying the synergistic effect of heat and flow recirculation on the flame dynamics. This paper systematically illustrates and discusses the blow-off mechanisms of a holder-stabilized laminar premixed flame in a preheated mesoscale combustor via three-dimensional numerical simulation. The flame blow-off processes that have been observed experimentally at various Reynolds number (Re) are well reproduced. Pinch-off appears at large Re but is absent at small Re. First, the moving characteristics of the flame root and flame top in the blow-off process at small Re are discussed in detail. Then, the flame pinch-off mechanisms are carefully analyzed in terms of the flow field, conjugate heat exchange, and stretch effect. The results indicate that the influence of conjugate heat exchange, which exists between the flame holder and flame root, is not significant in the flame pinch-off phenomenon, but a larger recirculation zone (which helps to stabilize the flame root) and flame stretch (which splits the flame) both induce flame pinch-off. This work can aid in gaining insight into the flame blow-off dynamics when the effects of heat and flow recirculation work synergistically in many practical combustors.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
A novel combustor with a flame holder and preheating channels, which takes advantage of heat and flow recirculation, was developed to improve the stability of the micro flame in our previous work. ...The results demonstrated that the lean methane/air premixed flame could be stabilized within a wide operating range. Certainly, the flame blows off when the equivalence ratio of the fuel mixture becomes too small or too big or the Reynolds number is too large. The present work reports and analyzes the blow-off dynamics of a methane/air laminar premixed flame at a relatively large Reynolds number under the synergistic effect of heat and flow recirculation. Interestingly, a flame with local extinction and re-ignition appears, which is first found in the micro combustor. The underlying mechanisms of this interesting phenomenon are systematically discussed in terms of the flow recirculation effect, stretch effect, conjugate heat exchange, and preferential transport effect. We find that the counter flow in the recirculation zone and stretch effect induce the local extinction of the flame. The smaller heat loss from the residual flame to the flame holder and the better preferential transport effect contribute to the re-ignition of the fuel mixture. The preheating effect on the unburned fuel mixture is not expected to be insignificant for the local extinction and re-ignition phenomenon. These results mean that the generated mechanism of the flame with local extinction and re-ignition in the micro combustor is different from that in the conventional combustors.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
•The anomalous blow-off phenomenon is first found for CH4/air premixed flame.•A novel blow-off mode (low speed blow-off) is reported for the first time.•The distributions of blow-off mode at lean ...fuel and rich fuel are different.
The present work experimentally studies the blow-off limit of methane/air premixed flame in a novel combustor with a flame holder and preheating channels. The previously reported normal and anomalous blow-off phenomena of lean H2/CH4/air premixed flame at Lewis number Le < 1.0 was first observed for the lean CH4/air premixed flame of Le ≈ 1.0. In addition to the similar normal and anomalous blow-off at fuel lean condition, a novel low speed blow-off at fuel rich condition was discovered. This finding indicated that the kinds and distribution characteristics of the blow-off modes with varied Reynolds number were different at fuel lean and rich conditions. But the ratio of flammable ranges between fuel rich and lean conditions at each Reynolds number Re nearly kept the same for moderate and large Reynolds numbers. It could be the intrinsic unification of the anomalous and normal blow-off phenomena at fuel lean and rich conditions.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
The present study investigates the blow-off limits of a laminar methane–air premixed flame under the synergistic effect of heat and flow recirculation by means of experimentation and ...three-dimensional numerical simulation. An anomalous blow-off phenomenon (the blow-off limit decreases with the decrease in Reynolds number (Re)) is experimentally observed and reproduced via simulation. First, the flame base structures for various Re values are studied. Subsequently, the underlying mechanisms are revealed in terms of flow field, stretch effect, preferential transport effect, and conjugate heat transfer. The main causes of the present anomalous blow-off phenomenon are the flow recirculation effect, preferential transport effect, and conjugate heat transfer, rather than the stretch effect. Specifically, a weaker flow recirculation effect, a weaker preferential transport effect, a larger heat loss ratio from the combustor to the ambient environment, and a lower preheated temperature of the fresh mixture altogether lead to a smaller blow-off limit at a smaller Re. In addition, the sharp reductions in both the local equivalence ratio and preheated temperature of the fresh fuel-oxidizer mixture explain why the blow-off limit sharply decreases from Re = 80 to 40. This work offers insights into flame blow-off behaviors under the synergistic effect of heat and flow recirculation in many practical combustors.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
The study presents the laminar methane-air non-premixed flame patterns under the synergistic effect of heat and flow recirculation in a radial microchannel, as obtained through a numerical ...simulation. A regime diagram of the flame behaviors for various operating conditions was obtained. It was observed that the flame always remained stable until it was extinguished, which implies that the flame stability was strong. In addition, the flame patterns at small, moderate, and large Reynolds numbers (Re) were revealed. For a small Re, the flame could be well-anchored in a very upstream channel at a large global equivalence ratio (ϕ). At a moderate ϕ, the upstream part of the flame was anchored by a minor recirculation zone (RZ) near the inlet of the combustion chamber. At a small ϕ (i.e., 0.6), the flame became completely flat, and its upstream part was anchored by the major RZ behind the annular step. For a moderate Re, the flame roots were remarkably anchored by the minor and/or major RZs. For a large Re, for the first time. It was observed that the whole stable flame split into two parts in the radial channel at ϕ = 0.8 and 0.7, namely the upstream minor flame engulfed in the minor RZ and the downstream major flame anchored by the major RZ. However, when ϕ was further decreased to 0.6, the minor flame was extinguished and only the large major flame existed. The transformation mechanism of the aforementioned flame behaviors was also revealed in detail. Furthermore, this study has yielded abundant information on flame patterns in the current radial microchannel with the annular step and the radial preheated channel, which could help us gain further insights into the flame propagation characteristics in a radial channel.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
