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In the present study, the influences of three operating parameters on the mixing performance of H 2 /CH 4 blends and air in a miniature combustor with a rectangular cross section were numerically investigated. The results show that: (1) The mixing process of H 2 and air is faster than CH 4 and air. (2) The mixing process of H 2 and air is diffusion-dominated under low velocities, while convection-dominated under medium and high velocities; However, the mixing of CH 4 and air is diffusion-dominated under both low and high average velocities, but dominated by convection under medium velocities. (3) An increase in the nominal equivalence ratio creates a larger fuel concentration gradient perpendicular to the mainstream, which leads to an increase in the diffusion rate. (4) A larger hydrogen blending ratio intensifies the fuel stratification at the combustor entrance, which deteriorates the mixing performance. (5) An empirical correlation is established between the mixing performance evaluation criterion and the three parameters. The nominal equivalence ratio is the most significant factor to the mixing performance. This figure presents the variations of non-uniformity index of CH 4 and H 2 at the combustor outlet with an increasing average velocity, which reveals the corresponding dominant mechanism for the mixing process within different velocity ranges. [Display omitted] • Mixing performance of H 2 /CH 4 and air in a narrow channel was numerically studied. • The dominant mixing mechanism differs for H 2 and CH 4 under an identical velocity. • Mixing is enhanced at large equivalence ratios by increasing concentration gradient. • High H 2 blending ratio deteriorates mixing performance due to fuel stratification. • Empirical correlation of the mixing performance index was obtained. [ABSTRACT FROM AUTHOR]

A meso-scale jet flame model was established for the flame ports of domestic gas stoves. The influences of hydrogen addition ratio (β = 0%–25%) on the combustion limits were explored. The results show that with the increase of hydrogen addition ratio, the blow-off limit increases obviously, while the extinction limit decreases slightly, namely, the combustible range expands significantly. Quantitative analysis was carried out in terms of chemical effect and thermal effect. It was found that hydrogen addition will reduce O 2 fraction in the pre-mixture for a constant equivalence ratio. Under near-extinction limit condition, since the flame is located at the nozzle exit, the external O 2 cannot be entrained into or diffuse into the upstream of the flame, which leads to the decrease of reaction rate. However, for the near-blow-off cases, the external O 2 can be entrained and diffuse into the flame, which compensates the difference of O 2 content in the pre-mixture. Therefore, the combustion reaction is enhanced by hydrogen addition because more H radicals can be produced. In addition, as the flame is located closer to the tube with the increase of hydrogen addition ratio, heat transfer between flame and tube wall is augmented and the preheating of fresh mixture is strengthened by the inner tube wall. This heat recirculation effect becomes especially notable in low velocity cases. In conclusion, the extension of extinction limit by hydrogen addition is attributed to the thermal effect, while the increase of blow-off limit is mainly due to the intensification of chemical effect. • A micro-jet flame model was established for the ports of domestic gas stove. • Effects of H 2 addition ratio (β = 0%–25%) on the combustion limits were explored. • With the increase of β , the blow-off limit increases obviously while the extinction limit decreases slightly. • The extension of extinction limit by hydrogen addition is attributed to the thermal effect. • The increase of blow-off limit is mainly due to the intensification of chemical reactions. [ABSTRACT FROM AUTHOR]

A planar micro-combustor with a slotted bluff-body was developed for non-premixed H 2 /air combustion. Numerical simulation was conducted and the flame blow-off limit under blockage ratio of ξ = 0.45, 0.55 and 0.64 was obtained (corresponding half height of the bluff-body is 0.25, 0.30 and 0.35 mm, total height of the channel is 1.1 mm). It was found that the blow-off limit peaks at ξ = 0.55. The simulation results of flame blow-off dynamics demonstrate that when the blockage ratio is small, the flame root cannot be anchored by the recirculation zone and the flame is entirely blown off the bluff-body. However, if the blockage ratio is relatively large, flame splitting occurs due to excess strain rate before flame being blown out. Analysis reveals that increasing the blockage ratio of the bluff-body has both positive and negative impacts. On the one hand, a large blockage ratio can increase the turbulence kinetic energy in the recirculation zone, which promotes fuel-air mixing and flame stability. On the other hand, the augmented stretch effect leads to local extinction and flame splitting. As a result, the combustor with a medium blockage ratio of the bluff-body can gain a tradeoff between the positive and negative aspects and achieve a largest blow-off limit. A planar micro-combustor with a slotted bluff-body was developed for non-premixed H 2 /air combustion. It is found that increasing the bluff-body size has both positive and negative impacts. Therefore, a medium blockage ratio can gain a tradeoff between the positive and negative aspects and achieve the largest blow-off limit. Image 1 • A micro-combustor with slotted bluff-body was designed for non-premixed H 2 /air flame. • Effect of bluff-body blockage ratio on flame blow-off limit was numerically studied. • The flame blow-off limit peaks at a medium blockage ratio of the bluff-body. • Increasing the blockage ratio of bluff-body has both positive and negative impacts. • A medium blockage ratio gains a tradeoff between the positive and negative effects. [ABSTRACT FROM AUTHOR]

In this work, buoyancy effects on hydrogen jet flames confined in a small tube without air co-flow were numerically investigated. The results show that the extinction limit of fuel velocity under buoyant condition is much lower than that without buoyancy. Moreover, hydrogen flames under buoyant condition attatch the nozzle exit for all fuel velocities investigated; however, the flames without buoyancy surround the lower wall at low fuel velocity. In addition, combustion is nearly complete in the presence of buoyancy, whereas the combustion efficiency under non-buoyant condition is below 45%. Furthermore, flame temperature under buoyant condition is much higher compared to the counterpart under non-buoyant condition at low and moderate fuel velocities. Analysis reveals that in the case without buoyancy, the negative gauge pressure in the annular space is unable to entrain sufficient air from the ambient. Hence, hydrogen has to diffuse downwards to sustain the flame and complete combustion is unrealizable. Hydrogen flames under buoyant condition attatch the nozzle exit for all fuel velocities investigated; however, the flames without buoyancy surround the lower wall at low fuel velocity. Analysis reveals that in the case without buoyancy, the negative gauge pressure in the annular space is unable to entrain sufficient air from the ambient. Hence, hydrogen has to diffuse downwards to sustain the flame. Image 1 • Extinction limit under buoyant condition is much lower than that without buoyancy. • The flames under non-buoyant condition surround the lower wall at low fuel velocity. • Combustion efficiency under non-buoyant condition is below 45%. • Flame temperature under buoyancy is much higher relatively low fuel velocities. • Without buoyancy, negative pressure in the tube is unable to entrain sufficient air. [ABSTRACT FROM AUTHOR]

Graphical abstract Combustion efficiency of lean H 2 /air flames (left figure) can be notably improved at high inlet velocity in a double-layered micro-combustor compared to single-layered ones. This is attributed to the enhancement of heat recirculation through inner wall and the reduction of heat loss from outer wall, which can alleviate flame tip opening phenomenon and fuel leakage (right figure). Highlights • A micro-channel combustor with double-layered walls and cavities was proposed. • The double-layered combustor can achieve higher efficiency than single-layered ones. • Heat recirculation is enhanced and heat loss is reduced in double-layered combustor. • Flame tip opening and fuel leakage can be alleviated in double-layered combustor. Abstract In this study, a micro combustor with double-layered walls was proposed for micro energy and power systems. SiC and quartz were selected for the inner and outer layers of combustor walls, respectively. Numerical investigation demonstrates that the double-layered micro combustor can achieve a higher combustion efficiency at large inlet velocity than single-layered SiC and quartz micro combustors. It is revealed that the heat recirculation effect of double-layered micro combustor is greatly enhanced compared to quartz combustor; meanwhile, the heat loss rate is decreased to some extent compared to SiC combustor. Moreover, the negative stretch effect at flame tip is reduced in the double-layered combustor. Consequently, combustion process can be intensified and flame tip opening can be alleviated, leading to a higher combustion efficiency in the double-layered micro combustor. [ABSTRACT FROM AUTHOR]

In this study, the flame structures of a fuel-rich premixed H 2 –air mixture issued from a divergent micronozzle were numerically studied and compared with their counterparts. The flame dynamics of the divergent micronozzle at relatively low jet velocities were also analyzed. A flame pattern of "dual flames with a smaller one surrounded by a larger one" was discovered for both the straight and divergent micro-nozzles at high jet velocities. This flame structure began to appear at a lower jet velocity (V in) as the divergence angle (θ) decreased. Specifically, it first appeared at V in = 7.5 and 20 m/s for θ = 0° and 5°, respectively. In addition, a flame mode consisting of dual flames with a smaller one inside the nozzle occurred only for divergent micronozzles under low and medium jet velocities. This flame structure could emerge at a higher jet velocity as the divergence angle increased. For example, it could occur until V in = 5 m/s and 17.5 m/s for θ = 1° and 5°, respectively. A global map of all flame patterns was drawn based on the results obtained by varying the jet velocity and divergence angle. Moreover, periodic flame dynamics appeared when the jet velocity was less than the critical value (<1.7 m/s). Specifically, a small flame (i.e., secondary flame) split from the jet flame (i.e., main flame) and propagated towards the nozzle inlet; however, its propagation direction was inverted and it finally extinguished owing to the heat loss to the nozzle wall. The next cycle was initiated after a short duration. The analysis revealed that the periodic flame dynamics are a result of the thermal interactions between the flame and nozzle wall. Using H 2 or natural gas/H 2 blends as the fuel of domestic gas stoves is a promising technology to reduce CO 2 emission. In this study, the flame characteristics of pure H 2 jet flame issued from a micro divergent nozzle was numerically investigated. As shown in the figure above, new flame dynamics have been observed. A small flame splits from the bottom of H 2 jet flame and propagates towards the inlet of divergent nozzle, then alters its direction and extinguishes owing to the heat loss to the wall. Such flame dynamics occur periodically. [Display omitted] • A smaller flame surrounded by a larger one appears at high jet velocities. • A smaller flame occurs in the divergent nozzle at low and medium jet velocities. • A global flame pattern map is drawn by varying jet velocity and divergence angle. • Periodic flame dynamics appear under low jet velocities for divergent nozzles. • The flame dynamics are a result of thermal coupling of the flame and nozzle wall. [ABSTRACT FROM AUTHOR]

A micro Swiss-roll combustor with a bluff-body was proposed for the first time. It is demonstrated that this improved design can greatly extend flame blow-off limit by reducing the flame stretch effect. The flame stabilization ability of this new combustor made of quartz, stainless steel (SS) and silicon carbide (SiC) was numerically investigated using a three dimensional, steady state, k-epsilon turbulence model. The results show that at an equivalence ratio of 0.5, the blow-off limits of quartz combustor, SS combustor and SiC combustor are 40 m/s, 50 m/s and 35 m/s, respectively. Comprehensive analyses were conducted under an inlet velocity of 20 m/s. It was revealled that the heat loss rates for quartz, SS and SiC combustors were 153 W, 108 W and 185 W, respectively. The SS combustor has the best heat-recirculation effect, followed by quartz combustor and SiC combustor. The lengths of recirculation zones are 4.35 mm, 3.4 mm and 3.37 mm for SiC combustor, quartz combustor and SS combustor, respectively. In summary, the heat loss rate is the chief factor responsible for the flame blow-off limit of this novel Swiss-roll combustor. It acts together with the heat-recirculation effect and flow-recirculation effect to determine the flame blow-off limit. [ABSTRACT FROM AUTHOR]

This figure shows the temporal evolution of flame position of uninterrupted flame with repetitive extinction and ignition (UFREI) under different fuel compositions. The period of UFREI will be shortened with the increase of CO/H 2 ratio. Meanwhile, the downstream flame is constrained within x = 27 mm for all these cases. [Display omitted] • Effect of CO-to-H 2 ratio on flame behaviors in a micro flow reactor was studied. • Five flame regimes were numerically discovered and regime diagram was drawn. • A higher H 2 proportion promotes OH production in the upstream normal flame. • A larger H 2 fraction causes more CO residual which enhances downstream weak flame. • Flame propagation distance and period are decreased for a larger CO/H 2 ratio. Flame characteristics of CO/H 2 /O 2 /N 2 mixture (O 2 :N 2 = 1:7) with different CO/H 2 ratios in a micro-channel with a controlled wall temperature gradient were investigated through numerical simulation. Five flame regimes were identified and their global diagram was drawn. The influence of CO/H 2 ratio on two typical flame dynamics (one is stable and the other is unstable), i.e., "coexisting upstream normal flame (UNF) and downstream weak flame (DWF)" and "uninterrupted flame with repetitive extinction and ignition (UFREI)", were discussed. It is revealed that in the UNF-DWF regime, a higher H 2 proportion can strengthen OH generation in the UNF through H 2 -OH and H-HO 2 -OH reaction paths, which causes the UNF to propagate upstream. However, the upstream lower wall temperature results in weaker near-wall reaction intensity for the UNF, which brings about more CO residual and thus enhances the DWF. For the UFREI regime, both the period and the propagation distance of flame dynamics are reduced with the augmentation of CO/H 2 ratio because of the weakened flame intensity. Moreover, the characteristic flame positions are also affected by the CO/H 2 ratio. In details, the farthest position of UFREI moves upstream while the transition position slightly shifts downstream as the CO/H 2 ratio is increased, which is substantially affected by the transport of residual fuel after the extinction of UFREI. In summary, the present investigation unraveled the complicated influence of CO/H 2 ratio on the syngas flame dynamics in a micro-channel with a controlled wall temperature gradient. [ABSTRACT FROM AUTHOR]

It is challenging to maintain a stable flame in miniature combustors. In the present work, a mesoscale bluff-body combustor was developed. The flame blow-off limits of CH 4 /air mixtures were experimentally obtained, which showed that the bluff body can effectively expand the flame stabilization limit. The flame anchoring mechanisms of this mesoscale bluff-body combustor were studied through 3D numerical simulation. It is revealed that a flow recirculation zone is formed behind the bluff body and its area increases with the increase of inlet velocity. Moreover, the incoming fresh mixture can be preheated by both the upstream channel wall and the front and side walls of the bluff body, which leads to an earlier initiation of chemical reactions. Furthermore, the local equivalence ratio grows larger than that of the incoming pre-mixture due to preferential transport effect, which favors the flame stabilization, especially for relatively leaner mixtures. Besides, the dynamic process of flame blow-off was numerically reproduced, which demonstrates that the excessively large strain rate is responsible for flame extinguishment at high inlet velocity. In conclusion, the present work revealed the complex interactions between flow field, mass and heat transfer processes, and chemical reactions in the mesoscale bluff-body combustor. [ABSTRACT FROM AUTHOR]

The effect of reduced pressures (i.e., p = 1.0 atm, 0.8 atm and 0.5 atm) on the combustion efficiency of lean H 2 /air flames in a micro-combustor was investigated in this work. We found that the combustion efficiency increases as the pressure is reduced from 1.0 atm to 0.8 atm, and then decreases when the pressure is further reduced from 0.8 atm to 0.5 atm. Numerical analyses reveal that when the pressure is reduced to 0.8 atm, the laminar burning velocity, the heat release rate and the effective Lewis number are increased, while the stretch rate is decreased. The combined effects of these aspects retard the occurrence of “flame tip opening” and lead to a higher combustion efficiency. When the pressure is further reduced to 0.5 atm, the fuel supply and heat release amount are significantly reduced, resulting in much lower temperature level and weaker reaction intensity. Meanwhile, the heat-loss ratio is almost doubled. Therefore, the combustion is completely extinguished in the downstream channel and a large amount of fuel leaks out without burning. As a consequence, the combustion efficiency decreases drastically. [ABSTRACT FROM AUTHOR]

It is challenging to achieve a large blowout limit for micro-combustors due to the increased heat loss ratio and reduced residence time. For this, we recently developed a micro-combustor with a plate flame holder and two preheating channels. In this paper, the effect of the plate length (L b = 0.5, 1.0, 2.0, 3.0, 4.0, 5.0 and 6.0 mm) on the blowout limit of CH 4 /air flames was numerically investigated. The results show that the flame blowout limit increases firstly and then decreases with an increasing plate length. The largest blowout limit is obtained at L b = 1.0 mm. Three neighboring cases, i.e., L b = 0.5, 1.0 and 2.0 mm, are taken to analyze the underlying mechanisms responsible for this non-monotonic trend. The flame blowout process demonstrates that, the flame is extinguished due to "pinch-off phenomenon" at high inlet velocity, and the shorter the distance between the upper and lower flame fronts, the smaller the blowout limit will be. Numerical analysis reveals that the differences in flame blowout limits are a result of the combined effects of heat recirculation and local flow field at the entrance of the combustion chamber. The heat recirculation effect grows stronger with a decreasing length of flame holder, which results in a more obvious volumetric expansion at the entrance of the combustion chamber. Meanwhile, the plate flame holder has a redirection effect on the local flow field. As a result, the gaseous mixture enters the combustion chamber with a smallest acute angle in the case of L b = 1.0 mm, which leads to a largest distance between the upper and lower flame fronts and a longest recirculation zone. Consequently, the flame blowout limit reaches a peak at L b = 1.0 mm. The present work provides an important guideline to design such kind of micro combustors. [ABSTRACT FROM AUTHOR]

Filtration gas combustion in a mesoscale combustor filled with inert fibrous porous media was numerically investigated. Downstream propagating combustion wave and standing combustion wave were obtained in the simulation. The predicted flame stability diagram agrees well with experimental results in the literature. Moreover, it was found that the downstream propagating flame changes its shape and transforms from one stable state to another stable state in a short duration after ignition, while the flame shape of the standing wave remains unchanged. Furthermore, combustion efficiency of both combustion waves increases with time and a nearly complete conversion can be achieved spontaneously due to the strong heat recuperation. It is also demonstrated that heat conduction in the tube wall has a crucial effect on flame behaviors and combustion efficiency. On one hand, for a bigger wall thermal conductivity, the downstream propagating wave moves with a larger speed; on the other hand, a high combustion efficiency can be expected for a smaller thermal conductivity. [ABSTRACT FROM AUTHOR]

Recently, we developed micro- and meso-scale combustors with wall cavities, which have been verified to be a simple and effective way for flame anchoring. In the present paper, the effect of wall thermal conductivity on flame blow-off limit was investigated numerically. It is very interesting to find that the flame blow-off limit is a non-monotonic function of the wall thermal conductivity, i.e., it achieves smaller values at moderate thermal conductivities. Our previous study has demonstrated that the occurrence of flame blow-off is due to the flame splitting phenomenon at the transition point between the ramped cavity wall and downstream channel wall. This is because the heat loss rate and strain rate at this point is very large which lead to local extinction and flame blow-off. Therefore, if the flame front approaches the transition point earlier, the flame blow-off limit will be smaller. For the present work, the differences in wall thermal conductivity result in different heat recirculation effect, which has a significant influence on the shape of flame front. Three typical thermal conductivities ( λ s = 0.5, 1.05 and 50 W/m K) were taken to analyze the non-monotonic trend of the flame blow-off limit. It is revealed that, under the same condition, the distance between flame front and transition point is the shortest at λ s = 1.05, which leads to the smallest flame blow-off limit among the three cases. Therefore, it can be concluded that the heat recirculation effect is of crucial importance to the flame blow-off limit of the mesoscale cavity-combustor. [ABSTRACT FROM AUTHOR]

The blow-off limits of CH 4 /air flames in a mesoscale cavity-combustor under various pressures (P = 1.0–3.0 atm) are investigated numerically. The results show that the flame blow-off limit increases first and then decreases with an increasing pressure. Three typical pressures (P = 1, 2 and 3 atm) are selected to perform numerical analysis with a detailed reaction mechanism. The analysis demonstrates that the reaction intensity in the cavity is enhanced as the pressure is raised, which is beneficial for flame stability. On the other hand, the flame front is prolonged at a higher pressure. This leads to more intense stretching effect, which is detrimental for flame stability. Therefore, the flame blow-off limit depends on the competition between the positive and negative sides. When the pressure is increased from 1 atm to 2 atm, the enhancement of anchoring ability in the cavity overwhelms the augmentation of stretching effect, which leads to an increase in flame blow-off limit. However, as the pressure is further raised from 2 atm to 3 atm, the stretching effect becomes the dominated side, which results in a decrease in flame blow-off limit. In summary, these complicated interactions determine that the flame blow-off limit is a non-monotonic function of the pressure. [ABSTRACT FROM AUTHOR]

It is challenging to achieve a wide operational range in micro combustors due to the increased heat loss. In the present work, a micro combustor with a plate flame holder and preheating channels is developed. Three kinds of solid materials (i.e., stainless steel, SiC and copper) are chosen to investigate the heat recirculation effect on the flame blow-off limit of this micro combustor. The results show that, even for very lean CH 4 /air mixtures, this micro combustor can achieve a very large flame blow-off limit, which confirms its excellent flame stabilization ability. Moreover, the copper combustor has a widest operational range while the SiC combustor has the poorest performance. The analysis reveals that a large blow-off limit is mainly attributed to a good heat recirculation effect, which relies upon both the thermal conductivity and the surface emissivity of the solid material. It is also demonstrated that the differences in the recirculation-zone lengths are small, which is not expected to be a dominant factor that responsible for the differences in the blow-off limits of the three combustors. In conclusion, to obtain a large flame blow-off limit, a solid material with a large thermal conductivity and a small emissivity is suggested to fabricate this kind of micro combustor. [ABSTRACT FROM AUTHOR]

Our recent work demonstrated that flame blow-off limit of microscale and mesoscale channels can be greatly extended by using wall cavity. In the present paper, we numerically investigate the flame-anchoring mechanisms of H 2 /air flame in terms of interplays between flow field, heat recirculation, chemical reaction and preferential transport of species. The results show that obvious recirculation zone and low velocity zone are formed in the cavity, and the flame-anchoring location is near the vertical cavity wall. Moreover, the incoming fresh mixture is gradually heated by the upstream inner wall (i.e., heat recirculation effect), which leads to earlier initiation of chain reactions and more intense reaction near the flame-anchoring location. Furthermore, the preferential transport effect, which is caused by the differences in mass diffusivities of various species and two-dimensionality of the flow field, can change the local equivalence ratio ( ϕ local ). It is shown that ϕ local increases in the recirculation zone and low velocity zone in the cavity. More importantly, ϕ local is much larger in the vicinity of the flame root, which is beneficial for flame-anchoring. In summary, the cavity can simultaneously provide a recirculation zone and low velocity zone, a better heat recirculation effect, and a high ϕ local region, which are the main flame-anchoring mechanisms of the micro cavity-combustor for premixed H 2 /air flames. [ABSTRACT FROM AUTHOR]

The development of simple and effective flame stabilization methods is crucial for micro- and mesoscale combustors. In the present paper, flame stability in a mesoscale channel with cavities was experimentally investigated, and the effect of the channel gap distance on the flame blow-off limit was examined. Experimental results demonstrate that the stable operational range of the combustor is extended in the presence of cavities, which act as a flame holder. Meanwhile, the flame blow-off limit increases as the channel gap distance is increased. Numerical simulation was conducted to facilitate analysis of the underlying mechanisms responsible for the differences in flame blow-off limits. Analyses reveal that effects of heat loss and heat recirculation are the two dominant factors that lead to different blow-off limits. First, the heat recirculation effect is more prominent for a larger channel gap distance, which exhibits much more intense initiation reactions. In addition, with the decrease of the channel gap distance, the heat loss rate from the flame root becomes larger, and the flame root is liable to be thermally quenched by the cavity walls. In conclusion, the fresh mixture is better preheated by the recirculated heat, and the heat loss rate from the flame root is also lower when the channel gap distance is larger. Hence, a larger flame blow-off limit can be expected for a channel with a wider gap. [ABSTRACT FROM AUTHOR]

Curved micro-channels are frequently used in micro-Swiss roll combustors and other applications. The secondary flows (i.e., Dean vortices) play an important role in both the mixing performance of fuel and oxidant and the flame propagation characteristics in curved micro-channel combustors. In the present study, the helicity method was adopted and a mixing performance evaluation criterion (MPEC) was proposed to investigate the impacts of inlet velocity, nominal equivalence ratio (φ) and curvature radius on the Dean vortices and mixing performance of H 2 and air in curved micro-channels. First, it is found that with the increase of inlet velocity, the intensity of Dean vortices increases and their shape grow asymmetrical. When the inlet velocity is high enough, another pair of smaller vortices appear near the outer wall. Meanwhile, the mixing performance of H 2 and air becomes worse due to the reduced residence time of gas flow. Second, as the nominal equivalence ratio is decreased, the Dean vortices are intensified and the vortices shape become asymmetrical. Moreover, the mixing process is improved owing to the enhanced secondary flows. Thirdly, the intensity of Dean vortices increases significantly as the curvature radius is decreased. However, the mixing performance becomes worse due to the shortened length of the curved micro-channel. Finally, an empirical correlation between MPEC and the Reynolds number (Re), Dean number (De) and φ was obtained based on the numerical results, which may provide a guidance for the design and operation of curved micro-channel combustors. • Mixing performance in curved micro-channels depends on intensity of Dean vortices and flow time. • Dean vortices are intensified at higher velocity but mixing is worsened due to reduced residence time. • Dean vortices are enhanced under a lower φ because of increased centripetal pressure gradient. • A smaller curvature radius leads to intensified Dean vortices but worse mixing due to insufficient mixing time. • An empirical correlation between MPEC and Re , De and φ was obtained for practical design. [ABSTRACT FROM AUTHOR]

[Display omitted] • Partially submerged flames or surface flames were formed in most cases. • A center convex started to appear in the flame front from certain inlet velocity. • Flame blow-off limit rose almost linearly with the increase of equivalence ratio. • The wall temperature level was the highest under the equivalence ratio of 1.1. • The variation of exhaust gas temperature versus equivalence ratio was very small. Liquid fuels are good candidates for combustion-based miniature power sources due to their large specific energy. In this study, combustion characteristics of n-C 4 H 10 /air mixtures in a tube (ID = 6 mm) partially filled with wire mesh was experimentally explored over an equivalence ratio of ϕ = 0.5–1.5. In most cases, partially submerged flames and surface flames were observed on the downstream side of the porous media. An external diffusion flame was established under high inlet velocities for fuel-rich mixtures of 1.2 ≤ ϕ ≤ 1.5. The flame with a center convex started to appear at a certain inlet velocity due to the relatively higher local velocity, which was a result of the rolling up method of the wire mesh. Moreover, a lower extinction limit of 0.1 m/s was achieved over 0.9 ≤ ϕ ≤ 1.2, whereas the blow-off limit rose almost linearly up to 0.8 m/s with the equivalence ratio even on the fuel-rich side. The underlying mechanisms for this tendency were comprehensively analyzed from the preheating effect, heat loss effect, volumetric expansion effect as well as the support effect from the external diffusion flame. Furthermore, for an identical inlet velocity, the wall temperature first rose until ϕ = 1.1 and then decreased when ϕ > 1.1. Under a constant equivalence ratio, both the maximum wall temperature and the exhaust gas temperature increased with an increasing inlet velocity. However, for an identical inlet velocity, the exhaust gas temperature at low and moderate inlet velocities remained almost invariable versus the equivalence ratio. This was speculated to be the relatively small variation of heat loss ratio versus equivalence ratio. In summary, these findings enriched the knowledge of n-C 4 H 10 /air combustion in small tubes partially filled with porous media. [ABSTRACT FROM AUTHOR]

Flame characteristics of CH 4 /H 2 /air (CH 4 :H 2 =1:1) mixture in a cylindrical micro flow reactor with a controlled wall temperature distribution were numerically studied. Bifurcation phenomena were captured in both stable flame regime and FREI (Flame with Repetitive Extinction and Ignition) regime. For the stable flame regime, a normal flame and a weak flame co-exist in upstream and downstream of the channel, respectively. Analysis of reaction paths demonstrates that H 2 competes with CO for OH radical in upstream normal flame, which causes incomplete combustion and produces CO residual. When the residual CO moves to farther downstream with a higher wall temperature, CO and OH react completely and a weak flame is formed. For the unstable flame regime, twice bifurcations occur during one period of FREI. The first bifurcation mainly results from residual O 2 and CO after the re-ignition point, while the formation mechanism of the second bifurcation is similar to that of the co-existing weak flame which bifurcates from upstream stable flame. In a word, the present study discovered complex flame characteristics of CH 4 /H 2 blended fuels in a micro flow reactor with wall temperature gradient and revealed the underlying mechanisms responsible for these flame phenomena. [ABSTRACT FROM AUTHOR]

We recently developed a micro cavity-combustor which has a strong ability in flame stabilization and is promising to be used in micro power generation apparatus for MEMS and micro propulsion systems. In the present paper, the effect of wall thermal conductivity on the combustion efficiency of lean H 2 /air flame was numerically investigated. It is shown that the decrease of combustion efficiency is due to flame-tip opening at high velocity. For the convenience of quantitative comparison, a “flame-splitting limit” was defined as the critical velocity when the combustion efficiency drops to 80%. It is very interesting to find that the flame-splitting limit exhibits a non-monotonic variation with the wall thermal conductivity. The analysis reveals that, for a larger thermal conductivity, the heat recirculation effect on the fresh mixture is better, which leads to higher flow velocity at the cavity exit; therefore, the flame front suffers stronger stretching effect and is splitted at a lower velocity. However, for a smaller thermal conductivity, the flow velocity in the central region of far downstream becomes faster, which results in stronger flame-stretching effect and smaller flame-splitting limit. In summary, a moderate thermal conductivity is advantageous to achieve high combustion efficiency. [ABSTRACT FROM AUTHOR]

Behaviors of premixed CH 4 /air flame in mesoscale channels with and without cavities were experimentally investigated. No stable symmetric flame was observed in the channel without cavities and flame is prone to inclining and pulsating. In contrast, flame can be effectively anchored in the presence of cavities. When the inlet velocity is increased sufficiently high, curved fluctuating flame front appears. Blow-off limits of the channel with cavities are several times larger than the corresponding burning velocity of incoming CH 4 /air mixture, while the flashback limits are almost the same as the straight channel counterparts. These indicate that the cavities have a strong ability to extend the operational range of inlet velocity. Numerical simulation demonstrates that combined effects, i.e., the formation of recirculation zone and low velocity zone in the cavities, preferential diffusion effect, as well as the preheating effect of upstream inner walls, are major mechanisms responsible for flame stabilization. Furthermore, numerical result reveals that large strain rate and heat loss rate exist at the transition point between the ramped cavity wall and the downstream inner wall, which results in flame splitting at high inlet velocity due to local extinction, and eventually leads to flame blow-off. In summary, the combustion behaviors in the mesoscale channel with cavities strongly depend on the interactions between the reaction zone, conjugate heat exchange and flow field. [ABSTRACT FROM AUTHOR]

Our previous investigations demonstrated that conical strip inserts have good thermo-hydraulic performances based on evaluation criteria of the First Law of Thermodynamics. The present work is dedicated to further analyze the performances of these inserts from a viewpoint of entropy generation. Effects of alignment method and geometrical parameters on entropy generations of laminar heat transfer in the tubular flow are investigated. Local entropy generations are presented for discussion. Results show that entropy generation rates caused by non-staggered strips are about 81.1% that of staggered alignment, while the heat transfer rates and PECs of the former are about 33.8% and 13.5% larger than the latter counterparts, respectively. Moreover, the entropy generation rate (and thus irreversible loss) caused by heat transfer process overwhelms the counterpart by viscous flow. The total entropy generation number of enhanced tube, ranging between 0.0657 and 0.0975, is most sensitive to geometry angle with a maximum averaged relative variation of 32.2%. Its variation trend with Re is concave with a pit at Re = ∼600. In brief, non-staggered inserts with a larger geometry angle and smaller strip-wall gap and pitch, facilitate a better thermo-hydraulic performance at Re = ∼600 from the viewpoint of exergy loss reduction. [ABSTRACT FROM AUTHOR]

Abstract: We recently confirmed the “tip open phenomenon” of lean H2/air flame in a microchannel with cavities. The critical inlet velocity when fuel conversion ratio drops to 80% was defined as “flame splitting limit”. In the present work, we numerically studied the impact of channel gap distance. Results showed that corresponding limits for 1.0-mm, 0.8-mm and 0.6-mm channels are 26 m/s, 33 m/s and 16 m/s respectively, exhibiting a non-monotonic dependence. The analysis reveals that when the gap distance is decreased from 1.0 mm to 0.8 mm, the proportion of fuel that involved into the cavities is increased, flame length is reduced simultaneously, and better preheating of the fresh mixture is attained. These positive effects lead to an increase in flame splitting limit. As the gap distance is further reduced to 0.6 mm, the excessive stretch effect results in complete extinction of downstream flame, causing a decrease of the splitting limit. [Copyright &y& Elsevier]

Highlights: [•] A design method of heat exchangers motivated by constructal theory is proposed. [•] A genetic algorithm is applied and the TEMA standards are rigorously followed. [•] Three cases are studied to illustrate the advantage of the proposed design method. [•] The design method will reduce the total cost compared to two other methods. [Copyright &y& Elsevier]

Abstract: We recently developed a micro bluff body combustor. Both experimental and numerical investigations demonstrated that the bluff body can significantly extend the blow-off limit. In the present paper, the effect of solid materials (i.e., quartz, stainless steel, and SiC) on the blow-off limit of this micro-combustor was investigated numerically. The results show that the blow-off limit of the quartz combustor is the largest, while that of the SiC combustor is the smallest. The underlying mechanisms were analyzed in terms of the interactions between the flow field, heat transfer processes and flame stabilization. It is demonstrated that when the thermal conductivity is small (i.e., quartz), less heat is conducted to the upstream walls, the fresh mixture is not sufficiently preheated and the gaseous volume does not expanded so significantly. Therefore, the flame stretching effect is weaker than the other two cases and thus a larger blow-off limit is achieved. Moreover, for the stainless steel and SiC micro-combustors, a larger thermal emissivity (i.e., SiC) results in a bigger ‘total heat loss ratio’ and a smaller blow-off limit. In summary, a solid material with relatively low thermal conductivity and emissivity is beneficial to obtain a large blow-off limit for the micro bluff body combustor. The present study also demonstrates that both flow and heat transfer processes, as well as their interactions, play an important role in flame stabilization of the micro bluff body combustor. [Copyright &y& Elsevier]

Abstract: 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. [Copyright &y& Elsevier]

Highlights: [•] Theoretical models are constructed for heat exchangers with various recycles. [•] Effects of reflux ratio, capacitance ratio, recycle length, etc., were studied. [•] A basic internal recycle increases heat transfer rate by 27% than no recycle. [•] Recycles of half length increase overall performance by 65% than full-length ones. [ABSTRACT FROM AUTHOR]

Abstract: 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. [Copyright &y& Elsevier]

Abstract: CFD simulation has become a powerful and popular tool for the thermal hydraulic design and analysis of heat exchangers. However, the computation load is usually too heavy to simulate a whole shell-and-tube heat exchanger (STHX) applying the traditional modeling method. In the present study, a numerical model based on the concepts of porosity and permeability is developed to obtain the shell-side thermal hydraulic performances. In this model, the distributed resistances and heat sources, as well as the distributed turbulence kinetic energy and its dissipation rate are introduced to account for the impacts of tubes on the fluid. The numerical model is solved over Re =6813–22,326 for the shell side of a STHX with flower baffles, and reasonable accuracy is demonstrated by the comparison with test data (maximum relative deviation within 15%). With this model, the velocity and temperature fields, together with the distribution of convective heat transfer coefficient, are obtained and presented to help analyzing the underlying mechanism of shell-side thermal augmentation. The present work shows that this model is economic and effective in the thermal hydraulic design and analysis of a whole device. [Copyright &y& Elsevier]

Abstract: Our previous study has demonstrated that the conical strip inserts have a good thermo-hydraulic performance in the tubular turbulent flow regime. This paper investigates its thermo-hydraulic performance of laminar flow numerically. Effects of some geometrical parameters are studied. Computation results show that the average Nusselt number of enhanced tube is augmented by 3.70–5.51 times (average Nu = 16.15–24.05), while the average friction factor increases by 5.31–14.77 times (average f = 0.59–1.51) those of plain tube. The values of performance evaluation criterion (PEC) range between 1.17 and 2.97. Numerical results indicate that larger strip size (characterized by a geometry angle), smaller strip-wall gap and smaller strip pitch can effectively enhance the heat transfer rate, but also increase the flow resistance. The impact of slant angle depends on the magnitude of Reynolds number. Moreover, it is shown that the Nusselt number and friction factor are sensitive to the geometry angle, while the most sensitive factor of PEC value is the strip pitch. Relatively high PEC value is obtained at a parameter combination with small slant angle, strip-wall gap, strip pitch, and moderate geometry angle (α = 30°, δ/D = 0.10, s/D = 1.5, β = 60°) among all the investigated conditions. [Copyright &y& Elsevier]

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. [Copyright &y& Elsevier]

Abstract: Twisted tape is a widely used technique for heat transfer enhancement. In the present paper, we proposed a center-cleared twisted tape aiming at acxhieving good thermohydraulic performance. A comparative study between this type and the short-width twisted tape was performed numerically in laminar tubular flows. The computation results demonstrated that the flow resistance can be reduced by both methods; however, the thermal behaviors are very different from each other. For tubes with short-width twisted tapes, the heat transfer and thermohydraulic performance are weakened by cutting off the tape edge. Contrarily, for tubes with center-cleared twisted tapes, the heat transfer can be even enhanced in the cases with a suitable central clearance ratio. The thermal performance factor of the tube with center-cleared twisted tape can be enhanced by 7–20% as compared with the tube with conventional twisted tape. All these demonstrated that the center-cleared twisted tape is a promising technique for laminar convective heat transfer enhancement. [Copyright &y& Elsevier]

Abstract: Flame pattern formations of premixed DME–air mixture in a heated radial channel with a gap distance of 2.5mm were experimentally investigated. The DME–air mixture was introduced into the radial channel through a delivery tube which connected with the center of the top disk. With an image-intensified high-speed video camera, rich flame pattern formations were identified in this configuration. Regime diagram of all these flame patterns was drawn based on the experimental findings in the equivalence ratio range of 0.6–2.0 and inlet velocity range of 1.0–5.0m/s. Compared with our previous study on premixed methane–air flames, there are several distinct characteristics for the present study. First, Pelton-wheel-like rotary flames and traveling flames with kink-like structures were observed for the first time. Second, in most cases, flames can be stabilized near the inlet port of the channel, exhibiting a conical or cup-like shape, while the conventional circular flame was only observed under limited conditions. Thirdly, an oscillating flame phenomenon occurred under certain conditions. During the oscillation process, a target appearance was seen at some instance. These pattern formation characteristics are considered to be associated with the low-temperature oxidation of DME. [Copyright &y& Elsevier]

Flame characteristics of stoichiometric syngas/diluted-air mixture (CO:H 2 :O 2 :N 2 =1:1:1:7) in a micro-flow reactor with controlled wall temperature profile were numerically studied. In addition to single normal flame and single weak flame that respectively occur at high and low inlet velocities, two new combustion regimes, i.e., "coexistence of upstream normal flame and downstream weak flame (UNF-DWF)" and "uninterrupted flame with repetitive extinction and ignition (UFREI)", were found at moderate inlet velocities. It is revealed that the coexisting downstream weak flame of UNF-DWF is attributed to incomplete consumption of CO by the upstream normal flame. Analysis of reaction path further demonstrates that the downstream weak flame of UNF-DWF is partially sustained by H 2 O through H 2 O+O=2OH due to H 2 O production in the upstream normal flame. To elucidate the dynamic process of UFREI, it is artificially divided into "upstream flame" and "downstream flame (actually an oscillating weak flame)", and an entire period of UFREI is divided into four sub-regions. The temporal variations of CO mole fraction profile and heat release rate profile along the centerline of each sub-region are exhibited and analyzed. Briefly speaking, the formation of UFREIs is essentially a result of early re-ignition of residual CO from the upstream flame before the occurrence of its extinction. In summary, the present study sheds some light on the complicated combustion characteristics of syngas in the micro flow reactor with a prescribed wall temperature profile. [ABSTRACT FROM AUTHOR]

Abstract: Comprehensive regime diagrams of flame pattern formation in radial microchannels with temperature gradients were drawn based on experimental findings. A premixed methane–air mixture was introduced at the center of microchannels formed by two parallel circular quartz plates that were heated with an external porous burner to create a positive temperature gradient condition in the direction of flow. Combustion behavior in those microchannels at channel widths of 0.5, 1.0, 1.5, 2.0, 2.5, and 3.0 mm were experimentally investigated. Regime diagrams of various stable and unstable flame patterns were obtained, confirming that the flame pattern is a strong function of mixture equivalence ratio, inlet mixture velocity, and channel width. Furthermore, some combustion characteristics, such as the rotating frequency of the single pelton-like flame and the triple flame, the radius of the stable circular flame front, and comparison between the major combustion products of the single and double pelton-like flames, were also investigated. [Copyright &y& Elsevier]

[Display omitted] • Effect of wall temperature profile on DME weak flame structure was investigated. • Three-stage DME oxidation consists of one cool flame and two hot flames. • Under large temperature gradient, the cool flame moves to high temperature zone. • The second-stage oxidation disappears with under large wall temperature gradient. • Maximum wall temperature affects the heat release rate of hot flame significantly. Combustion characteristics of stoichiometric dimethyl ether (DME)/air mixture in a cylindrical micro-channel with linear wall temperature profiles were numerically investigated under an inlet velocity of 2.0 cm/s. The effects of wall temperature gradient and maximum wall temperature on flame structure were scrutinized. Stabilized three-stage oxidation of weak flame, which consists of one cool flame and two hot flames, was observed. With the increase of wall temperature gradient, the cool flame moves to higher temperature location because the residence time at local temperature becomes shorter. Moreover, the second-stage oxidation of hot flame disappears when the wall temperature gradient is great enough. Detailed analysis of reaction pathways and heat release rate demonstrates that under elevated wall temperature gradient, the reaction delay makes the cool flame to sustain at higher wall temperature position. As for the hot flame, the intensification of HCO decomposition under a larger wall temperature gradient produces more H and CO. The H radical accelerates the consumption of CH 3 OCH 3 and CH 2 O, which can further strengthen the total reaction and result in the mergence of the two heat release rate peaks of hot flames. When the wall temperature gradient is fixed, the heat release rate of hot flame is significantly influenced by the maximum wall temperature. The second heat release peak of hot flame will disappear if the maximum wall temperature is low enough because the low wall temperature could not provide enough energy for the direct decomposition of HCO and initiation of the chain reaction of CO oxidation. [ABSTRACT FROM AUTHOR]

In this paper, a model of partially premixed jet flames that sustained above a meso-scale short tube was established for an individual flame port of domestic gas stoves. The effects of hydrogen addition (volume ratio β = 0%, 10%, 20%, 30%) on the extinction dynamics of CH 4 -air jet flames were numerically investigated. It is found that flame oscillation occurs once (β = 10% and 20%) or twice (β = 30%) in the extinction process. Moreover, the larger of β , the longer the extinction process can sustain. Analysis was performed in terms of both chemical effect and thermal effect. As to the chemical effect, in the first place, the reaction rate decreases as the inlet velocity is reduced. As a result, the consumption rate of O 2 will be less than the supply rate from the incoming mixture, which makes the O 2 concentration in the flame center increase. On the other hand, the amount of H radicals increases with the increase of β , and when the O 2 content at the flame center reaches a "critical point", the key elementary reaction "H + O 2 ↔ O + OH" will be enhanced and consequently the total reaction rate will also be intensified. After that, the consumption rate of O 2 will be larger than the supply rate due to the reduced flow rate of incoming mixture. The total heat release rate will decrease sharply and extinction occurs. As regards the thermal effect, it is revealed that heat recirculation effect (indirect preheating effect) lags behind the variation of the reaction zone (i.e., flame), thus, it has a negligible impact on flame oscillation. In contrast, the preheating temperature in the vicinity of flame front (named as "direct preheating effect") exhibits a similar variation tendency with the total heat release rate of the flame. And the larger of β , the more remarkable of the direct preheating effect can be. In summary, due to the chemical effect and thermal effect caused by hydrogen addition, the flame can survive for a longer time with fluctuation during the extinction process. [Display omitted] • A micro-jet flame model was established for the ports of domestic gas stove. • Extinction dynamics was numerically investigated for partially premixed CH 4 /H 2 /air blends. • Flame oscillation occurs once or twice in the cases with H 2 addition ratio. • Key elementary reactions containing H and O 2 contribute greatly to the chemical effect by hydrogen addition. • Direct preheating by the flame front on unburned mixture plays a dominant role in thermal effects of hydrogen addition. [ABSTRACT FROM AUTHOR]

• Unstable CH4/H2/air flames are completely suppressed by a 1-mm long pt segment. • Response of flame position to inlet velocity consists of double C-shaped curves. • Regimes A, B, and c are divided for high, medium, and low inlet velocity cases. • Flame bifurcation occurs in regime b with an inlet velocity range of 21–41 cm/s. • Reaction pathway of OH(s) alters if the threshold of h adsorption rate is reached. Effects of a Pt segment on flame stability of CH 4 /H 2 /air mixtures in a micro-flow reactor were numerically scrutinized, which demonstrated that flame instabilities were completely suppressed. In addition, the response of flame position to inlet velocity exhibited two C-shaped curves, which was divided into regimes A, B and C for high, medium and low inlet velocities, respectively. Flame bifurcation occurred in regime B. A case study of regime B showed that the increased generation rate of H(s) caused the main H 2 O(s) formation reaction shift from OH(s)+OH(s)=H 2 O(s)+ O (s) to H(s)+OH(s)=H 2 O(s)+PT(s). This led to a decreased O(s) coverage and an increased PT(s) coverage, which prevented C(s) from reacting with sufficient O(s) to form CO 2 (s). As a result, CO(s) accumulated on the catalytic surface and flame bifurcation occurred. The dynamic processes from regime B to A and C were also examined, which manifested that a threshold value existed for the adsorption rate of H atoms. Once the threshold value was exceeded, the reaction pathway of OH(s) has undergone a transformation. In conclusion, this study uncovered the influence of heterogeneous reactions on gas combustion of CH 4 /H 2 blends, and provided an effective and economic technology for flame stability enhancement in micro-flow reactors. Flame dynamics of CH 4 /H 2 /air mixtures are completely suppressed by a 1-mm long Pt segment on the wall. The overall response of flame position to inlet velocity consists of double C-shaped curves. [Display omitted] [ABSTRACT FROM AUTHOR]

• Inhaled air mixes with the fuel jet in the suction section through two routes. • The entrainment ratio increases with an increasing fuel jet velocity. • Diameters of nozzle and mixing section affect the entrainment ratio significantly. • Variation of the mixing section length has little effect on the entrainment ratio. • The entrainment ratio decreases drastically at increased total area of air inlets. In the present study, an integrated micro-ejector was proposed to provide a mixture of n-butane and air for micro-combustors. The effects of fuel jet velocity and geometrical parameters of the ejector on the entrainment ratio were numerically investigated and theoretically analyzed. The results show that the incoming air mixes with the fuel jet flow through two routes. The entrainment ratio increases with an increasing fuel jet velocity due to the increased magnitude of negative pressure. Moreover, increasing the nozzle diameter leads to a sharp decrease in the entrainment ratio. However, the entrainment ratio can be drastically augmented by increasing the diameter of the mixing section. In addition, an increase in the length of mixing section will slightly decrease the entrainment ratio of the ejector. Furthermore, increasing the total area of air inlets gives rise to a drastic decrease in the entrainment ratio. These variation tendencies have a close relationship with the negative pressure contours. The findings of this study provide a guidance to the optimal design of this kind of micro-ejectors. [ABSTRACT FROM AUTHOR]

• 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. [ABSTRACT FROM AUTHOR]

Numerical study was performed on diffusion H 2 /air in Y-shaped mesoscale combustors for. After the ignition of fuel/air mixture at the combustor exit, the flame propagates towards the intersection, first in premixed flame mode in downstream and shifts to edge flame mode in the upstream. During the transition of flame propagation modes, the change of flame inclination direction was observed. • Flame propagates first in premixed flame mode and later shifts to edge flame mode. • The propagation speed of edge flame mode is faster than that of premixed flame mode. • The direction alters in upstream if the flame inclines to the air side in downstream. • The edge flame length decreases as the channel width is reduced. • In fuel lean cases the edge flame length is much shorter in the longer combustor. Recently, we experimentally investigated the combustion characteristics of non-premixed H 2 and air in Y-shaped mesoscale combustors. In the present study, we aim at numerically explaining some experimental phenomena and tendencies, and revealing more details of flame propagation which cannot be well captured in the experiments. The numerical results show that the flame propagates in premixed flame mode in the downstream and shifts to edge flame mode in the upstream, which is a result of the fuel/air mixing effectiveness associated with the longitudinal distance. The inclination direction of flame might change during the propagation process, depending on its original direction after ignition. Moreover, the flame propagates faster in edge flame mode than in premixed flame mode. In addition, the edge flame length increases with the increase of combustor width because a longer distance is needed for a well mixing in the wider channel. Furthermore, the edge flame in the combustor of L = 100 mm is evidently longer compared to that of L = 200 mm when the nominal equivalence ratio is below unity (e.g., ϕ = 0.7). This is because the heat loss ratio under L = 100 mm is less than that of L = 200 mm. [ABSTRACT FROM AUTHOR]

This left figure presents the variations of non-uniformity index of CH 4 and H 2 at the combustor outlet with an increasing average velocity, which reveals the corresponding dominant mechanism for the mixing process within different velocity ranges. The right graph demonstrates that severe fuel stratification phenomenon occurs in the vertical plane of the curved channel. [Display omitted] • Mixing performance of H 2 is better than CH 4 due to preferential diffusion effect. • The dominant mechanism was identified for H 2 and CH 4 under different velocities. • The formation of Dean vortices significantly enhances the mixing of CH 4 and air. • Increasing the equivalence ratio leads to an elevation in the diffusion rate. • A higher H 2 ratio enhances fuel stratification and worsens the mixing performance. Mixing performance of H 2 /CH 4 blends and air in a curved miniature combustor was numerically studied. The results indicate the following. (1) The mixing performance of H 2 is much better than that of CH 4 due to the preferential diffusion effect. (2) Mixing of CH 4 and air is dominated by diffusion at low velocities, and convection at medium and high velocities. Mixing of H 2 and air is dominated by diffusion at low and medium velocities, and convection at high velocities. (3) Increasing the nominal equivalence ratio leads to an increased diffusion rate due to the generation of large fuel concentration gradient perpendicular to the mainstream. (4) A higher hydrogen blending ratio intensifies fuel stratification at the combustor entrance, decreases the Dean vortices intensities, and deteriorates the mixing performance. (5) Among the factors investigated, average velocity and hydrogen blending ratio have the most and least significant effects, respectively, on the mixing performance. [ABSTRACT FROM AUTHOR]

Two-dimensional (2D) numerical models are frequently adopted to investigate combustion and thermal performances in rectangular micro-channels for micro-thermophotovoltaic and thermoelectric devices. However, large error may exist by applying a simple 2D model. In the present work, the outer wall temperature distributions predicted by 3D model and simple 2D model were compared. The results showed that the maximum relative error of the simple 2D model depends significantly on the aspect ratio (α) of the micro-channel. To be specific, the maximum relative error was >30% for α = 1 and > 10% for 2≤ α ≤ 4. However, it was <5% for α ≥ 9. A new 2D model was proposed to modify the underestimated heat loss ratio. The new computational results demonstrated that the maximum relative error of α = 1 decreased to 8.07% and for micro-channels with α ≥ 2, all the maximum relative errors are <5%. In summary, the modified 2D numerical model can achieve a satisfactory prediction with low computation cost. • Prediction error using simple 2D numerical model was assessed for micro-channels. • The maximum error of the simple 2D model was above 30% for the square channel. • The maximum error of the simple 2D model is <5% when the aspect ratio is ≥ 9. • The simple 2D numerical model underestimates the heat loss ratio by up to 22.8%. • A modified 2D model was proposed which reduced the maximum error to 8.07%. [ABSTRACT FROM AUTHOR]

A novel miniature Swiss-roll combustor (left figure) was designed for non-premixed combustion. Experimental investigation demonstrated that the flame structure and outer wall temperature distribution (right figure) are almost symmetric under most conditions. Second, the combustor can run at very low equivalence ratio and fuel flow rate. As a result, the wall temperature level is suitable for micro thermoelectric devices using low-temperature materials. • A novel miniature Swiss-roll combustor was developed for non-premixed combustion. • Stable flames can be sustained under a nominal equivalence ratio less than 0.3. • This combustor can run stably under very small flow rates. • The flame structure and outer wall temperature distribution are symmetric. • The new combustor can be used as a heat source for micro thermoelectric generators. In the past decades, combustion in miniature Swiss-roll combustors has widely investigated for applications to micro power generation systems. However, all the existing Swiss-roll combustors are designed for premixed combustion mode, in which the flame is prone to flash back or become extinguished at low flow rates. In the present work, a miniature Swiss-roll combustor was designed specifically for non-premixed combustion, which has two inlets for counterflows of methane and air, and two outlets for exhaust gas. Preliminary experimental investigation was conducted on a model combustor with a transparent quartz cover, which demonstrated that this new configuration brings about several merits. First, the flame structure and outer wall temperature distribution under most conditions are almost symmetric. Second, this combustor can run at a very low equivalence ratio (<0.3) and a very small fuel flow rate (˜0.029 L/min). As a result, the wall temperature level is relatively low, which makes this combustor a potential heat source for miniature thermoelectric devices using low-temperature materials such as Bi 2 Te 3. Furthermore, the exhaust gas can be controlled at a very low temperature level (˜390–450 K), which means this micro combustor has a good heat recirculation performance. [ABSTRACT FROM AUTHOR]

Abstract Hydrogen is an idea fuel for micro energy conversion devices. This study scrutinizes the influence of cavity aft ramp angle (θ = 90° and 135°) on the conversion ratio of lean hydrogen/air combustion in a micro-cavity combustor. It is shown that the combustion efficiencies are 95.8% and 93.5% for θ = 90° and 135° at V in = 24 m/s, while the counterparts are 56.9% and 84.6% at V in = 28 m/s. Numerical analysis demonstrates that at V in = 24 m/s the intensity of elementary reactions as well as the heat release rate at flame tip is larger for θ = 90° owing to a shorter flame height and smaller stretch rate. Therefore, flame tip opening is alleviated and higher hydrogen conversion efficiency is achieved in the combustor with θ = 90° at low inlet velocities. At V in = 28 m/s, the stretch effect becomes the key factor affecting flame stability. The maximum strain rate on the cavity ramp wall is 1.37 × 10 6 s − 1 for θ = 90°, which is much larger than the counterpart for θ = 135° (1.08 × 10 6 s − 1 ). The downstream flame is completely extinguished for θ = 90° at high inlet velocities. As a result, the combustion efficiency shows a sharp decrease for the combustor with θ = 90°. Highlights • The effect of cavity aft ramp angle on combustion efficiency was numerically studied. • Combustion efficiency for θ = 90° is higher than that for θ = 135° at low velocities. • Combustion efficiency for θ = 135° is higher than that for θ = 90°at high velocities. • Flame height for θ = 90° is shorter than that for θ = 135°. • Strain rate at the cavity transition point is larger for θ = 90°than that for θ = 135°. [ABSTRACT FROM AUTHOR]

Abstract A deep understanding of the ignition characteristics of syngas in O 2 /CO 2 and O 2 /H 2 O atmospheres is essential for the application of oxy-syngas combustion. In the present work, ignition properties of a syngas with a typical H 2 -to-CO ratio (1:2) in O 2 /N 2 , O 2 /CO 2 and O 2 /H 2 O atmospheres were investigated numerically. The ignition temperatures were determined by a 1-D model of a micro flow reactor with a controlled wall temperature profile, demonstrating that CO 2 and H 2 O can lead to an increase in the ignition temperature compared to N 2 , and the increase is more pronounced for the O 2 /H 2 O atmosphere. The analysis manifests that CO 2 and H 2 O can suppress OH production at the region with relatively lower wall temperature by promoting R10: H + O 2 (+M) = HO 2 +(M) to compete with R11: H + HO 2 = 2OH for H radical. Moreover, the direct reaction effect (directly take part in reactions as reactants) and third-body effect of CO 2 and H 2 O on ignition temperature were numerically isolated by adopting artificial species. The computation results reveal that the increase in ignition temperature mainly results from the enhanced reaction rate of R10 by the third-body effects of CO 2 and H 2 O. Highlights • The ignition properties of syngas in O 2 /H 2 O and O 2 /CO 2 atmospheres were studied. • The ignition temperature of syngas in the O 2 /H 2 O atmosphere is the highest. • The third-body effects of CO 2 and H 2 O can suppress the OH production. • The increase in ignition temperature is mainly due to the third-body effects. [ABSTRACT FROM AUTHOR]

Abstract In the present study, the stability of methane/air flames in a micro cylindrical channel with a prescribed wall temperature profile and a platinum (Pt) segment was investigated numerically. Two dimensional simulation was performed by the commonly used computational fluid dynamics software Fluent 6.3, incorporated with detailed gas combustion chemistry GRI 3.0 and catalytic reaction mechanism of methane/air on platinum. The results demonstrate that all the unstable flames become stationary in the presence of a 3-mm Pt segment located at x = 49–52 mm (total length of 80 mm) on the wall surface, and the response of flame position to inlet velocity exhibits an S-shaped curve. Detailed analysis was conducted to reveal the underlying mechanisms responsible for the existences of two turning points, which showed that the catalytic segment exerts a negative influence on homogeneous combustion at high and medium velocities, whereas it can enhance the homogeneous combustion at low velocities. In summary, the present study revealed the complicated effects of heterogeneous reaction on homogeneous combustion and suggested an effective and economic method to suppress flame instabilities in micro combustors. Highlights • Flame instability in a micro combustor is completely suppressed by a Pt segment. • The response of flame position to inlet velocity exhibits an S-shaped curve. • The first turning point mainly results from early reactants consumptions by catalyst. • The second turning point is owing to pathway change of homogeneous reactions. [ABSTRACT FROM AUTHOR]

Micro combustor is the core component of combustion-based micro power generation systems. Our recent studies showed that the combustion efficiency of lean H 2 /air flames in a micro-cavity combustor deceased rapidly at high inlet velocities due to the emergence of flame tip opening. To improve this undesirable phenomenon, oxygen enrichment was employed in this study. Numerical investigations of lean H 2 /O 2 /N 2 flames with an equivalence ratio of 0.4 were conducted under four different oxygen proportions, i.e., x O2  = 21%, 24%, 27% and 30%. The results demonstrate that at x O2  = 30%, the combustion efficiency can reach up to 98.2% even at an inlet velocity of 32 m/s. The analysis reveals that as the oxygen concentration is increased, the elementary reactions and heat release are intensified considerably, whereas the stretch effect is weakened essentially at the flame tip. As a result, the occurrence of flame tip opening can be eliminated and a high combustion efficiency is thus attainable at high inlet velocities for lean H 2 /N 2 /O 2 flames. [ABSTRACT FROM AUTHOR]

The impact of the cavity depth on the combustion efficiency of lean hydrogen/air flames in a micro-combustor with dual cavities was numerically investigated. The results show that under lower and higher inlet velocities, the combustion efficiency varies in opposite tendencies with an increasing cavity depth. Namely, under lower inlet velocity, the combustion efficiency decreases as the cavity depth is increased, while it increases with an increasing cavity depth under higher inlet velocity. To reveal the underlying mechanisms responsible for these variation tendencies, we performed comprehensive analysis in terms of heat transfer from the flame to cavity walls, heat regeneration ratio via upstream wall, residence time in the cavity, and stretch rate at the flame tip. It can be summarized that the total heat flux at the cavity walls and the heat regeneration ratio via upstream wall are the two dominant factors under lower inlet velocity, while the stretch rate at the flame tip and the residence time in the cavity overwhelm the other two factors under higher inlet velocity. [ABSTRACT FROM AUTHOR]