Flame Blow-off Limit Research Articles

Hydrogen-enriched flames in a novel miniature double-layer disc-combustor with annular step and Swiss-roll preheated channel

A novel miniature double-layer disc-combustor with annular step and Swiss-roll preheated channel is developed for the first time, which can act as a heater or the miniature heat source for a miniature power source. The premixed flame dynamics of 80%H2-20%CH4-air are experimentally studied in this combustor. The results show that the flame blow-off limit increases first and then sharply decreases with the decreased value of Reynolds number Re. It is the first time to find the anomalous blow-off phenomenon in the radial channel. The analysis indicates that the decreases of flow recirculation effect, heat recirculation effect, and preferential diffusion effect lead to the decreased blow-off limit with the decreased Re value. The flames can remain stable in a wide operating range. At the large Re value, the flame diameter obviously increases with the decreased value of equivalence ratio ϕ, and the flame anchors right behind the annular step at the small ϕ value. At the small Re value, the flames all reside around the exit of Swiss-roll preheated channel, which results in the small flame diameters. The magnitude of the upper wall temperature decreases with the decreased ϕ value. At large ϕ values, the distribution characteristics of high temperature zone of the upper wall are similar, and the peak value of temperature appears at its center. At small ϕ values, the peak value and magnitude of wall temperature are both larger for a bigger Re value, and the location of peak value slightly shifts upstream with the decreased Re value. The upper wall temperature of the combustor can be expediently managed by adjusting the operating conditions. In addition, the mean temperature of the upper wall and heat transferred from the upper wall to the environment almost linearly decrease with the decreased Re value. The present study expands our insights into the flame dynamics in the radial channel.

Combustion and thermal characteristics of a miniature double-layer disc-combustor with porous media and Swiss-roll preheated channel

A novel miniature double-layer disc-combustor with porous media and Swiss-roll preheated channel is developed for the first time, which can act as a heater or the miniature heat source for the miniature power source. The combustion characteristics of 60%CH4-40%H2-air in this combustor are investigated by the means of experiment. The result shows that the flame blow-off limit increases first and then decreases with the decreased Reynolds number Re value. It is the first time to observe the anomalous blow-off phenomenon in the radial channel. The flame can always remain stable until the flame extinguishes. Unexpectedly, at a constant Re value, the flame diameter slightly decreases with the decreased equivalence ratio, which is mainly caused by the thermal expansion effect. In summary, the present double-layer disc-combustor can run well in a wide operating range and its thermal performance is good.

A novel Swiss-roll micro-combustor with double combustion chambers: A numerical investigation on effect of solid material on premixed CH4/air flame blow-off limit

A novel Swiss-roll micro-combustor with double combustion chambers is proposed to improve flame stability and extend blow-off limits. This study is aimed to numerically investigate the effect of solid material (i.e., SiC, stainless steel and copper) on premixed CH4/air flame blow-off limit and reveal the flame stability mechanism. The simulated results show that this developed novel Swiss-roll micro-combustor not only can significantly anchor the flame owing to the flow recirculation behind the flame holders and the backward-facing steps, but also can further extend CH4 blow-off limits owing to heat recirculation in the long Swiss-roll preheating channels. The three solid material micro-combustors present the relatively slight difference in the recirculation-zone size but the remarkably difference in heat recirculation and heat loss. Good heat recirculation and low heat loss rate are the dominant reason that is responsible for the differences of the blow-off limits in this micro-combustor. The stainless steel micro-combustor achieves the highest blow-off limits while the copper micro-combustor achieves the lowest blow-off limit. These deep insights can give some useful information to design a similar Swiss-roll micro-combustor.

A numerical study on non-premixed H2/air flame stability in a micro-combustor with a slotted bluff-body

A planar micro-combustor with a slotted bluff-body was developed for non-premixed H2/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 novel flame-anchorage micro-combustor: Effects of flame holder shape and height on premixed CH4/air flame blow-off limit

Maintaining stable combustion is one of the major challenges for microscale combustors. In this study, a novel premixed CH4/air flame micro-combustor combining a flame holder, backward-facing step and preheating channel is developed to enhance the flame anchorage and extend the blow-off limits. The effects of flame holder shape (triangle and rectangle) and height (H = 0.0, 0.5, 1.0, 1.5 and 2.0 mm) on the combustion stability are studied numerically by FLUENT 16.0. The results show that this micro-combustor not only can considerably anchor the flame root owing to flow recirculation but also can further improve CH4 flammability owing to heat recirculation. Compared with the rectangle flame holder, the triangle flame holder achieves the higher blow-off limits owning to the larger recirculation zone and better preheating effect. As the flame holder height increases, (i) the recirculation zone sizes near the flame root decrease, weakening the flame-root anchorage ability, and (ii) the heat recirculation effect becomes stronger, improving CH4 flammability. Under the combined effects of flow recirculation and heat recirculation, the highest flame blow-off limits are attained when H = 0.5 mm. The flame holder with triangle shape and H = 0.5 mm attains the highest flame blow-off limits (10.3 m/s and 6.2 m/s at Φ = 0.7 and 0.5).

Experimental investigation on gliding arc discharge plasma ignition and flame stabilization in scramjet combustor

Ignition and flame stability in supersonic flow have always been the key problems of research in scramjet. In addition, ignition is difficult and the cavity flameholder is susceptible to support the flame stability under extreme conditions such as low equivalent ratio. In recent years, gliding arc plasma is recognized to expand ignition and extinction limit with lower energy consumption in the field of plasma assisted ignition due to its heating and chemical effects. In this paper, a gliding arc igniter has been designed and compared with the traditional spark plug in order to quantify the ignition ability. The igniter has the same size with the spark plug, using low-power AC gliding arc to carry out ethylene ignition test in Ma = 2.52 Ma supersonic flow. The average power of gliding arc discharge is 1199 W. A high-speed camera and CH⁎ chemiluminescence were used to make combustion diagnosis. Founded in the same discharge period, the lean ignition limit of the gliding arc is lower than the ignition limit of the spark. The average expansion of ethylene ignition limit is 17%. The ignition process is that gliding arc continues to generate the initial flame kernels during the discharge period, but it is extinguished continuously due to the strong convection. Until generating an initial flame kernel which can successfully propagate the flame. The ignition process can be divided into four stages. It continues to generate new flame kernels in flame propagation process. Gliding arc reignites the fuel and generates the new flame kernels after forming a stable flame, appearing intermittent ignition in the cavity. The high equivalent ratio can make ignition delay time shorter, generating initial flame kernels more frequently. The heating effect of the gliding arc and reignition character make the thermal product and ethylene occur intermittent combustion more often in the cavity, increasing the area of the combustion reaction. Gliding arc plasma can achieve combustion enhancement during the flame stabilization process. The shear layer of flame thickness increased by the average of 2 mm on S-B-1 and G-B-1 conditions. Compared with the traditional spark ignition, gliding arc broaden the lean blow-off limit in different stages may be the significant reason for broadening lean ignition limit. It concludes that gliding arc makes the flame's ignition limit closer to the flame's blow-off limit.

Numerical investigation on mixing performance and diffusion combustion characteristics of H2 and air in planar micro-combustor

In the present work, the effects of inlet velocity and channel height (H0 = 0.6 mm, 1.0 mm and 1.4 mm) on the mixing performance, flame stability limit and combustion efficiency of H2 and air in a 2D planar micro-combustor with a separating plate were studied numerically. The results demonstrate that improved mixing can be achieved with a decrease in inlet velocity and channel height. Moreover, the flame blow-off limit is the largest for a micro-combustor with H0 = 0.6 mm; the flame becomes inclined at a high velocity and the direction varies with the inlet velocity. Furthermore, a micro-combustor with a medium height (H0 = 1.0 mm) can achieve the largest blowout limit among the three cases. Finally, for identical inlet velocities, the combustion efficiency increases with decreasing combustor height. In summary, these findings can provide a guideline for the optimal design of such micro-combustors.

Numerical investigation on flame blow-off limit of a novel microscale Swiss-roll combustor with a bluff-body

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.

Effect of pressure on the blow-off limits of premixed CH4/air flames in a mesoscale cavity-combustor

The blow-off limits of CH4/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.

A non-monotonic variation of blow-off limit of premixed CH4/air flames in mesoscale cavity-combustors with different thermal conductivities

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 (λs=0.5, 1.05 and 50W/mK) 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 λs=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.

Effect of solid material on the blow-off limit of CH4/air flames in a micro combustor with a plate flame holder and preheating channels

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 CH4/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.

Effect of channel gap distance on the flame blow-off limit in mesoscale channels with cavities for premixed CH4/air flames

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.

Flame-anchoring mechanisms of a micro cavity-combustor for premixed H2/air flame

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 H2/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 (ϕlocal). It is shown that ϕlocal increases in the recirculation zone and low velocity zone in the cavity. More importantly, ϕlocal 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 ϕlocal region, which are the main flame-anchoring mechanisms of the micro cavity-combustor for premixed H2/air flames.

Determining factor for the blowoff limit of a flame spreading in an opposed turbulent flow, in a narrow solid-fuel duct

Abstract This study clarified the blowoff mechanism for a flame spreading in an opposed turbulent flow in narrow solid fuel ducts. To clarify this mechanism, two experiments were conducted. The first experiment was to investigate the influence of ambient pressure and fuel duct size on the blowoff limit. The results indicated that the flow velocity at the point when blowoff occurred, V g , t , increased with ambient pressure. This tendency could not be confirmed by a well-known expression for the Damkohler number, which is defined as the ratio of the characteristic flow time to the characteristic chemical time. Subsequently, to clarify the determining factor for the blowoff, the second experiment, which observed the flow field near the flame leading edge, was conducted. The results show that the flow separation in front of the flame leading edge, which provided sufficient residence time of oxidizer and gaseous fuel, is necessary for the flame to spread in an opposed oxidizer flow. From the results, it is found that the oxidizer friction velocity, u ∗ , which is an indicator of the turbulent momentum transfer, is the determining factor for the flame blowoff limit. When the friction velocity is larger than a critical value, flame blowoff occurs in the fuel duct, due to the absence of flow separation.

Experimental Augmentation of Turbulent Flames Through Free Radicals Delivered In Situ

Abstract Experiments are conducted with the aim of augmenting turbulent flames with specific applications to high-speed combustion (for example, Scramjets). Augmentation includes extension of rich and lean limits, extension of aerodynamic limits, and augmentations in efficiency (also, signature minimization). While the concept of free radicals use for these purposes is NOT new, the method of delivery is. Free radicals are very short lived and highly reactive; they are needed in small concentrations at specific sites at requisite times. We use stable compounds that are known to decompose at specific temperatures; their decomposition products are rich in specific free radicals that are delivered in situ. The method thus obviates the problems of storage, delivery, and loss. A methane/enriched-air turbulent flame system is used. The apparatus and flame studies have been extensively reported in the open literature. Dilute hydrogen peroxide is used as the free radicals source. The flame blow-off limit is determined, for example, in the lean limit. The same configuration is used, now with the H202 injection upstream. Extensions of the lean limit from 1 to 50 percent are seen.The results are found to be consistent with a simple turbulence theory.