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In order to develop a burner with uniform temperature field, the combustion characteristics and thermal performance of partially premixed methane/air jet flames were experimentally studied by using micro jet array burners. The circular tubes of 1.0-mm inner diameter and 1.5-mm outer diameter were used as nozzles. The effects of nozzle spacing and equivalence ratio on flame phenomenology, temperature distribution and pollutant emissions were respectively investigated by camera photography, thermocouple measurement and sampling analysis. Results show that there are two clean flame patterns: clean merged-flame and clean non-merging flames. The flame patterns depend on the strength of flame interaction, the equivalence ratio of the mixture and the quantity of air entrainment through the gap between nozzles. The burners with small nozzle spacing such as 2 mm and 2.5 mm tend to produce fully merged flame with low equivalence ratio limit and the corresponding temperature fields are very uniform with fluctuations less than 0.3%, but a small increase in equivalence ratio will lead to rapid deterioration of combustion property. The burner with a medium spacing of 3 mm can produce partially merged flame in a wide equivalence ratio range with low emissions, and the temperature fluctuation can be less than 0.5% (

A new skeletal mechanism of n-butane is developed for describing its ignition and combustion characteristics applicable over a wide range of conditions: initial temperature 690–1430 K, pressure 1–30 atm, and equivalence ratio 0.5–2.0. Starting with a detailed chemical reaction kinetic model of 230 species and 1328 reactions (Healy et al., Combust. Flame, 2010), the directed relation graph method is applied as the first step to derive a semi-detailed mechanism with 134 species. Then, the reaction path analysis in conjunction with temperature sensitivity analysis is used to remove the redundant species and reaction paths simultaneously under the condition of low-temperature and moderate-to-high temperatures, respectively. Finally, a skeletal n-butane mechanism consisting of 86 species and 373 reactions can be obtained. Mechanism validation indicates that the new developed skeletal mechanism is in good agreement with the detailed mechanism in predicting the global ignition and combustion characteristics. The new skeletal mechanism is further validated using extensive available literature data including rapid pressure machine ignition delay time, shock-tube ignition delay time, laminar flame speed, and jet-stirred reaction oxidation, covering a large range of temperatures, pressures, and equivalence ratios. The comparison results demonstrate that a satisfactory agreement between predictions and experimental measurements is achieved.

Experimental studies on premixed flame propagation characteristics in a sub-millimeter-scale closed chamber are performed. Propane/air mixtures are ignited in the center of the visualized chamber a...

Experimental and computational results on the combustion performances and their formation mechanisms were presented and compared in two types of tubular flame burners by using methane as fuel. Resu...

To improve the lifetime and the flame stability of combustion-based micro devices, two coating materials (AlCrN and alumina) are deposited on STS 304 substrate to investigate the interactions between the premixed n-butane/air flame and the solid walls heated to different temperatures in a slit burner. The flame-wall interaction is characterized by the measurements of quenching distance, flame pulsation and OH intensity distribution. Results show that the alumina coating sustains a shorter quenching distance compared with the STS 304 plate at an identical wall temperature, whereas the AlCrN coating exhibits a larger one. The highest near-wall OH intensity is observed in the alumina-coated wall, while the lowest is observed in the AlCrN-coated wall, indicating a correlation between the near-wall OH distribution and the quenching distance. When the channel gap is reduced to a critical value, the stable flame is converted into a pulsating flame, and the pulsation frequency monotonously increases as the wall temperature increases. Surface analysis reveals that the adsorbed oxygen on the surface may play an important role in flame quenching characteristics by affecting the OH intensity close to the wall surface. In addition, the flame exerts varying degrees of influence on the coating stability, surface structure, and elemental composition. The alumina coating maintains excellent thermal and chemical stability and can be used for surface optimization of static components. In contrast, although the AlCrN coating undergoes slight changes in structure and composition under the action of flame, it may still be a surface optimization option for moving parts without a significant increase in quenching distance.

A three-dimensional numerical simulation of a localized stratified tubular flame burner is carried out by using propane as fuel. The characteristics of the flow field, species distribution, flame s...

From the perspective of the thermal/chemical performance management of a slot burner, three kinds of blended coatings with the low thermal conductivity and certain chemical activities, including 30wt.%Y2O3/ZrO2, 30wt.%MgO/ZrO2 and 30wt.%Al2O3/ZrO2, were prepared on stainless steel walls plated by atmospheric plasma spraying. The effects of coating material, wall temperature, and equivalence ratio on fuel-rich methane flame characteristics were investigated experimentally. The OH planar laser-induced fluorescence (OH-PLIF) technique was used to obtain OH intensity and flame anchoring location. The physical and chemical properties of different coatings before and after spraying were analyzed by a series of material and surface characterization techniques. Results show that compared with pure ZrO2 coating, the quenching distances of three ZrO2-based blended coatings with Y2O3, MgO, and Al2O3 decrease to some extent, resulting in the minimum quenching distance of Al2O3/ZrO2 coating, and the maximum one for Y2O3/ZrO2 coating. Meanwhile, the blended coating walls result in the higher flame anchoring location than that of pure ZrO2 in most cases, in which the coating of Y2O3/ZrO2 displays the highest location. The blended coatings have higher thermal conductivity, which is beneficial to balance part of the heat loss through the axial heat recirculation of wall structures, and thus expanding stability limits at low temperatures. In addition, partial lattice solid solutions are formed in mixture systems after the ultra-high-temperature thermal spraying, leading to more surface lattice oxygen and stronger chemical promotion. In summary, reasonable allocation of ZrO2-based ceramic components as coating wall is an effective alternative strategy to control flame stabilization in small-scale combustion devices.

The combustion characteristics of non-premixed hydrogen micro-jet flames in co-flow air were investigated in a wide fuel flow velocity range experimentally and numerically to extend the knowledge on non-premixed micro flame. The direct flame images were captured by a digital reflex camera with long exposure time, and the OH radical distributions in the flames were measured by the laser induced fluorescence technique. The experimental observation shows that the flame height and shape change with the deceasing fuel flow velocity. A detailed chemical reaction mechanism was employed in the numerical computations with considering the heat transfer between gas and solid micro-tube. The computed distributions of OH concentrations agree qualitatively with those obtained by the experiments. The flame structure and thermal interaction between the flame and solid tube were inspected in detail. It was found that the thermal interactions at high, intermediate and low fuel flow velocities are different. The details of the thermal interactions between flames and solid tube are depended on the shape of the micro flame, which essentially affects the temperature distribution.

The combustion characteristics of non-premixed methane micro-jet flame in coflow air were investigated experimentally and numerically. A detailed reaction mechanism was employed in the two-dimension numerical computation. The thermal interaction between the flame and solid tube was considered by using fluid-solid coupled thermal boundaries in the numerical computation. Four typical flames, lifted flame, attached flame, hemisphere flame and umbrella flame, were observed in different fuel flow velocity ranges in experiments. The flame heights, blow off and extinction limits in coflow airs at different velocities were measured. The OH and CH distributions of non-premixed micro-jet flames were obtained by experiments, and the computational results agree well with those from the experiments. The computational temperature distributions show that there are thermal interactions between flames and solid tube. The heat exchanges through surfaces of the solid tube were analyzed in detail. The fresh fuel gas in the solid tube is preheated by the thermal recirculation. Consequently, the combustion intensity of the micro-jet flame is enhanced. The thermal interaction is essentially affected by the flame shape, and it is significant for the hemisphere flame.

The chemical catalytic performance of functional ceramics materials of rare earth is of great technological and scientific significance to actively regulate the flame-wall interaction in high-temperature combustion. The premixed methane-air flames in a slot burner with different wall materials were characterized by OH-PLIF technique to gain an insight into the effects of heterogeneous chemistry at gas–solid interfaces on the flame characteristics. The 304STS wall coated with ZrO2 and Y2O3-ZrO2 respectively through atmospheric plasma spray were investigated at the wall temperatures of 473 K and 773 K in this paper. The flame stability was quantitatively analyzed through the dynamic flame morphology of OH-PLIF images. The absolute maximum OH intensities in the flame cores were extracted to investigate the effects of different gap distances, wall temperatures and surface materials. The characterization of chemical interactions between flame and wall was also carried out based on the spatial distribution of OH radicals near the wall. Lastly, the heterogeneous chemistry between the gas-phase radicals in flame with the electrophilic oxygen species on the surface of Y2O3-ZrO2 coating was discussed to reveal the dominant pathways of the reduction and re-oxidation of surface lattice oxygen. Results show that a marked improvement in combustion stability was observed with using Y2O3-ZrO2 coated walls compared to the uncoated 304STS walls under the same conditions. The maximum OH intensity in flame would increase with the decrease of channel gap at higher temperature due to the unique chemical promoting effect of Y2O3-ZrO2 coating. Overall, this work indicates that coating with rare earth composite metal oxides such as Y2O3-ZrO2 can potentially serve as an effective strategy for the optimization of combustion chambers.

The concentrations of CO2 and H2O in the combustion environment are important for the combustion characteristics of oxygen-enriched flames using hot oxidizers with large amount of CO2 and/or H2O. To understand the variations of flame temperature and stretch extinction limit of oxygen -enriched flames with H2O and CO2 concentrations, the CH4/CO2 versus O-2/H2O/CO2 and O-2/H2O counterflow non-premixed flames with the oxygen mole fraction of 0.35 were investigated numerically and compared with those of the CH4/CO2 versus O-2/CO2 (XO2 = 0.35) flames at the oxidizer temperatures of 500, 700 and 1000 K. Two O-2/H2O/CO2 mixtures with different H2O and CO2 concentrations were used as the oxidizers in the study. The results show that the flame temperature and the extinction strain rate are increased when the CO2 in the oxidizer has been replaced by H2O. The analysis based on the theoretical solution and computational result suggests that the total amount of OH radicals of the flame can be used as a criterion for the comparison of stretch extinction limit. The computational results show that the production rate of OH radicals via the reaction O + H2O = 2OH is comparable to that of H + O-2 = O + OH when H2O is added in the oxidizer. And the amount of OH radicals generated by O + H2O = 2OH increases with increasing H2O molar fraction of the oxidizer, resulting in a significant increase of the extinction strain rate. (C) 2016 Elsevier Ltd. All rights reserved.

A novel vortex-tube combustor with axial fuel injection for NOx reduction was proposed to overcome the instability of ultra-lean combustion. The stability limit, flame configuration, NOx and CO emissions, and flame structure were investigated experimentally under various global equivalence ratios and fuel flow rates. The mechanisms of NOx emission reduction were analyzed by numerical simulation. Results show that the limit of global equivalence ratio can be as low as 0.01 and the amplitude of pressure fluctuation is always less than 1300 Pa, indicating fairly good performance in combustion stabilization of the combustor. In the operation range, there is a trade-off region with low NOx and CO simultaneously. The diffusion-like flame structure in this combustor can enhance the local equivalence ratio, whilst the flow field structure can also promote the transport of chemical enthalpy to the flame front, thus facilitating the stabilization. The enhanced stabilization enables the ultra-lean combustion and the ensued small area of the high-temperature zone to conduct, as well as the low NOx formation through reducing thermal NO. The local fuel-rich region and the flow field structure can promote the NOx to be reduced further via the Fenimore mechanism.

The operating limit and pressure fluctuations of a localized stratified vortex-tube combustor (LSVC) are investigated experimentally by taking methane as fuel at the inlet temperature of 300 K and the combustor pressure of 1 atm over a range of equivalence ratios (0.1–1.0). The combustion modes are distinguished according to the properties of pressure fluctuations and the driving mechanisms of each combustion mode are explored in combination with numerical simulation. The results show that the LSVC can realize stable combustion with pressure fluctuation amplitudes always less than 4 kPa in a large operating range, and the flame front is always continuous. The operating flammability limit experimental range can be divided into five regimes with different combustion modes. The first and fifth combustion modes are steady combustion with pressure fluctuation amplitudes less than 1 kPa, and the second to fourth ones are quasi-steady combustion with pressure fluctuation amplitudes between 1–4 kPa. The spectrum analyses of pressure fluctuations show that there are one low-frequency peak around 300 Hz and one high-frequency peak around 1500 Hz, which are dominated by the first and third axial natural acoustic modes of resonant oscillation combustion, respectively. In the first and fifth combustion modes, the resonances are both weak due to the influences of low flow rate and laminarization respectively. In the second mode, the thermo-acoustic coupling oscillation and the resonance are excited simultaneously, yielding the highest pressure fluctuation amplitude in the entire operating range. The high-frequency resonance causes the high-frequency pressure fluctuation of the third mode. Both the unsteady heat release and flow field affect the pressure fluctuation in the fourth mode. The former produces the low-frequency fluctuation, which can resonate with the natural frequency and excite a weak thermal-acoustic coupling.

The aim of this work is to understand the ignition and combustion behaviors of n-butane/air mixtures under glow plug ignition in the start-up condition of miniature IC engines. We utilized a centimeter-scale constant-volume chamber setup to investigate the effects of glow plug heating power, equivalence ratio, and initial pressure on the characteristics of ignition and flame propagation of n-butane/air mixtures. Results indicated that the ignition time and ignition temperature decrease as the heating power increases and the flame propagation time is weakly sensitive to that. Increasing the initial pressure can reduce the ignition time, decrease the ignition temperature and slow flame propagation; except for the flammable limit, the ignition time and ignition temperature increase as the equivalence ratio increases, in addition, the flame propagation is fastest at φ = 1.2. Furthermore, an obvious two-stage combustion phenomenon was observed under fuel-rich flame propagation conditions. Overall, the order of influencing factors on the ignition characteristics shows heating power > initial pressure > equivalence ratio, and that on the flame propagation characteristics shows equivalence ratio > initial pressure > heating power in present experimental conditions.

The combustion characteristics in a localized stratified tubular flame burner (LSTFB) were experimentally studied under lean conditions. The stability limit and pressure fluctuation were obtained under various global equivalence ratios and fuel flow rates. The mechanisms driving the combustion instability were analyzed with the combination of numerical simulation. Results show the lean stability limit of the burner can be as low as 0.12 of the global equivalence ratio. On the basis of the local stratification of species, a kind of unique binary tubular flame is formed with both the premixed and non-premixed flame properties. Meanwhile, the internal recirculation heats the incomplete combustion gases distributed inside the tubular flame, thereby guaranteeing the balance between the flow velocity and the local flame speed. The amplitudes of pressure fluctuation are less than 4 kPa in the entire experimental range because of the flow laminarization caused by the large body force and density gradient. The pressure fluctuation characteristics depend on the relative magnitude of viscous force, inertial force, body force, and density gradient. The Taylor number Ta and stratification parameters Ri∗ are key parameters in evaluating the flame stability in the LSTFB.

A micro gas turbine combustor with a sintered porous chamber wall as inlet was tested and simulated. When burning methane in the 2.14 cm3 chamber volume combustor, its maximum power density reached 336 M W/m3. The combustion efficiency was above 90% when producing 850–1100 K exit gas. The pressure drop ratio of the porous inlet was below 5 kPa with 6 L/min premixed gas flow rate at hot condition. Compared with a conventional solid-wall micro combustor with heat recovery channel, the outside wall temperature of the porous-wall micro combustor decreased 150–200 K, and the heat loss ratio reduced from 40–80% to 20–40%. Direct numerical simulation based on the skeletal chemical kinetics model was used to elucidate the flame structure and heat loss reducing mechanism of the porous-wall micro combustor.

Behaviors of premixed CH4/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 CH4/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. (C) 2014 The Combustion Institute. Published by Elsevier Inc. All rights reserved.

Combustion characteristics of H2/air mixture in a micro-combustor with wall cavities were investigated numerically. The effects of inlet velocity, equivalence ratio, and the length–depth ratio of the cavity were studied. The results show that at a high enough velocity the flame splits in the middle which leads to a large amount of fuel leakage and a sharp decrease in the conversion rate of hydrogen. Meanwhile, the flame splits at the inner wall which gives rise to two high temperature regions and double temperature peaks at outer wall. Moreover, the flame-splitting limit is extended at a higher equivalence ratio due to a more intensive reaction. Furthermore, the flame-splitting limit increases for a larger length–depth ratio of the cavity, whereas the wall temperature level decreases. Therefore, excessive large length–depth ratios are not beneficial for this type of micro-combustors if the combustor walls are used as heat sources of thermoelectric or thermal photovoltaic devices.

To examine the effects of the wall on flame quenching, an OH-PLIF investigation of a premixed methane/air flame was conducted with a slit burner between two parallel walls. Three types of materials, i.e., stainless steel 304, silicon, and zirconia ceramics, were tested at wall temperatures of 300 and 600 degrees C. The quenching process, captured by an intensified charge-coupled device (ICCD) camera, showed different critical distances for stable flames that became unstable and were quenched for the three materials at the same temperature. It is interesting to note that, at a higher wall temperature, the flame is lifted before finally quenching, while this did not happen at 300 degrees C. By analyzing the maximum OH fluorescence intensity in the flame extracted from the OH image, we have not found a distinct relation between the maximum OH fluorescence intensity and the quenching distance. In some cases, the flame can sustain a very weak OH intensity. Conversely, we also obtained the OH fluorescence intensity close to wall and within 0.6 mm from the surface. We found that the trend of the OH fluorescence intensity close to wall correlates with the quenching characteristics very well. At the same wall temperature, a greater OH intensity close to wall results in a shorter quenching distance and vice versa. The wall made of zirconia ceramics demonstrates the greatest OH intensity close to the wall and, thus, the shortest quenching distance, while of the quenching distance in the case of STS 304 demonstrates the opposite trend. Additionally, we also calculated the non-dimensional chemical action of the surface based on the OH-PLIF data, which can demonstrate the differences in the chemical quenching characteristics for the three types of materials. (C) 2012 The Combustion Institute. Published by Elsevier Inc. All rights reserved.

A centimeter magnitude thermoelectric (TE) power generation system based on a plat-flame micro combustor burning DME (dimethyl ether) has been developed. The chamber wall of this micro combustor was made of two parallel sintered porous plates which acted as mixture inlet. The main virtue of this combustor is that it can keep combustor wall at lower temperature for reducing heat loss when sustaining a stable flame. Experimental test results showed it was feasible to obtain stable DME/air premixed flame at lean combustion situations in the micro combustor. The combustion load of this 0.48 cm3 chamber capacity was 20–200 W at equivalence ratio Ф = 0.6. Though the flame temperature was above 1000 °C, the combustor’s wall temperature was near 600 °C lower than flame temperature. In the demonstrated TE power generation system which integrated the plat-flame micro combustor, a heat spreader had good effect on uniforming the hot side temperature field of TE modules. Cooled by water and with 150 W input power at Ф = 0.7, the system produced 10 V output at open circuit and 4 V at 10 Ω load. The maximum power output was above 2 W, and the maximum overall chemical-electric energy conversion efficiency was 1.25%.

Numerical analysis was used to study the deposition and burning characteristics of combining co-combustion with slagging combustion technologies in this paper. The pyrolysis and burning kinetic models of different fuels were implanted into the WBSF-PCC2 (wall burning and slag flow in pulverized co-combustion) computation code, and then the slagging and co-combustion characteristicsespecially the wall burning mechanism of different solid fuels and their effects on the whole burning behavior in the cylindrical combustor at different mixing ratios under the condition of keeping the heat input samewere simulated numerically. The results showed that adding wood powder at 25% mass fraction can increase the temperature at the initial stage of combustion, which is helpful to utilize the front space of the combustor. Adding wood powder at a 25% mass fraction can increase the reaction rate at the initial combustion stage; also, the coal ignitability is improved, and the burnout efficiency is enhanced by about 5% of suspension and deposition particles, which is helpful for coal particles to burn entirely and for combustion devices to minimize their dimensions or sizes. The results also showed that adding wood powder at a proper ratio is helpful to keep the combustion stability, not only because of the enhancement for the burning characteristics, but also because the running slag layer structure can be changed more continuously, which is very important for avoiding the abnormal slag accumulation in the slagging combustor. The theoretic analysis in this paper proves that unification of co-combustion and slagging combustion technologies is feasible, though more comprehensive and rigorous research is needed.

In a slagging combustor or furnace, the high combustion temperature makes the molten slag layer cover the wall and capture the particles. If these particles contain combustible matter, they will continue to burn on the running slag. As a result, the total amount of ash deposition will be much greater than that in dry-wall combustors and the total heat flux through the deposition surface will change greatly. Considering the limitations of existing simulation methods for slagging combustion, this paper introduces a new wall burning model and slag flow model from the analysis of particle deposition phenomena. Combined with a conventional combustion simulation program, the total computational frame is introduced. From comparisons of simulation results from several kinds of methods with experimental data, the conclusion is drawn that the conventional simulation methods are not very suitable for slagging combustion and the wall burning mechanism should be considered more thoroughly.

This paper presents the results of fundamental research on gas turbine cogeneration systems with steam injection that attains high thermal efficiency and low pollutant emissions. The concentration of OH radical may be affected significantly in these systems. To prove the behavior of OH radical in the combustion zone and to clarify the relationship between OH radical production and NOx emission, the characteristics of methane-air counterflow flames with steam addition have been simulated numerically by applying GRI-Mech detailed chemical kinetics. To investigate the chemical reaction effects of steam addition on the OH radical and NO formation with the effect of the temperature decrease because of steam addition excluded, the maximum flame temperature was adjusted to the same value by varying the initial temperature. The calculated results clearly show that with steam addition, the decomposition reaction of steam is suppressed and the production rate of the OH radical is decreased regardless of the increase in steam concentration. These results can be explained by the significant decrease in flame temperature for the same initial temperatures of fuel and oxidizer. For the same flame temperature, the OH concentration becomes higher. The increase in the OH concentration because of steam addition in itself results in the increase in NOx emissions. However, because of the decrease in CH concentration with steam addition, the production rates of HCN and N radicals decrease dramatically, and NO formation is suppressed as a whole. These results may be useful to predict NOx emissions in a high-temperature gas turbine system with steam or water addition.