多孔燃烧器中氨/空气燃烧特性数值研究

氨具有氢密度高、生产成本低、基础设施完善等优点,作为一种潜在的可再生替代燃料受到了广泛的关注.目前,仅有少数研究关注氨气燃烧喷嘴的研究,针对氨气稳定燃烧喷嘴的研究尤其不足.为实现氨燃料的稳定燃烧和低污染物排放,本研究提出了一种氨用多孔介质燃烧器.对氨用多孔介质燃烧器建立了二维数值模型,并对预混氨/空气在多孔介质燃烧器中...     

Experimental investigation of combustion in porous heating burners

The combustion characteristics of liquefied petroleum gas inside porous heating burners have been investigated experimentally under steady-state and transient conditions. Cooling tubes were embedded in the postflame region of the packed bed of a porous heating burner. The flame speed, temperature profile, and [NO_x] and [CO] in the product gases were monitored during an experiment. Due to the heat removal by the cooling tubes, a phenomenon termed metastable combustion was observed; this is that only one flame speed exists at a particular equivalence ratio for maintaining stable combustion within the porous bed of the porous heating burner. This behavior is quite different from that of porous burners without cooling tubes, in which an extended range of flame speeds usually is found for maintaining stable combustion. After metastable combustion has been established in a porous heating burner, a change in the equivalence ratio will stop the metastable combustion and drive the flame out of the packed bed. From the steady-state results, the porous heating burner was shown to maintain stable combustion under fuel-lean conditions with an equivalence ratio lower than the flammability limit of a normal free-burning system. The flame speed in a porous heating burner was found to decrease with an increase in the length of the porous bed. Combustion within a porous heating burner has the features of low flame temperature, extended reaction zone, high preheating temperature and low emissions of NO_x and CO. The flame temperature ranged from 1050 to 1250 °C, which is ∼200 °C lower than the adiabatic flame temperature at the corresponding equivalence ratio. The length of the reaction zone could be more than 70 mm and the preheating temperature ranged from 950 to 1000 °C. Both [NO_x] and [CO] were low, typically below 10 ppm.     

Fuel‐lean VOCs combustion in a porous burner stacked with alumina balls: A case for ethylene combustion

A porous burner stacked in turn with 3‐ and 9‐mm alumina pellets was established to perform C₂H₄ combustion experiments by acquiring the flammable limits, temperature variation characteristics, combustion wave velocity, pollutant emissions, and treatment efficiency. The burner operated well at equivalence ratios within 0.3 to 0.7. Larger alumina pellets widened the burner's lower flammable limit. As the flame propagated downstream, the higher premixed gas flow velocity and larger alumina pellets, the higher combustion wave velocity, whereas the circumstances were opposite as the flame spread upstream. The combustion temperature increased with the equivalence ratio and premixed gas flow velocity. In response to the effect of the alumina pellet dimension, 3‐mm alumina pellets corresponded to higher combustion temperatures, lower CO emissions, and higher treatment efficiency than those less than 9‐mm conditions.     

Simulation of thermally‐enhanced combustion in a porous medium burner

Premixed combustion in a porous medium burner is investigated numerically. A two‐dimensional steady, laminar flow model is used. A single‐step reaction of methane is used for the chemical kinetic model. The model also includes thermal radiation transport of the porous media that is placed inside the burner. The radiative transport equation is solved by using the discrete ordinate method. The results show that, for each equivalence ratio, the flame can be stabilized at various axial locations with different flame speeds. The flame temperature increases with the equivalence ratio and flame speed. Furthermore, the energy release rates are much higher than that of a free flame for the same equivalence ratio as a result of higher flame speed. © 2005 Wiley Periodicals, Inc. Heat Trans Asian Res, 35(1): 75–88, 2006; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20088     

有序堆积床内预混气体燃烧特性实验研究

为研究预混气体在多孔介质燃烧器中的火焰燃烧特性,设计了一种新型多孔介质燃烧器,其中多孔介质区域由氧化铝圆柱体有序堆积而成.分别研究了当量比和入口速度对甲烷/空气预混气体在多孔介质燃烧器中的火焰温度分布、火焰最高温度以及火焰传播速度的影响.结果 表明:在当量比0.162～0.324、入口速度0.287～0.860 m/s...     

Characterization of confined hydrogen-air jet flame in a crossflow configuration using design of experiments

Hydrogen combustion has many industrial applications and development of new hydrogen burners is required to fulfil new demands. A novel configuration of hydrogen burner utilizing crossflow injection of fuel jets into swirling combustion air is characterized empirically in this work. It is intended as a first step in the development of new burner technologies having reduced emission levels and improved efficiency. Experiments were designed using the full factorial design method. Operating parameters were varied simultaneously and the NO_X emissions from the flame stabilized on the burner were measured. Statistical analysis of the experimental data showed that overall equivalence ratio is the dominant factor and lower NO_X emissions are observed at low equivalence ratios, irrespective of the burner power level. The analysis yielded an empirical relationship among NO_X emission, overall equivalence ratio, and power level that is useful in the design activity for a future combustion system based on the proposed configuration.     

Investigation of the structural and reactants properties on the thermal characteristics of a premixed porous burner

Porous burners offer attractive features such as competitive combustion efficiency, high power ranges, and lower pollutant emissions. In the present study, the thermal characteristics of a porous burner are numerically investigated for a range of operating conditions and design specifications within a practical range. The premixed flame propagation of a methane/air mixture in a ceramic porous medium is simulated through an unsteady, one-dimensional model. The combustion process is modeled using a suitable single-step chemical kinetics. The reaction location is not predetermined, thus the flame is allowed to float within the solid matrix or to run off from either side of the porous medium. The numerical results indicate that flame stability and thermal characteristics of the burner are strongly dependent on the inlet mixture specifications and the solid matrix structural properties. For a fixed value of the inlet firing rate, the combustion products temperature will increase by an increase in the inlet gas temperature, an increase in the matrix porosity, or by a decrease of the matrix pore density. Among the geometrical properties, the burner length has virtually no effect on the burner performance. An increase in the solid matrix porosity or burner firing rate will increase the efficiency of the preheating zone, while increasing the inlet gas temperature or matrix pore density will cause a reduction in this efficiency. Simulation results also suggest that in order to prevent flame blow-out or flash-back, critical values of the burner settings and design parameters must be avoided.     

Heat transfer analysis of a two-dimensional rectangular porous radiant burner

In the present work, a 2-D rectangular porous burner is considered for investigation. Methane–air combustion with detailed chemical kinetics is used to model the combustion part. 164 chemical reactions with 20 species are considered. Separate energy equations for gas and solid phases are solved. The radiative part of the energy equation is modeled using the collapsed dimension method. The effects of the power density, equivalence ratio, extinction coefficient and volumetric heat transfer coefficient on temperature and concentration profiles are studied.     

Experimental investigation of the performance of a liquid fuel-fired porous burner operating on kerosene-vegetable cooking oil (VCO) blends for micro-cogeneration of thermoelectric power

Studies related to porous burner for thermoelectric (TE) power generation have mainly focused toward achieving a specific range of power output for various applications. However, detailed analyses on the performance and emission aspects of the porous burner are lacking. In addition, physical integration between the burner and TE modules has added further complexity in this research area. Thus, this work aims to comprehend the effects of fuel–air equivalence ratio on the performance and emission characteristics of a liquid fuel-fired porous burner for micro-cogeneration of TE power. A catalytically inert Al₂O₃ porous medium was incorporated into a liquid fuel-fired porous burner operating on four mixtures of kerosene-vegetable cooking oil (VCO) blends: 100 kerosene, 90/10 KVCO, 75/25 KVCO, and 50/50 KVCO. Ten bismuth-telluride TE cells were arranged in a ten-sided polygon that, together with finned dissipators, formed a TE module electrically connected in series but thermally connected in parallel. The performance aspects at various fuel–air equivalence ratios were thoroughly evaluated with the corresponding temperature profiles, voltage, current, power output, and electrical efficiency. Results indicated that the surface temperature of the porous media was generally higher than the developed and exit flame temperature of the burner. Varying the fuel-air equivalence ratio significantly affected the electrical efficiency, with a maximum and minimum value of 1.94% and 1.10%, respectively. The power output steadily increased in the lean region, but stabilized as the fuel–air equivalence ratio slowly increased beyond the stoichiometric ratio. The CO emission was relatively lower at the lean region; however, significant amount was recorded in the rich combustion region. Moreover, NOx fluctuated between 1 ppm and 4 ppm over the entire range of fuel–air equivalence ratio.     

Increasing the efficiency of radiant burners by using polymer membranes

Gas-fired radiant burners are used to convert fuel chemical energy into radiation energy for various applications. The radiation output of a radiant burner largely depends on the temperature of the combustion flame. In fact, the radiation output and, thus, the radiant efficiency increase to a great extent with flame temperature. Oxygen-enriched combustion can increase the flame temperature without increasing fuel cost. However, it has not been widely applied because of the high cost of oxygen production. In the present work, oxygen-enriched combustion of natural gas in porous radiant burners was studied. The oxygen-enriched air was produced passively, using polymer membranes. The membranes were shown to be an effective means of obtaining an oxygen-enriched environment for gas combustion in the radiant burners. Two different porous radiant burners were used in this study. One is a reticulated ceramic burner and the other is a ceramic fibre burner. The experimental results showed that the radiation output and the radiant efficiency of these burners increased markedly with rising oxygen concentrations in the combustion air. Also investigated were the effects of oxygen enrichment on combustion mode, and flame stability on the porous media.     

基于多孔介质燃烧的端部辐射器的实验研究

设计了基于多孔介质燃烧技术的端部辐射器,研究不同预混气体流速(功率)下当量比对燃烧器燃烧稳定性、多孔介质内部温度、辐射器表面温度及其均匀性、污染物排放、辐射效率等特性的影响.结果表明,燃烧器辐射表面的温度均匀性较好.最大相对温差小于3%:多孔介质燃烧器可实现最低当量比0.33的稳定可持续燃烧;小功率燃烧时.多孔介质内部温度及端部辐射表面温度都随当量比增大而增加,且流量越大增加程度越大,可据此提出实现更高辐射表面温度的方案.实验工况范围内.最大辐射效率达23%;NO<,x>排放体积分数低于25×10<'6>,在当量比大于0.45时,CO排放体积分数均低于10×10<'6>.     

多入口多孔介质燃烧器的数值模拟

在多入口燃烧器内加入多孔介质,以甲烷/空气为燃料,采用非预混燃烧的数值模拟方法,探究多入口燃烧器的燃烧情况.对比多孔介质燃烧与空间自由燃烧,分析了"超焓燃烧"现象;在多孔介质燃烧基础上,探究不同当量比对燃烧温度的影响;在多孔介质燃烧和不同当量比的基础上探究污染物CO和CO_2的排放情况.结果表明:多孔介质燃烧可以实现"超焓燃烧"特性,燃烧火焰温度高于自由空间燃烧温度;当量比对燃烧温度影响很大,随着当量比的增大,燃烧器内最高燃烧温度升高,但燃烧过程存在一个最佳当量比0.6,超过该当量比后最高温度将不再变化;多入口多孔介质燃烧有助于减少CO和CO_2的生成量.     

天然气在渐变型多孔介质中的预混燃烧

设计了孔径沿程变化的渐变型多孔介质(GVPM)燃烧器，为了解天然气在其中的预混燃烧特性，对燃烧室气体、固体温度分布和CO、NO；污染物排放进行了测量．试验研究了渐变型多孔介质中燃烧的温度场分布、火焰移动、污染物排放、稳定性及多孔介质孔径结构对燃烧特性的影响规律．将研究结果与几种均匀型多孔介质(HPM)中的燃烧进行比较，发现渐变型多孔介质中的燃烧可以有更多的优点，包括均匀温度场分布、极低污染物排放、高火焰速度、高稳定性、宽燃烧极限和有很大的负荷调节范围等．     

A Numerical and Experimental Investigation of Performance of a Nonsprayed Porous Burner

The performance of a nonsprayed porous burner (NSPB) is investigated through both numerical and experimental studies. The major requirement of liquid fuel combustion systems is excellent fuel vaporization, which is accomplished by using porous medium. Instead of heterogeneous combustion, which occurs in free space of a conventional sprayed burner, a homogeneous combustion of vaporized kerosene and air takes place within a porous medium. The liquid kerosene is preheated and completely vaporized in the first porous medium before being mixed with preheated air in the mixing chamber (i.e., a small space between two porous media). Then the combustion occurs in the second porous medium. A subcooled boiling, single global reaction combustion, and local nonthermal equilibrium between fluid and solid phases with phase change under complex radiative heat transfer are considered. The model accuracy is validated by the experimental data before parametric study—that is, equivalence ratio and firing rate are performed. Result show that a self-sustaining evaporation without atomization and matrix-stabilized flame can be achieved in the NSPB by providing the radiant output efficiency in the same range as a conventional premixed gaseous porous burner. This indicates that the NSPB is one possible technology to replace conventional spray burners for future requirements.     

Lean flammability limits for stable performance with a porous burner

Applications of porous burners are of high interest due to many advantages such as extended lean flammability limit in comparison with free flame structures. In this work, laminar premixed flame propagation of methane/air mixture in a porous medium is numerically investigated. An unsteady one-dimensional physical model of a porous burner is considered, in which the flame location is not predetermined. The computational domain is extended beyond either side of the porous medium to accurately model reactions close to the edges of the solid matrix. After validation of the model and performing a baseline simulation, a parametric study is carried out to investigate the lean flammability limits of the burner and the unstable flash-back/blow-out phenomena. Stable performance diagrams are given for two controlling parameters of turn–down ratio and porous medium porosity. The simulation results indicate that the stable performance range of the burner is extended when the equivalence ratio increases; however, the blow-out region expands with an increase in the firing rate. For constant values of porosity and firing rate, increasing the equivalence ratio can change the operating regime of the burner from blow-out to a stable condition. It is observed that by the variation of porosity in the range of 0.6–0.9, and for the equivalence ratios of more than 0.6, the flame flash-back cannot occur. An equivalence ratio of 0.43 is found to be the lower limit at which the flame stabilizes in the matrix.     

Effect of elliptic burner geometry and air equivalence ratio on the nitric oxide emissions from turbulent hydrogen flames

An experimental study on turbulent hydrogen flames from circular and elliptic burners with varying degrees of premixedness (diffusion, fuel-rich, stoichiometric, and fuel-lean) is presented. Flame stability, visible flame height, flame radiation, global nitric oxide (NO) concentration, and inflame temperature and NO concentration profiles were measured. We found that the elliptic burner flames had lower liftoff velocity, were shorter, and radiated less heat to the surrounding as compared to circular burner flames. Global NO concentration decreased with an increase in air equivalence ratio for both circular and elliptic burner flames. Peak in-flame NO concentration along the flame centerline increased with a decrease in air equivalence ratio. Elliptic burner flames produced higher peak in-flame temperatures. Overall, the elliptic burner flames produced less peak NO as compared to circular burner flames at all air equivalence ratios except zero (diffusion flames) in accordance with the global emission measurements.     

多孔介质燃烧器的辐射输出效率和污染物

使用双温度体积平均模型、详细化学反应机理GRI3．0,对甲烷,空气预混气在多孔介质燃烧器内的预混火焰进行模拟．分析不同当量比和质量流量下的预热效率、辐射输出效率以及污染物排放情况,并对辐射输出效率随多孔介质热物性参数的变化进行敏感性分析．结果表明,增大相间对流换热系数或减小当量比、质量流量及固相消光系数都可以提高辐射输出效率,减小当量比或质量流量可以减少污染物排放．在所有的影响因素中,当量比的影响最大,发展超贫燃燃烧技术是获得高效低污染多孔介质燃烧器的关键．     

Flame stabilization mechanisms and shape transitions in a 3D printed,hydrogen enriched,methane/air low-swirl burner

Flame shapes and their transitions of premixed hydrogen enriched methane flames in a 3D-printed low-swirl burner are studied using simultaneous OH×CH₂O planar laser induced fluorescence and stereoscopic particle image velocimetry. Three different flame shapes are observed, namely bowl-shape, W-shape, and crown-shape. The bowl-shaped flame has its base stabilized through flame-flow velocity balance and its sides stabilized in the inner shear layer. While the bulges of the W-shaped flame rely on a similar stabilization mechanism in the central flow, its outer edges are stabilized by large-scale eddies in the outer shear layer. The crown-shaped flame is also aerodynamically stabilized in the center, but its outer edges are anchored to the burner hardware. At a fixed equivalence ratio, the statistical transitions between flame shapes across test conditions are jointly dominated by hydrogen fraction and bulk velocity. Dynamically, W-to-crown transition is attributed to the upstream propagation and attachment of the flame outer edges.     

A review of enhancing micro combustion to improve energy conversion performance in micro power system

With the development of microfabrication technology and micro devices, the demand for Power Micro Electro Mechanical System (Power MEMS) is ever-increasing. However, traditional chemical batteries are not suitable for Power MEMS due to their low energy density. The combustion of hydrogen and hydrocarbon fuels offers a more promising alternative to conventional batteries. However, micro combustion faces the problems of flame instability and low combustion efficiency. Therefore, improving flame stabilization and combustion efficiency in micro combustions is necessary. Studies have made considerable progress in these aspects over the last decade. This paper summarized these studies and classified the optimization schemes according to flame stabilization and combustion efficiency. Besides, research on the Field Synergy Principle was discussed. The synergy between the flow field and temperature gradient field in the micro-scale domain will become a key research area in the future. It is proposed to insert porous media in MTES and MTPVS and adopt catalytic combustion. Adding hydrogen to the mixed gas was recommended. The equivalence ratio of the mixed gas in the range of 0.9–1.1 would be best. The equivalence ratio is the ratio of the theoretical requirement of air with complete combustion to the actual supply of air.     

Burning velocities of chlorinated hydrocarbonmethaneair mixtures

Results are presented for the variation in burning velocities with equivalence ratio and reactant gases preheat temperature for a number of chlorinated hydrocarbon compounds in methane-air mixtures of different concentrations at atmospheric pressure. Flame velocity of the mixture is determined with a Bunsen burner by measuring the unignited mixture approach flow rates and the area of the flame front. The method provides acceptable results and compares favorably with widely published methane flame data. Activation energy for a particular chlorinated compound was calculated by relating the flame velocity to the overall combustion reaction rate. Results are compared with nonchlorinated compounds and the available data in the literature. The reasons for discrepancies are discussed. The results show that increasing chlorine content decreases flame velocity and shifts the maximum flame velocity from fuel rich toward fuel lean. The flame velocity increases with increasing gas preheat temperature.