多孔介质特性参数对燃烧温度及压力影响的数值模拟研究

考查了两段式多孔介质内预混气燃烧的温度与压力分布情况。建立了甲烷／空气预混气体在多孔介质内燃烧的二维数学模型,运用FLUENT软件求解瞬态控制方程的方法计算出燃烧稳定后多孔介质内的温度、与压力分布,并考查了不同当量比、多孔介质辐射衰减系数和导热系数对温度和压力分布的影响。结果表明,甲烷／空气预混气体在多孔介质中燃烧,当量比越大温度峰值越高,压力梯度越大;小孔介质辐射衰减系数的改变对温度分布和压力分布没有明显的影响,而大孔介质辐射衰减系数对温度分布和压力分布有较大的影响;增加多孔介质的导热系数,会使固相与气相温度均有所升高,燃烧区域压力降低。     

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.     

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.     

Numerical and experimental investigation of a mild combustion burner

An industrial burner operating in the MILD combustion regime through internal recirculation of exhaust gases has been characterized numerically. To develop a self-sufficient numerical model of the burner, two subroutines are coupled to the CFD solver to model the air preheater section and heat losses from the burner through radiation. The resulting model is validated against experimental data on species concentration and temperature. A 3-dimensional CFD model of the burner is compared to an axisymmetric model, which allows considerable computational saving, but neglects some important burner features such as the presence of recirculation windows. Errors associated with the axisymmetric model are evaluated and discussed, as well as possible simplified procedures for engineering purposes. Modifications of the burner geometry are investigated numerically and suggested in order to enhance its performances. Such modifications are aimed at improving exhaust gases recirculation which is driven by the inlet air jet momentum. The burner is found to produce only 30 ppm_v of NO when operating in MILD combustion mode. For the same air preheating the NO emissions would be of approximately 1000 ppm_v in flame combustion mode. It is also shown that the burner ensures more homogeneous temperature distribution in the outer surfaces with respect to flame operation, and this is attractive for burners used in furnaces devoted to materials' thermal treatment processes. The effect of air excess on the combustion regime is also discussed.     

Porous burners for lean-burn applications

We review research on lean methane combustion in porous burners, with an emphasis on practical aspects of burner design and operation and the application of the technology to real-world problems. In particular we focus on ‘ultra-lean’ combustion, where the methane concentration is actually at or below the lean flammability limit for a free flame (5% methane by volume in air). Porous burners are an advanced combustion technology whereby a premixed fuel/air mixture burns within the cavities of a solid porous matrix. They are capable of burning low-calorific value fuels and very lean fuel/air mixtures that would not normally be flammable, potentially allowing the exploitation of what would otherwise be wasted energy resources. Possible lean-burn applications include the reburn of exhaust gases from existing combustion systems, and the mitigation of fugitive methane emissions. Porous burners operate on the principle that the solid porous matrix serves as a means of recirculating heat from the hot combustion products to the incoming reactants. This results in burning velocities higher than those for a free flame, as well as extended lean flammability limits. Burner performance is also characterised by low emissions of combustion related pollutants and stable operation over a wide range of fuel concentrations and flow rates. Stable combustion of methane/air mixtures below the conventional lean limit has been observed by a number of researchers; in one study the combustion of a mixture with a fuel concentration of only 1% was reported. A number of design considerations are important as regards optimising burner performance for lean-burn applications. Foremost among these is the selection of a suitable material for the porous matrix. Possibilities include packed beds of alumina spheres or saddles, and reticulated foams made of silicon carbide or high temperature metal alloys. Other potentially significant design issues include the length of the porous bed, the use of ‘multi-section’ designs where different porous materials are used in each section, the incorporation of external heat exchangers to supplement the heat recirculation provided by the porous matrix, and the ability to operate the burner at elevated pressures. There is an extensive body of research relating to porous burners, comprising experimental and numerical investigations. However the majority of previous studies have been directed towards the use of porous burners for radiant heating applications rather than for the combustion of low-calorific value fuels. Consequently there is a lack of reliable data relating specifically to ultra-lean combustion. We identify specific areas where further research is required to progress this field. These include the influence on burner performance of the design considerations listed above, the stability of the combustion process to fluctuations in fuel concentration and flow rate, the development of reliable models specifically for ultra-lean combustion in practical burners, and the investigation of issues relating to scale-up and commercial application.     

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的燃烧性能进行了评价,考察了不同进口速度u₀、当量比Φ和多孔介质导热系数对氨/空气火焰特性和NO排放的影响。结果表明,多孔介质燃烧器能在u₀ = 3 ~ 7 m/s和Φ = 0.9 ~ 1.2条件下稳定燃烧;随着多孔介质导热系数的增大,火焰最高温度下降且火焰位置向上游移动;减小进口速度和增大当量比能够显著降低NO的排放。     

预热对陶瓷燃烧器燃烧过程的影响

应用计算机模拟技术研究了不同空气和煤气预报温度对陶瓷燃烧过程的影响,通过研究发现不同的预热度对燃烧火焰的长度影响显著,配置合理的温度是热风炉操作一个有效的调节手段。     

Numerical simulation of turbulent combustion in porous materials

This paper presents one-dimensional simulations of combustion of an air/methane mixture in porous materials using a model that explicitly considers the intra-pore levels of turbulent kinetic energy. Transport equations are written in their time-and-volume-averaged form and a volume-based statistical turbulence model is applied to simulate turbulence generation due to the porous matrix. Four different thermo-mechanical models are compared, namely Laminar, Laminar with Radiation Transport, Turbulent, Turbulent with Radiation Transport. Combustion is modeled via a unique simple closure. Preliminary testing results indicate that a substantially different temperature distribution is obtained depending on the model used. In addition, for high excess air peak gas temperature is reduced and the flame front moves towards the exit of the burner. Also, increasing the inlet flow rate for stoichiometric mixture pushes the flame out of the porous material.     

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.     

Numerical study on methane/air combustion characteristics in a heat-recirculating micro combustor embedded with porous media

Micro-combustor is a portable power device that can provide energy efficiently, heat recirculating is considered to be an important factor affecting the combustion process. For enhancing the heat recirculating and improving the combustion stability, we proposed a heat-recirculating micro-combustor embedded with porous media, and the numerical simulation was carried out by CFD software. In this paper, the effect of porous media materials, thickness and inlet conditions (equivalence ratio, inlet velocity) on the temperature distribution and exhaust species in the micro combustor are investigated. The results showed that compared with the micro combustor without embedded porous media (MCNPM), micro-combustor embedded with porous media (MCEPM) can improve the temperature uniformity distribution in the radial direction and strengthen the preheating capacity. However, it is found that the embedding thickness of porous media should be reasonably arranged. Setting the thickness of porous media to 15 mm, the combustor can obtain excellent comprehensive capacity of steady combustion and heat recirculating. Compared the thermal performance of Al₂O₃, SiC, and ZrO₂ porous media materials, indicating that SiC due to its strong thermal conductivity, its combustion stabilization and heat recirculating capacity are obviously better than that of Al₂O₃ and ZrO₂. With the porous media embedded in the micro combustor, the combustion has a tempering limit of more than 10 m/s, and the flame is blown out of the porous media area over 100 m/s. The reasonable equivalence ratio of CH₄/air combustion should be controlled within the range of 0.1–0.5, and “super-enthalpy combustion” can be realized.     

Primary air entrainment characteristics for a self-aspirating burner: Model and experiments

Experimental and theoretical investigations of primary air entrainment characteristics of a self-aspirating burner are presented. Emphasis was made on experiments, which were performed using both hot and cold tests; and a correlation between them is proposed. The level of primary air entrainment is measured using an oxygen sensor and a particle image velocimetry system. Experimental results are used to validate the predicted ones, which are obtained by constructing a theoretical model basing on simple momentum and energy conservation principles. It is found that the model predictions agree with the experimental data for a similar system. Primary air entrainment is a function of fuel gas flow rate, fuel gas type, injector geometry, mixing tube geometry, and burner port geometry. The level of primary air entrainment increases with increasing momentum rate of the fuel gas. The hot test gives about a 22 percentage point (37% relative) lower PA value than that of the cold test because of the preheating effect caused by combustion. A first correlation between the hot test and the cold one for primary air entrainment is proposed. It is recommended that the preheating effect caused by combustion in a self-aspirating burner not be neglected when designing the burner.     

A numerical evaluation of combustion in porous media by EGM (Entropy Generation Minimization)

Combustion in Porous Media provides interesting advantages compared with the free flame combustion due to the higher burning rates, increased power dynamic range, the extension of lean flammability limits, and the low emissions of pollutants. A numerical code is developed in order to evaluate the effects of different parameters of combustion in porous media. The governing equations including Navier–Stokes, the solid and gas energy and the chemical species transport equations are solved using a multi-step reduced kinetic mechanism. Flame stabilization and the burner optimization are studied by EGM (Entropy Generation Minimization) method considering the effects of chemical affinities and reaction. It is found that the flames occurring at the upstream half of the porous layer are more stable and more efficient, producing less emissions than those occur at the downstream half of porous layer. Also at a specified equivalence ratio both the heat recirculation efficiency and the Merit number have similar trend by changing the flame location. For a FFL (Fixed Flame Location), there is an optimum value of equivalence ratio at which the burner efficiency is a maximum.     

蓄热式钢包烘烤器气体混合特性数值模拟

利用CFD通用商业软件模拟了蓄热式钢包烘烤器的流场和气体混合状况,比较了不同空气喷入速度、不同空煤气烧嘴间距、不同空气预热温度等不同工况对空煤气混合状况的影响,并定量分析了这些影响的大小和趋势.为蓄热式钢包烘烤器燃烧装置的合理设计提供依据.     

往复流多孔介质燃烧器的二维数值模拟与结构改进

对往复式惰性多孔介质燃烧器进行了二维数值模拟,模型的有效性通过实验数据进行验证.在燃烧器中分别填充4孔/cm泡沫陶瓷或小球,研究其内部的燃烧温度和压力损失.结果表明,由相同材料制成但结构不同的多孔介质对燃烧器内的高温区域和压力损失有显著的影响.孔隙率较大的泡沫陶瓷适合于布置在燃烧区,而孔隙率较小的小球适合于布置在热交换区域.改进燃烧器结构,即在燃烧器的中间布置泡沫陶瓷,而在两端布置小球.对于当量比为0.1的甲烷与空气混合气,得到了更为宽广的高温区域和适度的压力降.     

Studies on porous radiant burners for LPG (liquefied petroleum gas) cooking applications

This paper deals with the performance tests of a PRB (porous radiant burner) used for LPG (liquefied petroleum gas) domestic cooking stoves. The burner consists of a two-layer porous media. The combustion zone is made up of silicon carbide, and alumina balls forms the preheating zone. For a given burner diameter, the performances of the burner, in terms of thermal efficiency and emission characteristics, are analysed for different equivalence ratios and thermal loads (wattages). The water boiling test as prescribed in the BIS (Bureau of Indian Standard): 4246:2002 was used to calculate the thermal efficiency of both the conventional LPG cooking stoves and the PRB. The maximum thermal efficiency of the LPG cooking stoves with a PRB was found to be 68% which is 3% higher than that of the maximum thermal efficiency of the conventional domestic LPG cooking stoves. Unlike the conventional LPG stoves, for which the CO and NO_X emissions were found in the ranges 400–1050 mg/m³ and 162–216 mg/m³, respectively, for the one with PRB, the same were in the ranges of 25–350 mg/m³ and 12–25 mg/m³. The axial temperature distribution in the burner showed that the reaction zone was close to the interface of the two zones and at a higher thermal load, it shifted towards the downstream. The surface temperature of the PRB was found to be uniform.     

Combustion Wave Propagation of a Modular Porous Burner with Annular Heat Recirculation

A numerical investigation of the different arrangements of porous media in a combustor with annular heat recirculation is conducted.The effect of annular heat recirculation and porous block arrangement on the characteristics of combustion wave propagation is numerically studied.Results show that power input,heat capacity of porous matrix,arrangement of porous blocks,and annular heat recirculation are major factors that influence the propagation of combustion wave.The overall temperature of ceramic porous burner is higher than that of ceramic-metal type burner due to the lower heat storage capacity of the former,especially for the temperature downstream.The flame temperature is higher upstream and lower downstream with metal foams in the annulus than that without metal foams.The flame temperature of uniformity type burner is more uniform than that of gradually-varied and modular type burners.The flame front moves more slowly with metal foams in the annulus than that without metal foams due to the better preheating effect of metal foams.The flame position moves downstream,and the flame temperature gradually decreases and is eventually extinguished due to the low preheating temperature.     

Optimization of heat transfer performances of a heat-recirculating ceramic burner during methane/air and low-calorific-fuel/air combustion

An experimental burner of the heat-recirculating type was constructed and its thermal characteristics were investigated for steady methane/air and low-calorific-fuel/air combustion. Longitudinal temperature distributions of air and burned gas flowing in the passes of the burner were determined by means of both experiments and numerical simulations. Using the heat recirculation rate and the thermal efficiency as criteria for the heat transfer performances of the burner, the optimal design of the burner was examined in terms of a chemical parameter (the equivalence ratio), a fluid-mechanical parameter (the Reynolds number) and a geometrical parameter (the number of passes).     

3D numerical modelling of turbulent biogas combustion in a newly generated 10 KW burner

This study concentrates on the 3D numerical modelling of combustion of different biogases in a generated burner and combustor. The main goal of this study is to investigate the combustion characteristics (such as temperature and emissions) of biogases through a combustor due to depletion of natural gas. Moreover, the effect of the preheated air on flame temperatures of biogases have been studied in the present study. Finally, the effect of H₂S amount in biogas on SO₂ emissions has been investigated within these predictions. The numerical modelling of turbulent diffusion flames has been performed by using the standard k–ε model of turbulent flow, the PDF/Mixture Fraction combustion model and P-1 radiation model in the combustor. A CFD code has been used for all predictions. Temperature gradients have been determined on axial and radial directions for better understanding combustion characteristics of biogases. Modelling has been studied for thermal power of 10 kW and excess air ratio of λ = 1.2 for each biogas combustion. The first finding is that combustion of biogases is possible via the newly generated burner. Moreover, the results show that the one of biogas is very close to methane in terms of temperature distributions in the combustor due to including high amount of methane compared to other biogases. It is also concluded that the flame temperatures of biogases increase with preheating the combustion air as expected. It is finally revealed that SO₂ emissions increase as amount of H₂S in biogas is increased through the combustor.     

多孔介质中预混式燃烧的数值研究

本文考虑向燃烧室中插入高孔隙率的多孔介质的燃烧过程,根据气固两相局部非热平衡假设,建立了混合气体在惰性多孔介质中预混燃烧的一维数学模型,模拟了不同条件下甲烷-空气的预混合气在多孔介质中燃烧时的温度分布及气体流速、当量比和吸收系数对燃烧室气体温度峰值的影响.结果表明,多孔介质的存在明显改善了燃烧室的换热性能,强化了对新鲜混合气的预热,加速了燃烧反应的进行,燃烧室利用率提高.     

Combustion characteristics of a heat-recirculating ceramic burner using a low-calorific-fuel

A new type of the heat recirculating burner was constructed and its combustion characteristics during steady low-calorific-fuel/air combustion were investigated. Flammability limits of the burner were measured by experiments, and longitudinal temperature distributions of air and burned gas flowing in the passes of the burner were determined by means of both experimental measurements and numerical simulations. Using the heat recirculation rate and the thermal efficiency as criteria for the performance of the burner, the optimal design of the burner was examined in terms of a chemical parameter (the equivalence ratio), a fluid-mechanical parameter (the Reynolds number) and a geometrical parameter (the number of passes).