Tomographic data fusion with CFD simulations associated with a planar sensor

Tomographic techniques have great abilities to interrogate the combustion processes, especially when it is combined with the physical models of the combustion itself. In this study, a data fusion algorithm is developed to investigate the flame distribution of a swirl-induced environmental (EV) burner, a new type of burner for low NOx combustion. An electric capacitance tomography (ECT) system is used to acquire 3D flame images and computational fluid dynamics (CFD) is applied to calculate an initial distribution of the temperature profile for the EV burner. Experiments were also carried out to visualize flames at a series of locations above the burner. While the ECT images essentially agree with the CFD temperature distribution, discrepancies exist at a certain height. When data fusion is applied, the discrepancy is visibly reduced and the ECT images are improved. The methods used in this study can lead to a new route where combustion visualization can be much improved and applied to clean energy conversion and new burner development.     

Effects of hydrogen enhancement on mesoscale burner array flame stability under acoustic perturbations

Partial substitution of hydrocarbon fuel with hydrogen can effectively improve small-scale combustion system stability and performance, potentially opening the way for novel compact power generation and/or propulsion systems in the future. In this study, the effects of hydrogen enhancement between 0% and 40% hydrogen volumetric fractions in methane fuel were experimentally observed in a mesoscale burner array subjected to external acoustic perturbations. The mesoscale burner array utilizes an array of swirl-stabilized burner elements and their interactions with neighboring elements to improve the overall flame stability and simultaneously reduces the combustor length scale. OH1 chemiluminescence and OH planar laser-induced fluorescence (OH-PLIF) were used to image various hydrogen-enriched flames at an equivalence ratio of 0.7, subjected to transverse acoustic perturbations at 320 Hz. Two acoustic modes were imposed by controlling the phase difference between two speakers perturbing the flow. OH1 chemiluminescence images exhibited flame length scale reduction, leading to a denser flame array. Also, flame arrays with higher hydrogen enrichment were found to be more robust against transverse acoustic perturbations, demonstrated by reduced fluctuations in the global heat release rate. OH-PLIF images showed that flames with higher hydrogen enrichment initiated V- to M-shaped flame shape transition even under fuel lean conditions, thereby improving the combustion stability. OH-PLIF images were also used for flame stability analysis through spectral proper orthogonal decomposition (SPOD). The SPOD analysis showed hydrogen enrichment diminished flame fluctuation structures under fuel lean operation.     

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.     

Frontiers in combustion techniques and burner designs for emissions control and CO2 capture: A review

The use of fossil fuel is expected to increase significantly by midcentury because of the large rise in the world energy demand despite the effective integration of renewable energies in the energy production sector. This increase, alongside with the development of stricter emission regulations, forced the manufacturers of combustion systems, especially gas turbines, to develop novel combustion techniques for the control of NO_x and CO₂ emissions, the latter being a greenhouse gas responsible for more than 60% to the global warming problem. The present review addresses different burner designs and combustion techniques for clean power production in gas turbines. Combustion and emission characteristics, flame instabilities, and solution techniques are presented, such as lean premixed air‐fuel (LPM) and premixed oxy‐fuel combustion techniques, and the combustor performance is compared for both cases. The fuel flexibility approach is also reviewed, as one of the combustion techniques for controlling emissions and reducing flame instabilities, focusing on the hydrogen‐enrichment and the integrated fuel‐flexible premixed oxy‐combustion approaches. State‐of‐the‐art burner designs for gas turbine combustion applications are reviewed in this study, including stagnation point reverse flow (SPRF) burner, dry low NO_x (DLN) and dry low‐emission (DLE) burners, EnVironmental burners (including EV, AEV, and SEV burners), perforated plate (PP) burner, and micromixer (MM) burner. Special emphasis is made on the MM combustor technology, as one of the most recent advances in gas turbines for stable premixed flame operation with wide turndown and effective control of NO_x emissions. Since the generation of pure oxygen is prerequisite to oxy‐combustion, oxygen‐separation membranes became of immense importance either for air separation for clean oxy‐combustion applications or for conversion/splitting of the effluent CO₂ into useful chemical and energy products. The different carbon‐capture technologies, along with the most recent carbon‐utilization approaches towards CO₂ emissions control, are also reviewed.     

Development and characterization of a low-NOx partially premixed hydrogen burner using numerical simulation and flame diagnostics

The utilization of hydrogen as a fuel in free jet burners faces particular challenges due to its special combustion properties. The high laminar and turbulent flame velocities may lead to issues in flame stability and operational safety in premixed and partially premixed burners. Additionally, a high adiabatic combustion temperature favors the formation of thermal nitric oxides (NO). This study presents the development and optimization of a partially premixed hydrogen burner with low emissions of nitric oxides. The single-nozzle burner features a very short premixing duct and a simple geometric design. In a first development step, the design of the burner is optimized by numerical investigation (Star CCM+) of mixture formation, which is improved by geometric changes of the nozzle. The impact of geometric optimization and of humidification of the combustion air on NO_x emissions is then investigated experimentally. The hydrogen flame is detected with an infrared camera to evaluate the flame stability for different burner configurations. The improved mixture formation by geometric optimization avoids temperature peaks and leads to a noticeable reduction in NO_x emissions for equivalence ratios below 0.85. The experimental investigations also show that NO_x emissions decrease with increasing relative humidity of combustion air. This single-nozzle forms the basis for multi-nozzle burners, where the desired output power can flexibly be adjusted by the number of single nozzles.     

Performance and analysis by particle image velocimetry (PIV) of cooker-top burners in Thailand

Cooker-top burners are used extensively in Thailand because of the rapid combustion and high heating-rates created by an impinging flame, which is characteristic of these types of burners. High thermal efficiency with low level of CO emissions is the most important performance criteria for these burners. The wide variation in reported performances of the burners appears to be due to the ad hoc knowledge gained through trial and error of the local manufacturers rather than sound scientific principles. This is extremely undesirable in view of safety, energy conservation and environmental protection. In the present work, a nationwide cooker-top burner performance survey and an implementation of a PIV technique to analyze the burner performance as well as advising local manufacturers were carried out. Experimental data were reported for the base line value of thermal efficiency of all the burners. The thermal performance parameters and dynamic properties of the flow field at a flame impingement area, i.e. velocity magnitude, turbulent intensity, vorticity and strain rate were also reported as a function of burner type, which was categorized into four types based on the configuration of the burner head: radial flow burners, swirling flow burners, vertical flow burners and porous radiant burners.     

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.     

An experimental study on the high frequency oscillatory combustion in tubular flame burners

In this study, high frequency oscillatory combustion in tubular flame burner was experimentally investigated using large scale 8- and 12-in. diameter tubular flame burners. The conditions for the high frequency oscillatory combustion were determined, and the pressure fluctuations were measured, on which spectral analyses were made. The results showed that a smooth laminar tubular flame could be established, however, high frequency combustion sound was emitted from the 8- and 12-in. burners when the air flow rates exceeded 650 and 1200 m³/h, respectively. Pressure fluctuation measurements and spectral analyses showed that high frequency pressure fluctuations occurred simultaneously during the high frequency oscillation. The flame images were also obtained with a high speed video camera. The flame surface was found to be notably corrugated and the symmetry was broken during the oscillation. By assuming that the tubular flame burner is a simple tube, the natural frequencies of the burners were determined using the fundamental theory of the acoustic resonance in a cylindrical cavity, and the experimental peak frequencies in the spectra were compared with the natural frequencies. As a result, it was found that the high frequency oscillations in both the burners were identified as the tangential/radial mode acoustic resonant oscillations. It was further found that the tangential first mode of oscillation, which had an asymmetric structure, preceded the higher modes of oscillation which indicated that the mode dominated the occurrence of the high frequency oscillation. To verify the occurrence of the tangential first mode oscillation, the pressure fluctuations were measured with two pressure sensors installed at opposed locations. The results showed that the phase of the oscillation was 180° difference, and the asymmetric structure confirmed the occurrence of the tangential first mode of oscillation.     

单元熔窑燃烧过程数值模拟

对单元熔窑燃烧空间内的流动,燃烧及辐射传热等过程进行数值模拟研究,比较燃烧布置方式对火焰形状及传热过程中的影响,结果表明,对于所研究的宽度为3．2m的窑炉,燃烧器的布置应采用错排方式。     

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.     

Comparison of conventional and metal fiber burners in a compact methane reformer using CFD modeling

Compact reformers can be used to produce hydrogen for fuel-cell automobiles. The heat of the mehane seam reforming reaction is provided by methane burning. Generally, conventional burners have been used in combustion chambers. The Computational Fluid Dynamic (CFD) approach was used for the comparison of conventional burners with metal fiber burners and their locations for the first time. The rate of steam reforming reactions and methane combustion reactions were introduced to the CFD model and the Finite Rate/Eddy Dissipation model was used for reactions on the reforming and combustion sections. After validation of the compact reformer results by available experimental data, metal fiber was modeled using the porous-jump interior boundary condition. The results show that the best burner position for the metal fiber is the Bottom (near the catalyst) and for the conventional burner is the Top (far from the catalyst). The results show that the conventional burner in both the Middle and Bottom positions leads to an increase in the reaction zone temperature above 1200 K, which is higher than the catalyst tolerance, but placing a simple burner on the Top of the reactor does not have an out-of-range temperature problem. The hydrogen mass yield for a conventional burner at the Top position is 27.75% relative to methane. Due to the thermal uniformity in the metal fiber burner, the temperature does not exceed the catalyst limitation in the three positions (Top, Middle, and Bottom). The metal fiber burner at the Bottom of the combustion chamber shows the best performance with a hydrogen mass yield of 40.82%. The results indicate that metal fiber burners can distribute the flame more uniformly than conventional burners and increase the available heat for the reformer side.     

摆动式水平浓淡风煤粉燃烧器在670 t/h锅炉的应用研究

我厂2号锅炉(DG 670/13.7-8A)为四角切圆布置,一、二次风间隔布置,分上下两组,设计煤种为晋中贫煤.在实际运行中,煤质不稳定,锅炉着火和燃烧稳定性差.采用"摆动式水平浓淡风煤粉燃烧器"技术对燃烧器改造后,燃烧效率提高0.84%、在300 t/h时能稳定燃烧、炉膛无结渣,锅炉参数稳定.     

Influence of hydrogen addition to pipeline natural gas on the combustion performance of a cooktop burner

Displacing pipeline natural gas with renewable hydrogen is a promising way to reduce the emission of carbon dioxide, which is a major greenhouse gas. However, due to significantly differing characteristics of hydrogen and natural gas, such as flame speed, adiabatic flame temperature and stability limits, the combustion performance of hydrogen/natural gas mixture differs from pure natural gas. From the perspective of residential end users, a key question is: how much hydrogen can be injected into the pipeline natural gas without influencing the performance of the residential burners? A representative cooktop burner is selected to study the influence of hydrogen addition on the combustion and cooking performance. Flashback limits, ignition time, flame characteristics, cooking performance, combustion noise, burner temperature, and various emissions (NO, NO₂, N₂O, CO, unburned hydrocarbon (UHC), NH₃) are evaluated for different levels of hydrogen addition. According to the experimental results, the combustion performance of the cooktop burner is not significantly affected with up to about 15% hydrogen addition by volume, which shows the feasibility of utilizing hydrogen on existing cooking appliances without any modification. The experiment methodologies and results in this study will serve as a reference for future test and emission regulation standards on domestic burners.     

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.     

Combustion dynamics of a low-swirl combustor

The methodology for the measurement of dynamic combustor behavior has never been clearly established, due to the complexities associated with unsteady premixed flames and the difficulties in their measurement. The global and local distribution of Rayleigh index and the flame response functions are the main parameters normally employed to quantify and describe combustion dynamics. The Rayleigh index quantifies the thermoacoustic coupling, while the flame response function is a measure of the response of the system to outside disturbances. The primary objective of this work is to investigate the combustion dynamics of a commonly used low-swirl burner and to develop tools and methods for examining the dynamics of a combustion system. To this end, the effect of acoustic forcing at various frequencies on flame heat release behavior has been investigated. The current work uses OH-PLIF imaging of the flame region to produce phase-resolved measurements of flame behavior at each frequency. The response of the flame to the imposed acoustic field over the range of 22-400 Hz is then calculated from the processed images. This provides a starting point for an extension/extrapolation to practical acoustic ranges (∼5000 Hz). It was found that the thermoacoustic coupling was mainly evident in the shear mixing zone, producing a toroidal Rayleigh index distribution pattern. The phase shift of the flame fluctuation from the imposed acoustic wave seems to be very closely coupled to the vortices generated at the flame boundary due to shear mixing (Kelvin-Helmholtz instability), thus inducing the alternating toroidal structures. The peak value of the flame response function coincides with the peak absolute value of the Rayleigh index.     

多孔介质稳焰器对旋流火焰振荡燃烧特性影响的试验研究

为研究多孔介质稳焰器孔密度变化对贫预混旋流火焰振荡燃烧特性的影响,通过光电倍增管测量全局火焰热释放率,采用双麦克风方法测量旋流器入口速度脉动,获得不同孔密度多孔介质稳焰器火焰传递函数;并通过高速相机测量不同孔密度多孔介质稳焰器振荡火焰结构的变化。试验结果表明：多孔介质能够改变燃烧室声模态,有效抑制燃烧振荡,但孔密度对受迫燃烧火焰热释放率和压力脉动影响具有非线性;高频入口扰动对火焰响应特性影响较弱,火焰受迫响应呈现低通滤波特性;火焰传递函数增益峰值对应入口激励频率存在差异,但相位分布斜率基本一致;多孔介质导致火焰向稳焰器中心轴线聚拢,相干结构更加明显;宽频扰动范围内的火焰张角分布趋势与火焰传递函数增益曲线的分布趋势相反。     

The behaviour of lifted oxy-fuel flames in burners with separated jets

Oxy-fuel combustion in separated-jet burners has been proven to increase thermal efficiency and to have a potential for NOx emission reduction. This paper presents an investigation into confined, turbulent, oxy-flames generated by a burner consisting of a central natural gas jet surrounded by two oxygen jets. The study is focused on the identifying the influence of burner parameters on the flame characteristics and topology, namely stability, lift-off height and flame length. The effects of the natural gas and oxygen jet exit velocities, the distance separating the jets and the deflection of oxygen jets towards the natural gas jet are examined. The OH chemiluminescence. Results show that the lift-off heights increase when jet exit velocities and the distance separating the jets are increased. The deflection of oxygen jets decreases the lift-off height and increases the volume of flame in the transversal plane. The flame length increases principally with the oxygen exit velocity and the separation distance, and decreases considerably when the angle of oxygen jets is increased.     

附壁射流煤粉燃烧器试验研究

附壁射流煤粉燃烧器是根据燃烧空气动力学原理,利用壁面对射流的非接触式导流特性和射流组合特性而研制出的一种新型燃烧装置。本文科述了该燃烧器稳定煤粉火焰和不易结焦的原理及特性。试验室试验表明,它可在冷风条件下脱油稳燃V~(?)=10%的无烟煤,并有防焦功能,现在进行推广应用。     

Counterflow diffusion flames

Laminar counterflow diffusion flames are generally referred to as the pure diffusion flame. This paper reviews experimental studies of these counterflow diffusion flames and presents a detailed summary of our present understanding of them. The paper begins by classifying these laminar counterflow diffusion flames into four types: (I) the counterflow diffusion flame between two opposed jets, (II) the flat, counterflow diffusion flame between two opposed matrix burners, (III) the counterflow diffusion flame in the forward stagnation region of a spherical or hemispherical porous burner, and (IV) the counterflow diffusion flame in the forward stagnation region of a cylindrical porous burner. Next, the characteristics of these counterflow diffusion flames are presented, and these flames are then shown as being suitable for a fundamental study of diffusion flame and its combustion characteristics, especially, (1) for a detailed study of the structure of a laminar diffusion flame and of high-temperature flame kinetics, (2) for an accurate study of combustion characteristics closely related to flame-extinction phenomena (e.g. dilution limits of diffusion flames, the overall reaction rates for fuel-oxidant combinations, and the effectiveness of inhibitors), and (3) for a careful study of the various properties of a flame in an electric field.