Experimental and numerical investigation on performance of a porous medium burner with reciprocating flow

Experimental and two-dimensional numerical investigations on the performance of an inert porous media burner with reciprocating flow are presented. Attention was focused on the combustion temperature and pressure loss in the burner, which was, respectively, packed with 4PPC (Pores Per Centimeter) ceramic foams or alumina pellets with various sizes. Results show that material and structures of porous media have significant influence on the burner performance, and that ceramic foam with high porosity is suitable for using in the combustion region whereas alumina pellets should be placed in the heat exchange zone. In addition, the highly two-dimensional characteristics of the porous media burner are validated by the numerical model, which include temperature distributions, species profile and flame structure. Numerical results were validated against experiment data.     

Premixed gas combustion stabilized in fiber felt and its application to a novel radiant burner

Premixed gas combustion stabilized in a unique ceramic fiber felt has been investigated. Our aim was to better understand the flame structure and flame stabilization mechanisms in the porous felt medium in order to develop a new radiant burner. A novel recuperative radiant burner was designed and constructed. A flame stabilizes near the downstream interface of the porous medium that is an excellent selective thermal emitter. The burner was developed for use as a gas-fired light source. The combustion performance of the burner at various operating conditions and the effect of heat recuperation are presented. Combustion modes on the fiber felt were examined carefully. An optimal flame structure for the premixed gas combustion is attained and the flame stabilizes in the porous fiber felt at radiant mode combustion over a wide range of firing rates. The burner emits desired spectral radiation and generates fairly intense light at the conditions of heat recuperation. The light radiant burner could be used as an alternative low-glare light source in an integrated heat and light system in which the light is distributed through light pipes.     

Stability of lean combustion in mini-scale porous media combustor with heat recuperation

In miniaturization of burners, it is very difficult to organize stable self-sustained combustion. A mini-scale porous media combustor with heat recuperation was set up to study the stability of lean combustion and its emission. The diameter of the porous media was only 20 mm and the burner was about 140 mm in length. Experimental results showed that when the mass flow rate of the premixed gas was 0.163 g/s, the extinction limit was extended to Φ = 0.40 in the methane combustion and Φ = 0.39 in the propane combustion. For most cases, the emission of CO was lower than 100 ppm in both methane and propane combustion. The maximal concentration of NO_x was 63 ppm in the methane combustion. The ultra-lean combustion was also predicted by a numerical simulation with a 2D two-temperature model. The heat recuperation efficiency η, as high as 40%, made the ultra-lean combustion extremely stable. Although the maximal flame temperature in the porous media reached above 2000 K, the exhausts temperature was lower than 900 K.     

A numerical investigation of the flame stability in porous burners employing various ceramic sponge-like structures

In order to optimize the porous burner for the application as a pilot burner of stationary gas turbines aiming to reduce NO_x emissions a fundamental study investigating the influence of the thermo-physical properties of the porous structure on the flame stabilization in a porous burner was conducted. This work presents a numerical study of the stability of one-dimensional laminar premixed flame in porous inert ceramic sponge structure. A set of steady computations are considered, using a numerical model that takes into account solid and gas energy equations as well as detailed chemistry. The model considers additionally the enhancement of both, thermal and species diffusivity by the flow dispersion, whereas the dispersion coefficients of the investigated structures have been determined from three-dimensional flow simulations using MRI (magnet resonance imaging) and CT (computer tomography) data to regenerate the real sponge structures. Hence, it was possible to calculate a thickened flame front as it was detected in experiments, too. The computations were conducted for different operational conditions and different burner configurations in respect to geometrical and material properties of the porous inert media. The numerical predictions showed very good agreement with the corresponding experimental stability data. The obtained numerical results were used for the formulation of a simple stability model based on the Pe number that enables a prediction of the lean blow-off limits in the combustion systems employing porous burner concept.     

Adiabatic premixed combustion in a gaseous fuel porous inert media under high pressure and temperature: Novel flame stabilization technique

This work presents an experimental investigation to study the characteristics of combustion using a premixed methane-air mixture within a non-homogeneous porous inert medium (PIM) under high pressure and temperature. In order to obtain a stable flame under these operating conditions within PIM, a novel flame stabilization technique in porous inert media (PIM) combustion under high pressure and temperature has been developed and evaluated. The proposed technique avoids the draw backs of the hitherto developed techniques by properly matching the flow and flame speeds and, consequently, ensuring a stable combustion, for a wide range of operating pressure and temperature. The success of this technique permits the extension of PIM combustion to new applications such as gas turbines. The validity of this new technique has been assessed experimentally in detail by analyzing combustion inside a prototype burner. The effects of various operating conditions, such as initial preheating temperature and elevated pressure, have been examined for an output power range between 5 and 40 kW. The experiments covered a broad spectrum of operating conditions ranging from a mixture inlet temperature of 20 °C and pressure ratio of 1 up to a temperature of 400 °C and a pressure ratio of 9. Evaluation of the results revealed excellent flame stability with respect to both flashback and blow-out limits throughout all the operating conditions studied, including relative air ratios far beyond the normal lean limit. While the blow-out stability showed no significant dependence on pressure, it was strongly determined by the preheating mixture inlet temperature. A remarkable broadening of the stability range from 0.6 to 1.0 on preheating to 400 °C was observed. This reveals the potential of pre-heat temperature to improve the dynamic modularity of the burner.     

Pore-level numerical simulation of methane-air combustion in a simplified two-layer porous burner

A simplified two-dimensional model of two-layer porous burner based on pore level is developed.The heat transfer of solid phase in porous burner is seen as the synergistic effects of conduction through con-necting bridges and surface radiation between the solid particles in the model.A numerical simulation study on the characteristics of flow,combustion and heat transfer in the two-layer porous burner is car-ried out using the pore level model,and the effects of the control parameters such as the inlet velocity and solid thermal conductivity on thermal non-equilibrium are investigated.The results show that the flame structure is highly two-dimensional based on pore level.Obvious thermal non-equilibrium in the burner for the two phases and solid phase are observed,the largest temperature difference between the gas and solid phases is observed in combustion zone,while the temperature difference inside the solid particles is largest near the flame front.The results also reveal that thermal non-equilibrium of por-ous burner is much affected by the inlet velocity and solid thermal conductivity.     

Combustion characteristics in oil-vaporizing sustained by radiant heat reflux enhanced with higher porous ceramics

Liquid vaporizing combustion in porous ceramic burner has fine flame stability and characteristic of low emission. On the other hand, vaporization control has been seldom mentioned. In this work, kerosene vaporizing type combustor equipped with a porous ceramic plate, which has the porosity of 85%, is developed in order to enhance a rate of vaporization of the liquid fuel. The stability of combustion and NO_x emission characteristics were investigated in fuel vaporizing ceramic combustion. The plate burner is made of Al₂O₃ ceramic which has an optical-thickness of 0.54. The optically thin ceramics improved flame stability and enhances more fuel vaporization rate than optically thick ceramics. The thermal radiation energy from flame and the furnace walls can penetrate easily through the large pore of the ceramic plate. It is found possible to dispense the electric power for the fuel vaporization and the stable combustion is self-sustained by enhancement of vaporization, where the reflux rate of radiant heat was no less than 2% of the heating value.     

An overview of ceramic materials and their composites in porous media burner applications

The alarming rate at which the available fossil fuel-based resources are depleting has raised grave concerns among all stakeholders about the future of the world. These limited energy resources play a very significant role in conventional energy generation techniques and processes. Traditional energy consumption devices must be made more efficient to prolong the availability of fossil fuels. Along with achieving a substantial increase in the efficiency of these devices, it is the need of the hour to focus on minimizing the hazardous emissions resulting from improper combustion and inefficient design, leading to the ever-growing issue of climate change. Porous burners are one of the few energy consumption devices that are known to facilitate efficient combustion along with low emissions of detrimental combustion products. In the porous burner technology, ceramic-based porous media deliver better performance due to their superior physical, chemical, and thermal characteristics compared to metal-based media. The present paper, therefore, provides an overview of the application of different ceramic-based porous media in burner technology. Advancements, particularly over the last twenty years, in the development and use of novel ceramic composites for porous burner applications, have also been discussed.     

Parametric investigations of premixed methane-air combustion in two-section porous media by numerical simulation

Motivated by detailed designs of industrial porous burners published in patents, the combustion of methane-air mixtures in a two-section porous burner has been studied numerically. The software FLUENT is used to solve a two-dimensional transient mathematical model of the combustion. In order to reveal the reality of the combustion in porous media, the user defined function (UDF) is used to extend the ability of FLUENT and enable two-dimensional distributions of temperature and velocity to be obtained. Some operating or property parameters, which mainly affect the functions and quality of the industrial burner design, such as the inlet velocity of the reactants, the equivalence ratio, the extinction coefficient and the thermal conductivity of porous media, have been investigated. The results show that the contours of temperature and velocity change considerably at the interface of the porous media and near the wall, the gas temperature at the low inlet velocity limit is higher than that for the high velocity limit, the thermal conductivity in the upstream section has more influence on the temperature than that in the downstream section and finally, the temperature profiles of both the gas and the porous skeleton vary considerably with changes of the radiative extinction coefficient of the large-pore porous media.     

Modeling of combustion of premixed mixtures of gases in an expanding channel with allowance for radiative heat losses

Characteristics of the radiative heat flux from an expanding microchannel with combustion of a premixed mixture of gases are theoretically studied within the framework of a onedimensional diffusion-thermal model. The results obtained are generalized to the case of gas combustion in a porous medium consisting of a number of individual regularly packed microchannels. It is demonstrated that the radiative heat flux from subsurface layers of the porous body should be taken into account in calculating the efficiency of radiative porous burners and in modeling flame stabilization inside the porous medium.     

Flame Propagation in a Variable–Section Channel with Gas Filtration

A one–dimensional unsteady model is proposed, which describes gas–flame propagation in a narrow variable–section channel with a gas counterflow and takes into account heat propagation over the channel walls. The case of the channel cross section changing slowly at a distance of the order of the thermal thickness of the combustion wave is considered. It is shown that various regimes of flame propagation are possible in such a system: a regime of flame propagation with a high velocity (of the order of the burning velocity of the flame), a regime of flame propagation with a low velocity as in the case of filtrational gas combustion in a porous medium, and an intermittent regime of combustion, where the flame has a high velocity in the wide section of the channel and a low velocity in the narrow section. A simple analytical model of flame oscillations in such a system is constructed. The possibility of these oscillations was predicted by numerical simulation results. The simple model considered is an attempt to take into account the large–scale inhomogeneity of the porous medium in simulation of filtrational combustion of gases.     

Models of Filtration Combustion of Gases with Allowance for Flame Turbulence

The basic system of equations describing the combustion process inside a porous body as well as in the wave regime of filtration combustion of gases is refined on the basis of the Damköhler theory for turbulent burning rate. Better agreement of the models and available experimental data on the mass burning rate in such systems is obtained. The correction for turbulence of the interstitial flow becomes significant for filtration rates higher than 0.5 m/sec. The data analyzed are presented in the form of Borghi's diagrams. It is found that a weakly turbulent regime of a folded flame is typical of filtration combustion in gases in porous media with a mean pour size of 2–3 mm. Key words: turbulence, thermal wave, turbulent combustion regime, filtration wave, superadiabatic effect.     

Aging of reticulated Si-SiC foams in porous burners

Abstract

Abstract

Si-SiC open cell foams with porosity >87% and high pore sizes (4-7?mm) are commonly employed as active zone in porous burners for heat radiation applications. In a porous burner, the solid porous body let the heat recirculate from the hot combustion products to the incoming reactants. The result is that the flame is confined within the foam, meaning high thermomechanical loadings on its constituent material. A set of commercial Si-SiC foams from the same production batch was aged with flat porous burners. Thermal cycles ramp-up, dwell and cooling, as well as burner set-up (power: 15?kW, fuel/air ratio: 1·5), were chosen based on previous experience. Before aging, each foam was first cut in bars ready for bending tests, reassembled into the burner foam configuration and operated. As produced and aged samples were physically, mechanically and chemically analysed and results compared.     

Combustion of residual steel gases: laminar flame analysis and turbulent flamelet modeling

In recovery combustion systems operating in the steel industry, energy is provided by boilers burning residual gases of blast furnace and coke oven. To help understand combustion of this particular type of fuels, a numerical study is conducted where the major chemical properties of steel gas flames are collected. The chemical composition of representative fuel and oxidizer steel gas is varied over a large range in calculations using detailed chemistry and complex transport properties. The chemical equilibrium compositions, premixed flame speeds and diffusion flame extinction strain rates are determined. The advantages and shortcomings of the use of vitiated air emerge, and its introduction into the boiler appears as an interesting alternative to reduce NO_x emission. The detailed information obtained with laminar flame calculations is also introduced in flamelet turbulent combustion modeling. Reynolds Averaged Navier Stokes (RANS) simulations of a test case burner are performed and some comparisons between numerical predictions and experimental results are presented.     

Performance of a sonic burner system firing coal-oil mixtures

The development of coal—oil mixture technology is progressing at an accelerated rate and the aim of this work has been the development of a new burner system (termed the sonic burner) to increase lignite loadings in air-atomizing burner systems. The burner system developed incorporates a sonic velocity at the atomizing air throat and a fuel oil gun with a single, relatively large opening at its exit. Lignite loadings with No. 6 fuel oil have reached 60.0 wt%. The flame produced at these loadings was stable, compact and characteristic of a No. 6 fuel oil flame. Fuel line plugging did not occur at these high lignite loadings; however, pumping problems did preclude increased lignite loadings. Results of this study are directly applicable to higher rank coals. Although feasible from a combustion viewpoint, no inference should be made from these results concerning erosion, ash deposition, particulate control or pollution characteristics, the work reported being based solely on combustion characteristics.     

Investigation of the effects of several porosity variation profiles on performance and pollutants emission of the porous media burners

This paper presents results of numerical investigation on the effect of using variable porosity porous media burner on its performance and pollutant emission. A two‐dimensional axisymmetric model for premixed methane/air combustion in porous media has been developed. This code solves the continuity, Navier Stokes, solid and gas energies, and chemical species transport equations using the finite volume method. The pressure and velocity have been coupled with the SIMPLE algorithm. In this paper, the results of applying two different profiles of porosity instead of constant porosity for two zones of burner have been presented. The results showed that by applying porosity variation along the burner, the peak temperature can be decreased about 4.5%, and subsequently, the amount of exhaust pollutants such as NO_x can also be decreased while increase in pressure losses along the burner is negligible. In addition, the effects of excess air ratio, volumetric heat transfer coefficient, inlet velocity, chemical kinetics, conductivity coefficient, and wall temperature on the porous media burners with variation of porosity are investigated. Copyright © 2014 John Wiley & Sons, Ltd.     

旋转流中预混合火焰的高速传播现象——Ⅰ.稳燃特性及燃烧效率的提高

旋转流中预混合火焰高速传播特性的研究为强化低热值燃气的稳定燃烧提供了新的开发思路.针对圆管内强制涡作用下的甲烷/空气预混合火焰,本工作采用数值模拟方法分析了混合气的进口速度和旋转角速度对预混合火焰稳定燃烧的影响.结果表明,在不同当量比条件下,使火焰稳定的混合气进口速度和旋转角速度之间存在线性关系,但随着旋转角速度的增大,火焰半径变小,燃烧效率减小.改变混合气进口速度的分布形式是提高燃烧效率的有效方法.研究结果为实际的稳燃燃烧器设计提供了理论指导.     

旋转流中预混合火焰的高速传播现象——I.稳燃特性及燃烧效率的提高

旋转流中预混合火焰高速传播特性的研究为强化低热值燃气的稳定燃烧提供了新的开发思路.针对圆管内强制涡作用下的甲烷/空气预混合火焰,本工作采用数值模拟方法分析了混合气的进口速度和旋转角速度对预混合火焰稳定燃烧的影响.结果表明,在不同当量比条件下,使火焰稳定的混合气进口速度和旋转角速度之间存在线性关系,但随着旋转角速度的增大,火焰半径变小,燃烧效率减小.改变混合气进口速度的分布形式是提高燃烧效率的有效方法.研究结果为实际的稳燃燃烧器设计提供了理论指导.     

多孔介质内H2S超绝热燃烧制氢的数值模拟

为探索H₂S在多孔介质内超绝热燃烧裂解制硫制氢的机理,采用计算流体力学(CFD)与CHEMKIN相结合的方法,使用标准k-ε湍流模型和一个17组分、57步复杂化学反应机理,模拟了H₂S在直径为3 mm的Al₂O₃圆球堆积成的多孔介质内的燃烧,模拟结果与实验数据基本吻合.模拟结果显示：多孔介质内H₂S的燃烧温度超过了绝热燃烧温度,为H₂S的裂解制硫制氢提供高温环境,富燃条件下H₂S部分地裂解生成单质硫和氢气.另外,对采用的复杂化学反应机理是否适用于多孔介质内H₂S燃烧时各向异性火焰的模拟作了有意义的探索.     

自由堆积多孔介质内预混燃烧火焰传播

为了解多孔介质内预混燃烧火焰前沿的传播特性,对不同化学当量比（=0.7～1.0）的甲烷/空气预混气体在不同孔隙率（ε为0.37和0.42）的多孔介质内的火焰前沿传播特性进行了研究,多孔介质采用3 mm和6 mm直径的Al₂O₃小球在陶瓷管中堆积而成。结果表明,预混气体在多孔介质中能够形成低速燃烧的稳定燃烧波;其火焰传播速度随化学当量比增大而加快,最大的火焰传播速度为3.52×10^-3 cm·s^-1;多孔介质的结构对火焰前沿传播速度影响很大,即使在孔隙率差别不大的情况下,大球堆积而成的多孔介质比小球具有更高的火焰前沿传播速度。