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.     

Study on the effect of micro geometric structure on heat conduction in porous media subjected to pulse laser

Based on fractal theory, different two-dimensional fractal structures were constructed to simulate the practical porous media. Effective thermal conductivity for porous media was calculated by means of the finite volume method. Theoretical analysis of thermal response in the porous media under various heating conditions was performed with a multi-layer hyperbolic heat conduction model with volumetric heat generation. The results obtained in this paper indicate that pore size and micro distribution have a far-reaching impact on the heat conduction in porous media. If we assumed that both the thermal conductivity and the heat capacity of the solid phase is larger than those of liquid phase, decreasing the pore size and porosity is helpful to enhance the heat transfer in porous media and the peak of temperature increases with pore size and porosity. With the same pore size and porosity, the effect of the pore micro-geometric distribution on heat conduction in porous media is obvious. The method presented in this paper may suggest a valuable approach to theoretically evaluate the effect of pore micro-geometric structure on heat conduction in porous media.     

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.     

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.     

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.     

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.     

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.     

Laser-based investigations of flow fields and OH distributions in impinging flames of domestic cooker-top burners

A thermal performance of impinging flames of widely-used cooker-top burners depends on a complex interaction between flow field and combustion. The critical situation is placed on two typical cooker-top burners having almost the same configurations, but yielding about 10% point difference in thermal efficiency. Therefore, in situ flame measurements of averaged velocity using particle image velocimetry (PIV) and image of averaged OH radicals using planar laser-induced fluorescence (OH-PLIF) are required to obtain insight into the coupling mechanisms during the combustion and heat transfer, and to develop a comprehensive experimental data base to support a phenomenological understanding of the differences in flames of the two burners. A combined study of flow field and combustion forms the basis of a much better understanding of the casual links that couple fluid mechanics, combustion and heat transfer in the two burners. This can provide guidelines for efficient burner design to meet future regulatory requirements.     

Numerical calculations of the thermal conductivity of porous ceramics based on micrographs

The overall thermal conductivity of a porous material is strongly sensitive to the volume fraction and spatial distribution of the pores. For this second aspect analytical models predicting thermal conductivity as a function of pore volume fraction are obliged to make a simplifying assumption concerning the pore shape. In order to describe the effects of the microstructure on heat transfer in greater detail, we have developed a method involving 2D finite element calculations based on real micrographs of the porous solid. The approach was tested on micrographs of tin oxide samples with pore contents from 10% to 50%. Quantitative results obtained for pore contents up to 20% give very good agreement to Rayleigh's model. Higher pore contents lead to a number of difficulties but the qualitative results are used to support the choice of Landauer's effective medium expression as an appropriate general analytical model for the thermal conductivity of a porous ceramic material.     

Combustion characteristics and flame stability at the microscale: a CFD study of premixed methane/air mixtures

A two-dimensional elliptic, computational fluid dynamics (CFD) model of a microburner is solved to study the effects of microburner dimensions, conductivity and thickness of wall materials, external heat losses, and operating conditions on combustion characteristics and flame stability. We have found that the wall conductivity and thickness are very important as they determine the upstream heat transfer, which is necessary for flame ignition and stability, and the material's integrity by controlling the existence of hot spots. Two modes of flame extinction occur: a spatially global type for large wall thermal conductivities and/or low flow velocities and blowout. It is shown that there exists a narrow range of flow velocities that permit sustained combustion within a microburner. Large transverse and axial gradients are observed even at these small scales under certain conditions. Periodic oscillations are observed near extinction in cases of high heat loss. Engineering maps that delineate flame stability, extinction, and blowout are constructed. Design recommendations are finally made.     

High porous yttria-stabilized zirconia with aligned pore channels: Morphology directionality influence on heat transfer

High porous yttria stabilized zirconia with unidirectionally aligned channels is used in engineering applications with extremely low thermal conductivity. This property is strongly influenced by microstructure features such as pore volume fraction, pore size distribution, random porous microstructure and pore morphology directionality. Although several models are reported in the available literature, but their analytical formulas are formalised for homogeneous structures or they are based on proportion between solid and fluid phases. These differences from real microstructures cause significant computational errors especially when thermal conductivity changes as the function of the measurement direction (parallel or perpendicular). In this context, the application of an intermingled fractal unit's procedure capable of reproducing porous microstructure as well as predicting thermal conductivity has been proposed. The results are in agreement with experimental ones measured for parallel and perpendicular directions and suggest improving the formalisation of fractal modelling in order to obtain an instrument of microstructure design.     

固体火箭发动机的热安全性研究

采用带源项的热传导方程,对固体火箭发动机在外界热源作用下的加热过程进行了数值模拟,分析了固体发动机内推进剂在外界热源作用下的燃烧特点,并确定了发动机产生热危险性的临界温度和起始燃烧时间。研究结果表明,在热传导方程中加入化学反应源项,可以有效地模拟发动机在外界热源作用下的加热过程;推进剂产生热危险性的临界温度为520～525K;在外界火焰作用下,发动机内的推进剂将点火燃烧,随着外界火焰温度的上升,推进剂起始燃烧的延迟时间减少。     

Modelling of batch fluidised bed drying of pharmaceutical granules

This paper presents a mathematical model based on a three-phase theory, which is used to describe the mass and heat transfer between the gas and solids phases in a batch fluidised bed dryer. In the model, it is assumed that the dilute phase (i.e., bubble) is plug flow while the interstitial gas and the solid particles are considered as being perfectly mixed. The thermal conductivity of wet particles is modelled using a serial and parallel circuit. The moisture diffusion in wet particles was simulated using a numerical finite volume method. Applying a simplified lumped model to a single solid particle, the heat and mass transfer between the interstitial gas and solid phase is taken into account during the whole drying process as three drying rate periods: warming-up, constant rate and falling-rate. The effects of the process parameters, such as particle size, gas velocity, inlet gas temperature and relative humidity, on the moisture content of solids in the bed have been studied by numerical computation using this model. The results are in good agreement with experimental data of heat and mass transfer in fluidised bed dryers. The model will be employed for online simulation of a fluidised bed dryer and for online control.     

Heat transfer in high porous alumina: Experimental data interpretation by different modelling approaches

Advanced porous ceramics are a remarkable class of materials with important applications in engineering fields. Porosity features have received wide attention for their capability to influence all properties. In this paper, the correlation between pore structure and heat transfer has been studied. Different analytical procedures found in literature as well as an Intermingled Fractal Units’ model are proposed. Models predictions are compared with experimental data. It has been observed that IFU is particularly suitable to predict thermal conductivity values very close to experimental ones. This fact is related to its capability to replicate porous microstructures in terms of pore volume fraction, pore size range and pore size distribution.     

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.     

Analysis of solar‐driven gasification of biochar trickling through an interconnected porous structure

The efficient transfer of high‐temperature solar heat to the reaction site is crucial for the yield and selectivity of the solar‐driven gasification of biomass. The performance of a gas‐solid trickle‐bed reactor constructed from a high thermal conductivity porous ceramic packing has been investigated. Beech char particles were used as the model feedstock. A two‐dimensional finite‐volume model coupling chemical reaction with conduction, convection, and radiation of heat within the packing was developed and tested against measured temperatures and gasification rates. The sensitivity of the gasification rate and reactor temperatures to variations of the packing's pore diameter, porosity, thermal conductivity, and particle loading was numerically studied. A numerical comparison with a moving bed projected a more uniform temperature distribution and higher gasification rates due to the increased heat transfer via combined radiation and conduction through the trickle bed. © 2014 American Institute of Chemical Engineers AIChE J, 61: 867–879, 2015     

单相流体通过多孔金属换热器换热性能的理论分析

泡沫金属具有非常大的比表面积和良好的导热性能,在强迫对流情况下,具有很强的换热能力.通过建立泡沫金属在强迫对流的情况下的换热模型,得到了泡沫金属高度、孔密度、孔隙率和空气流速的变化对其换热性能的影响.分析结果显示:增大泡沫金属换热器的高度、孔密度、空气流速和减小孔隙率,都能提高换热器的换热性能;当这4个参数各自变化到一...     

Influence of pore distribution on the equivalent thermal conductivity of low porosity ceramic closed-cell foams

The microstructures of porous alumina materials with different porosities were established by introducing the departure factor of pore position and acentric factor of pore diameter to describe the distribution of pores in space and in size, respectively. The contribution of radiation and influence of pore distribution on the equivalent thermal conductivity were discussed based on numerical simulations by the finite volume method (FVM) considering both thermal conduction and radiation. When the pore diameter was less than 10?µm, the radiation component was less than 2%, and radiation could be neglected. Radiative heat transfer played a dominant role for materials with high porosity and large pore size at high temperatures. For micro pore materials (<?100?µm), broad pore size and non-uniform pore space distribution decreased the thermal conductivity across the entire temperature range. For materials with macro pores (>1?mm), broad pore distribution decreased the thermal conductivity at low temperatures and increased it at high temperatures. The basic prediction model of effective thermal conductivity for a two-component material, the Maxwell–Eucken model (ME1) and its modified model were corrected by introducing the pore structure factor. The results from experiments prove that the numerical values were satisfactory.     

Temperature excursions during the transient behaviour of high temperature catalytic combustion monoliths

Spatio-temporal temperature profiles experimentally observed during the operation of a structured perovskite-based catalytic monolith for the combustion of methane were simulated by means of a transient heterogeneous one-dimensional model which accounts for heat losses to the surroundings and thermal conductivity inside the monolith substrate. In fact it is shown that such thermal phenomena are essential to correctly reproduce the slow warm-up transients and the wrong-way behaviour observed in a low conductivity ceramic monolith. Model simulations reveal that at higher values of solid conductivity the catalyst does not exhibit hot spot both in time and space, but needs higher inlet gas temperatures to reach complete methane conversion. Bifurcation analysis of the catalytic monolith reactor model were carried out to study steady-state multiplicity. We show that the origin of multiplicity is purely thermal as it is mainly ruled by the heat transfer through the external surface.     

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.