Multimode heat transfer in cyclic flow reversal combustion in a porous medium

Experimental and numerical studies of combustion and multimode heat transfer in a porous medium, with and without a cyclic flow reversal of a mixture through a porous medium, were performed. Parametric studies were done in order to understand combustion characteristics such as maximum flame temperature and radiative heat flux using a one‐ dimensional conduction, convection, radiation and premixed flame model. The porous medium was assumed to emit and absorb radiant energy, while scattering is ignored. Non‐local thermodynamic equilibrium between the solid an d gas is taken into account by introducing separate energy equations for the gas and the solid phase. As a prelimina ry study, the combustion regime was described by a one‐step global mechanism with an internal heat source uniformly dist ributed along the reaction zone. The effects of the flame position, cyclic flow reversal, period of the cyclic flow rever sal, the optical thickness and the flow velocity on the burner performance were clarified by a rigorous radiation analysis. Th e model was validated by comparing the theoretical results with the experiments. It was shown that, for maximizing the fl ame temperature and the net radiative heat flux feedback, the flame should be stabilized near the centre of the po rous medium with a cyclic flow reversal, the period of which should be as small as possible. A high optical thickness prod uced a high flame temperature and a high net radiative feedback. Also, a high flow velocity at low period of the cyclic f low reversal of mixture yielded a high value of both the flame temperature and the net radiative feedback. Thermal structure predictions in terms of the gas‐phase and the solid‐phase temperature distributions along the axis of the combustor show good agreement with the experimental ones. Copyright © 1999 John Wiley & Sons, Ltd.     

惰性多孔介质内天然气预混燃烧的数学模拟评述

天然气在惰性多孔介质内的预混燃烧是一个包含燃烧、辐射、对流及导热的复杂过程，从数学模拟的角度，比较了几种不同的甲烷-空气化学反应模型，研究了多孔介质内辐射传递方程的不同求解方法，并且分析了多孔介质的导热系数、对流换热系数等对燃烧器性能的影响。     

Radiation effects on flame stabilization on flat flame burners

This study investigates the impact of radiative heat transfer on the behavior of flat flame burners within the framework of a simplified one-dimensional model. Flat flame burners stabilize planar premixed flames downstream of a porous plug. Within this study, the porous plug is modeled as a thermally conducting, optically thick medium, allowing for both conductive and radiative heat transfer. Based on the simplified model, the impact of radiative heat exchange between the porous plug exit and the downstream environment is investigated. In “surface” combustion, flame stabilization occurs due to heat transfer between gas phase and porous solid. Results demonstrate that radiative heat transfer from a hot downstream environment to the porous plug significantly increases maximum attainable mass fluxes. For a cold downstream environment, plug properties do not affect the maximum supportable mass flux, although plug porosity and heat transfer between gas and solid have a significant impact on the “stand-off” distance between flame and plug exit. In addition, the model provides insight to a second “submerged” combustion mode, where the flame is stabilized within the porous plug of the burner. Here, increased flame temperatures lead to a dramatic increase of the maximum supportable mass flux. Overall, results show that radiative heat losses play a critical role in both combustion modes: in surface combustion, they are an important mode of heat dissipation, where they can prevent “flash-back” conditions with the flame moving into the porous matrix; in submerged combustion, they prevent flame stabilization close to inlet and exit faces and enable a “slow” solution branch that does not exist without radiative losses.     

Combustion of suspended fine solid fuel in air inside inert porous medium: A heat transfer analysis

Present work is a numerical analysis of combustion of submicron carbon particles inside an inert porous medium where the particles in form of suspension in air enter the porous medium. A one-dimensional heat transfer model has been developed using the two-flux gray radiation approximation for radiative heat flux equations. The effects of absorption coefficient, emissivity of medium, flame position and reaction enthalpy flux on radiative energy output efficiency have been presented. It is revealed that in porous medium the combustion of suspended carbon particles is similar to premixed single phase gaseous fuel combustion except the former has shorter preheating temperature zone length. Use of porous ceramic having high porosity and made of Al₂O₃ or ZrO₂ with stabilized flame position operated nearer to downstream end will ensure radiative output maximum and minimum at downstream and upstream end, respectively.     

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

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

Modeling of a conceptual self-sustained liquid fuel vaporization – combustion system with radiative output using inert porous media

The present model is based on a combined self-sustained liquid fuel vaporization – combustion system, where the liquid fuel vaporization occurs on a wetted wall plate with energy transferred through the plate from the combustion of vaporized oil. The vaporization energy has been derived through the radiative interaction of the vaporizing plate and an upstream end surface of the porous medium. The inert porous medium, used in the flow passage of combustion gas, is allowed to emit and absorb radiant energy. The radiative heat flux equations for the porous medium have been derived using the two-flux gray approximation. The work analyzes the effect of emissivities of vaporizing plate and porous medium, the optical thickness of medium and equivalence ratio on the kerosene vaporization rate, combustion temperature and radiative output of the system. Combination of low and high emissivities of vaporizing plate and porous medium respectively with low optical thickness of medium makes the system operable over a wide range of power. The study covers the data concerning the design and operating characteristics of a practical system.     

A universal model of opposed flow combustion of solid fuel over an inert porous medium

In this study, a universal model is developed to examine the behavior of combustion wave observed in porous solid matters (e.g., smoldering, self-propagating high-temperature synthesis (SHS), diesel particulate filter (DPF) regeneration process). Analytical expressions of the combustion characters of solid combustible (e.g., diesel particulate matters trapped in a DPF) deposited over an inert porous medium are obtained employing large activation energy asymptotic taking into account the sensible transport processes; namely, heat transfer between the porous medium and gas phases, radiation heat transfer from the porous medium, heat loss from the porous medium to the environment, mass transfer of oxygen from the gas stream to the surface of solid fuel and the effective diffusion in modeling the species diffusion. Then it has been validated that the present model is applicable and adaptable for predicting the characteristics of smoldering combustion and thus SHS process. As a result, the features of combustion wave of the present phenomena would be useful to other processes. From practical point of view and for deep understanding of the behavior of combustion wave of these processes, we investigate the effects of various physical parameters over a wide range of conditions. We observe that the moving speed of the reaction front increases with the increase of porosity of the porous medium, mass transfer coefficient and initial fuel mass fraction; while it decreases owing to the increase of heat transfer rate from the porous medium to the gas, heat loss to the environment and radiative heat transfer. Furthermore, the results reveal that extinction tends to occur due to lower porosity of the porous medium, higher radiative heat transfer from the porous medium, higher heat transfer rate from the porous medium to the gas and higher heat losses from the porous medium to the environment. Even the observed near-extinction behavior in reaction front speed versus heat loss diagram is found to be similar what we got in gaseous premixed flame propagating through the porous medium. An extinction limit diagram has been presented as a function of radiation-conduction parameter and the gas flow velocity. In addition to, the impact of radiation and the combined effect of the inclusion of Knudsen diffusion and tortuosity are demonstrated in terms of the spatial temperature and species profiles to examine how these two parameters modify the reaction front structure. Furthermore, the governing equations have been solved numerically and it is observed that asymptotic analysis gives a good agreement with the numerical solution.     

切圆燃烧锅炉炉膛传热过程综合模型及模拟计算

本文针对大容量煤粉锅炉的特点,对炉内各过程的数值模拟提出切合实际简化模型,用假想面有效辐射分析法模型计算内辐射传热,应用理想反应器的串并联网络模拟炉内宏观流动－燃烧过程,从而建立了适合于任意形状炉膛的传热计算综合模型,通过对一台６７０Ｔ／Ｈ锅炉进行的实例计算,对模型的准确性和实用性进行了初步验证,得到了较满意的结果。     

Nonequilibrium in the transport of heat and reactants in combustion in porous media

Combustion in inert, catalytic and combustible porous media occurs under the influence of a large range of geometric length scales, thermophysical and thermochemical properties, and flow, heat and mass transfer conditions. As a result, a large range of phenomenological length and time scales control the extent of departure from local thermal and chemical nonequilibrium. The use of intraphase and interphase nonequilibria have allowed for the design of new combustion processes and systems, such as, catalytic reactors and converters, porous radiant burners, direct energy and gas conversion devices and systems, chemical sensors, and material synthesis processes. Improvement of current and design of yet newer and more innovative systems requires further investigations into the gas-phase and surface chemistry, solid-state and condensed-phase physics, transport in disordered structures, and mathematical and numerical methods. Here we summarize the processes leading to thermal and chemical nonequilibrium, their role in the combustion in porous media, their innovative uses and effects on applications, the current modeling of these processes and the modeling techniques that may allow for further improvements and developments.     

Coupled large eddy simulations of turbulent combustion and radiative heat transfer

Radiative heat transfer is known to play an important role in combustion processes but is often neglected in simulations because of its complexity and the related numerical costs. An original approach is proposed here to perform large eddy simulations of turbulent combustion including radiation: unsteady combustion and radiative heat transfer are computed by two independent codes that exchange data when needed through a specialized language, CORBA, working on an internal computer network or over the Internet. The radiation code gets temperature and mass-fraction fields from the combustion code and returns radiative energy source terms. This coupling technique is easy to implement, portable, flexible, and versatile. Each code keeps its own structure and may be developed and optimized independently, especially when running on massively parallel machines. Preliminary results show that taking radiative heat transfer into account strongly modifies the flame dynamics, probably because the burnt gas temperature decreases, making the flame stabilization weaker and increasing the flame sensitivity to turbulent motions. Comparisons with experimental data are very encouraging.     

Radiative heat transfer from potassium seeded water gas combustion plasma

Radiative heat transfer calculations from a potassium seeded water gas combustion plasma have been made to estimate the radiative heat losses through the walls of a MHD channel. Both molecular combustion products and seed contribute significantly to the total radiation loss from a plasma. The spectral emission properties of CO₂, H₂O, CO and potassium have been taken into account. It has been shown that the contribution of CO to heat flux is very small and, thus, can be neglected. CO₂ and H₂O are the primary contributors to the radiation from the combustion products. At MHD temperatures, 55–80% of the contribution to heat flux from the combustion products comes from bands lying up to 2.7 μm in the near infrared. It has been shown that accurate knowledge of absorption cross-section data is essential to predict the radiative heat transfer from potassium. It has been estimated that 25–30% of the total radiative heat flux is from the potassium seed.     

Simultaneous estimation of four parameters in a combined-mode heat transfer in a 2D porous matrix with heat generation

This article reports a study on simultaneous estimation of four parameters for combined-mode conduction and radiation heat transfer in a 2D rectangular porous matrix with a localized volumetric heat generation source. Air flows at uniform velocity through the conducting and radiating porous matrix. In the heat generation zone, and its downstream, the gas temperature is higher than that of the solid, and in the upstream the reverse situation occurs. This temperature difference between gas and the solid results in heat transfer by convection between the two phases, and the analysis thus requires consideration of separate energy equations for the two phases. The solid being involved radiatively, the volumetric radiative source term, in the form of the divergence of radiative heat flux, appears only in the solid-phase energy equation. The two equations are coupled through the convective heat transfer term. Four parameters—scattering albedo, emissivity, solid conductivity, and heat transfer coefficient—are simultaneously estimated based on the solid and gas temperature distributions, and convective and radiative heat fluxes at the outer surface of the porous matrix. In both direct and inverse approaches, the energy equations are solved using the finite volume method. For a test case, determining the genetic algorithm is much more time-consuming than the global search algorithm; in other cases, parameter estimations are done using the global search algorithm. Parameters are found to be estimated accurately.     

Modeling of trickle flow liquid fuel combustion in inert porous medium

Present work is a numerical analysis of fuel oil combustion inside an inert porous medium where fuel oil flows through the porous medium under gravity wetting its solid wall with concurrent movement of liquid fuel and air under steady state conditions. A one-dimensional heat transfer model has been developed under steady state conditions using a single step global reaction mechanism. The effects of optical thickness, emissivity of medium, flame position and reaction enthalpy flux on radiation energy output efficiency as well as the temperature, position and thickness of vaporization zone have been presented using kerosene as fuel. Low values of optical thickness and emissivity of porous medium will ensure efficient combustion, maximize downstream radiative output with minimum upstream radiative loss.     

Combustion of liquid fuel inside inert porous media: an analytical approach

This paper presents a numerical analysis of combustion of liquid fuel droplets suspended in air inside an inert porous media. A one-dimensional heat transfer model has been developed assuming complete vaporization of oil droplets prior to their entry into the flame. The effects of absorption coefficient, emissivity of medium, flame position on radiative energy output efficiency and optimum oil droplet size at the entry, defined as the maximum size for complete vaporization before entering the combustion zone, have been presented. The inert porous medium with low absorption coefficient will produce high downstream radiative output with large oil droplet sizes.     

Experimental research of radiative heat transfer in fluidized beds

A new method to measure the radiative heat transfer in fluidized beds was presented. Experiments were carried out on a 0.8 th⁻¹ fluidized bed combustion boiler. The residual slag of fired coal was operated in a fluidized bed at room temperature. As the radiative heat transfer at room temperature is insignificant, its contribution at high temperatures might be obtained by the comparison of experimental results at high and low temperatures. On experimental study, a radiative contribution was given as a function of bed temperature and particle size. The results were compared with those in other references.     

Enhancement of heat transfer: Combined convection and radiation in the entrance region of circular ducts with porous inserts

Using porous ceramic inserts in high temperature equipment has been proven to be an effective means to enhance combined convective–radiative heat transfer. The porous ceramic insert was referred to as a convection-to-radiation converter (CRC) by previous investigators. We consider a novel application of CRC cores in a partial by pass flow system for heat transfer enhancement. Both hydrodynamically and thermally developing laminar flow is considered in the entrance region of a circular pipe with a porous insert located at the center. The momentum and Darcy–Brinkman equations are applied to the flow field in the annular gas layer and central porous layer respectively. The energy equation is coupled with the radiative transfer equation by the radiation source term. The radiative transfer is simulated by the newly developed integral equations [X.L. Chen, W. Sutton, Radiative transfer in finite cylindrical media using transformed integral equations, J. Quant. Spectrosc. Radiat. Transfer 77 (3) (2003) 233–271; W. Sutton, X.L. Chen, A general integration method for radiative transfer in 3-D non-homogeneous cylindrical media with anisotropic scattering, J. Quant. Spectrosc. Radiat. Transfer 84 (2004) 65–103] to avoid singularity problem and give high accuracy. The working fluid and porous medium are both considered as participating media. Finally, this highly non-linear system of equations is solved by a mixed iteration method. The results are compared between the cases with and without the porous insert. The porous insert enhances both convective and radiative transfer by about 35% and 105% respectively at the most. The effects of important parameters on this enhancement are discussed in detail.     

Effect of thermal boundary layer on radiative heat transfer from combustion plasma

The effect of thermal boundary layer on radiative heat transfer considering nongray nonisothermal plasma has been calculated for potassium seeded watergas combustion plasma. The effect of combustion species concentration and seed concentration on radiative flux under the equilibrium flow and frozen flow condition has been studied. It has been estimated that reduction in radiative flux due to cold boundary layer may be upto 25%.     

Thermal analysis of a 2-D heat recovery system using porous media including lattice Boltzmann simulation of fluid flow

The present work deals with the fluid flow simulation and thermal analysis of a two-dimensional heat recovery system using porous media. A basic high-temperature flow system is considered in which a high-temperature non-radiating gas flows through a random porous matrix. The porous medium, in addition to its convective heat exchange with the gas, may absorb, emit and scatter thermal radiation. It is desirable to have large amount of radiative heat flux from the porous segment in the upstream direction (towards the thermal system). The lattice Boltzmann method (LBM) is used to simulate fluid flow in the porous medium. The gas and solid phases are considered in non-local thermal equilibrium, and separate energy equations are applied to these phases. Convection, conduction and radiation heat transfers take place simultaneously in solid phase, but in the gas flow, heat transfer occurs by conduction and convection. In order to analyze the thermal characteristics of the heat recovery system, volume-averaged velocities through the porous matrix obtained by LBM are used in the gas energy equation and then the coupled energy equations for gas and porous medium are numerically solved using finite difference method. For computing of radiative heat flux in the porous medium, discrete ordinates method is used to solve the radiative transfer equation. Finally the effect of various parameters on the performance of porous heat recovery system is studied.     

Prediction of radiative heat transfer between two concentric spherical enclosures with the finite volume method

The radiative heat transfer between two concentric spheres separated by an absorbing, emitting, and isotropically scattering gray medium is investigated by using the finite volume method (FVM). Especially, a mapping that simplifies the solution of spherically symmetric radiative heat transfer problems is introduced, thereby, the intensity depending on spatial one-dimension and angular one-dimension is transformed into spatial two-dimensional one. By adopting this mapping process, angular redistribution, which appears in such curvilinear coordinates as cylindrical or spherical ones, is treated efficiently without any artifice usually introduced in the conventional discrete ordinates method (DOM). After a mathematical formulation and corresponding discretization equation for the radiative transfer equation (RTE) are derived, final discretization equation is introduced by using the directional weight, which is the key parameter in the FVM since it represents the inflow or outflow of radiant energy across the control volume faces depending on its sign. The present approach is then validated by comparing the present results with those of previous works by changing such various parameters as temperature ratio between inner and outer spherical enclosure, wall emissivity, and optical thickness of the participating medium. All the results presented in this work show that the present method is accurate and valuable for the analysis of spherically symmetric radiative heat transfer problems between two concentric spheres.     

Radiative heat transfer during turbulent combustion process

We investigate the radiative heat transfer in a co-flowing turbulent nonpremixed propane-air flame inside a three-dimensional cylindrical combustion chamber. The radiation from the luminous flame, which is due to the appearance of soot particles in the flame, is studied here, through the balance equation of radiative transfer which is solved by the Discrete Ordinates Method (DOM) coupling with a Large Eddy Simulation (LES) of the flow, temperature, combustion species and soot formation. The effect of scattering is ignored as it is found that the absorption dominates the radiating medium. Assessments of the various orders of DOM are also made and we find that the results of the incident radiation predicted by the higher order approximations of the DOM are in good agreement.