Numerical investigation of the combustion performance in inert porous media (original) (raw)
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Combustion and Flame, 1999
A two-dimensional model of two simple porous burner geometries is developed to analyze the influence of multidimensionality on flames within pore scale structures. The first geometry simulates a honeycomb burner, in which a ceramic is penetrated by many small, straight, nonconnecting passages. The second geometry consists of many small parallel plates aligned with the flow direction. The Monte Carlo method is employed to calculate the viewfactors for radiation heat exchange in the second geometry. This model compares well with experiments on burning rates, operating ranges, and radiation output. Heat losses from the burner are found to reduce the burning rate. The flame is shown to be highly two-dimensional, and limitations of one-dimensional models are discussed. The effects of the material properties on the peak burning rate in these model porous media are examined. Variations in the flame on length scales smaller than the pore size are also present and are discussed and quantified. © 1998 by The Combustion Institute NOMENCLATURE A channel cross sectional area, m 2 a constant in heat capacity expression B constant in heat capacity expression c solid heat capacity, J/kg/K c p gas heat capacity at constant pressure, J/kg/K d porous medium square unit cell side length, m D mass diffusivity, m 2 /sec E chemical reaction activation energy, J/ mol F surface to burner end viewfactor h gas specific enthalpy, J/kg h c heat of combustion, J/kg fuel J radiosity, W/m 2 k thermal conductivity, W/m/K K surface to surface viewfactor ṁ mass flux, kg/m 2 /sec P pressure, Pa qЉ conduction/convection heat flux to surface, W/m 2 qٞ volumetric heat flux to solid, averaged over pore, W/m 3 R ideal gas constant, 8.314 J/mol/K s stoichiometric fuel/oxygen ratio t time, sec T temperature, K u axial gas velocity, m/sec U burning rate or volume flux, m/sec v ជ gas velocity, m/sec x axial distance y transverse distance y f fuel mass fraction y N nitrogen mass fraction y o oxygen mass fraction y p combustion products mass fraction Greek ␣ thermal diffusivity, m 2 /sec ⑀ surface emissivity equivalence ratio absolute viscosity, kg/m/sec density, kg/m 3 reaction rate, kg fuel/m 3 /sec
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This work analyses theoretically the excess enthalpy for laminar free flames and for flames within porous media in order to estimate the amount of excess temperature originated by the presence of the porous matrix and the factors that affect it. The analysis is based on the excess enthalpy function previously defined in the literature applied to the non-dimensional volumeaveraged equations for the combustion within an inert porous medium. Approximations for the reactants and the solid temperature profiles are assumed and the dependence of the excess enthalpy function on the problem parameters is accessed. The results obtained are in good qualitative agreement with numerical results. The excess enthalpy is shown to be a function of the gas Lewis number, the ratio of the solidand the gas-phasic effective thermal conductivities and the porosity of the medium.
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Progress in Energy and Combustion Science, 2010
Porous media combustion (PMC) has interesting advantages compared with free flame combustion due to higher burning rates, increased power dynamic range, extension of the lean flammability limits, and low emissions of pollutants. Extensive experimental and numerical works were carried out and are still underway, to explore the feasibility of this interesting technology for practical applications. For this purpose, numerical modeling plays a crucial role in the design and development of promising PMC systems. This article provides an exhaustive review of the fundamental aspects and emerging trends in numerical modeling of gas combustion in porous media. The modeling works published to date are reviewed, classified according to their objectives and presented with general conclusions. Numerical modeling of liquid fuel combustion in porous media is excluded.
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Numerical Computation of Reacting Flow in Porous Burners With an Extended CH
Volume 2, Parts A and B, 2004
The present paper presents, numerical computations for flow, heat transfer and chemical reactions in an axisymmetric inert porous burner. The porous media re-radiate the heat absorbed from the gaseous combustion products by convection and conduction. In the present work, the porous burner species mass fraction source terms are computed from an 'extended' reaction mechanism, controlled by chemical kinetics of elementary reactions. The porous burner has mingled zones of porous/nonporous reacting flow, i.e. the porosity is not uniform over the entire domain. Therefore, it has to be included inside the partial derivatives of the transport governing equations. Finite-difference equations are obtained by formal integration over control volumes surrounding each grid node. Up-wind differencing is used to insure that the influence coefficients are always positive to reflect the real effect of neighboring nodes on a typical central node. Finite-difference equations are solved, iteratively, for U, V, p'(pressure correction), enthalpy and species mass fractions, utilizing a grid of (60X40) nodes. The sixty grid nodes in the axial direction are needed to resolve the detailed structure of the thin reaction zone inside the porous media. The porous burner uses a premixed CH 4-air mixture, while its radiating characteristics are computed numerically, using a four-flux radiation model. Sixteen species are included, namely CH 4 , CH 3 , CH 2 , CH, CH 2 O, CHO, CO, CO 2 , O 2 , O, OH, H 2 , H, H 2 O, HO 2 , H 2 O 2 , involving 49 chemical reaction equations. It was found that 900 iterations are sufficient for complete conversion of the computed results with errors less than 0.1%. The computed temperature profiles of the gas and the solid show that, heat is conducted from downstream to the upstream of the reaction zone. Most stable species, such as H 2 O, CO 2 , H 2 , keep increasing inside the reaction zone staying appreciable in the combustion products. However, unstable products, such as HO 2 , H 2 O 2 and CH 3 , first increase in the preheating region of the reaction zone, they are then consumed fast in the postreaction zone of the porous burner. Therefore, it appears that their important function is only to help the chemical reactions continue to their inevitable completion of the more stable combustion products..
Combustion and Flame, 2015
This work is a comprehensive study of combustion in porous inert media (PIM) with focus on the effects of pressure and air/fuel equivalence ratio. Experiments on lean flame stability limits, flame stabilization and temperature profiles in porous inert media burners of different geometry have been carried out at nearly adiabatic conditions. The experiments were complemented by numerical simulations of combustion in porous inert media, employing a volume-averaged 1D model and 3D direct pore level simulation (3D DPLS) on real geometries of sponge like structures. A definition of the burning velocity is proposed to compare the geometrically different burners investigated. The numerically obtained macroscopic thermal flame thicknesses and the burning velocities are compared with that of free flames. The results show a non-monotonic dependence of the burning velocity on pressure considerably affected by the air/fuel equivalence ratio, which is well depicted by the models used in this work with some identified limitations. By performing a parametric study with the 1D model, dispersion of heat and mass has been identified as the major responsible mechanism of increasing of burning velocity with pressure. The DPLS results showed considerable temperature variations along the space coordinate, which were not considered up to now in 1D models when calculating reaction rates and which could be the reason of the observed limitations.