Stabilization and extinction of diffusion flames in an inert porous medium
01 Jan 2017-Vol. 36, Iss: 1, pp 1485-1493
TL;DR: In this paper, the authors studied the extinction conditions for diffusion flames in inert porous media and analyzed the effects of the heat exchange between gas and solid phases on the flame structure and found that when the heat removed from the flame by the solid matrix is large, the flame can extinguish because the lowering in the flame temperature leads to increasingly large leakage of reactants through the flame sheet.
Abstract: Diffusion flames established in inert porous media have been reported to present temperatures lower than a comparable gaseous mixture. Therefore, the study of the flame structure, temperature and extinction limits of confined diffusion flames is of importance. In the present work we discuss extinction conditions for such flames. Using an asymptotic model that accounts for the excess/deficient enthalpy at the reaction region, we study the multiscale problem and analyze the effects of the heat exchange between gas and solid phases on the flame structure. When the heat removed from the flame by the solid matrix is large, the flame can extinguish because the lowering in the flame temperature leads to increasingly large leakage of reactants through the flame sheet. We show that this occurs when the porosity or the mass injection rate is small enough. The extinction limit associated with a small value of the mass injection rate adds to the kinetic extinction limit (which is associated with a large value of the mass injection rate) to characterize a dual-extinction-point behavior for this problem. When the porosity of the medium reaches a minimum critical value, these two distinct extinction points collapse, such that for porosities lower than the critical porosity no flame can be established inside the porous chamber. Then, it is possible to construct a flammability map for the confined diffusion flame, where the critical porosity defines an absolute flammability limit.
TL;DR: In this article, a numerical model was established to analyze the combustion characteristics of blast furnace gas and solved by using the finite volume method, which was found that as increase of the distance between the porous body and the burner inlet, the temperature of the gas and the flame length increased and then decreased.
Abstract: This paper studied the combustion of blast furnace gas in a porous media burner. A numerical model method was established to analyze the combustion characteristics of blast furnace gas and solved by using the finite volume method. The temperature distribution, the position of the ignition, the flame structure, the recirculation zone and the concentration of the fuel were analyzed in the furnace. It was found that as increase of the distance between the porous body and the burner inlet, the temperature of the gas and the flame length increased and then decreased. When the porous media was located at 0.14 m in the burner, the peak temperature of the smoke gas was 1598 K at 0.36 m and the flame length was up to 0.41 m. With the porous media away from the burner nozzle, the ignition position was first away from the nozzle and then stabilize. The heat recirculation area increased and then stabilized. The position of the ignition was stable at 0.11 m. The flame length and flame radiation surface were wider and the oxygen concentration was the lowest at the exit, only 0.0043, when porous media was at 0.14 m.
TL;DR: In this paper, the influence of bed length on CO and NOx emissions for different excess air ratios (α ) and gas mixture velocities (ug,in) was investigated in a plane-parallel packed bed filled with 2.5mm alumina pellets.
Abstract: Gas diffusion combustion in a plane-parallel packed bed filled with 2.5 mm alumina pellets is experimentally and numerically investigated. Special attention is focused on the influence of bed length (h) on the CO and NOx emissions for different excess air ratios ( α ) and gas mixture velocities (ug,in). It is found that h has significant influence on NOx emission when h 120 mm. Increase in h always leads to an linear increases in CO emission over the bed length range from 40 mm to 200 mm regardless of α and ug,in. For the lowest α (1.88), good performance regarding CO emission is observed, CO emission increases from 2 ppm to 49 ppm when h increases from 40 mm to 200 mm. Moreover, wide high temperature zone of the external combustor walls indicates that it is feasible to develop radiative burner based on diffusion filtration combustion.
TL;DR: In this article, the effects of porous media porosity and gas velocity on temperature distribution and CO and NOx emissions were investigated in a round-jet burner with 3 mm alumina pellets.
Abstract: Methane-air diffusion filtration combustion in a radiative round-jet burner was numerically investigated in this work. The purpose of this study was focused on the effects of porous media porosity and gas velocity on temperature distribution and CO and NOx emissions. Reduced chemical kinetics was used where air and methane were assumed to be at their stoichiometric ratio, while thermo-physical properties were varied per the solid matrix porosity variation. Combustion characteristics were evaluated based on conduction and radiation as the two primary heat transfer modes within the solid matrix. Numerical simulations were carried out based on a packed bed with 3 mm alumina pellets. Results show that combustion temperature increases while the temperature gradient decreases with the increase in porosity, yielding higher NOx, and lower CO emissions. Furthermore, the combustion temperature is the lowest and most uniformly distributed with 1 m/s and 3 m/s gas velocities, wherewith 3 m/s gas velocity, combustion occurs outside of the porous zone. The corresponding NOx and CO emissions are the lowest with 1 m/s gas velocity and increase with the increase in gas velocity from 1 m/s to 10 m/s.
TL;DR: Results show that both immersed and surface flames coexist in the combustor, and although porous media enhance the mixing and diffusion processes, the diffusion flame shape is still observed from the side and top views of the combustors, and the diffusion filtration retains properties of diffusion combustion.
Abstract: Experimental and numerical studies were conducted to determine the combustion characteristics of gas diffusion combustion in a porous combustor packed with 2.5 mm or 3.5 mm alumina pellets, special...
TL;DR: In this article, a numerical study is carried out to investigate the stabilization and dynamic properties of the edge flame formed in the wake of two merging streams, one containing fuel and the other oxidizer, separated by a splitter plate.
Abstract: A numerical study is carried out to investigate the stabilization and dynamic properties of the edge flame formed in the wake of two merging streams, one containing fuel and the other oxidizer, separated by a splitter plate. Several plates are considered to illustrate the effects of their thermo-physical properties on the edge flame for low- and high-Lewis-number mixtures. The objective is to provide a comprehensive understanding of the effects of the heat recirculation cycle, from the edge flame through the splitter plate and back to the fresh reactants, on the edge flame. A diffusive-thermal model is adopted, with the flow field determined by solving the incompressible Navier–Stokes equations in the vicinity of the plate trailing edge, and the combustion field determined by solving the transport equations with a constant density. Two distinct modes of flame stabilization are identified and examined: a stationary mode, where the edge flame is held stationary at a well-defined distance, whether attached to or lifted from the tip of the plate, and an oscillatory mode where the edge flame undergoes sustained oscillations relative to an (unstable) equilibrium position. To characterize the heat recirculation effect under varying flow/mixture conditions, we introduce the thermal sensing distance, as a heuristic parameter that determines whether substantial thermal interaction between the edge flame and the plate occurs, and the average output heat flux, as a universal measure characterizing the efficiency of the heat recirculation cycle.
TL;DR: In this paper, the structure of steady state diffusion flames is investigated by analyzing the mixing and chemical reaction of two opposed jets of fuel and oxidizer as a particular example, and an Arrhenius one-step irreversible reaction in the realistic limit of large activation energies.
Abstract: The structure of steady state diffusion flames is investigated by analyzing the mixing and chemical reaction of two opposed jets of fuel and oxidizer as a particular example. An Arrhenius one-step irreversible reaction has been considered in the realistic limit of large activation energies. The entire range of Damkohler numbers, or ratio of characteristic diffusion and chemical times, has been covered. When the resulting maximum temperature is plotted in terms of the Damkohler number (which is inversely proportional to the flow velocity) the characteristic S curve emerges from the analysis, with segments from the curve resulting from: 1. (a) A nearly frozen ignition regime where the temperature and concentrations deviations from its frozen flow values are small. The lower branch and bend of the S curve is covered by this regime. 2. (b) A partial burning regime, where both reactants cross the reaction zone toward regions of frozen flow. This regime is unstable. 3. (c) A premixed flame regime where only one of the reactants leaks through the reaction zone, which then separates a region of frozen flow from a region of near-equilibrium. 4. (d) A near-equilibrium diffusion controlled regime, covering the upper branch of the S curve, with a thin reaction zone separating two regions of equilibrium flow. Analytical expressions are obtained, in particular, for the ignition and extinction conditions.
TL;DR: In this paper, a simple idea is proposed to produce an excess enthalpy flame by inserting a porous solid of high thermal conductivity into the one-dimensional flame zone, where heat is recirculated internally through the solid from the downstream high temperature region to the upstream low temperature region.
Abstract: A simple idea is proposed to produce an excess enthalpy flame by inserting a porous solid of high thermal conductivity into the one-dimensional flame zone. Heat is recirculated internally through the solid from the downstream high temperature region to the upstream low temperature region and large excess enthalpy is produced at the head of the reaction zone. The potentiality of the proposed artificially modified flame is analyzed on the basis of the simplified one-dimensional flame theory. It is found that in this flame the mass flow rate is not the eigenvalue but becomes a mere parameter. The heat transfer coefficient between the solid and the reacting gas is another parameter. With these additional two parameters the controllability over the flame characteristics increases remarkably. The analysis reveals several attractive characteristics of the flame and that the proposed idea is promising to burn mixtures of low heat content in a simple combustion system.
TL;DR: In this article, the volumetric heat transfer coefficients between cellular ceramics and a stream of air were measured using the single-blow transient experimental technique in conjunction with an inverse analysis.
Abstract: The volumetric heat transfer coefficients (hv) between cellular ceramics and a stream of air were measured using the single-blow transient experimental technique in conjunction with an inverse analysis. Test specimen made of mullite, YZA, SiC, cordierite and cordierite with LS-2 coating was studied. The number of pores per centimeter (PPC) ranged from 4 to 26 and the specimen thickness ranged from 6 to 12 mm. Based on the experimental data, the volumetric heat transfer coefficients were generalized by developing Nusselt number vs. Reynolds number correlations of the form Nu v =C Re m for the materials studied. The effects of pore length-scale and specimen thickness on the volumetric heat transfer coefficients are presented and discussed.
TL;DR: In this paper, a general asymptotic formulation for diffusion flames of large-activation-energy chemical reactions is presented, where the reaction is confined to a thin zone which, when viewed from a much larger diffusion scale, is a moving two-dimensional sheet.
Abstract: A general asymptotic formulation is presented for diffusion flames of large-activation-energy chemical reactions In this limit chemical reaction is confined to a thin zone which, when viewed from the much larger diffusion scale, is a moving two-dimensional sheet The formulation is not restricted to any particular configuration, and applies to conditions extending from complete combustion to extinction The detailed structure of the reaction zone yields jump conditions that permit full determination of the combustion field on both sides of the reaction zone, as well as the instantaneous shape of the reaction sheet itself The simplified system is subsequently used to study the intrinsic stability properties of diffusion flames and, in particular, the onset of cellular flames We show that cellular diffusion flames form under near-extinction conditions when the reactant in the feed stream is the more completely consumed reactant, and the corresponding reactant Lewis number is below some critical value Cell sizes at the onset of instability are on the order of the diffusion length Predicted cell sizes and conditions for instability are therefore both comparable with experimental observations Finally, we provide stability curves in the fuel and oxidant Lewis number parameter plane, showing where instability is expected for different values of both the initial mixture strength and the Damkohler number
TL;DR: In this article, a down-flow compact porous burner system was developed for burning kerosene without the need of using a spray atomizer, and the effect of the introduced packed bed emitter with suitable bed length and its installation location was investigated as an efficient method for enhancement of evaporation and combustion of the liquid fuels.
Abstract: Existing designs of most conventional liquid fuel burners have relied solely on spray atomizers, with a large amount of very fine droplets forming in a relatively large combustion chamber, resulting in a relatively low combustion intensity. Against this background, a novel down-flow compact porous burner system was developed for burning kerosene without the need of using a spray atomizer. Successive development on this burner research is important in view of the need to create energy by an efficient device based on simple technology. The focus has been on the introduction of the packed bed emitter installed downstream of the porous burner. The evaporation process and combustion phenomena that occurred are described through the coupled interaction of the solid phase (porous burner), the liquid phase (kerosene) and the gas phase. Enhancement of evaporation and combustion are evaluated through the measured thermal structures in terms of temperature distribution along the burner length and emission characteristics at the burner exit. Stable combustion with low emission of pollutants was realized even though the combustion flame was confined in-between the porous burner and the packed bed emitter with an increase in the back-pressure. The effects of various parameters including heat input and equivalence ratio on the combustion characteristics were clarified to confirm improvement in mixing of the fuel vapor/air mixture and turn-down ratio of the burner. The effect of the introduced packed bed emitter with suitable bed length and its installation location is investigated as an efficient method for enhancement of evaporation and combustion of the liquid fuels without a spray atomizer. Future applications of this type of burner system are suggested.
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