Burning Rate Of Particles Research Articles

Predictions of laminar flame in aluminium dust clouds with a two-dimensional analytical model

In the present study, with the assumption of a laminar, steady and uniform flow and with the purpose of gaining flame speed, quenching distance, lean flammability limit, and temperature distribution, a two-dimensional (2D) analytical model of micro-sized aluminium dust cloud combustion in a channel is presented. In the previous analytical models, energy conservation equations of laminar flame had been solved one-dimensionally and flame speed and quenching distance had been obtained assuming that heat loss to the wall is linear. This model is developed with the assumption that the particle burning rate in the flame front is controlled by the process of oxygen diffusion, and a channel with constant temperature boundary conditions, similar to those of quenching plates and experimental aluminium dust cloud combustion channels, is assumed as well. In order to examine more precisely the impact from the amount of heat loss originating from the channel wall on combustion parameters including flame speed, energy equations are written in two-dimensions. The equations are written in two limiting cases: lean and rich mixtures. Flame structure consists of preheat, reaction, and post-flame zones for the lean mixture and preheat and reaction zones for the rich mixture. Although equations in the lean mixture are solved using the separation of variables method, Laplace method is used for their solution in the rich mixture. By solving the energy equations in each zone and matching the temperature and heat flux at the interfacial boundaries, algebraic equations of flame speed are obtained. The obtained gas temperature distribution in different flame zones in the channel and also flame speed changes in terms of particles' diameter, equivalence ratio, and channel width in both lean and rich mixtures are presented in the results section. Generally, the results of the study shows that solving equations in 2D models leads to higher agreements between the theoretical and experimental results in flame speed, lean limit, and quenching distance terms.

Effect of particle size on combustion of aluminum particle dust in air

The combustion of aluminum particle dust in a laminar air flow is theoretically studied under fuel-lean conditions. A wide range of particle sizes at nano and micron scales is explored. The flame speed and temperature distribution are obtained by numerically solving the energy equation in the flame zone, with the particle burning rate modeled as a function of particle diameter and ambient temperature. The model allows for investigation into the effects of particle size, equivalence ratio, and chemical kinetics on the burning characteristics and flame structures of aluminum-particle/air mixtures. In addition, the flame behavior with ultra-fine particles in the sub-nanometer range is examined by asymptotically treating particles as large molecules. Calculated flame speeds show reasonable agreement with experimental data. As the particle diameter decreases from the micron to the nano range, the flame speed increases and the combustion transits from a diffusion-controlled to a kinetically controlled mode. For micron-sized and larger particles, the flame speed can be correlated with the particle size according to a d − m relationship, with m being 0.92. For nano-particles, a d −0.52 or d −0.13 dependence is obtained, depending on whether the d 1.0 - or d 0.3 -law of particle burning time is implemented in the flame model, respectively. No universal law of flame speed exists for the entire range of particle sizes.

Modeling char oxidation behavior under Zone II burning conditions at elevated pressures

For accurate modeling of the coal combustion process at elevated pressures, account must be made for variations in char-particle structure. As pressure is increased, particle swelling increases during the devolatilization of certain bituminous coals, yielding a variety of char-particle structures, from uniform high-density particles to thin-walled non-uniform low-density particles having large internal void volumes. Since under Zone II burning conditions the char conversion rate depends upon the accessibility of the internal surfaces, the char structure plays a key role in determining particle burnout times. In our approach to characterize the impact of char structure on particle burning rates, effectiveness factors appropriate for thin-walled cenospherical particles and thick-walled particles having a few large cavities are defined and related to the effectiveness factor for uniform high-density particles that have no large voids, only a random distribution of pores having a mean pore size in the sub-micron range. For the uniform case, the Thiele modulus approach is used to account for Zone II type burning in which internal burning is limited by the combined effects of pore diffusion and the intrinsic chemical reactivity of the carbonaceous material. In the paper, the impact of having a variety of char structures in a mix of particles burning under Zone II burning conditions is demonstrated.

Simulation on Combustion Flow with Diesel Particle in Metal Honeycomb

Diesel engines have been used in a wide variety of commercial vehicles. However, several problems must be solved. One of the major concerns is that diesel exhaust gas has more particulate matters (PM) including soot, which are suspected to be linked to human carcinogen. Then, more strict diesel emissions standards are being set in many countries, and diesel particulate filters have been developed to reduce PM in the after-treatment of exhaust gas. In this study, we conduct Lattice Boltzmann simulation on combustion in metal honeycomb filter, where metal obstacles with square shape are inserted in 2-D duct flow with the porosity of 0.8. To examine the filter properties, we change the arrangement and size of metal obstacles in the filter. Results show that the combustion reaction is well simulated to discuss the burning rate of soot particles in the exhaust gas.

303 ディーゼル排ガス焼却システムにおけるすす粒子の燃焼解析(燃焼排ガス処理,大気・水環境保全技術)

Diesel engines have been used in a wide variety of commercial vehicles. However, several problems must be solved. One of the major concerns is that diesel exhaust gas has more particulate matters (PM) including soot, which are suspected to be linked to human carcinogen. Then, more strict diesel emissions standards are being set in many counties. Recently, in order to reduce PM in the after-treatment of exhaust gas, DEGPBS (diesel exhaust gas perfect burning system) has been developed by COTEC Company, Ltd. In this study, we conducted numerical simulation of soot combustion in DEGPBS by the Lattice Boltzmann method. The inner structure of the filter was obtained by a 3D X-ray CT technique. Results show that the combustion reaction is well simulated to discuss the burning rate of soot particles in exhaust gas.

On the burning behavior of pulverized coal chars

A model that predicts the physical changes that pulverized coal char particles undergo during combustion has been developed. In the model, a burning particle is divided into a number of concentric annular volume elements. The mass loss rate, specific surface area, and apparent density in each volume element depend upon the local particle conditions, which vary as a consequence of the adsorbed oxygen and gas-phase oxygen concentration gradients inside the particle. The model predicts the particle's burning rate, temperature, diameter, apparent density, and specific surface area as combustion proceeds, given ambient conditions and initial char properties. A six-step heterogeneous reaction mechanism is used to describe carbon reactivity to oxygen. A distributed activation energy approach is used to account for the variation in desorption energies of adsorbed O-atoms on the carbonaceous surface. Model calculations support the three burning zones established for the oxidation of pulverized coal chars. The model indicates two types of zone II behavior, however. Under weak zone II burning conditions, constant-diameter burning occurs up to 30% to 50% conversion before burning commences with reductions in both size and apparent density. Under strong zone II conditions, particles burn with reductions in both size and apparent density after an initial short period ( < 2 % conversion) of constant-diameter burning. Model predictions reveal that early in the oxidation process, there is mass loss at constant diameter under all zone II burning conditions. Such weak and strong burning behavior cannot be predicted with the commonly used power-law model for the mode of burning employing a single value for the burning mode parameter. Model calculations also reveal how specific surface area evolves when oxidation occurs in the zone II burning regime. Based on the calculated results, a surface area submodel that accounts for the effects of pore growth and coalescence during combustion under zone I conditions was modified to permit the characterization of the variations in specific surface area that occur during char conversion under zones II conditions. The modified surface area model is applicable to all burning regimes. Calculations also indicate that the particle's effectiveness factor varies during conversion under zone II burning conditions. With the adsorption/desorption mechanism employed, a near first-order Thiele modulus–effectiveness factor relationship is obeyed over the particle's lifetime.

Combustion of a coal char particle in a stream of dry gas

The burning rate, surface temperature, drag, and extinction conditions of a single char particle moving in a gas are computed numerically. The effects of the size and velocity of the particle and of the temperature and composition of the gas are examined in the framework of a simple model that includes O 2 and CO 2 heterogeneous reactions and, in some cases, a diffusion-controlled CO oxidation flame in the gas around the particle. In agreement with known results, the burning rate is found to increase with the velocity of the particle when the Reynolds number of the gas flow ceases to be small. The temperature of the particle increases with the temperature and oxygen mass fraction of the gas and is little affected by the size and velocity of the particle, except in the vicinity of extinction. The drag coefficient is a decreasing function of the particle size and velocity in the range of Reynolds numbers that has been analyzed. The presence of CO 2 in the gas may have an important effect on the gasification of small particles.

Development of Particle Concentration Distributions and Burn Rate Gradients upon Shear-Induced Particle Migration during Processing of Energetic Suspensions

Shear-induced particle migration is an important issue to be addressed during the processing of concentrated suspensions, which constitute various energetic formulations consisting of polymeric binders with a relatively high concentration of rigid energetic particles. Upon shear-induced particle migration during manufacturing, concentration gradients of the rigid particles are developed that lead to burn rate gradients to alter the overall burn rate behavior of the energetic grains. Here it is demonstrated that changes in the burn rates would occur both in drag and pressure-driven processing flows under conditions in which the particle radius over the gap of the processing geometry is appreciable. Typical results are provided.

The combustion of large particles of char in bubbling fluidized beds: The dependence of Sherwood number and the rate of burning on particle diameter

Particles of char derived from a variety of fuels (e.g., biomass, sewage sludge, coal, or graphite), with diameters in excess of ∼ 1.5 mm , burn in fluidized bed combustors containing smaller particles of, e.g., sand, such that the rate is controlled by the diffusion both of O 2 to the burning solid and of the products CO and CO 2 away from it into the particulate phase. It is therefore important to characterize these mass transfer processes accurately. Measurements of the burning rate of char particles made from sewage sludge suggest that the Sherwood number, Sh, increases linearly with the diameter of the fuel particle, d char (for d char > ∼ 1.5 mm ). This linear dependence of Sh on d char is expected from the basic equation Sh = 2 ε mf ( 1 + d char / 2 δ diff ) / τ , provided the thickness of the boundary layer for mass transfer, δ diff , is constant in the region of interest ( d char > ∼ 1.5 mm ). Such a dependence is not seen in the empirical equations currently used and based on the Frössling expression. It is found here that for chars made from sewage sludge (for d char > ∼ 1.5 mm ), the thickness of the boundary layer for mass transfer in a fluidized bed, δ diff , is less than that predicted by empirical correlations based on the Frössling expression. In fact, δ diff is not more than the diameter of the fluidized sand particles. Finally, the experiments in this study indicate that models based on surface renewal theory should be rejected for a fluidized bed, because they give unrealistically short contact times for packets of fluidized particles at the surface of a burning sphere. The result is the new correlation Sh = 2 ε mf τ + A cush A char d char δ diff for the dependence of Sh on d char , the diameter of a burning char particle. This equation is based on there being a gas-cushion of fluidizing gas underneath a burning char particle; the implication of this correlation is that a completely new picture emerges for the combustion of a char particle in a hot fluidized bed.

Motor and Plume Particle Size Prediction in Solid-Propellant Rocket Motors

Models are developed for 1) agglomeration of powdered aluminum on the burning surface of a metallized composite solid propellant in order to predict average diameter of aluminum agglomerates as a function of propellant composition, pressure, burning rate, and size distribution of oxidizer particles and 2) the processes of coagulation and aerodynamic shattering of liquid aluminum-oxide droplets in an accelerating gas flow. The critical upper value of droplet diameter in the throat, which is calculated theoretically, is in agreement with the mean-mass diameter D 4 3 obtained by numerical simulation of a two-phase flow with coagulation and particle breakup. A formula is proposed for the average particle diameter at the nozzle exit, which is confirmed by agreement with the measured values of D 4 3 of aluminum-oxide particles in plumes of small, medium-sized, and large solid rocket motors. The proposed formula is also in agreement with Hermsen's correlation of measured diameters. Based on a comparison between theoretical calculations and numerous experimental data, the formula is recommended for prediction of the aluminum-oxide particle size in plumes of various types of solid rocket motors.

Evolution and Distribution of Surface Voids Affecting Char Burning Rates at Diffusion-Limited Conditions

Research has indicated that the accurate calculation of char burning rates at diffusion-limited conditions requires consideration of the effects of voids (gaps in the continuous surface of burnable material that are deeper than their surface radius) on the particle burning area. Experimentally, void area distributions as a function of void size can be quantified using scanning electron microscopy and digital processing of quenched char particle images. Modeling void area distributions is problematic, even assuming voids behave as pores, because of initial surface imperfections and effects of combustion on surface morphology. This work examines the effects of factors that affect general pore formation for their effect on voids larger than 1 μm, such as heterogeneous combustion, devolatilization, and particle size and how these factors influence the calculation of diffusion limited burning rate from temperature and image data. Void distributions were predicted using an established pore distribution model, with and without the effects of pore combination. Results indicate that void distributions roughly follow expected pore behavior through devolatiltzation, but not after ignition, with void area fraction increasing with increasing coal volatile content and particle heating rate, but not significantly affected by oxygen concentration or initial particle size. Further, analytical pore models provide reasonable approximations for void distributions in the calculation of particle burning rates from temperature and image data.

98/02535 Heterogeneous combustion of porous graphite particles in microgravity : Chelliali, H. K. and Miller, F. J. NASA Conf. Publ., 1997, 10194, 511–516

Recent theoretical investigations on graphite particle combustion have employed several levels of heterogeneous reaction models, ranging from global to elementary models, to describe the oxidation of carbon to gaseous products. Unlike the counterpart homogeneous reaction models, these heterogeneous reaction models are not well developed because of the difficulties associated with decoupling the physical characteristics of the solid (e.g. surface area taking part in combustion) from the chemical kinetic data. This is certainly true for porous graphite particle combustion, where heterogeneous and homogeneous reactions occur within the pores and play an important role in the overall oxidation process. As a result, there are considerable uncertainties of physical phenomena predicted using different heterogeneous kinetic models available in the literature. A good example, discussed later in this paper, is the predicted critical particle size below which the mass burning rate becomes exponentially small. The main goal of this study is to understand the basic mechanism controlling such rapid changes in burning rates, by developing a model where physical contributions are decoupled from chemical rate constants in a consistent manner. Another important goal of the proposed study is to develop a truly intrinsic, detailed heterogeneous reaction model for porous graphite combustion at high-temperatures, and to derive a systematically reduced heterogeneous reaction model in terms of the elementary reaction rate constants of the detailed model. The validation of chemical kinetic models describing the heterogeneous and homogeneous combustion in and around a spherically symmetric porous graphite particle can be considerably simplified by experimental measurements obtained under microgravity conditions. A vital component of this study is to conduct such supporting experiments on particle burning rate and surface temperature using NASA microgravity facilities, in close coordination with the theoretical effort. The basic understanding obtained and models developed as part of this project will be useful for optimal design of coal combustion devices. These models can also be extended to investigate the role of heterogeneous chemistry on pollutant formation pathways in combustion devices. The theoretical approach developed here, with pore diffusion effects decoupled from the chemical effects, can also be extended to understand the heterogeneous combustion of other porous fuels, for example, combustion of magnesium in a CO2 environment for propulsion in the Martian atmosphere.

97/04910 Study on burning rate of carbon particle under forced convection: Xu, C. and Fu, W. Ranshao Kexue Yu Jishu., 1996, 2, (1), 1–7. (In Chinese)

97/04910 Study on burning rate of carbon particle under forced convection: Xu, C. and Fu, W. Ranshao Kexue Yu Jishu., 1996, 2, (1), 1–7. (In Chinese)

Studies on the Ignition and Burning of Levitated Aluminum Particles∗

Abstract An experimental set-up is developed to investigate the burning of levitated aluminum particles ignited by a C02laser in air under high pressure conditions. Residence times of aluminum particles burning in the electrodynamic levitator are long enough to observe the total burning process, under normal and high pressures. Experiments allow to measure the ignition delay and the burning times of aluminum particles. These measurements and the accompanying high speed images give useful information on the burning processes of aluminum particles under various regimes. A numerical model is also developed to predict the burning rates of aluminum particles and the sizes of the alumina residues.

Quenching distance of laminar flame in aluminum dust clouds

Quenching distances for aluminum dust flames have been measured in an improved flow system which can yield stable, controlled, uniform dust mixtures. Experiments were performed with fine atomized aluminum dust ( d 32 = 5.4 μm). The dust dispersion technique uses an annular high-speed jet which disperses dust continuously supplied via a piston-type dust feeding system. Laminarized dust flow ascending in a vertical Pyrex tube ( d = 0.05 m, L = 1.2 m) was ignited at the open tube end. Constant pressure flames propagating downwards were observed. A set of thin, evenly spaced steel plates was installed in the upper third part of the tube in order to determine the flame quenching distance. Three different stages of flame propagation were observed: laminar, oscillating, and turbulent accelerating flames. Flame speed and quenching distance as a function of dust concentration were determined during the laminar stage of flame propagation in dust-oxygen-nitrogen and in dust-oxygen-helium mixtures. It was found that the quenching distance and flame speed are very weak functions of dust concentration in rich mixtures. The minimum quenching distance is found to be about 5 mm in air and increases to 15 mm in mixtures of 11% O 2. The substitution of nitrogen for helium in air increases the minimum quenching distance from 5 to 7 mm. A simple analytical model of a quasi one-dimensional dust flame with heat losses was developed with an assumption that the particle burning rate in the flame front is controlled by the process of oxygen diffusion. Algebraic equations defining flame speed were obtained in two limiting cases: lean and rich mixtures. The model predicts wide plateaus in the flame speed and quenching distance versus dust concentration plots in rich mixtures. These plateaus were observed experimentally. Calculated values of minimum quenching distances are in good agreement with experimental data.

Burning velocities in fuel-rich aluminum dust clouds

A new experimental apparatus that can produce Bunsen-type premixed dust flames stabilized on a water-cooled ring has been contructed. The burning velocities in rich aluminum dust clouds of particle size d 32 =5.4 μ m have been measured by determining the surface area of the inner flame cone and the total volumetric flow rate. It was found that in rich Al−O 2 −N 2 mixtures, the burning velocity is a weak function of dust concentration. At the same time, the dependence of the burning velocity on the oxygen concentration is considerably stronger in rich mixtures than that reported in literature for the fuel-lean aluminum dust clouds. The burning velocity changes from 12–13 cm/s to about 32 cm/s when the oxygen content in the mixture increases from 11% to 30%. The burning velocity was also found to be a strong function of the molecular transport properties of the carrier gas. Thus, replacing the argon in an Al−O 2 −Ar mixture with helium increases the burning velocity from 20 to 65 cm/s. The oxygen limit for the flame propagation (9.5±0.5%) was also found by measuring the minimal oxygen concentration in the dust carrier gas when a premixed flame cone is formed inside the diffusive dust flame. A simple model of the dust flame with molecular heat transfer that has been proposed is based on the assumption that the particle burning rate in the flame front is controlled by the oxygen diffusion. It was shown that the weak dependence of the flame speed on dust concentration is a direct result of the weak dependence of the particle burning rate in the diffusive regime on the flame temperature. The model also predicts dependence of the burning velocity in rich dust mixtures on the Lewis number in accordance with the experimental observations.

95/00658 CHP makes gains as attitudes change

A North Dakota lignite has been demineralized and selectively reloaded with calcium, potassium, and sodium cations by an ion-exchange process. Chars produced from the treated samples were burned in a laminar-flow reactor and single-particle temperatures were determined by optical pyrometry. Results show that sodium and potassium cations present in the chars have little effect on the char particle temperatures or burning rates. Calcium cations enhance the char particle temperatures at low concentration (<5000 ppm). The results are consistent with carbon dioxide being produced at the char surface by catalytic action of the char mineral matter.

95/00657 CHP at De Montfort University

A North Dakota lignite has been demineralized and selectively reloaded with calcium, potassium, and sodium cations by an ion-exchange process. Chars produced from the treated samples were burned in a laminar-flow reactor and single-particle temperatures were determined by optical pyrometry. Results show that sodium and potassium cations present in the chars have little effect on the char particle temperatures or burning rates. Calcium cations enhance the char particle temperatures at low concentration (<5000 ppm). The results are consistent with carbon dioxide being produced at the char surface by catalytic action of the char mineral matter.

Combustion of Cylindrical and Spherical Random Clusters of Char/Carbon Particles

Abstract The interaction between burning char/carbon particles of different sizes is analyzed using a quasi-steady approximation. The method of solution is based on the expansion into irreducible multipoles developed in the earlier paper. It is assumed that the chemical reaction which occurs at the surface of particles is of first order. The method is applied for the analysis of the burning rate of spherical and cylindrical clouds of particles. The results are presented in terms of correction factor which is defined as a ratio of a burning rate of a particle located inside a cloud to an isolated particle burning rate. It is shown that the burning rate of a cloud of particles is determined by a combustion rate of a primary (central) particle with the radius which is equal to the average radius of particles in a cluster. The results obtained with the developed model are validated by comparison with the available experimental data

Kinetic model for hydrocarbon‐assisted particulate boron combustion

AbstractA kinetic model is presented to describe the high temperature (1800 K &lt; T &lt; 3000 K) surface oxidation of particulate boron in a hydrocarbon combustion environment. The model includes a homogeneous gas‐phase B/O/H/C oxidation mechanism consisting of 19 chemical species and 58 forward and reverse elementary reactions, multi‐component gas‐phase diffusion, and a heterogeneous surface oxidation mechanism consisting of ‘elementary’ adsorption and desorption reaction steps. Thermochemical and kinetic parameters for the surface reactions are estimated from available experimental data and/or elementary transition state arguments. The kinetic processes are treated using a generalized kinetics code, with embedded sensitivity analysis, for the combustion of a one‐dimensional (particle radius), spherical particle. Model results are presented for the oxidation of a 200 μm boron particle in a JP‐4/air mixture at ambient temperatures of 1400 K and 2000 K. These results include temperature and gas‐phase species profiles as a function of radial distance and particle burning rates. © 1994 John Wiley &amp; Sons, Inc.