Diffusion flame calculations for composite propellants predicting particle-size effects

The complexities of the flame structure above a composite propellant containing 86% ammonium perchlorate (AP) and 14% hydroxy-terminated-polybutadiene (HTPB) have been elucidated using a two-dimensional, detailed gas-phase kinetic mechanism diffusion flame model. The model utilizes a vorticity formulation of the transport equations, which essentially eliminates the pressure field calculation and speeds convergence. Mass and energy coupling between the condensed and gas phases are achieved through iteration with one-dimensional, premixed combustion models to dynamically update the inlet boundary conditions. The model uses a detailed gas-phase kinetic mechanism consisting of 37 species and 127 reactions. Numerical studies have been performed to examine the influence of particle size on the flame structure above the AP/HTPB propellant. Three different combustion zones, based on AP particle size, were predicted: the AP monopropellant limit, the diffusion flame region, and the premixed limit. The modeled flame structure changed dramatically with particle size and was found to be qualitatively similar to the Beckstead–Derr–Price Model. Mechanistic insights are presented to explain AP’s unique ability to modify a propellant burning rate based on particle size alone. The premixed limit at which decreasing the size of the AP particles no longer influences the burning rate was also predicted. Results show promise in predicting formulistic effects using fundamental calculations.     

The effect of Lewis and Damk?hler numbers on the flame propagation through micro-organic dust particles

In this study, the role of Lewis and Damköhler numbers on the premixed flame propagation through micro-organic dust particles is investigated. It is presumed that the fuel particles vaporize first to yield a gaseous fuel, which is oxidized in the gas phase. In order to simulate the combustion process, the flame structure is composed of four zones; a preheat zone, a vaporization zone, a reaction zone and finally a post flame zone, respectively. Then the governing equations, required boundary conditions and matching conditions are applied for each zone and the standard asymptotic method is used in order to solve these differential equations. Consequently the important parameters on the combustion phenomenon of organic dust particles such as gaseous fuel mass fraction, organic dust mass fraction and burning velocity with the various numbers of Lewis, Damköhler and the onset of vaporization are plotted in figures. This prediction has a reasonable agreement with experimental data of micro-organic dust particle combustion.     

Modeling of laminar flame propagation through organic dust cloud with thermal radiation effect

In this research, a mathematical model is performed to analyze the structure of flame propagation through a two-phase mixture consisting of organic fuel particles and air. In contrast to previous analytical studies, thermal radiation effect is taken into consideration, which has not been attempted before. In order to simulate of the dust combustion phenomenon, it is assumed that the flame structure consists of four zones: preheat, vaporization, reaction and post flame (burned). Furthermore, radiative heat transfer equation is employed to describe the thermal radiation exchanged between these zones. The obtained results show that the induced thermal radiation from flame interface into the preheat and vaporization zones plays a significant role in the improvement of vaporization process and burning velocity of organic dust mixture, compared with the case in which the thermal radiation factor is neglected. According to present results, flame structure variables such as the burning velocity, mixture temperature, mass fraction of volatile fuel particles and gaseous fuel mass fraction strongly depend on radiative heat transfer. These predictions have reasonable agreement with published experimental data.     

Multiphysics modeling of carbon gasification processes in a well-stirred reactor with detailed gas-phase chemistry

Fuel synthesis through coal and biomass gasification has the potential to provide a solution to the increasing demand for energy and transportation fuels. To theoretically understand the complex chemical processes in a gasifier and to identify the most influential parameters for syngas production, we developed a multiphysics model to simulate the gasification processes in a well-stirred reactor. This model is the first of its kind and considers detailed gas-phase chemistry, particle-phase reactions, radiative heat transfer, as well as full coupling between the two phases at various scales for mass, species, and energy exchange. The gas-phase reactions use the detailed chemistry GRI-Mech 1.2, including 177 elementary reactions and 31 species, as well as variable thermodynamic and transport properties. Four surface reactions were considered and the reaction rates were simulated by the diffusion-kinetics model with consideration of boundary layer diffusion. A random pore model was used to account for the evolution of the char porous structure and its impact on gasification rates. A numerical code was developed to solve the gas-phase and the particle-phase governing equations. Numerical simulations were conducted to understand the gasification process and the effects of particle size, porous structure, radiative heat transfer, pressure, O₂ concentration, and H₂ addition on gasification performance.     

Flame propagation behaviors and influential factors of TiH2 dust explosions at a constant pressure

The flame propagation through TiH₂ dust cloud at near constant pressure condition is studied in a series of experiments using an apparatus with transparent latex balloons. The influential factors for the combustion performance of TiH₂ dust cloud, including dust concentration, particle size, scale of isobaric space and oxygen content are investigated. Results show that the burning velocity increases with dust concentration in the fuel-lean mixtures, and then plateaus after crossing the stoichiometric condition, while the trend of flame speed changing with dust concentration varies for different mean particle sizes (D₅₀) of 48 and 106 μm. The flame propagation speed of dust cloud is positively correlated to the isobaric space scale and oxygen content. The burning mechanism of TiH₂ dust is thought to be mainly controlled by diffusion regime, the appearance of hydrogen gas accelerates the combustion rate of TiH₂ particles and also makes the TiH₂ dust changed from a discrete media to a continuum, which may account for the phenomenon that the flame speed in dust cloud of TiH₂ is larger than that of Ti at the same concentration no matter in air or oxygen atmosphere.     

Impacts of Synchronous Combustion of Small Amounts of Coal Particles with Natural Gas on Enhancing Radiative Characteristic and NOx Flame Pollutant Emission

Radiation is the most important regime of heat transfer of a flame which is directly affected by temperature and emissivity coefficient of the flame. Natural gas has a nonluminous flame, although, the flame temperature is high, but, the emissivity coefficient of the flame is small. In this paper the impacts of synchronous combustion of small amounts of anthracite coal particles with natural gas on the detailed emissivity coefficient of the flame, radiative species and pollutant emissions were investigated experimentally and numerically. A sprint CFD code was used in numerical solution and the pollutants were measured by a Testo 350XL gas analyzer. The results showed that a small amount of coal particles injected into the hot flame of natural gas increases the volume distribution and radiation view factor of high‐emissive intermediate solid soot particles in the flame which enhances the total flame emissivity coefficient. Also, coal particle injection leads to a decrease in the upstream flame temperature and an increase in the downstream region creating a more uniform temperature distribution and decreases the concentration of thermal NO pollutant of the natural gas flame. Furthermore, the role of solid soot particles on the total emissivity coefficient is remarkable, while an increase in CO₂ and H₂O concentrations has an insignificant effect on the flame emissivity coefficient.     

惰性颗粒抑制CH4/O2/N2爆炸过程的二维数值模拟与实验验证

使用带基元化学反应的两相Euler方程,对惰性颗粒抑制高温火团诱导的CH4/O2/N2爆炸过程进行了二维数值模拟.其中,采用全耦合二阶精度的TVD格式求解气相方程,采用MacCormack格式求解惰性颗粒相方程,同时用分步方法处理方程组的耦合刚性,用Gear隐式方法处理化学反应刚性.计算并讨论了不同颗粒相浓度条件下,气相压力场、气相密度和颗粒相浓度的分布、爆炸衰减的气相结构以及两相间的能量传递.结果表明,惰性颗粒相的堆积可阻碍气相爆炸波的传播,使其衰减为引导激波,引导激波继续向颗粒相传递能量,而进一步衰减最终使得爆炸波被抑制.随着颗粒相浓度的增大,爆炸波的抑制更明显.计算结果与大型水平管的实验结果进行了比较,定性证明了计算的合理性.     

A Comparative Exploration of Enhancing Thermal Characteristics of Natural Gas Flame by Synchronous Combustion Technique

This article is a comparative study of how the injection of micro kerosene droplets and pulverized anthracite coal particles affects soot particle nucleation inside natural gas flame and, subsequently, radiation. To this end, the yellow chemiluminescence of soot particles and IR photography were used to locate radiative soot particles and discover their qualitative distribution. The IR filter was tested with a Thermo Nicolet Avatar 370 FTIR Spectrometer for its spectral transmittance to be specified. Also, the spectral absorbance of soot particles, which are formed in flame, was measured by BOMEM FTIR. Furthermore, the variations of flame temperature, transient heat transfer, and thermal efficiency were investigated. The results indicate that, for equal heating values, kerosene droplets are more effective than coal particles in improving the radiation and thermal characteristics of natural gas flame. Also, kerosene droplets cause a higher rise in the temperature in flame downstream and make the axial flame temperature more uniform than coal particles do. In quantitative terms, when kerosene droplets were injected, the radiative heat transfer and thermal efficiency of flame were 93% and 35% higher than the corresponding values for the coal particles injection mode.     

Numerical simulation of the devolatilization of a moving coal particle

The devolatilization of an isolated coal particle moving relative to the surrounding gas is numerically simulated using a competing reaction model of the pyrolysis and assuming that the released volatiles burn in an infinitely thin diffusion flame around the particle or not at all. The temperature of the particle is assumed to be uniform and the effects of the heat of pyrolysis, the intraparticle mass transfer resistance, and the variation of the particle radius are neglected. The effects of the size and velocity of the particle and of the temperature and oxygen mass fraction of the gas on the particle and flame temperature histories, the devolatilization time and the yield of light and heavy volatiles are investigated. The motion of the particle may have an important effect on the shape and position of the flame of volatiles, but it has only a mild effect on the devolatilization process for the particle sizes typical of pulverized coal combustion. This effect increases for large particles or in the absence of radiation. The relative motion enhances the heat transfer between the particle and the gas, causing the devolatilization time to decrease at high gas temperatures and to increase at low gas temperatures. The numerical results are compared with a blowing-corrected Nusselt number correlation often used in heat transfer models of the process.     

Analysis of steady and oscillating flames fueled by biomass particles and syngases considering two-step pyrolysis and heterogeneous and homogeneous reactions

Under unprecedented environmental crisis associated with greenhouse gas emission, biomass has attracted a great deal of attention due to renewable and carbon neutral nature. In this study, the premixed combustion of various types of wood and its derived syngases are examined for steady and oscillating sates. For this purpose, the poplar, birch, beech and pin sawdust woods and syngases composing of H₂, CH₄ and CO are considered. To model dust cloud combustion, a novel and comprehensive flame structure consisting of drying, two-step pyrolysis and homogeneous and heterogeneous reactions is proposed. Afterward, the governing equations and their appropriate boundary conditions are derived and solved analytically-numerically. The oscillating combustion is also modeled by exerting an external perturbation on the velocity field. The results indicate that due to the occurrence of heterogeneous reactions in wood combustion, the flame propagation velocity of wood is higher than that of syngases which contributes to high oscillations amplitude of syngases. When the mixture initial temperature changes between 300 and 550 K, the flame velocities of woods and syngases vary in the ranges of 0.4–0.7 m/s and 0.1–0.27 m/s, respectively. The maximum amplitude of temperature oscillation of syngases is approximately 8 times more than that of woods.     

Droplet heat and mass transfer in a turbulent hot airstream

A three-dimensional numerical model is developed to investigate the effect of turbulence on heat and mass transfer rates of a droplet exposed to a hot airstream. The airstream turbulence, temperature and mean Reynolds number are varied to provide a wide range of test conditions. The ambient pressure is kept atmospheric. In addition, variable thermophysical properties, transient gas and liquid phases, and the effect of radiation are all considered in the numerical study. The turbulence terms in the conservation equations of the gas-phase are modelled by using the shear-stress transport (SST) model. A Cartesian grid based blocked-off technique is used in conjunction with the finite-volume method to solve numerically the governing equations of the gas and liquid phases. The numerical results indicate that the effect of freestream turbulence is persistent although it weakens as the airstream temperature increases. The effect of radiation becomes significantly important at elevated airstream temperatures. Comprehensive droplet heat and mass transfer correlations are proposed, which take into consideration all the aforementioned variables.     

Model of accelerating coal dust flames

A model describing turbulent coal dust flame propagation and accelaration is based on the transient, macroscopic equations of change. The turbulent flame velocity was obtained from a simple correlative technique combining turbulence and chemical effects. Predictions indicate that a deflegrating coal dust flame can accelerate to high velocity and pressure, with increasing turbulence a major cause of the acceleration. A parametric study was conducted to identify key parameters in the model. The need for turbulent flame velocity data for particle-laden systems was identified and the effects of duct diameter, coal particle diameter, and various model parameters were described. The model is useful for describing relative effects of various parameters.     

Heat transfer in a directly irradiated solar receiver/reactor for solid–gas reactions

Particle laden solar receivers can be used at high temperatures for efficient heat transfer and fuel generation via chemical reactions. A theoretical analysis of a directly irradiated, particle laden, solar receiver is presented here and compared with experiments. The radiation characteristics of the particles are approximated using a method, which adapts Mie theory to certain cases where a solar receiver is used with seeded particles of variable sizes and shapes. Based on this model carbon black particles whose effective radius, r_p, is less than 100 nm are inefficient in absorbing solar energy and the most suitable particle sizes is in the same range as the wavelengths of the radiation (100 nm < r_p < 1000 nm). The heat transfer coefficient between the particles and the gas was calculated using a refined limiting sphere model developed for the transition regime between molecular and continuum transfer. Previous models assume that there are no gas molecule collisions in the energy transfer layer and the mean free path of the gas molecules is equal to the thickness of this layer. The present model accounts for molecule collisions in the energy transfer layer and therefore enables the thickness of this layer to be larger than one mean free path length. The model was extended to estimate the Nusselt number for gases with several atoms as well as for monatomic gas. A code to simulate the flow and heat transfer in the receiver was developed, utilizing the models for heat transfer from sunlight to the particles and from the particles to the gas. The receiver simulations show good agreement with the wall temperature distribution measured in experiments, but the gas exit temperature in the model was significantly lower than the measured value. This discrepancy could be due to limitations of the simulation code and the particle heat transfer models. The simulation suggests that changing the Nusselt number and particle radius have a small influence on the receiver wall and gas temperatures. Increasing the particle cloud concentration improves the receiver heat transfer up to a threshold value; further increase of the particles concentration has only a marginal influence on the receiver’s heat transfer. This result from the receiver modeling was in a good agreement with solar experiments.     

Thermal creep flow around a spherical particle suspended near a plate

Thermophoresis of small particles, like mist, fumes, dust, and so on, floating in a gas, particularly in the vicinity of a solid surface, is of practical interest in aerosol technology and related fields of treating gas molecule-surface interactions. This paper deals with thermal creep flow of a gas around a spherical particle suspended near a plane plate; temperature and flow fields are analyzed by counting the discontinuous rarefaction effects of the gas. In a consequence, the thermal force, as well as the drift velocity of the particle at an arbitrary distance from the plate, but limited to lower Knudsen numbers, are clarified, with thermal and flow fields in the gas being presented for some selected schemes. The thermal force is enlarged by the temperature jump decreased by the momentum slip. It increases as the particle approaches the plate; this increase is very sharp when the particle is closer to the plate. The drift velocity, on the other hand, remains nearly the same as that far away from the plate, but shows a slight decrease in the very vicinity of the plate. © 1998 Scripta Technica. Heat Trans Jpn Res, 27(1): 57–73, 1998     

LES model for sooting turbulent nonpremixed flames

In this work, an integrated Large Eddy Simulation (LES) model is developed for sooting turbulent nonpremixed flames and validated in a laboratory scale flame. The integrated approach leverages state-of-the-art developments in both soot modeling and turbulent combustion modeling and gives special consideration to the small-scale interactions between turbulence, soot, and chemistry. The oxidation of the fuel and the formation of gas-phase soot precursors is described by the Flamelet/Progress Variable model, which has been previously extended to account for radiation losses. However, previous DNS studies have shown that Polycyclic Aromatic Hydrocarbons (PAH), the immediate precursors of soot particles, exhibit significant unsteady effects due to relatively slow chemistry. To model these unsteady effects, a transport equation is solved for a lumped PAH species. In addition, due to the removal of PAH from the gas-phase, alternative definitions of the mixture fraction, progress variable, and enthalpy are proposed. The evolution of the soot population is modeled with the Hybrid Method of Moments (HMOM), an efficient statistical model requiring the solution of only a few transport equations describing statistics of the soot population. The filtered source terms in these equations that describe the various formation, growth, and destruction processes are closed with a recently developed presumed subfilter PDF approach that accounts for the high spatial intermittency of soot. The integrated LES model is validated in a piloted natural gas turbulent jet diffusion flame and is shown to predict the magnitude of the maximum soot volume fraction in the flame relatively accurately, although the maximum soot volume fraction is shown to be rather sensitive to the subfilter scalar dissipation rate model.     

Modeling and Coupling Particle Scale Heat Transfer with DEM through Heat Transfer Mechanisms

The detailed heat transfer mechanisms particle interior, gas film around particles, gas gap between contact surfaces, and rough surface are considered to model heat transfer between particles. The validation of the heat transfer model is accomplished and the predicted results show good agreement with other experiments. From the quantitative comparison of four heat transfer paths, it is revealed that the heat transfer through gas gap and rough surface could be neglected for a particle diameter larger than 2 mm. Furthermore, the detailed heat transfer model is coupled with the discrete element method (DEM) to calculate macro effective thermal conductivity (ETC) of fixed beds, and the accuracy and applicability is verified by comparing with other estimated and experimental results. The influence of particle diameter, density, specific thermal capacity, and thermal conductivity on ETC is investigated. Results show that the proposed heat transfer model provides an effective and accurate way to couple with DEM in the particle system.     

Effect of turbulence on flame propagation in cornstarch dust-air mixtures

Following the quantitative determination of dust cloud parameters, this study investigated the flame propagation through cornstarch dust clouds in a vertical duct of 780 mm height and 160×160 mm square cross section, and gave particular attention to the effect of turbulence on flame characteristics. The turbulence induced by dust dispersion process was measured using a particle image velocimetry (PIV) system. Upward propagating dust flames were visualized with direct light and shadow photography. The results show that a critical value of the turbulence intensity can be specified below which laminar flame propagation would be established. This transition condition is about 10 cm/s. The measured propagation speed of laminar flames appears to be in the range of 0.45–0.56 m/s, consistent with the measurements reported in the literature. For the present experimental conditions, the flame speed is little sensitive to the variations in dust concentration. Some information on the flame structure was revealed from the shadow records, showing the typical heterogeneous feature of dust combustion process.      

Sensitivity analysis of a biomass gasification model in fixed bed downdraft reactors: Effect of model and process parameters on reaction front

A detailed sensitivity analysis is performed on a one-dimensional fixed bed downdraft biomass gasification model. The aim of this work is to analyze how the heat transfer mechanisms and rates are affected as reaction front progresses along the bed with its main reactive stages (drying, pyrolysis, combustion and reduction) under auto-thermal conditions. To this end, a batch type fixed-bed gasifier was simulated and used to study process propagation velocity of biomass gasification. The previously proposed model was validated with experimental data as a function of particle size. The model was capable of predicting coherently the physicochemical processes of gasification allowing an agreement between experimental and calculated data with an average error of 8%. Model sensitivity to parametric changes in several model and process parameters was evaluated by analyzing their effect on heat transfer mechanisms of reaction front (solid–gas, bed–wall and radiative in the solid phase) and key response variables (temperature field, maximum solid and gas temperatures inside the bed, flame front velocity, biomass consumption and fuel/air ratio). The model coefficients analyzed were the solid–gas heat transfer, radiation absorption, bed–wall heat transfer, pyrolysis kinetic rates and reactor-environment heat transfer. On the other hand, particle size, bed void fraction, air intake temperature, gasifying agent composition and gasifier wall material were analyzed as process parameters. The solid–gas heat transfer coefficient (0.02 < correction factor < 1.0) and particle size (4 < diameter < 30 mm) were the most significant parameters affecting process behavior. They led to variations of 88% and 68% in process velocity, respectively.     

流化床煤燃烧与气化过程中的辐射换热

流化床反应器中颗粒与颗粒之间的传热在一定程度上决定了化学反应的速率及反应的中间历程。本文通过对气固流化床乳化相中颗粒群结构的进一步认识，建立了颗粒间的辐射换热模型，比较了不同颗粒直径、不同床层温度水平及不同流化工况下颗粒间辐射换热与通过气膜导热份额的大小，并预测了流化床反应器中反应颗粒与惰性床料之间的温差，对于流化床反应器选择合理的运行工况和进行操作参数优化具有参考价值