Influence of pressure-driven gas permeation on the quasi-steady burning of porous energetic materials

A theoretical two-phase-flow analysis is developed to describe the quasi-steady propagation, across a pressure jump, of a multi-phase deflagration in confined porous energetic materials. The difference, or overpressure, between the upstream (unburned) and downstream (burned) gas pressure leads to a more complex structure than that which is obtained for an unconfined deflagration in which the pressure across the multi-phase flame region is approximately constant. In particular, the structure of such a wave is shown by asymptotic methods to consist of a thin boundary layer characterized by gas permeation into the unburned solid, followed by a liquid-gas flame region, common to both types of problem, in which the melted material is preheated further and ultimately converted to gaseous products. The effect of gas flow relative to the condensed material is shown to be significant, both in the porous unburned solid as well as in the exothermic liquid-gas melt layer, and is, in turn, strongly affected by the overpressure. Indeed, all quantities of interest, including the burn temperature, gas velocity and the propagation speed, depend on this pressure difference, leading to a significant enhancement of the burning rate with increasing overpressure. In the limit that the overpressure becomes small, the pressure gradient is insufficient to drive gas produced in the reaction zone in the upstream direction, and all gas flow relative to the condensed material is directed in the downstream direction, as in the case of an unconfined deflagration. The present analysis is particularly applicable to those types of porous energetic solid, such as degraded nitramine propellants that can experience significant gas flow in the solid preheat region and which are characterized by the presence of exothermic reactions in a bubbling melt layer at their surfaces.     

Analysis of the influence of nanometric aluminium particle vaporisation on flame propagation in bulk powder media

The combustion of nanometric aluminium (Al) powder with an oxidiser such as molybdenum trioxide (MoO₃) is studied analytically. The analysis was performed to correlate individual Al particle gasification rates to macroscopic flame propagation rates observed in flame tube experiments. Examination of various characteristic times relevant to propagation of a deflagration reveals that particles below about 1.7 nm in diameter evaporate before appreciable chemical reactions occur. Experimental studies used Al particles greater than 1.7 nm in diameter such that a diffusion flame model was developed to better understand the combustion dynamics of multiphase Al particles greater than 1.7 nm diameter relative to experimentally measured macroscopic flame propagation rates. The diffusion flame model predicted orders of magnitude slower propagation rates than experimentally observed. These results imply that (1) another reaction mechanism is responsible for promoting reaction propagation and/or (2) modes other than diffusion play a more dominant role in flame propagation.     

Gasification of porous carbon particle by steam

Diffusive-kinetic model of porous carbon particle gasification by steam is developed. The model considers the processes of heat and mass transfer both inside the porous particle and above it. Heat losses by radiation to the particle from furnace wall are taken into account. Heterogeneous reactions of carbon with steam and carbon with carbon dioxide and homogeneous reaction of carbon monoxide with steam are considered. Pressure variation caused by gas mass increasing inside the particle is considered too. The analysis of the model inside the porous particle made possible determining the correlation between the reaction rate of carbon with steam and the reaction rate of carbon with carbon dioxide. The homogeneous reaction is supposed to be equilibrium. It is considered that the kinetics of heterogeneous reactions is known, than the equations of the model may be solved; and consequently the dependences of the particle gasification rate and the composition of the gasification products vs. composition, pressure and temperature of ambient gas and the internal surface of the porous particle are determined.     

Realizing microgravity flame spread characteristics at 1 g over a bed of nano-aluminum powder

Nanoscale aluminum (nAl) powders demonstrate relatively fast counter-flow flame spread rates compared to typical fuels such as Poly(methyl methacrylate) or cellulose at similar conditions. This allows for the dominant forward heat transfer mechanism to be through the solid fuel at higher applied oxidizer velocities, and flame structure characteristics typically observed in microgravity to be realized at 1 g conditions. Because of the porosity of the nAl powder, the gaseous oxidizer can diffuse into the bed and reactions within the solid phase become important. Using an energy balance applied to only the solid phase, an analytical model is developed which predicts the experiments for flame spread over a nAl bed. Moreover, an explanation for fingering phenomenon is established based on the effective Lewis and Damköhler numbers. This allows for an explanation of why flame spread over a bed of nAl will demonstrate this fingering instability in a quiescent, 1 g environment without a top plate to hinder buoyant flows.     

An analysis of the transient combustion and burnout time of carbon particles

The various coupled and transient processes controlling the gasification mechanism and burnout time of carbon particles were analyzed, with emphasis on the influence of the initial particle size for the size range that is relevant to the firing of pulverized solid fuels. The formulation recognizes the suppression of the envelop gas-phase CO flame because of the small particle size, and allows for the three surface reactions of C + O₂, C + CO₂, and C + H₂O, as well as radiation heat transfer because of the potential high temperature attainable by the carbon particle. Results show that while the particle temperature continuously increases during the combustion of sufficiently large particles, the gasification actually consists of three phases: namely an initial particle heating period, an activation period for the surface reactions, and a diffusion-controlled, d²-law gasification period characterized by perpetually maximized surface reaction rates in spite of the continuously decreasing particle size. Radiation heat transfer is shown to have the same magnitude as those of reaction heat release and conduction, and actively affects the particle gasification response. For smaller particles, activation of the surface reactions is either substantially delayed subsequent to the initial heating period, or is completely suppressed, which respectively leads to either long burnout times or incomplete particle gasification. Influences due to the ambient oxygen concentration and the presence of CO₂ and H₂O as the oxidizer were also studied. Comparisons with literature experimental data show adequate agreement.     

双层多孔介质燃烧器的数值模拟

考虑多孔固体构架的辐射换热以及气固相间的对流换热，引入弥散效应及相间对流换热系数，使用GRI3．0机理和双通量辐射模型，数值求解双层多孔介质内燃烧过程．分析了双层多孔介质燃烧器内火焰稳定性和污染物排放，并与单层的进行比较．结果表明，双层多孔介质燃烧器能够在较宽的流量范围内将火焰稳定在它的交界面附近．     

A detailed chemical kinetic model of high-temperature ethylene glycol gasification

In recent experimental investigations, ethylene glycol is used as a model substance for biomass-based pyrolysis oil in an entrained flow gasifier. In order to gain a deeper insight into process sequences and to conduct parametric analysis, this study describes the development and validation of a detailed chemical kinetic model of high-temperature ethylene glycol gasification. A detailed reaction mechanism based on elementary reactions has been developed considering 80 species and 1243 reactions for application in CFD software. In addition to mechanism validation based on ignition delay times, laminar flame speeds and concentration profiles, simulation results are compared to experimental data of ethylene glycol gasification under complex turbulent reactive flow conditions.     

An assessment of large eddy simulations of premixed flames propagating past repeated obstacles

This paper presents an assessment of Large Eddy Simulations (LES) in calculating the structure of turbulent premixed flames propagating past solid obstacles. One objective of the present study is to evaluate the LES simulations and identify the drawbacks in accounting the chemical reaction rate. Another objective is to analyse the flame structure and to calculate flame speed, generated overpressure at different time intervals following ignition of a stoichiometric propane/air mixture. The combustion chamber has built-in repeated solid obstructions to enhance the turbulence level and hence increase the flame propagating speed. Various numerical tests have also been carried out to determine the regimes of combustion at different stages of the flame propagation. These have been identified from the calculated results for the flow and flame characteristic parameters. It is found that the flame lies within the ‘thin reaction zone’ regime which supports the use of the laminar flamelet approach for modelling turbulent premixed flames. A submodel to calculate the model coefficient in the algebraic flame surface density model is implemented and examined. It is found that the LES predictions are slightly improved owing to the calculation of model coefficient by using submodel. Results are presented and discussed in this paper are for the flame structure, position, speed, generated pressure and the regimes of combustion during all stages of flame propagation from ignition to venting. The calculated results are validated against available experimental data.     

Quantifying the effect of CO2 gasification on pulverized coal char oxy-fuel combustion

Previous research has provided strong evidence that CO₂ and H₂O gasification reactions can provide non-negligible contributions to the consumption rates of pulverized coal (pc) char during combustion, particularly in oxy-fuel environments. Fully quantifying the contribution of these gasification reactions has proven to be difficult, due to the dearth of knowledge of gasification rates at the elevated particle temperatures associated with typical pc char combustion processes, as well as the complex interaction of oxidation and gasification reactions. Gasification reactions tend to become more important at higher char particle temperatures (because of their high activation energy) and they tend to reduce pc oxidation due to their endothermicity (i.e. cooling effect). The work reported here attempts to quantify the influence of the gasification reaction of CO₂ in a rigorous manner by combining experimental measurements of the particle temperatures and consumption rates of size-classified pc char particles in tailored oxy-fuel environments with simulations from a detailed reacting porous particle model. The results demonstrate that a specific gasification reaction rate relative to the oxidation rate (within an accuracy of approximately +/- 20% of the pre-exponential value), is consistent with the experimentally measured char particle temperatures and burnout rates in oxy-fuel combustion environments. Conversely, the results also show, in agreement with past calculations, that it is extremely difficult to construct a set of kinetics that does not substantially overpredict particle temperature increase in strongly oxygen-enriched N₂ environments. This latter result is believed to result from deficiencies in standard oxidation mechanisms that fail to account for falloff in char oxidation rates at high temperatures.     

气流床撞击流水煤浆气化火焰光谱辐射特性实验研究

火焰光谱检测技术应用于气化炉有效监控,能实时反映气化炉工况,保障气化炉稳定运行。采用实验室规模的气流床撞击水煤浆气化装置,利用光纤光谱仪通过对气化炉不同部位进行探测,研究了水煤浆气化火焰在距离撞击平面不同轴向位置L处的光谱辐射特性,并利用不同自由基强度及分布对气化炉内各反应区进行表征,为气化炉运行工况提供依据。结果表明:在300~800 nm范围内可检测到明显的OH(306.7和309.8 nm),H 2(382 nm),CH(314.5和387 nm),Na(589 nm),Ar(671 nm)和K(404,768和770 nm)特征峰,而各种粒子激发方式及分布方式不同,可用于实现火焰宏观特征的表征。从紫外至可见光区域。水煤浆气化火焰中存在强烈的背景辐射,主要包括颗粒在高温下产生的黑体辐射及CO 2受热激发产生的350~600 nm的连续旋转辐射,强烈的背景辐射对自由基强度辐射测定形成干扰,需通过计算扣除背景辐射。利用检测到的各自由基强度分布可对气化火焰进行表征,OH分布可表征火焰反应区域,而CH存在范围相对较窄,仅存在于-10 cm相似文献   

Effect of non-zero relative velocity on the flame speed of two-phase laminar flames

A numerical study of one-dimensional n-heptane/air spray flames is presented. The objective is to evaluate the flame propagation speed in the case where droplets evaporate inside the reaction zone with possibly non-zero relative velocity. A Direct Numerical Simulation approach for the gaseous phase is coupled to a discrete particle Lagrangian formalism for the dispersed phase. A global two-step n-heptane/air chemical mechanism is used. The effects of initial droplet diameter, overall equivalence ratio, liquid loading and relative velocity between gaseous and liquid phases on the laminar spray flame speed and structure are studied. For lean premixed cases, it is found that the laminar flame speed decreases with increasing initial droplet diameter and relative velocity. On the contrary, rich premixed cases show a range of diameters for which the flame speed is enhanced compared to the corresponding purely gaseous flame. Finally, spray flames controlled by evaporation always have lower flame speeds. To highlight the controlling parameters of spray flame speed, approximate analytical expressions are proposed, which give the correct trends of the spray flame propagation speed behavior for both lean and rich mixtures.     

Experimental low-stretch gaseous diffusion flames in buoyancy-induced flowfields

The present study experimentally investigates the structure and instabilities associated with extremely low-stretch (1 s⁻¹) gaseous diffusion flames. Ultra-low-stretch flames are established in normal gravity by bottom burning of a methane/nitrogen mixture discharged from a porous spherically symmetric burner of large radius of curvature. OH-PLIF and IR imaging techniques are used to characterize the reaction zone and the burner surface temperature, respectively. A flame stability diagram mapping the response of the ultra-low-stretch diffusion flame to varying fuel injection rate and nitrogen dilution is explored. In this diagram, two main boundaries are identified. These boundaries separate the stability diagram into three regions: sooting flame, non-sooting flame, and extinction. Two distinct extinction mechanisms are noted. For low fuel injection rates, flame extinction is caused by heat loss to the burner surface. For relatively high injection rates, at which the heat loss to burner surface is negligible, flame radiative heat loss is the dominant extinction mechanism. There also exists a critical inert dilution level beyond which the flame cannot be sustained. The existence of multi-dimensional flame phenomena near the extinction limits is also identified. Various multi-dimensional flame patterns are observed, and their evolutions are studied using direct chemiluminescence and OH-PLIF imaging. The results demonstrate the usefulness of the present burner configuration for the study of low-stretch gaseous diffusion flames.     

Solid-phase reactions and the order-disorder phase transition in thin films

A comparative analysis was carried out of the initiation temperatures of solid-phase reactions in bilayer solid films and the Kurnakov temperatures of the phases forming in the reaction products. It has been shown that in superstructures where ordering is usually observed, the Kurnakov temperature coincides with the initiation temperature of the solid-phase reactions if no other solid-phase structural transformation precedes the order-disorder phase transition in the state diagram. A rule was proposed by which pairs of films capable of entering into solid-phase reactions and their initiation temperatures can be determined.     

热氧喷嘴水煤浆扩散火焰辐射光谱特性研究

火焰的辐射光谱可为燃烧诊断提供诸多信息,因此目前对简单的气态火焰自由基辐射特性已进行了大量研究,而关于非均相火焰的辐射光谱特性研究则相对较少。采用改进的热氧喷嘴技术在敞开空间下直接点燃水煤浆,并利用光纤光谱仪和紫外成像系统,着重对甲烷和水煤浆火焰的辐射光谱及OH*的二维分布特性进行研究。结果表明：与甲烷火焰的光谱辐射相比,水煤浆火焰不仅存在OH*,CH*和C₂*特征辐射,还产生了Na*,Li*,K*和H*的发射谱线,并出现了连续的黑体辐射,这些光谱辐射特征可作为水煤浆气化或燃烧的标志,也可作为水煤浆是否点燃的判据;通入水煤浆后,OH*强度明显下降,而CH*和C₂*强度增大。对比甲烷火焰OH*二维分布,水煤浆火焰OH*峰值强度明显下降,化学反应区域面积显著减小;沿着火焰传播方向,甲烷和水煤浆火焰轴向的OH*强度均呈先增大后减小的趋势;甲烷火焰径向的OH*在反应核心区出现了双峰形态分布,而水煤浆火焰OH*径向始终呈单峰分布。随着氧碳当量比增大,水煤浆火焰OH*的存在范围扩大,说明氧气的增加促进了OH*的产生;随水煤浆流量提高,OH*的反应核心区域缩小,峰值强度明显下降,CH*,C₂*,Na*,Li*,K*和H*的强度显著增强,连续的黑体辐射强度也明显增大,这些辐射光谱的变化可用于表征操作负荷的变化。     

Piloted ignition and extinction for solid fuels

Piloted ignition of solid fuels is investigated by simulating the transport and chemical reaction in a counter-flow arrangement where a known fuel (methane) is supplied through a porous burner and the power and the location of the igniter are varied. The porous burner arrangement simulates a pyrolyzing solid fuel at constant temperature by separating the gas phase from the solid conduction and pyrolysis phenomena. An Arrhenius one-step global reaction and a simplified transport model with Lewis number equal to one were used in the simulation. Only quasi-steady conditions are considered for the gas phase in this work because the response time for the solid phenomena is, in general, much larger than the response diffusion time for the gaseous phenomena. The relation of piloted ignition to extinction is also investigated. The effect of Damköhler number on ignition and extinction and the effect of the igniter on ignition are presented through a characteristic S curve obtained by plotting the evolving maximum temperature as a function of fuel mass flux. Based on the S-shaped curve (representing the maximum temperature in the system versus the mass flux of fuel), the relationship between the piloted ignition and extinction turning points and mass fluxes has been demonstrated in this paper. The piloted ignition turning point gradually approaches the extinction turning point with increasing Damköhler number and also with increasing power of the igniter. The ignition mass flux is found to depend basically on three parameters, Damköhler number, the location of the igniter and the power of the igniter all expressed in dimensionless forms.     

Flame structure and flame spread rate over a solid fuel in partially premixed atmospheres

We have investigated the downward flame spread over a thin solid fuel. Hydrogen, methane, or propane, included in the gaseous product of pyrolysis reaction, is added in the ambient air. The fuel concentration is kept below the lean flammability limit to observe the partially premixing effect. Both experimental and numerical studies have been conducted. Results show that, in partially premixed atmospheres, both blue flame and luminous flame regions are enlarged, and the flame spread rate is increased. Based on the flame index, a so-called triple flame is observed. The heat release rate ahead of the original diffusion flame is increased by adding the fuel, and its profile is moved upstream. Here, we focus on the heat input by adding the fuel in the opposed air, which could be a direct factor to intensify the combustion reaction. The dependence of the flame spread rate on the heat input is almost the same for methane and propane/air mixtures, but larger effect is observed for hydrogen/air mixture. Since the deficient reactant in lean mixture is fuel, the larger effect of hydrogen could be explained based on the Lewis number consideration. That is, the combustion is surely intensified for all cases, but this effect is larger for lean hydrogen/air mixture (Le < 1), because more fuel diffuses toward the lean premixed flame ahead of the original diffusion flame. Resultantly, the pyrolysis reaction is promoted to support the higher flame spread rate.     

Development of plasma pyrolysis/gasification systems for energy efficient and environmentally sound waste disposal

As more efficient and reliable torches for thermal plasma generation have become available in recent years, the use of thermal plasma as an energy source for pyrolysis/gasification has attracted much interest, and special attention has been paid to waste treatment for resource and energy recovery. Plasma pyrolysis/gasification systems have unique features such as the extremely high reaction temperature and ultra-fast reaction velocity compared to traditional pyrolysis/gasification systems. Plasma pyrolysis/gasification is therefore acknowledged as a novel pyrolysis/gasification technology with great potential in solid waste disposal. This paper gives a comprehensive review on the development of fundamental researches on plasma pyrolysis/gasification systems including direct current (DC) arc plasma system and radio frequency (RF) plasma system with an emphasis on reactor design such as plasma fixed/moving bed reactor system, plasma entrained-flow bed reactor system and plasma spout-fluid bed reactor system.     

A three-dimensional model of steady flame spread over a thin solid in low-speed concurrent flows

Athree-dimensional model of a steady concurrent flame spread over a thin solid in a low-speed flowtunnel in microgravity has been formulated and numerically solved. The gas-phase combustion model includes the full Navier-Stokes equations for the conservation of mass, momentum, energy and species. The solid is assumed to be a thermally thin, non-charring cellulosic sheet and the solid model consists of continuity and energy equations whose solution provides boundary conditions for the gas phase. The gas-phase reaction is represented by a one-step, second-order, finite-rate Arrhenius kinetics and the solid pyrolysis is approximated by a one-step, zeroth-order decomposition obeying an Arrhenius law. Gas-phase radiation is neglected but solid radiative loss is included in the model. Selected results are presented showing detailed three-dimensional flame structures and flame spread characteristics.

In a parametric study, varying the tunnel (solid) widths and the flow velocity, two important three-dimensional effects have been investigated, namely wall heat loss and oxygen side diffusion. The lateral heat loss shortens the flame and retards flame spread. On the other hand, oxygen side diffusion enhances the combustion reaction at the base region and pushes the flame base closer to the solid surface. This closer flame base increases the solid burnout rate and enhances the steady flame spread rate. In higher speed flows, three-dimensional effects are dominated by heat loss to the side-walls in the downstream portion of the flame and the flame spread rate increases with fuel width. In low-speed flows, the flames are short and close to the quenching limit. Oxygen side diffusion then becomes a dominant mechanism in the narrow three-dimensional flames. The flame spreads faster as the solid width is made narrower in this regime. Additional parametric studies include the effect of tunnelwall thermal condition and the effect of adding solid fuel sample holders.