镁颗粒群着火和燃烧过程数值模拟

建立了一维非稳态球形镁颗粒群的着火燃烧模型, 数值模拟镁颗粒群的着火和燃烧过程, 研究表明, 颗粒群着火首先发生在颗粒群边界, 随后初始的燃烧火焰会分离为两个, 一个向颗粒群内部传播, 一个向外部传播, 最终内部火焰消失, 外部火焰维持并控制着整个颗粒群的燃烧; 内火焰向颗粒群内部传播过程中, 传播速度会逐渐加快, 且火焰温度值呈逐渐降低趋势. 分析了颗粒群内部参数和环境参数对镁颗粒群着火燃烧的影响. 随颗粒浓度的增大, 颗粒群着火时间略有增长, 但火焰传播速度更快, 燃烧稳定时火焰球尺寸也更大. 颗粒群初温越高, 则颗粒群着火时间越短, 火焰传播速度也会加快, 但燃烧稳定时火焰球尺寸基本不变. 环境温度对颗粒群着火燃烧的影响较复杂, 环境温度越高, 颗粒群着火时间越短, 但火焰传播速度却越慢, 燃烧稳定时火焰球尺寸变化很小. 颗粒粒径和辐射源温度对颗粒群着火燃烧的影响较显著, 颗粒粒径越小或辐射源温度越高, 则颗粒群着火时间越短, 火焰传播速度越快, 燃烧稳定时火焰球尺寸也越大. 数值模拟结果与文献中试验结果相一致. 关键词： 粉末燃料冲压发动机 镁着火燃烧 颗粒群     

弱浮力下导线绝缘层着火先期特性的研究

本文在低压弱浮力环境中模拟了微重力过载电流时导线绝缘层的着火先期过程,研究了压力对绝缘层着火先期特性的影响。结果表明,2 A时,绝缘层的温升率和最终平衡温度随压力降低而不断增大,模拟出了微重力下绝缘层的温升特性;10 A时,随着压力降低,绝缘层存在三种破坏机制;在不同区域压力对着火延迟时间的影响不同。最后,提高环境氧气浓度修正了低压对化学反应速率的抑制。结果表明,当压力在一定范围内降低时,随着氧气浓度的提高,绝缘层的着火极限范围拓宽,着火延迟时间缩短,可以初步模拟微重力下绝缘层的着火先期过程。     

三维格子涡方法模拟自由下落颗粒群

本文发展了基于双向耦合的三维格子涡方法,采用涡方法模拟流场中的涡量变化及涡元运动,使用双势法求解速度场,采用拉格朗日方法跟踪颗粒相.利用该模型模拟了颗粒群的自由下落及下落过程引起的气相流动,模拟结果与实验吻合良好。结果显示空气相速度径向分布满足高斯分布,颗粒相流量对颗粒群的扩散影响不显著,而流量增大会引起颗粒速度的增大.     

颗粒趋壁沉积的直接数值模拟

气相采用直接数值模拟方法、颗粒相采用拉格朗日轨道模型方法对摩擦雷诺数为180的充分发展槽道内Stokes数为1～104的颗粒趋壁沉积现象进行了研究.研究表明,惯性颗粒在湍流涡的作用下趋向壁面运动,并以一定速率沉积.在"吸收壁面"条件下沉积速率首先随着颗粒弛豫时间增大而增大,在弛豫时间大于100时,沉积速率保持不变.     

考虑Stefan影响的单颗粒硼着火过程研究

针对含硼推进剂固体火箭冲压发动机内单颗粒硼的着火过程展开了系统研究. 考虑硼颗粒周围气相流动以及硼颗粒与周围环境间的传热传质过程, 建立了考虑Stefan流作用的一维硼颗粒着火模型, 研究了硼颗粒实现着火和未能实现着火两种典型情形下硼颗粒及周围气相的参数变化规律, 对两种情形下Stefan流的变化规律及其成因展开了详细分析. 研究表明, 在硼颗粒实现着火的过程中, 液态B₂O₃的蒸发及硼的 氧化均能在硼颗粒的反应自加热作用下急剧加速, 硼颗粒表面附近的氧气和气相B₂O₃分布变化剧烈; 在未能实现着火的过程中, 液态B₂O₃的蒸发和氧气消耗的质量流率相对较小, 并逐渐趋于稳定, 硼颗粒表面附近的氧气和气相B₂O₃分布相对变化很小.在两种典型情形下, 硼颗粒外表面的Stefan流都会经历先由周围空间流向颗粒表面, 而后变为由颗粒表面流向周围空间的过程. 关键词： 固体火箭冲压发动机 硼颗粒 着火过程 Stefan流     

煤粉再燃脱硝效率与着火状态之间的关系

在一维炉上采用煤粉作为再燃燃料进行了脱硝的实验研究,发现脱硝效率随再燃区氧浓度的增大呈现非单调的变化规律.采用煤粉气流均相着火模型和炭粒非均相热力着火模型对煤粉再燃脱硝效率与其着火状态的相互作用进行了研究.煤粉均相着火之前,脱硝效率随氧浓度的增加而上升,均相着火之后,脱硝效率明显下降并逐渐达到一个谷点.氧浓度进一步增大时,煤粉发生非均相着火,颗粒温度升高,异相脱硝效率升高,它的作用开始占优,总体脱硝效率再次上升.     

生物质超临界水制氢的数值模拟

本文对生物质在超临界水环境下气化制氢过程提出简化的两相流物理化学模型,并利用该模型进行数值模拟.着重讨论了温度、颗粒半径对生成气体摩尔百分比、气化率的影响.数值结果表明,颗粒的半径主要影响生物质颗粒气化分解的速率,而温度主要影响颗粒气化产物进一步生成氢气的过程.颗粒越小,气化分解的速率越快.温度的影响主要集中在气相反应上,使得CO进一步转化为H2.本文的理论和数值结果对实际的制氢过程中的参数控制具有实用价值.     

燃煤黄铁矿迁移转化模拟的研究

基于已有的动力学数据对单颗粒黄铁矿迁移转化进行了全过程模拟,分析讨论了各种因素对黄铁矿氧化过程和S释放率的影响,结果表明:随着颗粒尺寸的减小和炉膛温度的升高,黄铁矿颗粒的分解氧化过程加快,颗粒在炉内停留很短时间内温度即升高到共融点以上,使颗粒呈混熔态,进而导致灰沉积、结渣.颗粒氧化过程中S的释放主要受扩散控制,其释放速率与颗粒粒径成反比,与氧气浓度和炉膛温度成正比.     

基于分子运动对流换热和热辐射下煤粉颗粒群粒子的加热分析

本文建立了在烟气对流加热和辐射状态下的煤粉颗粒群加热模型,通过数值模拟,模拟研究了不同锅炉炉膛尺寸下不同煤粉粒子粒径的煤粉群粒子加热时间以及粒子温升的关系,对对流换热和辐射换热在着火热源中所占比重进行了分析,模型很好的模拟了粒子的升温,能够较好的反映出煤粉粒子加热升温机理,为煤粉射流微元加热及着火提供了计算方法。     

气体-颗粒掺混的数值模拟研究

通过数值模拟方法研究了圆管内湍流气体和压力旋流雾化喷嘴产生的液滴群的掺混规律.定义了掺混度、不均匀浓度因子和无量纲颗粒浓度定量评价掺混水平,根据这些特征参数,获得了掺混度曲线和截面等浓度线.分析了不同水气比、喷嘴雾化角度和颗粒直径情况下颗粒群对横向气流的影响以及气相对颗粒群掺混过程的作用.结果表明:颗粒群的加入使气相产生了尺度不同的涡,促进了气相湍流发展;近喷雾区,大尺度的涡造成了颗粒局部浓度较高,掺混度较低,但是经过较短的距离就能充分掺混,此时掺混度值较高,同时初始的喷雾形态和壁面处的较大速度梯度使得颗粒存在趋壁现象;水气比1:1、中等雾化角度90°和较大的颗粒更有利于掺混.     

煤低温氧化的微区组分分析与反应性研究

煤的低温氧化对着火性能、反应性、煤焦质量以及煤的自燃有重要影响．本文通过TGA-DSC、FSEM和EBSP研究了烟煤和无烟煤的低温氧化特性,研究表明:活性较高的煤表面固相氧的浓度增量比热重低温段的表观增重量大得多;根据最大吸氧量确定的着火温度随煤变质程度的加深和升温速率的增加而增加;升温速率小于1／12 K／s时,最大氧化速率和最大吸氧量能代表煤的氧化活性;含灰量低的中等变质程度的煤氧化活性最高;化学吸氧经历了从临界活化能、到活化能减小再增加到极大最后吸附终止的过程．     

Numerical study of coal particle ignition in air and oxy-atmosphere

The ignition and combustion of coal particles are investigated numerically under conventional and oxy-fuel atmospheres. Devolatilization is computed using the chemical percolation devolatilization (CPD) model. The CPD model is coupled with a Lagrangian particle tracking method in the framework of a multiphysics, multiscale Navier–Stokes solver. Combustion in the gas phase is described using finite rate chemistry. The numerical results for ignition are compared with available experimental data and a remarkably good agreement is observed. The effect on flame ignition of the different phases characterizing the release of volatile gases is assessed. These different phases manifest themselves in two distinct peaks in the devolatilization rate and it is observed that ignition can occur during the first volatile release or on the onset of the second, depending on the particle size and gas temperature. It is found that an increase of ignition delay time in oxy-atmosphere compared to the air case is related to the depletion of radicals that react with the abundant carbon dioxide of the oxy-atmosphere, while the increased heat capacity of the mixture does not play a role.     

Ignition of single coal particle in a hot furnace under normal- and micro-gravity condition

An experimental study on ignition and combustion of single particles was conducted at normal gravity (1-g) and microgravity (μ-g) for three high volatile coals with initial diameter of 1.5 and 2.0 mm, respectively. The non-intrusive twin-color pyrometry method was used to retrieve the surface temperature of the coal particle through processing the images taken by a color CCD camera. At the same time, a mathematical model considering thermal conduction inside the coal particle was developed to simulate the ignition process.Both experiments and modeling found that ignition occurred homogeneously at the beginning and then heterogeneously for the testing coal particles burning at μ-g. Experimental results confirmed that ignition temperature decreased with increasing volatile content and increasing particle size. However, contradicted to previous studies, this study found that for a given coal with certain particle size, ignition temperature was about 50–80 K lower at μ-g than that at 1-g.The model predictions agreed well with the μ-g experimental data on ignition temperature. The criterion that the temperature gradient in the space away from the particle surface equaled to zero was validated to determine the commence of homogeneous ignition. Thermal conduction inside the particle could have a noticeable effect for determining the ignition temperature. With the consideration of thermal conduction, the critical size for the phase transient from homogeneous to heterogeneous is about 700 μm at ambient temperature 1500 K and oxygen concentration 0.23.     

Particle imaging of ignition and devolatilization of pulverized coal during oxy-fuel combustion

Oxy-fuel combustion of coal is a promising technology for cost-effective power production with carbon capture and sequestration that has ancillary benefits of emission reductions and lower flue gas cleanup costs. To fully understand the results of pilot-scale tests of oxy-fuel combustion and to accurately predict scale-up performance through CFD modeling, fundamental data are needed concerning coal and coal char combustion properties under these unconventional conditions. In the work reported here, the ignition and devolatilization characteristics of both a high-volatile bituminous coal and a Powder River Basin subbituminous coal were analyzed in detail through single-particle imaging at a gas temperature of 1700 K over a range of 12–36 vol % O₂ in both N₂ and CO₂ diluent gases. The bituminous coal images show large, hot soot cloud radiation whose size and shape vary with oxygen concentration and, to a lesser extent, with the use of N₂ versus CO₂ diluent gas. Subbituminous coal images show cooler, smaller emission signals during devolatilization that have the same characteristic size as the coal particles introduced into the flow (nominally 100 μm). The measurements also demonstrate that the use of CO₂ diluent retards the onset of ignition and increases the duration of devolatilization, once initiated. For a given diluent gas, a higher oxygen concentration yields shorter ignition delay and devolatilization times. The effect of CO₂ on coal particle ignition is explained by its higher molar specific heat and its tendency to reduce the local radical pool. The effect of O₂ on coal particle ignition results from its effect on the local mixture reactivity. CO₂ decreases the rate of devolatilization because of the lower mass diffusivity of volatiles in CO₂ mixtures, whereas higher O₂ concentrations increase the mass flux of oxygen to the volatiles flame and thereby increase the rate of devolatilization.     

Ignition and volatile combustion behaviors of a single lignite particle in a fluidized bed under O2/H2O condition

O₂/H₂O combustion, as a new evolution of oxy-fuel combustion, has gradually gained more attention recently for carbon capture in a coal-fired power plant. The physical and chemical properties of steam e.g. reactivity, thermal capacity, diffusivity, can affect the coal combustion process. In this work, the ignition and volatile combustion characteristics of a single lignite particle were first investigated in a fluidized bed combustor under O₂/H₂O atmosphere. The flame and particle temperatures were measured by a calibrated two-color pyrometry and pre-buried thermocouple, respectively. Results indicated that the volatile flame became smaller and brighter as the oxygen concentration increased. The ignition delay time of particle in dense phase was shorter than that in dilute phase due to its higher heat transfer coefficient. Also, the volatile flame was completely separated from particles (defined as off-flame) in dense phase while the flame lay on the particle surface (defined as on-flame) in dilute phase. The self-heating of fuel particles by on-flame in dilute phase was more obvious than that in dense phase, leading to earlier char combustion. At low oxygen concentration, the flame in the H₂O atmosphere was darker than that in the N₂ atmosphere because the heat capacity of H₂O is higher than that of N₂. With the increase of oxygen concentration, the flame temperature in the O₂/H₂O atmosphere was dramatically enhanced rather than that in the O₂/N₂ atmosphere, where the diffusion rate of oxygen in O₂/N₂ atmosphere became the dominant factor.     

Effect of polymorphic phase transformations in alumina layer on ignition of aluminium particles

The mechanism of aluminium oxidation is quantified and a simplified ignition model is developed. The model describes ignition of an aluminium particle inserted in a hot oxygenated gas environment: a scenario similar to the particle ignition in a reflected shock in a shock tube experiment. The model treats heterogeneous oxidation as an exothermic process leading to ignition. The ignition is assumed to occur when the particle's temperature exceeds the alumina melting point. The model analyses processes of simultaneous growth and phase transformations in the oxide scale. Kinetic parameters for both direct oxidative growth and phase transformations are determined from thermal analysis. Additional assumptions about oxidation rates are made to account for discontinuities produced in the oxide scale as a result of increase in its density caused by the polymorphic phase changes. The model predicts that particles of different sizes ignite at different environment temperatures. Generally, finer particles ignite at lower temperatures. The model consistently interprets a wide range of the previously published experimental data describing aluminium ignition.     

The effects of oxygen concentration and gas temperature on coal stream ignition and particle surface temperature in reducing-to-oxidizing environments

In the near-burner region of pulverized coal burners, two zones exist, with very different oxygen concentrations. The first zone is a locally reducing environment, caused by the fast release of volatiles from a region of dense coal particles, and the second zone, which is surrounding the first zone, is a hot oxidizing environment. The transition of coal particles from the reducing zone to the oxidizing zone affects early stage coal combustion characteristics, such as devolatilization, ignition and particle temperature history. In this work, we used a two-stage Hencken flat-flame burner to simulate the conditions that coal particles experience in practical combustors when they transition from a reducing environment to an oxidizing environments. The composition of the reducing environment was chosen to approximate that of a typical coal volatile. Three oxygen concentrations (5, 10 and 15 vol%) in the “ambient” oxidizing environment were tested, corresponding to those at different distances downstream from a commercial burner. The corresponding gas temperatures for the oxidizing environments were adjusted for the different oxygen concentrations such that the “volatile” flame temperatures were the same, as this is what would be expected in a commercial combustor. High speed videography was used to obtain the ignition characteristics, and RGB color pyrometry was used to measure particle surface temperatures. Two different sizes of coal particles were used. It is found that when particles undergo a reducing-to-oxidizing transition at high temperatures, the particles are preheated such that the critical factor for ignition delay is point at which the particle is in the presence of oxygen, not the concentration of oxygen. The ignition delay of large particles is found to be 53% longer than that of small particles due to their higher thermal mass and slower devolatilization. The oxygen concentration in the ambient have a negligible effect on early-stage particle temperatures.