单颗粒无烟煤着火特性典型影响因素研究

为理解不同典型参数对无烟煤着火特性的影响,本文建立了单颗粒煤粉着火模型。基于主要的总包非均相反应和气相反应及相应的对流和传热传质规律,模拟O_2/N_2燃烧方式下煤粉颗粒的着火过程,研究了不同的气流温度、O_2浓度、对流条件等关键因素对无烟煤颗粒着火的影响特征,结果表明气流温度增加时煤粉颗粒着火延迟时间在不同对流条件下普遍变短,且温度较高时着火延迟时间对对流强度变化的响应有所减弱;在相同气流温度和O_2浓度条件下,气流对流强度处于较低水平时,其变化对着火延迟时间的影响相对明显;当气流的温度和对流条件一定时,O_2浓度增加则着火延迟时间变短,但影响较小。模型得到文献数据的有效检验。     

劣质烟煤和无烟煤燃烧生成飞灰的电镜研究

一、前言 粉煤燃烧是一个非常复杂的物理化学过程。当煤粉被喷入锅炉炉膛后,受到火焰的辐射加热和回流烟气的对流加热,温度急剧上升,加热速率非常快,可达10~5K/秒。在一个极短的时间内,在煤粉粒子与周围介质之间及粒子内部进行着一系列的热质交换和化学反应:煤粒析出挥发份;挥发份与周围氧化介质发生气相反应;其反应产物与剩余焦碳     

湍流条件下煤粉颗粒群着火特性实验研究

本文搭建了平面火焰携带流反应器系统,研究了不同煤粉条件下煤粉颗粒群湍流射流着火特性。实验表明,湍流射流速度下低挥发份煤着火表现出扩散状暗红色火焰形态;随挥发分含量的增大,火焰亮度增加,并且在一定高度处发展为颗粒群群燃着火。随给粉量的增大,着火延迟呈现先减小后增大的趋势,最佳给粉浓度为0.75~1.00 kg/m~3。随粒径的减小,加热速率更快,挥发分大量析出使得火焰亮度增加;并且颗粒间的相互作用增强,更早出现颗粒群的群燃现象。     

激光点火煤粒着火的模型分析

本文对颗粒煤在激光加热条件下的着火和燃烧进行了数值模拟。采用的是一个简单的煤粒着火与燃烧的一维模型。该模型采用了热解和双平行反应模型,考虑了煤粒表面的多相反应和气相的基元反应以及气相中的传热与传质。从获得的煤粒表面和气相空间的温度随时间的变化规律,可以判断不同煤种的着火方式。     

不同重力环境下辐射加热材料表面着火特性分析

研究外界辐射加热下，不同重力环境中热薄燃料的着火特性．探讨了重力、环境氧浓度、环境压力及外界辐射强度对着火的影响．结果表明，随着重力的变化，存在不同的着火机制．在微重力和在高的环境氧浓度中，材料的着火延迟时间变短．压力减小，着火延迟时间增大．随着辐射强度的增大，着火延迟时间变小．     

对冲火焰中煤粉着火的实验研究

本文发展并建立了一套适用于研究煤粉穿越气相火焰面着火特性的对冲火焰实验系统,基于碰撞解聚原理研制了全新的微小载气量给粉系统。通过统计ICCD拍摄的煤粉着火图像,结合数值计算分析煤粉颗粒的沿程温度经历,发现穿越扩散火焰面时颗粒升温速率可达10~5 K/s,火焰面温度及煤粉停留时间是决定煤粉穿越火焰面后着火模式的主要因素。     

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.     

辐射在CH泡沫中传输的数值模拟

 利用一维多群辐射输运程序对辐射在CH泡沫中的传输过程进行了数值模拟,给出一些细致的物理图像和定量结果。在一定的入射辐射流条件下，密度变化对辐射传输特征、辐射加热介质的热力学平衡弛豫过程有重要影响。随着密度下降，辐射由亚声速传输转变为超声速传输，辐射在传输过程中的能谱形状也不同。     

半透明随形粒子形貌对近场辐射的影响研究

本文以半透明粒子材料作为研究对象,通过变换粒子表面粗糙度建立了随形粒子近场辐射的传输模型,并引入时域有限差分偶极子法模拟粒子的辐射涨落电流和辐射传热,通过与圆球形粒子的解析解进行对比验证了本方法的正确性,并分析了不同随形粒子间的近场辐射特性.研究结果显示,在谱带1～10 μm范围内,随着粒子表面粗糙度的增大,粒子间辐射...     

辐射方向对木材引燃特性影响的实验研究

在有点火源的条件下,对榆木在两种不同辐射方向下的热解和着火特性进行了研究,测定了不同外加热辐射强度、不同辐射方向下榆木的着火时间、表面温度、质量损失的变化规律。实验结果表明,榆木在水平方向下较垂直方向更容易着火,临界热流更小,着火表面温度更高,质量损失速率更小。     

Prediction of soot formation characteristics in a pulverized-coal combustion field by large eddy simulations with the TDP model

In this study, the soot formation characteristics in a pulverized-coal combustion field formed by a 4 kW Central Research Institute of Electric Power Industry (CRIEPI) jet burner were predicted by large eddy simulation (LES) employing a tabulated-devolatilization-process model (TDP model) [N. Hashimoto et al., Combust. Flame 159 (2012) 353–366]. This model enables to take into account the effect of coal particle heating rate on coal pyrolysis. The coal-derived soot formation model proposed by Brown and Fletcher [A. L. Brown and T. H. Fletcher, Energy Fuels 12 (1998) 745–757] was employed in the LES. A comparison between the data predicted by LES and the soot volume fraction distribution data measured by laser induced incandescence confirmed that the soot formation characteristics in the coal combustion field of the CRIEPI burner can be accurately predicted by LES. A detailed analysis of the data predicted by LES showed that the soot particle distribution in this burner is narrow because the net soot formation rate is negative on both sides of the base of the soot volume fraction. At these positions, soot particles diffused from the peak position of soot volume fraction are oxidized due to a relatively high oxygen concentration. Finally, the effect of soot radiation on the predicted gas temperature distribution was examined by comparing the simulation results obtained with and without soot radiation. This comparison showed that the maximum gas temperature predicted by the simulation performed with soot radiation was over 100 K lower than that predicted by the simulation performed without soot radiation. From result strongly suggests the importance of considering a soot formation model for performing numerical simulations of a pulverized-coal combustion filed.     

煤颗粒热解的传热传质分析

采用分布活化能模型及能量守恒方程对煤颗粒热解的传热传质过程建立数学模型,模型考虑煤热解的吸热效应及挥发分逸出时对流换热的影响.与有关煤粉和大颗粒煤热解的实验数据对比,对模型进行验证.针对煤颗粒的温度变化过程和煤热解过程进行数值分析,研究煤热解的吸热效应、挥发分气体逸出的对流效应、颗粒尺寸等参数的影响.     

Heat and mass transfer at gas-phase ignition of grinded coal layer by several metal particles heated to a high temperature

Characteristics of gas-phase ignition of grinded brown coal (brand 2B, Shive-Ovoos deposit in Mongolia) layer by single and several metal particles heated to a high temperature (above 1000 K) have been investigated numerically. The developed mathematical model of the process takes into account the heating and thermal decomposition of coal at the expense of the heat supplied from local heat sources, release of volatiles, formation and heating of gas mixture and its ignition. The conditions of the joint effect of several hot particles on the main characteristic of the process–ignition delay time are determined. The relation of the ignition zone position in the vicinity of local heat sources and the intensity of combustible gas mixture warming has been elucidated. It has been found that when the distance between neighboring particles exceeds 1.5 hot particle size, an analysis of characteristics and regularities of coal ignition by several local heat sources can be carried out within the framework of the model of “single metal particle / grinded coal / air”. Besides, it has been shown with the use of this model that the increase in the hot particle height leads, along with the ignition delay time reduction, to a reduction of the source initial temperatures required for solid fuel ignition. At an imperfect thermal contact at the interface hot particle / grinded coal due to the natural porosity of the solid fuel structure, the intensity of ignition reduces due to a less significant effect of radiation in the area of pores on the heat transfer conditions compared to heat transfer by conduction in the near-surface coal layer without regard to its heterogeneous structure.     

高温电离气体中微细颗粒的加热

本文分析表明，自由分子流区高温电离气体中颗粒的团相加热、熔化、液相加热与强烈蒸发等加热阶段所需的时间，均正比于颗粒半径，其中蒸发阶段需时最长．颗粒表面温度高于约４０００Ｋ时，热电子发射开始显著影响颗粒的加热速率，辐射则在低得多的温度下（如２０００Ｋ）开始影响颗粒的加热．     

Phenomenological models of microwave heating of a flat coal mass with release of absorbed heat by the convection law

Phenomenological models of electrodynamics and heat transfer in application to microwave heating are constructed. Analytically rigorous solutions to problems of heating a flat coal mass under microwave radiation are obtained. The boundary conditions correspond to convection mechanism of absorbed heat release to the ambient medium. Mathematical models of dielectric heating for homogeneous boundary conditions are solved by a method of dual integral Laplace and Fourier transform. In the presence of inhomogeneities in the boundary conditions, a quite universal method of Green functions is used. The obtained formulas have a constraint associated with constancy of electro- and thermophysical characteristics of coal fuel, or when their piecewise constant approximation is admissible. The obtained dependences form the basis for scientific support of the microwave heating technology.     

沙尘大气物理约束方程研究

基于风沙两相流的相互作用和气块质量变化的观点,推导出了闭合的沙尘大气物理约束方程组.理想状况下对之分析发现:沙尘大气密度比同体积洁净大气大,一定程度上会减缓气块的运动速度;沙尘与空气间的速度差异会使得细粒子处于高流速区,而粗粒子位于低流速区;温度差异会使得沙尘在上升运动中充当热源,下沉运动中充当冷源,从而加强对流;沙尘大气质量定容热容会在等压面上诱发新的温度梯度,促进沙尘云边界处的夹卷;沙尘大气气体常数会在等温面上诱发新的气压梯度,促进沙尘云边界处的夹卷;质量变化会对气块密度、速度和温度造成较大影响.总之,沙尘云要比基于被动标量的方程组模拟结果更高大、内部对流更强、边界处夹卷更活跃、水平运动更迟缓.     

流化床内单个颗粒传热特性的模拟

单个颗粒的传热过程对于燃煤流化床锅炉中煤的着火和随后的燃烧过程具有重要的影响。本文建立了一个基于Euler-Lagrange方法的数学模型对其进行了数值研究。该模型对流体相的运动和传热规律以Euler。方法描述,对固体颗粒相则以近年来发展起来的离散单元方法(DEM)在颗粒层次上进行描述。研究结果表明颗粒与床层之间有很高的传热系数。另外,对单个颗粒温度变化过程的模拟研究表明,颗粒经过较短的时间便能接近床层的温度,且不同颗粒具有类似的温度变化特性。     

Model of heating and ignition of conductive polydisperse powder in electrostatic discharge

Heating of a conductive polydisperse powder by electrostatic discharge (ESD) is modelled numerically. Powder packing is described using a discrete element model; powder resistance is defined by geometry of particle contacts and properties of plasma produced by electrical breakdown between neighbour particles. A set of parametric calculations in combination with experimental data is used to determine necessary adjustable model parameters. The model predicts the temperature for each powder particle resulting from its heating by the ESD current. Location and packing of individual particles within the powder affects greatly their achieved temperatures and thus the likelihood of ignition. Consistently with experiments, a trend showing that smaller particles are generally heated to higher temperatures at a given ESD energy is detected for coarser powders; this trend becomes less clear for finer powders with particle sizes less than the breakdown distance given by the Paschen curve in air. Comparison of the experimental data and calculations suggests that the transition from single particle to cloud combustion occurs when the distance between the particles ignited by ESD becomes close to the flame size for the individual burning particle. This distance, inversely proportional to the number of ignited particles, is primarily determined by the ESD energy.     

煤燃烧过程中一次破碎的影响因素分析

本文从理论上分析了煤燃烧过程中的一次破碎过程,同时考虑热应力和内部压力积聚的作用,建立了简化的一次破碎模型,通过此模型分析了煤颗粒的粒径、炉膛温度、炉膛对煤颗粒的传热系数、煤颗粒的热扩散系数、煤颗粒脱挥发分的化学动力学参数、煤中对流孔直径、煤的孔隙率、挥发分的粘性系数以及挥发分含量这9个参数对燃烧过程中煤颗粒一次破碎的影响,分析所得结果与近年来报道的实验结论基本保持一致。     

双颗粒在溶解条件下沉降的多相流动特性

对牛顿流体内溶解双颗粒在垂直管道中的沉降运动进行了直接数值模拟. 流体运动由守恒方程计算, 密度和黏性的变化考虑流场温度变化的影响, 通过积分粘性应力和压力获得颗粒的受力跟踪颗粒运动, 溶解引起的相变及其形状的变化由溶解潜热、溶解质量与分散相边界处的温度梯度的关系建立的方程决定. 通过颗粒和流体间相互的作用力和力矩及边界条件的施加实现相间耦合. 对双颗粒在等温流体无溶解条件和非等温流体溶解条件下的沉降过程进行了计算. 结果表明, 在一定雷诺数内, 热对流产生的颗粒尾迹处涡的脱落以及溶解引起的颗粒质量、颗粒表面形态的变化引起了颗粒的横向摆动, 并使颗粒沉降速度发生了变化.