CSTR系统中甲烷燃烧过程的分岔特性分析

运用分岔理论对连续流动均匀搅拌反应器(CSTR)中甲烷燃烧过程的分岔特性进行了详细分析,采用了甲烷的详细化学反应机理.分别以系统温度、滞留时间为分岔参数,详细讨论了CSTR系统的各种工况(系统压力、混合气过量空气系数、系统温度及滞留时间)对甲烷燃烧过程分岔特性的影响.结果表明,在甲烷的整个燃烧过程中,出现了双着火点,即部分着火点和完全着火点;同时,出现了3个阶段,即部分着火阶段、完全着火阶段和熄火阶段.当以系统温度为分岔参数时,随着系统压力的升高、滞留时间的延长、过量空气系数的增加,甲烷的完全着火温度也随之降低.当以滞留时间为分岔参数时,随着系统温度、系统压力的升高,甲烷发生完全着火所需要的滞留时间随之缩短;过量空气系数对甲烷发生着火所需要的滞留时间影响较小.     

CSTR系统中氢气燃烧特性的分岔分析

运用分岔理论,采用了氧气的详细化学反应机理,对连续流动均匀搅拌反应器(CSTR)中氢气的燃烧特性进行详细的分析.分别以系统温度、滞留时间为分岔参数,详细讨论了CSTR系统的各种工况(系统压力、入口混合气过量空气系数、系统温度及滞留时间)对混合气着火特性的影响.结果表明,当以系统温度为分岔参数时,系统压力及滞留时间对混合气的燃烧特性影响较大,而过量空气系数影响较小;当以滞留时间为分岔参数时,系统温度对混合气燃烧特性有较大的影响.     

燃料床特性对炭粉阴燃过程影响的理论预测

建立了自然对流(静止空气)条件下炭粉床一维竖直向下阴燃的简化数学模型,预测了典璎条件下燃料床各特性参数对阴燃过程的影响.结果表明,床层收缩率、碳堆积密度、燃料床孔隙尺寸、氧气在多孔灰层中的扩散率对阴燃传播速度、床层内部温度和阴燃持续时间影响显著;而初始条件、环境条件及燃料和灰的比热容、导热系数等物性参数的影响可忽略不计.     

自然通风下煤堆积床阴燃过程的基本特性

为研究地下煤火的蔓延特性,根据真实煤火背景设计搭建了煤火阴燃的实验装置．采用加热棒进行强制点火,并根据工况需要改变实验装置的通风条件,揭示了自然通风下煤火阴燃过程中温度分布和质量损失的变化规律．结果表明：点火过程表面裂隙的发育具有随机性,导致点火阶段不可重复．水平方向上,上、中层峰值温度稳定在590℃和540℃,传播速度为0.274 mm/min和0.261 mm/min．根据升温速率和加速度曲线将煤-氧反应过程分为吸附热控制区、水分蒸发控制区、氧化控制区、热解控制区和氧化-热解反应竞争区5个区间．将质量损失分为4个阶段,随着反应的进行,质量损失速率不断减小．侧边通风增大边界处氧气浓度,氧气从侧面沿水平方向进入煤层有一个渗透距离,大约为6 cm.     

考虑转捩影响的压气机叶型流动特性数值模拟北大核心

通过采用在SST湍流模型基础上发展形成的γ-Reθt转捩模型对某型可控扩散叶型进行了变冲角的数值模拟研究,并与试验结果和LES计算结果进行了比对。结果表明,SSTγ-Reθt转捩模型对叶型型面静压分布的模拟较为准确;在负冲角流动状态下对叶型的气动性能模拟比LES的结果更为贴近试验实际所得,但在正冲角流动状态下的模拟结果仍有提高的空间。     

煤粉掺烧干化污泥的燃烧特性及能效分析

针对某电厂420 t/h四角切圆煤粉炉进行掺烧污泥的改造,根据煤与污泥掺混后在煤粉炉上的着火、稳燃、结渣等特性分析了煤粉炉掺烧干化污泥的可行性,并利用火用平衡分析方法对煤粉炉掺混不同比例、不同含水率污泥时锅炉效率及各受热面火用损情况进行了分析。结果表明,当污泥掺混比例小于1∶4时,泥煤混合燃烧特性与煤相似,且火用效率略大;污泥掺混大于该比例后,其灰熔点下降明显,有明显结渣倾向,且排烟损失显著增加。以上研究结果可为电厂掺烧干化污泥的可行性提供了必要的实验和理论依据。     

球窝的流动控制及分离转捩特性研究

流体经过一定的逆压梯度容易发生流动分离从而大大减小流动效率,球窝结构具有良好的流动控制效果,在机翼、航空发动机、汽轮机叶片等工业领域有较大的应用前景。对一典型收缩扩张通道的流动分离转捩状况进行了数值模拟,并提出了一种带有球窝结构的被动控制方法。研究结果表明:球窝结构作为一种被动流动控制方法,布置在具有明显逆压梯度的通道上能起到良好的流动控制作用,并且能诱导层流边界层提前向湍流边界层转捩,抑制了通道中的流动分离,减小分离泡的尺度,其中球窝的布置位置以及流动Re均对球窝的控制作用有重要影响。球窝的引入还将减小通道的总压损失系数,起到了流动减阻的效果,表明球窝结构是一种较优的流动控制方法。     

棉秆热解及燃烧特性试验研究

利用热重分析仪研究棉秆在N2及空气气氛下的热解及燃烧特性,通过动力学分析得到了棉秆热解及燃烧的动力学参数,相关系数在0.99以上.研究结果表明:随着升温速率提高,棉秆热解过程中的活化能增加.试验棉秆的着火温度为246℃.     

不同气体燃料燃烧特性及热特性分析

由于气体燃料容易实现燃烧过程精确控制及燃烧的清洁化,文章采用数值计算的方法,对6种常见气体燃料的燃烧特性及热特性进行理论计算,并通过改变空气过量系数、空气预热温度、富氧度等燃烧条件对其燃烧特性及热特性进行对比和分析.结果表明,预热温度每提高200℃,对于天然、焦炉煤气和沼气来说,可提高燃烧温度约90℃,高炉煤气和发生炉...     

石灰石在油页岩CFB燃烧中脱硫特性分析

石灰石作为脱硫剂,在油页岩CFB燃烧中能显著减少SO2的排放量,分析油页岩和石灰石特性及其脱硫机理的基础上,通过1 Mw的CFB试验台的试验研究,讨论了不同Ca/S和不同燃烧温度下石灰石对脱硫特性的影响.试验结果表明,温度在860℃左右,Ca/S为2.6时,SO2排放浓度能满足环保要求,而且脱硫效率较高.     

水平阴燃向有焰火转化的研究

讨论了在水平阴燃稳定传播情况下,随风速的增加向有焰火发生转化的内在机理,并得到了其转化的临界条件．影响阴燃向有焰火发生转化的因素是多样的．随着阴燃展开程度的不同,所得到的临界风速的数值也发生变化,但数量级都在10^-2m／s．但燃烧结构发生变化时,在自然对流情况下,也会导致阴燃向有焰火发生转化．建立的模型很好地解释了这些现象,所得到的结果与有关实验及文献的结果相一致．     

On the transition from smoldering to flaming

We consider the transition from smoldering to flaming. Though there have been numerous experimental studies of this topic, it has been virtually untouched theoretically. Rather than investigating the details of either the transition process or the subsequent flaming, we focus on determining the mechanism and the conditions that trigger the transition. We employ both computation and approximate analytical approaches. We consider a planar, forward smolder wave driven by a constant forced flow of gas containing oxidizer. The chemical kinetic scheme employed consists of three reactions, namely, pyrolysis, fuel oxidation, and char oxidation. There have been a number of speculations about the nature of the triggering mechanism, including the gaseous reactions, the char oxidation reaction, destruction of the porous matrix through which the smolder wave propagates, and others. However, no mechanism has, as yet, been theoretically demonstrated to be capable of acting as the triggering mechanism. We show that the char oxidation reaction hardly affects the characteristics of smolder wave propagation due to its small reaction rate, though under appropriate conditions, it can act as the trigger for the transition to flaming due to its ability to self-accelerate. Specifically, we introduce the concept of, and then compute, a quantity that we term the flaming distance, LF. This is the distance that a steadily propagating smolder wave initiated at the gas flux inlet travels inside the porous medium before the char oxidation reaction spontaneously self-accelerates, resulting in an eruption of the temperature at the smolder front. That is, the smolder wave propagates for a relatively long latent period of time until it reaches LF. A transition to flaming then occurs. The flaming distance LF depends on the physicochemical parameters of the fuel and the products as well as external parameters: the velocity and composition of the incoming gas, heat loss, etc. We show that smolder waves propagating in porous samples of length L do (do not) exhibit a transition to flaming if LF<L (LF>L).     

聚亚安酯材料阴燃转为有焰燃烧的实验研究

在聚亚安酯材料的稳定阴燃中,通过实验研究了阴燃向有焰燃烧转化的机制.在富氧条件下,阴燃向有焰燃烧转化还需具备两个条件:一是有足够高的温度,要求高于约400℃;二是有足够高的热解可燃气浓度.在阴燃达到热平衡稳定状态对应的最高温度———峰值温度时(>400℃),第一个条件是自动满足的.当温度高于约350℃且在氧气充足的情况下,聚亚安酯材料发生更快速的有氧热解反应,此时释放的热解可燃气满足第二个条件.因此,沿着阴燃方向加风,或者在自然对流条件下增大氧的扩散量,都可以导致向有焰燃烧的转化.对于逆向阴燃,风速增加也导致向有焰燃烧的转化,规律同正向阴燃一致.     

半封闭空间阴燃模型及向明火转化的研究

为了了解在半封闭空间中阴燃的机制,对水平放置在实验体中的聚氨酯泡沫进行了阴燃实验研究.结果表明,在半封闭空间中,阴燃反应区域温度低且变化幅度小.根据在此条件下阴燃的特点,建立了阴燃区域的模型.分析表明,该模型在未反应区(未燃材料区)中符合得很好,在碳层区中除了接近材料末端处的温度外,其余各处误差不大.从阴燃转明火的实验研究中发现,在半封闭空间中的阴燃很难形成明火,能否向明火转化主要由泡沫中氧气含量控制.     

Experimental study and asymptotic analysis of horizontally forced forward smoldering combustion

An experimental study and asymptotic analysis were conducted to investigate smoldering propagation in a horizontally oriented layer of flexible polyurethane foam. Experiments were done in a small-scale wind tunnel, where the air current is flush with the upper surface of the foam layer. The activation energy asymptotic approach was applied to analyze smoldering propagation. The experimental results show that increasing the air flow over the foam's surface not only enhances the mass transfer of oxygen to the smoldering reaction zone, but also increases the heat loss to the surroundings. The maximum smoldering propagation rate was observed at medium flow rates of air. The variation of the steady smoldering temperature with the flow rate followed the same trend as that of the smoldering propagation rate. Both adiabatic and non-adiabatic conditions were considered in the asymptotic analysis. For the adiabatic case, it is shown that the smoldering propagation rate and the smoldering temperature both increase with the flow rate of air. Results from the adiabatic analysis show that an increase in the density of the porous material lowers the smoldering propagation rate. Increasing the concentration of ambient oxygen, the porosity of the porous material or the mass transfer coefficient of the air stream increases the smoldering propagation rate. Results from the nonadiabatic analysis show a similar trend. Consistency is observed between the analytical results and the experimental measurements.     

Voltage response and two-phase flow during mode switching from fuel cell to water electrolyser in a unitized regenerative fuel cell

Mode switching operations between fuel cell (FC) and water electrolysis (WE) modes are indispensable to unitized regenerative FCs. Complicated electrochemical reactions and product transformations occur in the cell during mode switching. Thus, identifying dynamic behaviors during this procedure can improve cell durability and system design. In this study, the dynamic behaviors of cell voltage and electrochemical reaction during the switch from the FC mode to the WE mode are experimentally investigated. Reactant switching time significantly affects the electrochemical reactions. The water pumped into the cell in the FC mode reduces the cell voltage to a negative value and results in a hydrogen evolution reaction at the oxygen electrode side. Before FC mode voltage rapid decrease caused by supplied water, current transition could efficiently avoided the hydrogen evolution reaction at oxygen side. Ensuring that the moment water reaches the channels is close to the moment of current transition can improve the stability of unitized regenerative FCs.     

Experimental investigation of the Reynolds criterion for transition from the transition zone to complete turbulence in 3D internally finned tubes

The relations between the point for transition from the transition zone to complete turbulence and the fin height, fin width as well as fin density in 3D internally finned tubes have been investigated experimentally. At the same time, the performance of heat transfer and pressure drop in the tubes have been investigated when the flow is completely turbulent. Three expressions hale been obtained for (1) the Reynolds criterion for transition from the transition zone to complete turbulence, (2) the Nusselt number of complete turbulence in the tubes, and (3) the Fanning friction factor of complete turbulence in the tubes. These expressions can be used in engineering designs. © 1999 Scripta Technica, Heat Trans Asian Res, 28(3): 183–188, 1999     

Percolation theory for flame propagation in non- or less-volatile fuel spray: A conceptual analysis to group combustion excitation mechanism

A percolation theory for flame propagation in non- or less-volatile fuel spray is developed based on a cubic lattice model representing a local spray state. The interdroplet flame propagation characteristics found from microgravity experiments on flame spread along a linear droplet array are applicable to describing interdroplet flame propagation between neighboring droplets in any distribution of droplets because the effect of heat conduction from the flame front is shielded by the nearest unburned droplet, which acts as a heat sink. Thus, once the method by which the unburned droplet nearest to the flame front is ignited is identified and formulated into a simple algorithm rule, we can examine by computer simulation the statistical flame propagation behavior in a non- or less-volatile fuel spray in the framework of the percolation theory. In non- or less-volatile fuel, an unburned droplet swallowed by an envelope diffusion flame of other droplets is heated and becomes a new supplier of fuel vapor to the flame front, allowing the flame front to advance. For randomly distributed droplets, the flame front selects the path that minimizes its propagation time. These two phenomena occur when the grid spacing of the cubic lattice model is equal to the maximum flame radius of an isolated droplet immersed in the same air conditions as the local spray state. Furthermore, physical considerations reveal that the lattice size that leads to statistically meaningful information can be rather small, i.e., 20×20×20 vertices. Therefore, the proposed percolation theory is tractable and useful in finding the probability that a flame front propagates across a spray element and for exploring the mechanism of the excitation of group combustion for non- or less-volatile fuel sprays.     

管内爆燃转爆轰的热力学原理

本文首先说明了燃烧的两种传播机制，一种是燃烧自身的蔓延，国一种是由运动速度比火焰传播速度快的点火源引导而形成的传播，进而指出了可能存在的两种不稳定燃烧状态和两种极端物理过程的爆轰波，一种不稳定燃烧状态由爆燃加速到超过临界速度而致，另一种不稳定燃烧状态则由激波诱导燃烧引起，并采用简化理论计算了燃烧产物的压力和熵增随燃烧度的变化规律。由此出发，本文试图从热力学角度说明管内火中速及爆燃转爆轰的原理。爆燃