多孔纤维质颗粒阴燃特性及烟气粒度

针对多孔介质阴燃过程发烟的显著特征，研究了同向和逆向阴燃的传播特性以及烟气中颗粒的粒度分布特性．粒度谱的测量基于消光法，采用光电光谱仪测量消光系数，遗传算法被用做重建算法．实验结果表明，相同的氧化剂浓度下，逆向阴燃比同向阴燃稳定，阴燃传播过程中烟气浓度高，阴燃传播速度快，平均温度水平高．从烟气颗粒的粒度谱可以看出逆向阴燃的颗粒浓度高，平均粒度大，而同向阴燃烟气中气溶胶含量比逆向阴燃高．     

热表面上粉尘层阴燃的研究

采用自主开发的实验装置,以石松子粉尘为实验样品进行了热表面上的粉尘层阴燃实验,研究了自然对流环境下粉尘阴燃过程机理及粉尘厚度对阴燃过程的影响.实验结果表明,粉尘厚度对阴燃的起始时间和持续时间有影响,粉尘层越厚,阴燃起始时间越长且其持续时间也越长.采用简化的三步反应动力学机理,应用可燃固体颗粒通用热解模型计算软件Gpyro对热表面上的粉尘阴燃进行了二维数值模拟研究.模拟结果与实验结果符合较好,模拟结果较好地反映了粉尘阴燃的发展过程,粉尘升温过程和阴燃传播过程均在模拟结果中得到较好体现.     

自然流动条件下竖直阴燃的传播和气体成分

实验研究了在自然流动条件下竖直燃料床的阴燃过程，得出了阴燃的传播和气体成分与多种因素之间关系的规律，实验表明在自然流动的条件下生物质燃料阴燃的传播速度很慢，大约0．1－0．9mm/min；阴燃过程有大量的可燃气，特别是CO通常在10％左右；在阴燃向明火转捩前可燃气成分很高，可以超过40％。这些规律对改进燃池和分析生物质气化炉都是需要的。     

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.     

二维碳粒床中阴燃传播的数值模拟

本文对内部有受迫气流经过的二维碳粒填充床的阴燃，从点火开始到稳态传播整个过程进行了数值模拟。计算采用控制容积法，用ＰＩＳＯ算法求解。主要研究新鲜空气流动速度与阴燃波的传播速度和反应温度之间的关系，也探讨了正向阴燃与反向阴燃之间的异同。其结果为：在受迫气流经过的阴燃床中点火，将产生反向阴燃，且气流速度越大，阴燃波的传播速度越快，阴燃温度越高。在条件相同的情况下，反向阴燃的传播速度比正向阴燃的传播速度大；而反向阴燃的反应温度比正向阴燃的反应温度低。     

阴燃转捩为明火燃烧的分岔特性及气相反应临界状态消失分析

通过理论分析研究了燃料阴燃着火-熄火及向明火转捩的分岔特性.以β1为控制参数对燃料阴燃着火-熄火及向明火转捩的发生进行了的分岔分析.计算表明:分岔曲线明显地分为固相着火区和气相着火区,整个变化过程呈现出二次分岔特点.通过气相反应临界状态消失分析可知: ε2=0.05是气相反应的分岔曲线临界状态消失的转变点,ε2值越大,燃料在阴燃越容易发生气相反应,但温度降低;α2=0.53也是气相反应临界状态的一个转变点,α2越大气相反应的温度越接近于固相阴燃温度,而且当α2→ ∞时,只有固相阴燃反应存在而不会发生气相反应.     

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.     

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.     

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．     

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.     

碳粒填充床逆向阴燃传播渐近分析解

阴燃是发生在多孔介质内的缓慢燃烧过程。如果氧化剂能够在多孔介质中渗透进入孔内，则阴燃可以在固体内部形成稳定的波并自行维持传播。本文考虑一圆柱形状的填充床内装碳粒，气体从一端受迫流过，在另一端引起燃料，形成稳定的阴燃。由于此时阴燃传播方向与气体流动方向相反，因此为异向阴燃。本文采用大活化能渐近法，分别分析了氧气安全消耗和固体完全转化两种情形，计算出了反应区的参数值，如：阴燃的传播速率，燃烧温度，固体     

空隙率变化及填充床表面下沉对阴燃烧传播的影响

假定阴燃过程中,活性物,焦炭和灰三种物质的密度是恒定的,但它们各自占固体质量的份额随阴燃而改变,确定了阴燃过程固体颗粒的体积收缩速率及填充空隙率变化数学模型,对上方具有空气掠过的水平纤维质填充床,从点火到稳态传播的阴燃过程进行了模拟计算。计算结果表明,空隙率随阴燃过程增大,从而加快了阴燃传播速度,提高了其峰值温度,水平填充床表现上深所引起的固－固辐射换热在阴燃模拟计算中则可以忽略不计。     

Is there a transition to flaming in reverse smolder waves?

We previously considered the problem of forward smolder (Aldushin et al., 2006) [13] and showed, both analytically and numerically, that smolder waves propagating in porous samples of length L do (do not) exhibit a transition to flaming according as L exceeds (does not exceed) a critical length L_F, which we termed the flaming length. L_F, a concept which we introduced, is the distance that a steadily propagating smolder wave must travel inside the porous sample before the char oxidation reaction, which hardly affects the characteristics of the smolder wave itself, spontaneously self-accelerates, resulting in an eruption of the temperature in the smolder front. We plotted L_F as a function of the incoming gas influx g₀, based on both analytical and numerical considerations, and found it to be a U shaped curve. The temperature which was achieved after self-acceleration was approximately 800 K, depending on parameters, which is in the range where the transition to flaming has been observed to occur.It is generally believed that reverse smolder waves are unable to spontaneously burst into flaming. This is based on certain seemingly reasonable physical arguments and on experimental observations. However, we note that the experiments were generally performed on relatively short samples, perhaps too short to allow transition. Thus, there may be reason to revisit this view. We therefore consider the problem of reverse smolder, employing the same model which includes three step kinetics (fuel oxidation, pyrolysis, and char oxidation), which we previously considered in the forward smolder configuration (Aldushin et al., 2006) [13]. We show that under certain conditions self-acceleration of the char oxidation reaction may also occur in the reverse smolder configuration, leading to a rapid temperature rise, similar to what we observed for forward smoldering. Employing the same model (except for the boundary conditions, to reflect the reverse, rather than forward configuration), and the same set of kinetic and physical parameters employed in (Aldushin et al., 2006) [13], we find that there is again a flaming distance L_F, that a plot of L_F vs. g₀ exhibits a portion of a U shaped curve and that upon self-ignition, the temperature in the product region behind the smolder front could rise from 600 K in the reaction zone to a value as high as about 825 K, depending on parameters. In fact, we find that the temperatures to which the smolder front rises are comparable to those for forward smolder. Whether or not this temperature rise actually triggers the gaseous reactions leading to flaming ultimately will be settled to everyone’s satisfaction by experiments not yet performed, i.e., in sufficiently long samples. If the temperature rise is not sufficient to trigger a transition to flaming, then our results indicate the existence of a high-temperature smolder front following the low-temperature smolder front, provided that the sample length is sufficiently large, i.e., exceeds a critical value that depend on the incoming gas flux and the kinetic characteristics, to allow sufficient time for the transition to occur. For subcritical sample lengths the char oxidation reaction has essentially no effect on the characteristics of the smolder wave ignited by an igniter at temperature T_ign close to the smolder temperature T_b. However, if T_ign is above a certain critical value, we see either a rapid high-temperature smolder wave driven by the char oxidation reaction or a transition to flaming.We also consider the problem of extinction of reverse smolder waves. Specifically, we show that extinction occurs in the (solid) fuel deficient mode of smolder wave propagation. The smolder wave velocity achieves a maximum for gas fluxes near a certain critical value, in accord with experimental observations (Torero et al., 1993) [4]. For fluxes near the extinction limit we find oscillatory propagation of the smolder wave.     

Enhancement of condensate water quantity and coefficient of performance of an air conditioning system: An experimental study

The objective of the present work is to increase the quantity of condensate water from atmospheric air during the air conditioning (A/C) process by using two techniques: increasing moisture content in atmospheric air by adding steam and passing compressed air instead of atmospheric air, respectively. The experiment was performed with both capillary valve and thermostatic expansion valve operations for assessing the volume flow rates (VFRs) of condensate and coefficient of performance (COP) concerning the air A/C arrangement. The control valve operated for the steam supply case was at half valve opening and one-fourth opening, respectively. The R-22 refrigerant was used in this study. From this study, it was perceived that the condensate water quantity was increased by adding the moisture to the inlet air for the A/C system simultaneously with the usage of compressed air. Furthermore, the COP of the system was also compared to normal atmospheric air admission conditions. The compressed air and steam admission to the A/C system has shown a tremendous increase in COP together with the VFR of the system rather than atmospheric air admission conditions.     

An application of ultrasonic tomographic imaging to study smoldering combustion

An ultrasonic imaging technique has been developed and applied to examine smoldering combustion within a permeable medium. The technique provides information about local permeability variations within a smoldering sample, which can, in turn, be interpreted to visualize the propagation of the smolder reaction. The method utilizes the observation that transmission of an ultrasonic signal through a porous material increases with increasing permeability. Since a propagating smolder reaction leaves behind a char that is higher in permeability than the original (unburnt) material, ultrasonic transmission can be employed to monitor smolder progress. Additionally, the technique allows observation of the evolution of the char (i.e., material left by the smolder reaction), which, in certain circumstances, can continue to increase in permeability, due to secondary reactions (either oxidative or pyrolytic in nature). Experiments have been conducted, applying the technique to smoldering combustion in a two-dimensional geometry with line-of-sight imaging. For axisymmetric configurations, tomographic techniques have been implemented, providing three-dimensional mappings of the smolder front, as well as visualization of the smolder process itself. The results have furthered the understanding of two-dimensional smolder, and have been especially informative in identifying the controlling mechanisms leading to the transition from smoldering to flaming combustion.     

Effect of secondary air supply rate on the combustion of volatile gas in a controlled‐air incinerator

Experiments have been carried out to study the effect of secondary air supply rate on the combustion of volatile gas in a controlled‐air incinerator. The secondary air supply was varied at flow rates of 0.56, 1.026, 1.73 Nm³/min. The waste in this study was categorized into three types: namely those containing low, medium and high combustible matter. The significant results of the experimental investigation shown that increasing secondary air supply rate reduces the chamber temperature in both chambers and strong effect on CO reduction for waste with high combustible content. NO formation increases slightly as secondary air supply increases and SO₂ formation depends on the sulfur content in the waste and insensitive to secondary air for the range of flow rate tested. Copyright © 2005 John Wiley & Sons, Ltd.     

Comprehensive investigation of membrane sorption and CFD modeling of a tube membrane humidifier with experimental validation

The humidification of PEM fuel cells is critical for their performance and efficiency and for ensuring a long durability. In most PEM fuel cell systems for mobile applications membrane humidifiers are used to humidify the fresh air. In this process, the water contained in the cathode exhaust gas is used to increase the humidity of the supply air. Despite the simple design of membrane humidifiers, the simulation of the water transfer is difficult and so far there exist hardly any precise models to calculate the absorption and desorption processes. Common approaches that use the Sherwood number to determine the sorption rates cannot account for the influence of the local water content of the membrane. This ultimately leads to an inaccurate simulation of humidifier behavior, as these models cannot consider the fact that desorption is nearly ten times faster than absorption.In this study, an empirical formula for an accurate determination of the sorption rate is derived based on experimental data. This function accounts for the different absorption and desorption rates by finding a sorption rate coefficient as a function of the local membrane water content, temperature, pressure and flow velocity.Furthermore, a CFD model is derived from the geometry of a commercially available membrane humidifier, which is also investigated on a test bench. Using the experimental data, the CFD model is validated and it is shown that the developed sorption rate formula leads to good agreements between simulations and experiments at steady-state operating points of the humidifier.     

沈阳苏家屯地区某渠基土冻结条件下温度场及位移场分析

为研究东北季冻区的渠道冻胀问题,选取沈阳苏家屯地区渠基土为试验对象,将其冻胀情况简化为自上而下的一维冻胀模型,设计并制作了符合一维冻胀工况的试验装置与方法,分析了在封闭系统和开放系统条件下冻胀率随初始含水率、降温速率的变化规律,并应用SPSS统计分析软件建立了冻胀率与初始含水率、降温速率的多元线性回归方程。结果表明,封闭系统条件下人工冻结试样的温度场的整体发展趋势快于开放系统;较高含水率人工冻结试样的温度场稍滞后于低含水率试样。开放系统条件下人工冻结试样的冻胀量均大于封闭系统;较高含水率人工冻结试样的冻胀量大于低含水率试样。初始含水率是冻胀率的主要影响因数,多元线性回归模型拟合度较好,能很好地进行冻胀预报。