分形多孔介质传热传质过程的格子Boltzmann模拟

针对自然界中实际多孔介质具有的分形特性和随机性,利用中点替代算法和二值化处理构造统计上具有分形特性的随机多孔介质。分析了所构造的多孔介质盒维数与Hurst指数之间的关系。基于随机分形构造的原理,对二维实际多孔介质图像进行了重构。利用两点相关函数,分析了重构图像的结构相关性, 并与实际目标多孔介质的结构特征进行比较。在与解析解对比验证的基础上,将基于二元混合理论的格子Boltzmann模型（LBM）用于模拟多孔介质内流体扩散过程。通过计算不同分形特性的二维多孔介质的有效扩散系数,研究了重构多孔介质的分形维数与有效扩散系数的关系。利用热耦合LBM模型计算多孔介质内传热过程,分析了不同的分形特性对多孔介质蓄热过程的影响。     

分形多孔介质传热传质过程的格子Boltzmann模拟

针对自然界中实际多孔介质具有的分形特性和随机性,利用中点替代算法和二值化处理构造统计上具有分形特性的随机多孔介质。分析了所构造的多孔介质盒维数与Hurst指数之间的关系。基于随机分形构造的原理,对二维实际多孔介质图像进行了重构。利用两点相关函数,分析了重构图像的结构相关性,并与实际目标多孔介质的结构特征进行比较。在与解析解对比验证的基础上,将基于二元混合理论的格子Boltzmann模型(LBM)用于模拟多孔介质内流体扩散过程。通过计算不同分形特性的二维多孔介质的有效扩散系数,研究了重构多孔介质的分形维数与有效扩散系数的关系。利用热耦合LBM模型计算多孔介质内传热过程,分析了不同的分形特性对多孔介质蓄热过程的影响。     

轰燃机理的研究及在防、灭火中的应用

随着经济技术的发展,人们的生活质量越来越高,对室内的装修也越来越注重,可是当室内发生火灾时,人身伤亡和财产损失也愈加惨重。轰燃会使室内火灾更加严重,会导致室内全面燃烧。从消防角度来说,室内火灾一旦发生轰燃,不但灭火救援难以开展,还会导致消防官兵的人员伤亡,因此对轰然的研究和对轰燃机理的掌握能够使消防官兵成功开展救援;合理的防火工作能够将轰燃发生的可能性降到最低。     

改进BP神经网络在气液两相流流型识别中的应用

采用统计理论与分形理论相结合的方法对水平管内空气-水两相流的压差波动信号进行分析,得到了压差均值、标准差、偏斜度、能量份额、盒维数、关联维数和Hurst指数7个参数,并将上述参数作为流型的特征向量输入自适应学习率的改进BP神经网络,通过对训练样本的学习,网络可以实现对未知流型的客观识别.仿真结果表明：统计参数与分形参数相结合得到的流型特征向量可以很好地反映各流型之间的差异,网络识别率高达93%,并且改进后的BP网络具有收敛速度快、不易陷入局部极小的优点.     

中低负荷下脉动热管壁温信号的分形特性分析

运用功率谱、小波分解、相空间重构、关联维数计算等非线性分析方法对脉动热管壁面温度波动信号分析。表明:功率谱呈现连续谱线,时间序列存在自相似性,展现温度波动信号的混沌行为;小波分解表现出温度波动的分形特性;重构吸引子展现出4种不同的空间分形结构,说明温度信号波动属于分形下的混沌行为。计算分形维数与传热性能的关系,发现系统存在3~6个分形维;随嵌入维数的增加,分形速度关系为R134a大于丙酮大于去离子水,基本表现为分形维数越大,传热性能越好。并发现关联维和热阻成负相关关系。     

脉冲热管温度时间序列的非线性混沌分析

运用功率谱分析、自相关分析、关联维数和K熵、R/S分析等线性和非线性分析技术对以FC-72和水为工质的脉冲热管温度波动信号进行了定性和定量研究．研究表明：波动信号具有确定性混沌特性，其重构的动力学系统具有非整数值的关联维，范围在2.5~11之间，K熵为有限正值。Hurst指数在0.8~1之间，表明温度波动信号有强烈的长程相关性。传热特性和关联维之间有密切的联系，热阻越小时关联维数越大。     

分形理论在润滑油添加剂合成研究中的应用

本文基于分形理论,利用图形处理软件对釜式法和超重力法合成的环烷酸钙的TEM照片进行处理,使用网格法对其分形维数进行了计算,并根据计算结果对不同合成条件下生成的环烷酸钙颗粒的形貌特征以及合成工艺进行了分析对比。结果表明:将分形理论应用于添加剂的产品的形态分析及合成工艺中,分形维数不仅被证明可以作为表征颗粒形貌特征及评价添加剂清净性能的重要参数,同时也与添加剂产品的合成工艺密切相关,可以间接作为评价添加剂合成工艺优劣的重要指标,同时从反应动力学和反应器结构两方面出发分析了添加剂颗粒成核的分形特征以及运动变化规律,说明添加剂合成过程不仅与物料的微观混合情况有关,同时也与反应器的组成结构关系密切。     

室内液化石油气轰燃特性的实验模拟研究

通过对中小规模的火灾实验箱进行实验,研究了通风口尺寸、泄漏速率等因素对轰燃特性的影响。结果表明,随着通风口宽度、高度、泄漏速率、通风因子的增加,轰燃越容易发生;在一定范围内,通风口开口面积越小、位置越靠近下部,发生轰燃的可能性越大;最后进行数值模拟,并与实验数据进行了对比。     

分形理论在滑坡预测中的应用研究

滑坡是我国三大地质灾害之一,具有危害范围大,致灾性强的特点,一旦发生将严重危害人类的生命及财产安全。滑坡是一个复杂的非线性系统,可以用非线性理论分析研究,分形理论作为非线性理论的一个分支,是依据分形几何学,利用分形维数的变化来对事物进行客观分析,能够对事物进行属性和状态的描述与分析。本文就分形理论在滑坡预测中的应用进行了分析和研究,为滑坡预测提供了参考。     

室内通风开口高度对轰燃的影响

本文利用小规模火灾实验箱研究通风口高度对轰燃的影响。实验结果表明:通风开口处于下方时要比处于上方时更容易与新鲜空气接触,燃烧速率加快,室内达到最高温度更容易。     

Experimental study of the influence of interior lining distributions on flashover in the enclosure with different ceiling structures

Different distributions of interior linings in an enclosure have been studied to reveal the influence on time to flashover. Two kinds of ceiling structures, flat ceiling and flat ceiling with beams, were used for enclosure. Six full‐scale experiments were performed to investigate the occurrence of flashover in large enclosures. Heat release rate, gas temperature, and surface temperature of the enclosure were measured and analyzed. Experimental results show that time to flashover is extended for enclosure with large surface areas and large doors. Whether lining the interior linings on the ceiling or beams has a significant effect on flashover. Considering that ceiling fires have effect on the progress of the spread of flame on the wall, time to flashover is reduced for the tests that are fitted with the interior linings on the wall of the enclosure. In the enclosure with beams, the direction of spread of flame on the ceiling is changed, resulting in the change of time to flashover. Additionally, the strength of ceiling jets that is affected by the heat release rate of fire source and the area of the interior linings mounted on the fire source influenced region affects flashover. It is observed that flashover occurs when flame fronts are throughout the upper part of the back wall and the flame‐covered area on the back wall exceeds 0.2 times the area of the back wall.     

A comparison of fire retarded and non‐fire retarded wood‐based wall linings exposed to fire in an enclosure

Full‐scale fire experiments were carried out in an ISO room to study the behaviour of commonly used cellulosic lining materials in real fire conditions. In addition to the temperature measurements recommended by the ISO 9705, temperature recordings were made at each node of grid lines on the wall lining surfaces. Four lining materials were chosen to represent different types of products and the surface spread of classifications determined using the BS 476 Part 7 flame spread test environment. The linings included fire retarded, melamine faced and non‐fire retarded boards which facilitated a comparative study of the behaviour of these materials with respect to ignition, flame spread, heat release rate and time to flashover. Corner fire scenarios were used in all the experiments. A T shape flame spread pattern on the surface of the two adjacent walls was observed prior to flashover. Prior to the onset of flashover conditions, downward opposed flow surface flame spread to the wall/ceiling intersection. For the non‐retarded wood based materials, such as plywood and medium density fibre board, flashover conditions occurred approximately 4 min after the start of the experiment. However, the fire retarded chipboard ignition was delayed by some 11 min 45 s after which flame spread was very rapid with flashover occurring within a further 1 min 45 s. An explanation for this particular behaviour is the considerable pre‐heating which occurred during the pre‐ignition period. For the fire retarded linings, much higher surface temperatures were recorded compared with those for non‐fire retarded linings. It was found that the areas of the fire retarded linings facing the source flame suffered extensive pyrolysis and charring which penetrated to the rear surface of the lining. Copyright © 1999 John Wiley & Sons, Ltd.     

Smoke data from the Cone Calorimeter for comparison with the room fire test

Smoke production in the full–scale room fire test ISO 9705 (Commonly referred to as the Room Corner Test) and in the Cone Calorimeter ISO 5660 has been analysed for three sets of building products comprising a total of 28 products. The smoke production may be critical for the fire classification of surface products since some products produce large amounts of smoke in the room fire test even if they do not reach flashover within 20 min. Several smoke parameters in the Cone Calorimeter and the room fire test have been analysed. Good correlations have been obtained when the products are divided into two groups: products with more than 10 min to flashover in the room fire test and those with less than 10 min. These two time categories correspond to the two heat output levels in the room fire test: 100 kW for the first 10 min and then 300 kW up to 20 min. For products with more than 10 min to flashover the average rate of smoke production and the total smoke production seem to be useful parameters for predictions of smoke release in the room fire test. Both parameters have good correlations between data from the Cone Calorimeter and the room fire test. For products with less than 10 min to flashover no parameter seems to give useful predictions. For all products evaluated together, the correlations are not so good, but the same regression lines as for products with more than 10 min might be used as a first rough estimate. In this case the total smoke production in the Cone Calorimeter could be used to estimate the total smoke production in the room fire test for different building products, independent of their estimated time to flashover. It is suggested that the average rate of smoke production and the total smoke production from the Cone Calorimeter is reported in addition to the mass-based specific extinction area. This will be helpful in predicting smoke release in the room fire test and will also make the data on smoke release analogous to those on heat release.     

Full‐scale burning tests on double‐skin façade fires

Fire hazard of an architectural feature with double‐skin façade is a concern. An identified scenario of having a post‐flashover room fire adjacent to the façade was studied experimentally in a rig of 6 m tall. Eight tests of cavity depths including a wide value of 2 m, normal values of 1.5 m and 1 m and a narrow value of 0.5 m under a room flashover fire of 1200 kW were carried out. From the measured air temperature profiles inside the façade cavity, the chance of breaking the interior glass pane above the fire room is therefore very high. Copyright © 2012 John Wiley & Sons, Ltd.     

流化床内垂直向上射流脉动特性的试验研究

本文利用图像处理技术,得到了二维流化床内垂直向上射流深度的时间序列。基于Ｒ／Ｓ分析方法,得到了表征射流脉动强弱的Ｈｕｒｓｔ指数和分数维。研究了流化数、射流初始速度、颗粒平均粒径及静态床层高度对射流脉动特性的影响。结果表明：垂直向上射流的脉动随射流初始速度、流化数、静态床层高度以及颗粒平均粒径的增加而减弱。     

On the role of KρC of room lining materials in the growth of room fires

The role of the product of the thermal properties of thermal conductivity and thermal capacity per unit volume in fire growth is explored in relation to a simple model of a room fire. The calculated differences between flashover times for two different lining materials are plausible; the main conclusion is to show that the flashover time depends on K_ρC in a weaker relationship than simple proportionality.     

A two‐step method for predicting time to flashover in room corner test fires using cone calorimeter data

A method to predict time to flashover in ISO 9705 room corner fire tests based on cone calorimeter data is proposed in this paper. The method involves classification of materials and estimation of time to flashover as two steps in sequence. In the first step, the investigated material is classified into discriminating material groups. In the second step, the time to flashover is calculated with a derived formula for the corresponding material group, which contains material density, time to ignition and heat release rate collected from small‐scale experiments. Compared with two existing models, the proposed method has improved the prediction precisions in both the time to flashover and classification of material categories for a wide range of materials. Copyright © 2012 John Wiley & Sons, Ltd.     

A fractal approach for interpretation of local instantaneous temperature signals around a horizontal heat transfer tube in a bubbling fluidized bed

Temperature signals measured around a horizontal heat transfer tube in a bubbling fluidized bed have been analyzed using Hurst's rescaled range (R/S) analysis. This analysis estimates and identifies long-term persistence or correlation in measured time series. The Hurst exponent H, which is evaluated from R/S analysis, also provides the local fractal dimension of the time series. A new approach to analyze an air fluidized particle system is proposed based on the evaluation of the Hurst exponent. Two Hurst exponents can be evaluated from a single time series, one from the discrete time fractional noise (where the linearity of the signal is subtracted and short-term fluctuations are emphasized) and the other from the signal itself (without subtracting the linearity of the signal). The authors argue that the Hurst exponent obtained from discrete time fractional noise characterizes the particle motion, whereas the Hurst exponent obtained from the signal itself characterizes the bubble motion. Moreover, a comparison between these two Hurst components identifies the zones where an alternating type of contact between the tube surface and the bubble-emulsion phase occur. The results were interpreted in conjunction with the mutual information function. The mutual information function provides the relationship between the data points separated in time and uses only the statistical relationship between the data points. The mutual information functions and the Hurst exponents exhibited similar trends around the heat transfer tube.     

Evidence of a Fractal Percolating Network During Geopolymerization

This study focuses on the transition from the paste state to the solid state which occurs during the hardening of metakaolin‐based Na‐geopolymers. The occurrence of primary interactions and the structural properties were investigated using classical oscillatory rheology (OR) methods. A time‐frequency‐resolved rheology (TF2R) method was used in order to obtain more detailed information about the percolation and aggregation mechanisms. The results obtained show the occurrence of the following process: (i) Elastic behavior predominates initially over the viscous behavior at low frequency. (ii) A percolating process then takes place when the viscoelastic parameters become parallel over more than two frequency decades. At the gel point, a mass‐fractal dimension of approximately 2 was determined. (iii) During the formation of the porous network, the viscous behavior predominates over the elastic one due to the occurrence of polycondensation reactions. (iv) Lastly, a solid state is reached, where the elastic modulus shows a plateau and the viscous modulus decreases. These macroscopic mechanical results are compared using the small angle x‐ray scattering technique. Scattering experiments were in complete agreement with the rheological measurements and showed the presence of aggregated oligomers and a mass fractal dimension equal to 2.1 at the gel point, increasing up to a surface fractal dimension reflecting the formation of the mesoporous network.     

后处理对半芳香透明聚酰胺力学性能的影响

采用固相后缩聚工艺制备了一系列不同熔体流动速率的半芳香透明聚酰胺（SATPA）,研究了材料的吸水性能和调湿、退火后处理对材料力学性能的影响。结果表明,固相后缩聚SATPA的吸水性能不随熔体流动速率的减小而变化,但外界条件对吸收速率有一定影响,水温越高,环境湿度越大,试样吸水速率越大,吸水前后试样尺寸基本不变。调湿处理后,试样性能提高不明显,断裂伸长率下降;退火处理后,试样的拉伸强度、断裂伸长率和缺口冲击强度均有所提高。