Measuring Eddy Covariance Fluxes of Ammonia Using Tunable Diode Laser Absorption Spectroscopy

Field measurements of NH3 fluxes using an eddy covariance technique were made for a total of 60 days between July and September 2002 at an intensively managed grassland in Southern Scotland. The collected data demonstrate the suitability of a tuneable diode laser absorption spectroscopy (TDLAS) system coupled with a sonic anemometer for eddy covariance measurements. The novelty of these measurements is the application to ammonia, which has only become measurable using TDLAS techniques recently, because of its small ambient concentration. Data presented in this paper show typical features of the fluxes and concentration for the summer season. NH3 concentration and flux values are in a similar range to previous studies using flux gradient methods at the same field site, although the particularly wet season reduced the concentration of NH₃ in the air. For an example day, measured NH₃ fluxes ranged between –11 and 44 ng m^–2 s^–1 with an average value of 3.78 ng m^–2 s^–1 indicating a small net emission from the vegetation. Spectral analysis executed on the data shows the percentage of flux carried by the small eddies (from 0.2 to 2 m) suggesting that high detection frequency instruments are particularly suitable for estimating NH₃ fluxes between atmosphere and vegetation.     

计算机模拟在火灾科学与工程研究中的应用

计算机模拟技术尤其是场模拟技术是近年来发展完善的高新技术,在许多工程领域和基础研究方面得到广泛应用。但其在大灾科学和工程领域的应用研究刚刚起步。本文在介绍场模拟基本方法的基础上,重点介绍了场模拟方法中大涡模拟技术的基本理论和方法,并将其引入火灾动力学的研究中。列出了火灾动力学大涡模拟的基本方程、湍流模型、燃烧模型和幅射模型。并用此方法模拟了油罐区火灾动力学演化实例,据此可提出更有效的控制和补救措施。 通过利用场模拟方法中的大涡模拟技术研究典型工业重点火灾危险源——油罐区火灾动力学演化实例,可以看出场模拟技术在安全工程领域方面的发展前景。预计它将成为安全工程领域中的安全管理、危险性评估以及安全事故处理决策支持等的有效手段。     

Mesoscale influence on long-range transport — evidence from ETEX modelling and observations

During the first European Tracer Experiment (ETEX) tracer gas was released from a site in Brittany, France, and subsequently observed over a range of 2000 km. Hourly measurements were taken at the National Environmental Research Institute (NERI) located at Risø, Denmark, using two measurement techniques. At this location, the observed concentration time series shows a double-peak structure occurring between two and three days after the release. By using the Danish Emergency Response Model of the Atmosphere (DERMA), which is developed at the Danish Meteorological Institute (DMI), simulations of the dispersion of the tracer gas have been performed. Using numerical weather-prediction data from the European Centre for Medium-Range Weather Forecast (ECMWF) by DERMA, the arrival time of the tracer is quite well predicted, so also is the duration of the passage of the plume, but the double-peak structure is not reproduced. However, using higher-resolution data from the DMI version of the HIgh Resolution Limited Area Model (DMI-HIRLAM), DERMA reproduces the observed structure very well. The double-peak structure is caused by the influence of a mesoscale anti-cyclonic eddy on the tracer gas plume about one day earlier.     

巴丹吉林沙漠腹地夏季不同天气条件下陆-湖面辐射收支与能量分配特征对比

论文以巴丹吉林沙漠腹地陆面和湖面能量平衡系统的定位观测数据为基础,对比分析了夏季不同天气条件下两种典型下垫面的辐射收支和能量分配之异同,评价了典型晴天的辐射收支与能量分配对季节平均气候态的代表性。结果显示,按“晴天-多云-阴天-降水”的顺序,陆面与湖面的总辐射、反射短波辐射和净辐射之日均值呈逐渐减小趋势,大气逆辐射则逐渐增大。湖面长波辐射亦随这一天气变化顺序逐渐增大,暗示地下水携带的热量和局地平流作用对湖水表面温度皆有影响。非降水天气时,陆面感热通量占净辐射的主要部分。云和降水皆会直接影响陆面的能量分配,感热通量一般随云量的增加而减小,潜热通量则逐渐增大。湖面典型晴天的午后存在逆温现象,然降水天气时,湖面空气温度降低较快,破坏了湖面上空的逆温层,即感热通量皆为正值。湖面的潜热通量主要决定于风速,故其对天气变化响应不明显。在地下水携带的热量和局地平流共同作用下,湖面的感热通量和潜热通量之和大于湖面的净辐射。不同天气下陆面和湖面的波文比差异较大。陆面典型晴天的辐射收支和能量分配与季节平均气候态相差不大,湖面的典型晴天则不具有季节平均气候态的代表性。     

涡流热成像动态检测模式的缺陷信号重构方法研究

在涡流热成像检测中,缺陷信号重构可以有效评定结构的安全性和可靠性,及时预防生产中安全事故的发生,目前大多数信号重构方法都不适用于动态模式的涡流热成像检测。为了对动态模式涡流热成像检测的信号进行重构,提出了1种基于区域扫描的方法,选定区域大小及划分准则,在每帧图像相同位置划分区域,对各个区域内的温度数据进行相应的运算处理并提取结果,以此作为每个区域的重构温度,进而重构出包含所有缺陷温度信息的热像图。结果表明:该方法可以对动态模式下的涡流热成像温度信号进行重构,并能在一定程度上消除噪声,有利于后续的图像处理。     

风速对高层建筑火灾时环境中烟气分布的影响

为了探究风速对高层建筑火灾时环境中温度、烟气浓度、CO浓度分布状态的影响,以央视北配楼火灾为模型背景,应用火灾动力学软件FDS,对火灾进行模拟与分析。通过讨论不同风速下火源温度中心、烟气浓度中心、CO浓度中心离着火面距离与高度之间的关系,得到风速一定时各中心的位置与高度之间的变化规律,以及该变化规律与风速之间的关系,风速小于3m/s时各中心位置随风速变化较明显;风速越大,温度、烟气浓度、CO浓度越高,当风速小于2m/s时各值增量随风速增加明显;与其他因素相比,温度对防火间距的影响最大。     

Numerical simulation of flame acceleration and deflagration-to-detonation transition of ethylene in channels

Ethylene (C₂H₄) is a hydrocarbon fuel and widely used in chemical industry, however, ethylene is highly flammable and therefore presents a serious fire and explosion hazard. This work is initiated by addressing the hazard assessment of ethylene mixtures in different scale channels (d = 5 mm, 10 mm and 20 mm) from the aspect of flame acceleration (FA) and deflagration-to-detonation transition (DDT) by using large eddy simulation (LES) method coupled with the artificially thickened flame (ATF) approach. The fifth order local characteristics based weighted essentially non-oscillatory (WENO) conservative finite difference scheme is employed to solve the governing equations. The numerical results confirm that flame velocity increase rapidly at the beginning stage in three channels, and the flame acceleration rate is slower in the subsequent stage, afterwards, the flame velocity has an abrupt increase, and the onset of detonation occurs. Due to the fact that wall effect is significant in the narrow channel (e.g.,5 mm), especially in the ignition stage of the flame, flames have different shapes in wider channels (10 mm and 20 mm) and narrow channel (5 mm). Both the pressure and temperature profiles confirm DDT run-up distances are 0.251 m, 0.203 m and 0.161 m in 20 mm, 10 mm and 5 mm channels, respectively, which indicates that a shorter run-up distance is required in narrower channel. The cellular detonation structures for the ethylene-air mixture in different channels indicate that multi-headed detonation structures can be found in 20 mm channel, as the channel width decreases to 10 mm, detonation has a single-headed spinning structure, as the width is further reduced to 5 mm, only large longitudinal oscillation of the pressure can be observed.     

脉冲涡流不拆保温探头优化设计的数值模拟方法研究

通过比较分析国外脉冲涡流不拆保温探头的技术原理和设计范例,针对现有文献中探头的参数,进行上下梯度模拟,提出具有针对性的探头优化设计方法。通过理论计算和数值模拟,取灵敏度为优化指标,得到最优发射线圈的直径与高度数据;通过数值模拟和计算求解,得到发射线圈最优线径和最优匝数;通过数值模拟得到最优的接收线圈线径和匝数;通过实际测试和正交试验,得到实际应用化的发射线圈最佳线径和匝数,以及接收线圈的最佳线径和匝数。依据有限元模型,脉冲涡流不拆保温探头的参数在一定范围内都进行了优化。针对特有脉冲涡流系统的传感器探头,依据现有文献中参数进行理论计算和数值模拟提出的探头优化设计方法是可行的。     

Estimation of carbon dioxide flux and source partitioning over Beijing, China

The magnitude and partitioning of carbon dioxide emission from the urban area in Beijing, China was estimated based on a statistical approach. Results showed that the urban surface is a net source of CO2 to atmosphere. The main sources of CO2 are vehicles, which accounted for 75.5% and 38.9% of CO2 emission in summer and winter, respectively. At midday in summer, the CO2 uptake of-0.034 mg/（m^2.sec） indicated that vegetation is an important sink of CO2 in summer. Comparison between the annual emission rates of CO2 from the statistical approach and that directly measured by the eddy covariance technique implies that a bottom-up emission approach is a viable means to estimate CO2 emission in an urban area.     

一种乳状液破乳的新方法──涡旋电场法

由螺线管、磁芯构成的电感器与电容器串联组成谐振电路。在此电路中通入高频电流，使其在破乳器内产生高频磁场，从而在破乳器内产生高频涡旋电场。在此高频涡电场作用下，Ｗ／Ｏ乳状液滴内电解质缔合体极化，电矩增大，界面双电层破坏，离子电泳速度加快，液滴碰撞增加，从而加速了分散相的聚结，达到破乳的目的。内相电解质导电能力越弱，破乳所需的电磁场强度越大。