Unsteady extinction mechanism of nonpremixed flame interacting with a vortex

The extinction mechanism of a CH₄/N₂-air counterflow nonpremixed flame interacting with a single vortex was numerically studied. An augmented reduced mechanism was used to treat the CH₄ oxidation reactions. The contribution of each term in the energy and the OH species equations were evaluated to investigate the unsteady extinction mechanism of nonpremixed flame. The flame temperature began to decrease due to the convection heat loss when the flame interacted with a vortex. The investigation of the radical behavior during the flame-vortex interaction process also provided useful information on the unsteady extinction mechanism. The OH radical concentration could be used as a good tracer of the state of the unsteady extinction of nonpremixed flame. The reduction mechanism of OH concentration was confirmed by analyzing the contribution of each term in the OH species equation. At initial stage of flame-vortex interaction, the OH production and consumption rates increased gradually, while the OH concentration was kept nearly constant. Near the extinction limit, the OH production rate decreased rapidly due to the low flame temperature, and the balance between the OH production and OH consumption by diffusion could not be maintained. The unsteady nonpremixed flame interacting with a vortex under the conditions of regime (V) shown in the spectral combustion diagram [Thévenin, D., Renard, P.H., Fiechtner, G.J., Gord, J.R., Rolon, J.C., 2000. Regimes of non-premixed flame-vortex interactions. Proceedings of the Combustion Institute 28, 2101-2108.] was finally extinguished due to low reactivity, which was induced by the low flame temperature.     

低损耗、高消光比的Z切钛扩散铌酸锂调制器

为了获得低损耗、高消光比的光波导电光调制器,以钛扩散铌酸锂( Ti: LiNbO3 )波导折射率的分布和光波导的优化设计为基础,利用有限差分光束传播法( FD-BPM )和点匹配法,从理论上给出了一种光损耗为-0.53dB、消光比为30dB的Z切钛扩散铌酸锂调制器。并且,在10Gbit/s的系统中对其进行了测试,取得了在120 km长的光纤上传输无误码的效果和较清晰的眼图。     

抗毫米波制导的两栖装甲车辆人工水雾干扰

在分析两栖装甲车辆的毫米波暴露特征、常见毫米波干扰技术的基础上,利用Mie散射理论分析人工水雾干扰对抗毫米波制导的原理.推导出单个雾滴半径越大,毫米波消光截面就越大;温度越低,毫米波消光截面就越大.并编程模拟计算了人工水雾的毫米波衰减性能,结果证明该方法可行.     

Europe as Consumer of Exotic Biodiversity: Greek and Roman times

Landscapes at the edge of the city comprise vast areas that ‘lie fallow’, awaiting future urban development. During this time new landscape values evolve. It is argued that the complexity of urban fringe landscapes is not adequately considered either within landscape research or in the practice of spatial planning. A key to understanding landscapes at the inner urban fringe is to focus on landscape dynamics, and on the interactions between spatial plans and everyday activities. The study is divided into three parts. First, theoretical considerations about landscape dynamics and the character of the inner urban fringe are presented. Thereafter, the relationship between spatial planning and everyday places is analysed in a case-study area at the edge of the city of Malmö in southernmost Sweden. The case study demonstrates the complexity of landscape dynamics at the inner urban fringe, as well as problems regarding the handling of ephemeral and transitory aspects within spatial planning. The study concludes with a discussion concerning the importance of studies of landscape dynamics within landscape research.     

光学式颗粒计数装置的研制

设计了一种检测装置,用来对洁净介质中的颗粒粒径大小和数量进行统计。详细分析了用几何光学的光阻公式替代消光公式对大粒径颗粒的检测机理和前向光散射探测器检测小粒径颗粒时的计数技术。用该装置对标准颗粒物质进行了检测实验,结果表明：对于2μm以上的颗粒计数。效率能达到90％;对于1μm以下的颗粒也有较好的分辨能力,计数效率在70％左右。数值计算和实验结果证实了该装置具有较好的计数能力,能够表征洁净介质的污染程度。     

两栖装甲车辆对抗双红外制导伪装方法研究

在分析两栖装甲车辆的双红外暴露特征、双红外制导抗干扰原理和常见的红外干扰技术的基础上,得出了在两栖装甲车辆对抗双红外制导的最佳伪装方法——人工水雾红外遮蔽技术。并利用Mie散射理论分析了单位喷水量条件下,水雾滴径与3～5 μm、8～14 μm波段衰减的关系,从理论上给出了水雾滴径的最佳尺寸,最后编程模拟计算了人工水雾的双红外遮蔽性能。     

Diagnostic considerations for optical laser-extinction measurements of soot in high-pressure transient combustion environments

Laser-extinction diagnostics can provide spatially and temporally resolved measurements of attenuation from combustion-generated soot within the path of the beam. When laser-extinction techniques are utilized in high-pressure combustion environments, however, a number of complications may be encountered that are not present in low-pressure environments. Several of these experimental difficulties were investigated in diesel engine environments, and solutions that facilitated acquisition of reliable laser-extinction data were demonstrated. Beam steering due to refractive index gradients within the combusting gases was observed, and a full-angle beam divergence of over 100 mrad was measured. A spatial-filtering scheme was employed to reduce the collection of forward-scattered light and background combustion luminosity while ensuring full collection of the steered beam. To further reject combustion luminosity, a narrow-bandpass laser-line filter was employed, after diffusing the transmitted light sufficiently to avoid the effects of significant spatial non-uniformities of the filter. As the windows were subjected to thermal and mechanical stresses, dynamic etaloning effects due to the photoelastic properties of synthetic fused silica were observed. Dynamic changes in the polarization of the exit beam were also observed, as stress-induced birefringence in the windows caused dynamic phase retardation of the transmitted beam. Although these photoelastic effects could not be eliminated, they were mitigated by introducing curvature to the wavefronts in the laser-extinction beam and using polarization-insensitive elements in the detection optics. Soot deposits on window surfaces were removed ablatively using a coaxial, high-energy, pulsed Nd:YAG laser beam.     

Effect of the composition of the hot product stream in the quasi-steady extinction of strained premixed flames

The extinction of premixed CH₄/O₂/N₂ flames counterflowing against a jet of combustion products in chemical equilibrium was investigated numerically using detailed chemistry and transport mechanisms. Such a problem is of relevance to combustion systems with non-homogeneous air/fuel mixtures or recirculation of the burnt gases. Contrary to similar studies that were focused on heat loss/gain, depending on the degree of non-adiabaticity of the system, the emphasis here was on the yet unexplored role of the composition of counterflowing burnt gases in the extinction of lean-to-stoichiometric premixed flames. For a given temperature of the counterflowing products of combustion, it was found that the decrease of heat release with increase in strain rate could be either monotonic or non-monotonic, depending on the equivalence ratio φ_b of the flame feeding the hot combustion product stream. Two distinct extinction modes were observed: an abrupt one, when the hot counterflowing stream consists of either inert gas or equilibrium products of a stoichiometric premixed flame, and a smooth extinction, when there is an excess of oxidizing species in the combustion product stream. In the latter case four burning regimes can be distinguished as the strain rate is progressively increased while the heat release decreases smoothly: an adiabatic propagating flame regime, a non-adiabatic propagating flame regime, the so-called partially-extinguished flame regime, in which the location of the peak of heat release crosses the stagnation plane, and a frozen flow regime. The flame structure was analyzed in detail in the different burning regimes. Abrupt extinction was attributed to the quenching of the oxidation layer with the entire H-OH-O radical pool being comparably reduced. Under conditions of smooth extinction, the behavior is different and the concentration of the H radical decreases the most with increasing strain rate, whereas OH and O remain comparatively abundant in the oxidation layer. As the profile of the heat release rate thickens, the oxidation layer is quenched and the attack of the fuel relies more heavily on the OH radicals.     

Emissivity of substrate supported films

Results of experimental measurements of emissivity carried out on cobalt oxide and nickel sulphide films deposited on aluminum substrates are reported in this paper. Experimental results supported the general behaviour of variation of emissivity with film thickness as reported theoretically. The theoretical model is based on the interaction and attenuation of the emitted wavelengths from the substrate and film materials.     

Improvement of flame stability and NOx reduction in hydrogen-added ultra lean premixed combustion

Lean premixed combustion is a well known method in gas turbine combustors that can reduce fuel consumption and decrease flame temperature. In lean premixed flames, flame instabilities can occur because the combustion takes place near the lean flammable limit. For the purpose of increasing flame stability, a small amount of hydrogen was added into a fuel, which has ultra low lean flammable limit. The extinction stretch rate increased and total equivalence ratio at extinction decreased with hydrogen addition; consequently, ultra lean premixed combustion was possible and flame stability could be achieved at low temperature conditions. The NO_x emission increased with hydrogen addition for the same stretch rate and equivalence ratio, but the extinction stretch rate and lean flammability limit was enlarged. Consequently, NO_x emission decreased with hydrogen addition in the near extinction conditions. Hydrogen addition could improve flame stability and reduce NO_x emission in ultra lean premixed combustion. This paper was recommended for publication in revised form by Associate Editor Ohchae Kwon Dr. Eun-Seong Cho received his B.S. and M.S. degrees in Mechanical Engineering from Hanyang University, Korea, in 1996 and 1998, respectively. He then received his Ph.D. degree from Seoul National University, Korea, in 2005. He was a principal engineer of KD Navien research center and currently a research associate at Delft University of Technology, The Netherlands. His research interests include eco-friendly clean combustion technology, new and renewable energy systems. Prof. Suk Ho Chung received his B.S. degree from Seoul National University, Korea, in 1976 and Ph.D. degree in Mechanical Engineering from Northwestern University, USA, in 1983. He is a Professor since 1984 in the School of Mechanical and Aerospace Engineering at Seoul National University in Seoul, Korea. His research interests cover combustion fundamentals, pollutant formation, laser diagnostics, and plasma-assisted combustion.