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。     

Experiments of evacuation speed in smoke-filled tunnel

Among tunnel fire safety strategies, evacuation speed in smoke, which is the basic evacuation performance characteristic, is one of the most important factors when assessing safety. An evacuation experiment in a full-scale tunnel filled with smoke has been done in order to clarify the relation between extinction coefficient up to Cs = 1.0 m⁻¹, which includes Cs = 0.4 m⁻¹ as a Japanese road tunnel fire prevention standard, and evacuation speed. The maximum, minimum and mean values of normal walking speeds are almost constant regardless of the extinction coefficient. As for the emergency evacuation speeds, the maximum speed is largely influenced by extinction coefficient, decreasing rapidly from 3.55 m/s at Cs = 0.30 m⁻¹ to 2.53 m/s at Cs = 0.75 m⁻¹ while the minimum and mean speeds are almost constant with a slight decrease as Cs increases. The maximum evacuation speed trends in the present experiments and those in Frantzich and Nillson (2003, 2004) and Fridolf et al. (2013), lie on the same decreasing logarithmic curve as a function of extinction coefficient.     

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.     

Combustion stability limits and NOx emissions of nonpremixed ammonia-substituted hydrogen–air flames

The combustion stability (extinction) limits and nitrogen oxide (NO_x) emissions of nonpremixed ammonia (NH₃)–hydrogen (H₂)–air flames at normal temperature and pressure are studied to evaluate the potential of partial NH₃ substitution for improving the safety of H₂ use and to provide a database for the nonpremixed NH₃-substituted H₂–air flames. Considering coflow nonpremixed NH₃–H₂–air flames for a wide range of fuel and coflow air injection velocities (V_fuel and V_coflow) and the extent of NH₃ substitution, the effects of NH₃ substitution on the stability limits and NO_x emissions of the NH₃–H₂–air flames are experimentally determined, while the nonpremixed NH₃–H₂–air flame structure is computationally predicted using a detailed reaction mechanism. Results show significant reduction in the stability limits and unremarkable increase in the NO_x emission index for enhanced NH₃ substitution, supporting the potential of NH₃ as an effective, carbon-free additive in nonpremixed H₂–air flames. With increasing V_coflow the NO_x emission index decreases, while with increasing V_fuel it decreases and then increases due to the recirculation of burned gas and the reduced radiant heat losses, respectively. Given V_coflow/V_fuel the flame length increases with enhanced NH₃ substitution since more air is needed for reaction stoichiometry. The predicted flame structure shows that NH₃ is consumed more upstream than H₂ due to the difference between their diffusivities in air.     

Laminar flame speeds and extinction limits of conventional and alternative jet fuels

Experimental results of laminar flame speeds and extinction stretch rates for the conventional (Jet-A) and alternative (S-8) jet fuels are acquired and compared to the results from our earlier studies for neat hydrocarbon surrogate components, including n-decane and n-dodecane. Specifically, atmospheric pressure laminar flame speeds are measured using a counterflow twin-flame configuration for Jet-A/O₂/N₂ and S-8/O₂/N₂ mixtures at preheat temperatures of 400, 450, and 470 K and equivalence ratios ranging from 0.7 to 1.4. The flow field is recorded using digital particle image velocimetry. Linear extrapolation is then applied to determine the unstretched laminar flame speed. Experimental data for the extinction stretch rates of the nitrogen diluted jet fuel/oxidizer mixtures as a function of equivalence ratio are also obtained. In addition, the experimental data of Jet-A are compared to the computed values using a chemical kinetic mechanism for a kerosene surrogate reported in literature. A sensitivity analysis is further performed to identify the key reactions affecting the laminar flame speed and extinction stretch rate for this kerosene surrogate.     

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.