氢气/甲烷-空气爆轰波在含环形障碍物圆管内传播的试验研究

在内径48mm、长度5 800mm的含环形障碍物圆管内,进行了氢气-空气及氢气-甲烷-空气的爆轰波传播试验研究,确定了爆燃转爆轰(Deflagration-to-Detonation Transition,DDT)极限。环形障碍物阻塞比为0.56,间距分为两种,即S=D和S=2D,其中S为障碍物间距,D为管道内径。火焰的速度由安装在管道壁面上的光电二极管采集得到。试验测量得到的火焰为准爆轰或阻塞火焰。在S=2D情况下得到的火焰速度均比S=D情况下的火焰速度高,并且靠近DDT极限时速度波动更明显,表明在间距较大的情况下爆轰的重起爆循环周期更长,类似于"弛振爆轰"。对于氢气-空气,障碍物间距为D时在DDT极限处有d/λ1(富氧条件下d/λ=1.6,贫氧条件下d/λ=1.4),间距为2D时更容易形成爆轰的重起爆,在DDT极限处与准则d/λ≈1一致;对于氢气-甲烷-空气,甲烷的添加使爆轰更不稳定,对于两种间距的障碍物得到的DDT极限均有d/λ≈1(d和λ分别为障碍物内径和爆轰胞格尺寸)。说明障碍物间距对爆轰波传播有显著的影响,即间距的增大更有利于爆轰波的传播。为形成准爆轰,障碍物内径必须至少可以容纳一个爆轰胞格,同时障碍物间距足够大从而引起爆轰的重起爆。     

半开口管道中的氢/空气火焰加速和压力发展过程

本文研究了氢／空气预混火焰在半开口管道中的火焰加速现象和压力发展过程．结果表明,重复布置的障碍物对火焰速度和压力提升产生显著的影响．火焰传播状态随着氢气当量比的变化而发生改变,在氢气当量比约为０．３４时,火焰速度出现第一次跃变;随着氢气当量比进一步提高,火焰速度发生第二次跃变,即由爆燃转为爆轰．发生爆轰时氢气当量比的范围随着阻塞比的不同而发生变化．     

基于变传质强度因子Lee模型的流动沸腾模拟

本文基于不同流速下变传质强度因子的Lee模型,对低热通量下上升管内流动沸腾进行了数值模拟。通过分析气相分布、局部气泡行为解释了沸腾换热特性和局部传热恶化。结果表明：低热通量下,不同流速的平均气体体积分数沿管长线性分布,沿径向呈双峰状分布,峰值在壁面附近;高流速时,近壁处气体体积分数的增长速率最大。换热性能受流速影响显著,流速越大,换热系数越大;三种流速下换热系数最小值对应的气体体积分数相同,气相分布不同是换热特性产生差异的直接原因。低中流速下,壁温局部升高位置与换热系数骤降位置是一致的,表明壁面附近局部气泡附着会造成壁温的局部升高,导致传热恶化。     

THz干涉测量用于障碍物后振动传感的研究

为探索THz干涉技术用于障碍物后振动传感的可行性,采用工作波长214.58 μm （对应频率约1.4 THz）的CO2激光器泵浦气体太赫兹源搭建了一套基于迈克尔逊干涉仪结构的THz干涉测量装置,实验研究了薄纸板遮挡后敲击目标镜产生的微小振动,利用相位分析法和频谱分析法对振动干涉信号进行处理,得到了振动位移随时间的变化以及不同时段振动频率的分布情况,测得的峰峰值振幅最小为7.98 μm,最大为17.54 μm,振动峰值速度为2.7 mm/s,振动频率最小21 Hz,最大58 Hz.研究结果表明THz干涉测量技术能有效克服传统振动传感技术无法穿透障碍物的缺点,是一种简便有效的障碍物后振动传感的新型手段,预示了THz技术在振动检测相关领域的广阔应用前景.     

上升管中强烈段塞流消除方法试验研究

为了消除上升管中强烈段塞流,采用节流和扰动两种方法进行了试验研究。两种方法都能完全消除强烈段塞流, 能使强烈段塞流的发生范围显著减小。节流程度较小时将导致强烈段塞流压力波动更大、周期更长;但扰动法能使压力波动的幅度变小、周期变短。两种方法都使上游管道的压力显著增大,但扰动法产生的背压比节流法产生的背压小。     

喷粉压力及点火延迟时间对粉尘爆炸参数的影响

为进一步研究影响粉尘爆炸特性参数的因素,在5L柱形密闭爆炸容器中,以食用玉米淀粉为试样,利用高压放电火花点火,并通过压力采集系统记录容器内压力的变化,研究了不同吹粉压力下,点火延迟时间对粉尘爆炸压力参数的影响,并对实验现象进行了理论分析。实验结果表明:点火延迟时间对粉尘爆炸压力和压力上升速率影响显著;吹粉压力存在一个临界值,当吹粉压力大于临界值时,存在一个最佳点火延迟时间,使得爆炸压力峰值和压力上升速率峰值最大,且随着吹粉压力的增大,粉尘爆炸的最佳点火延迟时间缩短;当吹粉压力小于临界值时,点火延迟时间越长,粉尘爆炸压力越小。     

阀门关闭对低温盲支管水击的影响分析

采用SINDA/FLUINT软件建立了具有盲支管的低温加注管路模型,仿真计算得到了不同阀门关闭时间、盲支管长度、阀门关闭顺序、管径下的水击压力变化情况。结果显示,阀门关闭时间越短,造成盲支管内的压力变化越剧烈,形成较大的压力峰值,容易对管路造成损坏;在低温管路处于冷态的情况下,盲支管长度对盲支管内压力峰值影响不大;某一支路阀门关闭使得该阀前压力出现变化,对另一支路压力影响不大,在整个管路形成封闭腔后,阀门关闭造成系统压力急剧变化,影响较大;管径越大,阀门关闭时的水击压力峰值越大。     

指数型谐振管内非线性声场的模拟研究

采用气体动理学方法模拟研究五种不同驱动强度下,=0的圆直热声谐振管和=2 2的指数型热声谐振管内的非线性声场特征。研究发现:指数型管能抑制激波及声饱和现象,但管内会出现尖峰现象。尖峰处的压力波形发生了畸变;它们是非线性的能量传递与各种耗散的竞争过程中产生的不同形态的波特征;当活塞驱动位移幅值400μm时,指数型管内压力振幅增长速度明显下降。     

菌紫质光致各向异性动力学的理论和实验

对菌紫质(Bacteriorhodopsin,BR)薄膜的光致各向异性特性进行了理论模拟.由随机取向的极性BR分子构成的聚合物薄膜在宏观上是各向同性的,但在线偏振光的作用下,BR分子对激发光的选择性吸收,导致不同异构体分子取向的不均匀分布,从而使BR薄膜在宏观上呈现出各向异性.利用BR光循环的二能级模型,得到了B态分子取向分布随时间的变化关系,推导出了BR薄膜光致各向异性动力学的数学表达式,模拟了不同激发光强下BR薄膜的光致各向异性动力学曲线,得到了和实验结果一致的计算结果.     

窄带随机激励双稳压电悬臂梁响应机制与能量采集研究

具有中心频率的窄带随机振动是一种典型的环境振动,其振动特征与环境的变化密切相关.本文以双稳压电悬臂梁能量采集系统为研究对象,分析系统在不同磁铁间距下的等效线性固有频率特性,以带通滤波器输出一定带宽的窄带随机激励模拟环境振动,研究系统的响应和能量采集特征.研究表明,对于一定带宽的窄带随机激励,一方面系统始终存在一个固定的磁铁间距使其输出达到峰值,另一方面当激励中心频率在一定范围内变化时,系统还分别存在另外两个或一个不同磁铁间距也能使系统输出达到峰值,而且该峰值特性是系统在其等效线性固有频率处诱导双稳或单稳“共振”形成的.研究结果可为具有窄带随机激励特征的振动能量采集提供一定的理论和技术支持.     

High-speed visualization of flame propagation in explosions

Flow visualization data is presented to describe the structure of flames propagating in methane-air explosions in semi-confined enclosures. The role of turbulence is well established as a mechanism for increasing burning velocity by fragmenting the flame front and increasing the surface area of flames propagating in explosions. This area increase enhances the burning rate and increases the resultant explosion overpressure. In real situations, such as those found in complex process plant areas offshore, the acceleration of a flame front results from a complex interaction between the moving flame front and the local blockage caused by presence of equipment. It is clear that any localised increase in flame burn rate and overpressure would have important implications for any adjacent plant and equipment and may lead to an escalation process internal to the overall event. To obtain the information required to quantify the role of obstacles, it is necessary to apply a range of sophisticated laser-based, optical diagnostic techniques. This paper describes the application of high-speed, laser-sheet flow visualization and digital imaging to record the temporal development of the flame structure in explosions. Data is presented to describe the interaction of the propagating flame with a range of obstacles for both homogeneous and stratified mixtures. The presented image sequences show the importance of turbulent flow structures in the wake of obstacles for controlling the mixing of a stratified concentration field and the subsequent flame propagation through the wake. The data quantifies the flame speed, shape and area for a range of obstacle shapes.     

Numerical simulations of the flow field ahead of an accelerating flame in an obstructed channel

The development of the unburned gas flow field ahead of a flame front in an obstructed channel was investigated using large eddy simulation (LES). The standard Smagorinsky–Lilly and dynamic Smagorinsky–Lilly subgrid models were used in these simulations. The geometry is essentially two-dimensional. The fence-type obstacles were placed on the top and bottom surfaces of a square cross-section channel, equally spaced along the channel length at the channel height. The laminar rollup of a vortex downstream of each obstacle, transition to turbulence, and growth of a recirculation zone between consecutive obstacles were observed in the simulations. By restricting the simulations to the early stages of the flame acceleration and by varying the domain width and domain length, the three-dimensionality of the vortex rollup process was investigated. It was found that initially the rollup process was two-dimensional and unaffected by the domain length and width. As the recirculation zone grew to fill the streamwise gap between obstacles, the length and width of the computational domain started to affect the simulation results. Three-dimensional flow structures formed within the shear layer, which was generated near the obstacle tips, and the core flow was affected by large-scale turbulence. The simulation predictions were compared to experimental schlieren images of the convection of helium tracer. The development of recirculation zones resulted in the formation of contraction and expansion regions near the obstacles, which significantly affected the centerline gas velocity. Oscillations in the centerline unburned gas velocity were found to be the dominate cause for the experimentally observed early flame-tip velocity oscillations. At later simulation times, regular oscillations in the unburned streamwise gas velocity were not observed, which is contrary to the experimental evidence. This suggests that fluctuations in the burning rate might be the source of the late flame-tip velocity oscillations. The effect of the obstacle blockage ratio (BR) on the development of the unburned gas flow field was also investigated by varying the obstacle height. Simulation predictions show favorable agreement with the experimental results and indicate that turbulence production increases with increasing obstacle BR.     

Numerical simulations of flame propagation and DDT in obstructed channels filled with hydrogen–air mixture

We study flame acceleration and deflagration-to-detonation transition (DDT) in channels with obstacles using 2D and 3D reactive Navier–Stokes numerical simulations. The energy release rate for the stoichiometric H₂–air mixture is modeled by a one-step Arrhenius kinetics. Computations show that at initial stages, the flame and flow acceleration is caused by thermal expansion of hot combustion products. At later stages, shock–flame interactions, Rayleigh–Taylor, Richtmyer–Meshkov, and Kelvin–Helmholtz instabilities, and flame–vortex interactions in obstacle wakes become responsible for the increase of the flame surface area, the energy-release rate, and, eventually, the shock strength. Computations performed for different channel widths d with the distance between obstacles d and the constant blockage ratio 0.5 reproduce the main regimes observed in experiments: choking flames, quasi-detonations, and detonations. For quasi-detonations, both the initial DDT and succeeding detonation reignitions occur when the Mach stem, created by the reflection of the leading shock from the bottom wall, collides with an obstacle. As the size of the system increases, the time to DDT and the distance to DDT increase linearly with d². We also observe an intermediate regime of fast flame propagation in which local detonations periodically appear behind the leading shock, but do not reach it.     

障碍物扰动对预混气点火的影响

在一根半开口管道中,以乙炔为燃料,改变当量比和进气速度,实验研究了障碍物扰动对预混气点火的影响。实验表明,在小当量比情况下,在没有障碍物的光管中,随着进气速度的增加,点火所需要的当量比逐渐减小,回火区域逐渐增大。而添加障碍物后,回火区域的位置和点火所需要的当量比相对来说比较稳定,基本不随进气速度的增加而大幅度变化。并且由于障碍物引起的扰动使得点火所需要的当量比要比无障碍物时大得多。     

低真空管道列车表面压力变化规律研究

既有高速铁路进一步提速受限,构建低真空管道运行超高速列车的发展趋势日益明显.运用滑移网格技术,建立动车组列车和低真空管道的三维耦合模型,考虑管道气体的瞬态压缩效应,分析低真空管道横截面积、动车组列车运行速度、管道环境温度和环境压力对车体表面压力的影响.研究表明,低真空管道横截面积、动车组列车运行速度、管道环境温度和环境...     

Depinning of a discrete elastic string from a two-dimensional random array of weak pinning points

The present work is essentially concerned with the development of statistical theory for the low temperature dislocation glide in concentrated solid solutions where atom-sized obstacles impede plastic flow. In connection with such a problem, we compute analytically the external force required to drag an elastic string along a discrete two-dimensional square lattice, where some obstacles have been randomly distributed. Some numerical simulations allow us to demonstrate the remarkable agreement between simulations and theory for an obstacle density ranging from 1% to 50% and for lattices with different aspect ratios. The theory proves efficient on the condition that the obstacle-chain interaction remains sufficiently weak compared to the string stiffness.     

偏振X射线荧光系统测量痕量Cd元素的Geant4模拟研究

通过传统能量色散X射线荧光分析(EDXRF)对Cd元素进行痕量分析时,X光管发出的原级轫致辐射连续谱对Cd元素的分析具有严重的影响.为了减弱原级X射线对测量结果的影响,通过Geant4程序包模拟不同几何尺寸下偏振激发X射线荧光分析(P-EDXRF)中荧光靶的结构,研究其在减弱测量过程中原级X射线轫致辐射连续谱的影响.为...     

On the number of tube rows required to study cross-flow induced vibrations in tube banks

Careful experiments in which a triangular array of tubes with a pitch ratio of 1·375 was used have been undertaken to determine the minimum number of tube rows required to study flow induced vibration phenomena typical of a tube bank. Individual tube responses were monitored to examine the effects of different numbers of upstream and downstream tube rows. It was found that the critical tubes for fluid-elastic instability are in the third and fourth rows. From the point of view of studying vibrations induced by turbulence, vorticity phenomena and fluid-elastic instability, it is recommended that six tube rows be used.     

一种复杂环境下移动机器人避障新方法

基于传统人工势场法的机器人避障算法虽然计算量小、实时性好,但在陷阱区域或在障碍物前可能产生震荡,导致机器人不能到达目标点,为解决上述缺陷,提出一种带记忆功能的沿边法。通过记录和分析障碍物边缘人工势场法的局部最小点来判断目标点是否被障碍物包围,从而避免机器人来回震荡和围着目标点旋转的发生。该方法步骤分为沿边行为激活和退出条件、局部最小点记录条件以及目标点被障碍物包围的判断准则。通过实验,结果表明该方法解决了复杂环境下机器人的避障问题。该方法用于复杂环境下的机器人避障是可行的、有效的。