An Experimental Study on Flame Propagation in Cornstarch Dust Clouds

Following the quantitative determination of dust cloud parameters, this study investigates the flame propagation through cornstarch dust clouds in a vertical duct of 780 mm height and 160 x 160 mm square cross section, and gives particular attention to the effect of small scale turbulence and small turbulence intensity on flame characteristics. Dust suspensions in air were produced using an improved apparatus ensuring more uniform distribution and repeatable dust concentrations in the testing duct. The dispersion-induced turbulence was measured by means of a particle image velocimetry (PIV) system, and dust concentrations were estimated by direct weighing method. This quantitative assessment made it possible to correlate observed flame behaviors with the parameters of the dust cloud. Upward propagating dust flames, from both closed/open bottom end to open/closed top end of the duct, were visualized by direct light and shadow photography. From the observation of propagation regimes and the measurements of flame velocity, a critical value of the turbulence intensity can be specified below which laminar flame propagation would be established. This transition condition was determined to be 10 cm/s. Laminar flames propagated with oscillations from the closed bottom end to the open top end of the testing duct, while the turbulent flames accelerated continuously. Both laminar and turbulent flames propagated with steady velocity from the open bottom end to the closed top end of the duct. The measured propagation velocity of laminar flames appeared to be in the range of 0.45-0.56 m/s, and it was consistent with the measurements reported in the literature. In the present experimental study, the influence of dust concentration on flame propagation was also examined, and the flame propagation velocity was found weakly sensitive to the variations in dust concentration. Some information on the flame structure was revealed from the shadow records, showing the typical heterogeneous feature of the dust combustion process.

Chemical equilibrium analysis and CFD simulation of coal combustion and NOx formation

A full two-fluid model of reacting gas-particle flows with an algebraic unified second-order moment (AUSM) turbulence-chemistry model is used to simulate Beijing coal combustion and NOx formation. The sub-models are the k-epsilon-kp two-phase turbulence model, the EBU-Arrhenius volatile and CO combustion model, the six-flux radiation model, coal devolatilization model and char combustion model. The blocking effect on NOx formation is discussed. In addition, the chemical equilibrium analysis is used to predict NOx concentration at different temperature. Results of CID simulation and chemical equilibrium analysis show that, optimizing air dynamic parameters can delay the NOx formation and decrease NOx emission, but it is effective only in a restricted range. In order to decrease NOx emission near to zero, the re-burning or other chemical methods must be used.

What Do We Know about Long Laminar Plasma Jets?

Silent and stable long laminar plasma jets can be generated in a rather wide range of working parameters. The laminar flow state can be maintained even if considerable parameter fluctuations exist in the laminar plasma jet or if there is an impact of laterally injected particulate matter and its carrier gas. The attractive special features of laminar plasma jets include extremely low noise level, less entrainment of ambient air, much longer and adjustable high-temperature region length, and smaller axial gradient of plasma parameters. Modeling results show that the laminar plasma jet length increases with increasing jet inlet velocity or temperature and the effect of natural convection on laminar plasma jet characteristics can be ignored, consistent with experimental observations. The large difference between laminar and turbulent plasma jet characteristics is revealed to be due to their different laws of surrounding gas entrainment. Besides the promising applications of the laminar plasma jet to remelting and cladding strengthening of the metallic surface and to thermal barrier coating preparation, it is expected that the laminar plasma jet can become a rather ideal object for the basic studies of thermal plasma science owing to the nonexistence of the complexity caused by turbulence.

Direct initiation of detonation with ignition tube

Experiments of direct initiation of hydrogen-oxygen by means of a hot turbulent jet were made. Results indicate that the length of ignition tube is the dominant factor in determining the ignition capability of hot turbulent jet, and that the ignition capability of turbulence jet increases with the length of ignition tube. Because this ignition capability can meet the demands of a gas-detonation-driver shock tunnel and it doesn't require additional facilities, the hot turbulent jet initiation method can be applied to large hydrogen-oxygen detonation-driver shock tunnels. Influences of obstacles on the ignition capability were also studied. It was found that the presence of obstacles weakens the ignition capability of a hot turbulent jet.

Cone Effect in Astronomical Adaptive Optics System Investigated by a Pure Numerical Simulation

It is well-known that cone effect or focus anisoplanatism is produced by the limited distance of a laser guide star (LGS) which is created within the Earth atmosphere and consequently located at a finite distance from the observer. In this paper, the cone effect of the LGS for different vertical profiles of the refractive index structure constant Cn2 is numerically investigated by using a revised computer program of atmospheric propagation of optical wave and an adaptive optics (AO) system including dynamic control process. According to the practice, the overall tilt for the tilt-correction mirror is obtained from a natural star and the aberrated wavefront for phase correction of the deformable mirror is obtained from a LGS in our numerical simulation. It is surprisingly found that the effect of altitude of the LGS on the AO phase compensation effectiveness by using the commonly-available vertical profiles of Cn2 and the lateral wind speed in the atmosphere is relatively weak, and the cone effect for some Cn2 profiles is even negligible. It is found that the cone effect does not have obvious relationship with the turbulence strength, however, it depends on the vertical distribution profile of Cn 2 apparently. On the other hand, the cone effect depends on the vertical distribution of the lateral wind speed as well. In comparison to a longer wavelength, the cone effect becomes more obvious in the case of a shorter wavelength. In all cases concerned in this paper, an AO system by using a sodium guide star has almost same phase compensation effectiveness as that by using the astronomical target itself as a beacon. Effect of dynamic control process in an AO system on the cone effect is studied in this paper for the first time within our knowledge.

Numerical Study of Heat Transfer Mechanism in Turbulent Supercritical CO2 Channel Flow

Direct numerical simulation (DNS) of supercritical CO2 turbulent channel flow has been performed to investigate the heat transfer mechanism of supercritical fluid. In the present DNS, full compressible Navier-Stokes equations and Peng-Robison state equation are solved. Due to effects of the mean density variation in the wall normal direction, mean velocity in the cooling region becomes high compared with that in the heating region. The mean width between high-and low-speed streaks near the wall decreases in the cooling region, which means that turbulence in the cooling region is enhanced and lots of fine scale eddies are created due to the local high Reynolds number effects. From the turbulent kinetic energy budget, it is found that compressibility effects related with pressure fluctuation and dilatation of velocity fluctuation can be ignored even for supercritical condition. However, the effect of density fluctuation on turbulent kinetic energy cannot be ignored. In the cooling region, low kinematic viscosity and high thermal conductivity in the low speed streaks modify fine scale structure and turbulent transport of temperature, which results in high Nusselt number in the cooling condition of the supercritical CO2.

Analytical calculations of Eulerian and Lagrangian time correlations in turbulent shear flows

The Taylor series expansion method is used to analytically calculate the Eulerian and Lagrangian time correlations in turbulent shear flows. The short-time behaviors of those correlation functions can be obtained from the series expansions. Especially, the propagation velocity and sweeping velocity in the elliptic model of space-time correlation are analytically calculated and further simplified using the sweeping hypothesis and straining hypothesis. These two characteristic velocities mainly determine the space-time correlations.

Floating zone convection

The microgravity research, as a branch of the advanced sciences and a spe- cialized field of high technology, has been made in China since the late 1980's. The research group investigating microgravity fluid physics consisted of our col- leagues and the authors in the Institute of Mechanics of the Chinese Academy of Sciences (CAS), and we pay special attention to the floating zone convection as our first research priority. Now, the research group has expanded and is a part of the National Microgravity Laboratory of the CAS, and the research fields have been extended to include more subjects related to microgravity science. Howev- er, the floating zone convection is still an important topic that greatly holds our research interests.

陆面过程的模式与观测研究

谈镐生文集

本文集收入了著名力学家、应用数学家、中国科学院院士谈镐生先生在流体力学、稀薄气体动力学和应用数学研究领域的论文和研究报告26篇，谈镐生先生倡导和支持力学基础研究的有关文章和论述21篇，谈先生的学术活动和生活图片多幅，以及谈镐生先生生平，最后附有谈先生生平年表。

 

 

若干流体力学问题的数学性质研究

该文利用高智的扩散抛物化方程组理论及流体力学基本方程组的特征次特征理论,流体大小尺度（LSS）方程组理论以及摄动有限差分（PFD）方法,研究若干流体力学问题的数学性质.该文得到的主要结论有：1.利用湍流大小尺度（LSS）方程组推导出湍流大小尺度涡量（LSSV）方程组,并证明两个关于湍流大小尺度涡量的命题,从而得到湍流封闭大小尺度涡量（CLSSV）方程组,并对已有的近程相互作用命题进行推广.2.根据扩散抛物化方程组理论和流体力学层次结构方程组的特征和次特征方法,研究了抛物化稳定性方程组（PSE）的特征和次特征以及消除PSE的剩余椭圆特性的问题.3.利用摄动有限差分（PFD）方法得到对流扩散反应方程的变步长摄动有限差分格式,是等步长摄动有限差分格式的推广.

槽道湍流的直接数值模拟

通过直接数值模拟(DNS)研究槽道湍流的性质和机理。包含五个部分：1）湍流直接数值模拟的差分方法研究。2）求解不可压N－S方程的高效算法和不可压槽道湍流的直接数值模拟。3）可压缩槽道湍流的直接数值模拟和压缩性机理分析。4）“二维湍流”的机理分析。5）槽道湍流的标度律分析。1．针对壁湍流计算网格变化剧烈的特点，构造了基于非等距网格的的迎风紧致格式。该方法直接针对计算网格构造格式中的系数，克服了传统方法采用 Jacobian 变换因网格变化剧烈而带来的误差。针对湍流场的多尺度特性分析了差分格式的精度、网格尺度与数值模拟能分辨的最小尺度的关系，给出不同差分格式对计算网格步长的限制。同时分析了计算中混淆误差的来源和控制方法，指出了迎风型紧致格式能很好地控制混淆误差。2．将上述格式与三阶精度的Adams半隐格式相结合，构造了不可压槽道湍流直接数值模拟的高效算法。该算法利用基于交错网格的离散形式的压力Poisson方程求解压力项，避免了压力边界条件处理的困难。利用FFT对方程中的隐式部分进行解耦，解耦后的方程采用追赶法（LU分解法）求解，大大减少了计算量。为了检验该方法，进行了三维不可压槽道湍流的直接数值模拟，得到了Re＝2800的充分发展不可压槽道湍流，并对该湍流场进行了统计分析。包括脉动速度偏斜因子在内的各阶统计量与实验结果及Kim等人的计算结果吻合十分理想，说明本方法是行之有效的。3．进行了三维充分发展的可压缩槽道湍流的直接数值模拟。得到了 Re=3300，Ma=0.8的充分发展可压槽道湍流的数据库。流场的统计特征（如等效平均速度分布，“半局部”尺度无量纲化的脉动速度均方根）和他人的数值计算结果吻合。得到了可压槽道湍流的各阶统计量，其中脉动速度的偏斜因子和平坦因子等高阶统计量尚未见其他文献报道。同时还分析了压缩性效应对壁湍流影响的机理，指出近壁处的压力－膨胀项将部分湍流脉动的动能转换成内能，使得可压湍流近壁速度条带结构更加平整。4．模拟了二维不可压槽道流动的饱和态（所谓“二维湍流”），分析了“二维槽道湍流”的非线性行为特征。分析了流场中的上抛－下扫和间歇现象，研究了“二维湍流”与三维湍流的区别。指出“二维湍流”反映了三维湍流的部分特征，同时指出了展向扰动对于湍流核心区发展的重要性。5．首次对可压缩槽道湍流及“二维槽道湍流”标度律进行了分析，得出了以下结论：a）槽道湍流中，在槽道中心线附近较宽的区域，存在标度律。b）该区域流场存在扩展自相似性（ESS）。c）在Mach数不是很高时，压缩性对标度指数影响不大。本文结果同SL标度律的理论值吻合较好，有效支持了该理论。对“二维槽道湍流”也有相似的结论，但与三维湍流不同的是，“二维槽道湍流”存在标度律的区域更宽，近壁处的标度指数比中心处有所升高。

再入尾迹电磁特性的湍流效应研究

本文旨意在于通过探讨高超声速再入尾迹中的湍流等离子体与电磁波相互作用的机理，以及建立能反映此机理的应用性理论模型，从而提供一套可进行目标特性分析的方法，以便为工程部门的突防技术服务。本题目在再入气动物理现象研究中具有重要意义。综合分析指出，地面雷达观测到的非相干散射信号主要来源于再入尾迹的亚密湍流区产生的体积散射。因此，电磁散射特性分析主要针对尾迹亚密湍流等离子体。并且，这里所有的分析都是根据在工程应用中最成熟的一阶畸变波Born近似理论模型。再入尾迹电磁特性的湍流效应研究，着眼点就在于湍流等离子体场的研究。对湍流等离子体场理论模型，本文试图通过模式理论来表达，即求解平均化的全Navier-Stokes方程及其封闭方程k-ε-g模型，从而准确获得流动平均场和脉动场信息。这种表达方式较以前有了较大改进。注意到高超声速流动具有强烈可压缩性的特点，故使用的N-S平均方程由质量加权平均过程产生，湍流模型方程也经过可压缩性修正。方程的离散求解方法，都是运用带矢通量分裂的二阶TVD格式的有限体积法。再入尾迹湍流场的初始条件由近尾迹（底部）流动经N-S方程求解给定，初始值更加准确可靠。尾迹从层流到湍流的转捩过程采用相对成熟的半经验公式确定。飞行器的高超声速再入过程必然导致它周围的空气温度升高，使得流动表现出真实气体效应。对重点考察的湍流流动而言，真实气体效应主要表现为气体处于热化学平衡状态。就工程部门面临的实际问题，把一阶畸变波Born近似的解算方法做些改进，使其能够处理的范围从轴对称尾迹扩展到三维湍流等离子体场是必要的。这为深入的理论分析提供了有力的保障。在能够准确模拟湍流流动的刻划雷达散射截面的基础上，考察亚密湍流等离子体对电磁散射的影响。通过选择的几个有代表性的因素进行讨论，初步结果表明：湍流转捩方式、湍流尺度对尾迹雷达散射截面值计算影响不大，而电子组份脉动能初始值影响较明显，且在特定条件下湍流模型的影响亦不大。但由于湍流模型涉及脉动初始值，其影响需进一步确定。同时，一些今后开展继续此项研究工作的有益建议也提了出来。

障碍物扰动对半开口管道中火焰传播特性的影响

自从1926年Chanman和Wheeler率先开创有障碍物管道中的火焰传播研究工作以来，管道中障碍物扰动引起的火焰加速现象引起了广泛的关注。由于这类现象在燃烧科学上的学术意义及其在生产中诸如安全问题等方面的实际意义，人们相继进行了一系列的研究工作。归纳起来，可以分为两大类：1）封闭管道的火焰传播。人们在不同形状的管道或容器中研究了障碍物对火焰加速的影响，在理论分析和数值计算方面也作了一些有益的工作；2）开口管道的火焰传播。相对于闭口管道，开口管道中的火焰传播研究则逊色得多。这方面尚缺乏系统的实验数据，理论方面的分析也欠缺得多。但当前在实际中，并不乏开口体系的应用，如，现已广泛应用于电力系统的一种燃气除灰装置，是一燃烧气体燃料的半开口系统。因此，研究半开口管道中非稳态燃烧的加速机制具有重要意义。在本论文工作中，通过大量的实验研究，比较系统地研究了障碍物的扰动对预混火焰传播特性的影响。实验在一长L=5m、内径D＝80mm的一端封闭、一端开口的火焰传播管内进行，管内均匀布置障碍物，通过改变障碍物的形状、间距、阻塞比大小，同时选用五种不同的可燃气体，探索了障碍物结构对预混湍流火焰加速和管内压力上升的影响。实验表明，对于敏感气体如氢气和乙炔，由于障碍物扰动产生的影响，火焰不断加速，并最终达到一准稳定状态；在适当的条件下，火焰传播状态可由爆燃向爆轰转变，此时火焰速度发生跃变；而对干不敏感气体如甲烷，则爆燃转爆轰现象不容易发生。在不同的火焰传播状态，障碍物结构特性对火焰速度和压力产生的影响各不相同。在缓燃态，随着阻塞比的变化，最大火焰速度先上升后下降，在BR=0.3～0.4之间存在一最大值；在銮塞态，最大火焰速度受阻塞比变化的影响不明显，略低于燃烧产物的声速；在阻塞比BR＝0.5附近，压力达到最大值。而在爆轰态，随着阻塞比的增加，最大火焰速度和压力逐渐降低，爆燃转爆轰的浓度范围变小。由于在有障碍物的管道中，火焰速度很容易达到声速（变塞态）或超声速（爆轰态），必须考虑流体的马赫数效应。本文在前人研究成果的基础上，给出了湍流马赫数修正的可压缩性两方程湍流模型，模拟了半开口狭长管道中重复布置的障碍物引起的湍流火焰加速现象。最大火焰速度和管内压力的计算结果与实验测量值吻合良好，这表明用修正后的湍流模型能够比较真实地模拟障碍物管内预混火焰的发展过程。通过对管道内障碍物扰动引起的燃烧波加速的机理和技术研究，对流场扰动对燃烧波产生和发展的影响规律有了比较全面的了解，研究结果对气脉冲除灰技术的完善具有直接的指导意义。