地形因素对雷电定位的影响与分析

针对目前我国电力系统的雷电探测定位系统采用基于VLF-LF雷电探测器接收雷电波地波及实际计算模型中采用真空雷电波波速的精度问题,研究和改正了雷电定位计算中由于地形因素引起的雷电波传播距离误差,并对雷电波传播速度进行了分析、比较和探讨.通过实例计算表明,提出的距离改正计算及雷电波波速的取值大大提高了雷电定位计算精度.     

应用图像处理技术进行预混层流火焰传播速度的在线测量

利用图像处理技术，根据层流预混火焰稳定燃烧的条件结合预混火焰燃烧模型，设计了火焰传播速度在线实时测量系统，通过图像采集在线获取火焰的静态图像，然后自动对图像进行滤波，边缘提取，再进行计算得到火焰传播前沿面积，在线算出火焰传播速度，这种方法具有非接触式测量的优点，利用它不仅可以测量层流预混火焰的传播速度，同时还为层流火焰的燃烧机理的研究提供一种直接的手段．     

Diffusion equations based on generalized continuum mechanics

We propose that an increasing value in entropy inequality is not a local value of entropy but an average value in a mesodomain. From this standpoint, new balance equations are obtained by applying the concept of generalized continuum mechanics to mass transfer. In the present paper, it is clarified that the influence of a mesoscopic gradient of mass concentration should be included in Fick's first law and Fourier's law. Even if thermal effects on diffusion are neglected, the diffusion equations obtained here are simultaneous differential equations with two undetermined values, which include the conventional diffusion equation as a special case. The conventional diffusion equation describing infinite velocity of propagation associated with diffusion entails a contradiction. The velocity of propagation defined here is shown in the results of a numerical analysis for an extreme initial state. Consequently, it is indicated that the present theory gives an acceptable solution to the above problem in the conventional theory. © 1998 Scripta Technica, Inc. Heat Trans Jpn Res, 26(2): 84–96, 1997     

On the dispersion of waves for the linear thermoelastic relaxed micromorphic model

Abstract

We present the complete set of constitutive relations and field equations for the linear thermoelastic relaxed micromorphic continuum and investigate its variants for wave propagation. It is found that the additional thermal effects give rise to new waves and generate couplings with longitudinal waves which are not existing in the relaxed micromorphic continuum without thermal effects. However, transverse waves go unaffected by the thermal properties. Thermal effects do not create any band gap in the dispersion curves of the model with three curvature parameters. The dispersion curves have been computed numerically for a particular model and compared with those presented in earlier studies.     

Performance analysis of a HAT tidal current turbine and wake flow characteristics

Having very strong current on the west coast with up to 10 m tidal range, there are many suitable sites for the application of tidal current power (TCP) in Korea. The turbine, which initially converts the tidal energy, is an important component because it affects the efficiency of the entire system. To design a turbine that can extract the maximum power on the site, the depth and duration of current velocity with respect to direction should be considered. To extract a significant quantity of power, a tidal current farm with a multi-arrangement is necessary in the ocean. The interactions between devices contribute significantly to the total power capacity. Thus, the study of wake propagation is necessary to understand the evolution of the wake behind a turbine. This paper introduces configuration design of horizontal axis tidal current turbine based on the blade element theory, and evaluating its performance with CFD. The maximum efficiency of the designed turbine was calculated as 40% at a tip speed ratio (TSR) of 5. The target capacity of 300 kW was generated at the design velocity, and the performance was stable over a wide range of rotating speeds. To investigate the wakes behind the turbine, unsteady simulation was carried out. The wake velocity distribution was obtained, and velocity deficit was calculated. A large and rapid recovery was observed from 2D to 8D downstream, followed by a much slower recovery beyond. The velocity was recovered up to 86% at 18D downstream.     

Simulation of one method of laser welding of metal plates involving an SHS-reacting powder mixture

The mathematical model is based on the physical and chemical mechanism of the fusion penetration of welded joints involving the energy of chemical interaction in a powder mixture in the self-propagating high-temperature synthesis (SHS) mode. The reacting mixture located inside the joint between thermal-conducting metal plates is initiated with a laser beam moving simultaneously along the plate contact boundary. The minimum power of the laser radiation and physical and chemical properties of the powder mixture needed to provide the sufficient velocity of the welding and depth of plate joint penetration have been determined. Numerical simulation has enabled to ground theoretically the possibility of realization of the SHS welding of both homogeneous and heterogeneous plates from aluminum and titanium with no steam–gas channel in the stable mode, when the intrinsic velocity of the SHS propagation does not exceed the welding velocity.     

Three-dimensional numerical simulation of non-condensables accumulation induced by steam condensation in a non-vented pipeline

A substantial increase of the concentration of non-condensable gases in the mixture with steam can occur in a non-vented pipeline due to the condensation. This phenomenon is investigated with the thermal-hydraulic and physicochemical code HELIO. The hydrogen and oxygen accumulation is simulated and analyzed for a real non-vented steam pipeline of the nuclear power plant. The results show the propagation of non-condensables concentration front, the temperature and velocity field of the steam–non-condensables mixture, and the velocity and thickness of the condensate that drains on the pipeline’s inner walls. The gas mixture temperature is verified with measurements from a full size test facility. The presented modelling approach and numerical results are unique regarding the simultaneous solution of the heat and mass transfer in the system consisting of the steam–non-condensable gases mixture and the thin liquid film on the pipe’s wall.     

Numerical study of turbulent flame velocity

A premixed flame propagating through a combination of vortices in a tube/channel is studied using direct numerical simulations of the complete set of combustion equations including thermal conduction, diffusion, viscosity, and chemical kinetics. Two cases are considered, a single-mode vortex array and a multimode combination of vortices obeying the Kolmogorov spectrum. It is shown that the velocity of flame propagation depends strongly on the vortex intensity and size. The dependence on the vortex intensity is almost linear in agreement with the general belief. The dependence on the vortex size may be imitated by a power law ∝D2/3. This result is different from theoretical predictions, which creates a challenge for the theory. In the case of the Kolmogorov spectrum of vortices, the velocity of flame propagation is noticeably smaller than for a single-mode vortex array. The flame velocity depends weakly on the thermal expansion of burning matter within the domain of realistically large expansion factors. Comparison to the experimental data indicates that small-scale turbulence is not the only effect that influences the flame velocity in the experimental flows. Large-scale processes, such as the Darrieus-Landau instability and flame-wall interaction, contribute considerably to the velocity of flame propagation. Still, on small scales, the Darrieus-Landau instability becomes important only for a sufficiently low vortex intensity.     

预混合燃烧现象学紊流火焰速度模型

提出了一个用于预混合燃烧的现象学紊流火焰速度模型,描述了火焰从层流传播到充分发展的紊流传播的全过程。基于火焰瞬时尺度和基本的紊流特性参数,按照火焰生长的各个阶段,将紊流火焰速度的计算分为３个步骤,以有效紊流强度显示从层流传播到紊流传播的转化,以紊流积分标尺和梅尔莫哥洛夫标尺作为火焰皱折程度的度量,考虑了火焰表面扭曲对火焰速度的作用。计算结果与测量数据的比较显示了较好的一致性。     

Liftoff of turbulent jet flames—assessment of edge flame and other concepts using cinema-PIV

Three theories of the liftoff of a turbulent jet flame were assessed using cinema-particle imaging velocimetry movies recorded at 8000 images/s. The images visualize the time histories of the eddies, the flame motion, the turbulence intensity, and streamline divergence. The first theory assumes that the flame base has a propagation speed that is controlled by the turbulence intensity. Results conflict with this idea; measured propagation speeds remains close to the laminar burning velocity and are not correlated with the turbulence levels. Even when the turbulence intensity increases by a factor of 3, there is no increase in the propagation speed. The second theory assumes that large eddies stabilize the flame; results also conflict with this idea since there is no significant correlation between propagation speed and the passage of large eddies. The data do support the “edge flame” concept. Even though the turbulence level and the mean velocity in the undisturbed jet are large (at jet Reynolds numbers of 4300 and 8500), the edge flame creates its own local low-velocity, low-turbulence-level region due to streamline divergence caused by heat release. The edge flame has two propagation velocities. The actual velocity of the flame base with respect to the disturbed local flow is found to be nearly equal to the laminar burning velocity; however, the effective propagation velocity of the entire edge flame with respect to the upstream (undisturbed) flow exceeds the laminar burning velocity. A simple model is proposed which simulates the divergence of the streamlines by considering the potential flow over a source. It predicts the well-established empirical formula for liftoff height, and it agrees with experiment in that the controlling factor is streamline divergence, and not turbulence intensity or large eddy passage. The results apply only to jet flames for Re<8500; for other geometries the role of turbulence could be larger.     

Simulation of reactive nanolaminates using reduced models: II. Normal propagation

Transient normal flame propagation in reactive Ni/Al multilayers is analyzed computationally. Two approaches are implemented, based on generalization of earlier methodology developed for axial propagation, and on extension of the model reduction formalism introduced in Part I. In both cases, the formulation accommodates non-uniform layering as well as the presence of inert layers. The equations of motion for the reactive system are integrated using a specially-tailored integration scheme, that combines extended-stability, Runge–Kutta–Chebychev (RKC) integration of diffusion terms with exact treatment of the chemical source term. The detailed and reduced models are first applied to the analysis of self-propagating fronts in uniformly-layered materials. Results indicate that both the front velocities and the ignition threshold are comparable for normal and axial propagation. Attention is then focused on analyzing the effect of a gap composed of inert material on reaction propagation. In particular, the impacts of gap width and thermal conductivity are briefly addressed. Finally, an example is considered illustrating reaction propagation in reactive composites combining regions corresponding to two bilayer widths. This setup is used to analyze the effect of the layering frequency on the velocity of the corresponding reaction fronts. In all cases considered, good agreement is observed between the predictions of the detailed model and the reduced model, which provides further support for adoption of the latter.     

计及随机波动风速、风向的风电场建模方法研究

提出风电场建模方法建立风电场随机模型,对风电场运行特性进行仿真,为实现大规模风力发电的可预测、可控制目标服务。通过对风电场等值模型与详细模型的仿真比较,验证建模方法的合理性,并得出在研究风电场动态特性及其对电网影响时应考虑风速、风向的随机波动建立风电场模型。     

Combined power generation with wind and ocean waves

It is often advantageous to generate power with combinations of wind and ocean waves. In fact ocean waves, their generation, propagation, dissipation are directly related to wind velocity and its duration oven the sea. In this paper an attempt has been made to demonstrate statistically to present some advantages with combined wind and ocean wave power generation. Even though many conceptual techniques and methods are possible to harness combined power generation, it is important to test feasibility of combined out put as well as individual outputs mathematically. One of the major advantages of combined wind & wave power generation is to improve probability of continuous power supply (it minimises the interruptions and compensates power fluctuations with one another). Some of the major wave characteristics like wave Height (H), Time period (T), Wave length (L) significantly influence wave power generation. Interestingly, these ocean waves are dependent on wind velocity over ocean. To establish, a relation, a simple mathematical model has been developed to test different sets of combinations with wind velocities and wave characteristics. Statistical analysis has been made to estimate individual as well as combined probability density functions for a range of power outputs. Probability density functions at certain combinations showed promising results and it indicates that, combined power generation improves probability of continuous power supply (i.e. it minimises one of the major criticisms for renewable sources of energy).     

Numerical simulation study and dimensional analysis of hydrogen explosion characteristics in a closed rectangular duct with obstacles

The influence of obstacles on hydrogen explosion is studied by numerical simulation and dimensional analysis. The numerical simulation is conducted based on the premixed model in a closed rectangular duct with rectangular obstacles, and ten variables that affect the flame propagation velocity are analyzed by dimensional analysis. Continuous acceleration of flame and collision annihilation of flame were successfully realized through triangular obstacles in simulation. The result shows that with the number of obstacles changes, the flame invariably converts to hemispherical flame, finger flame, tongue flame, quasi-plane flame, and mouth flame in turn. But the flame front is more twisted in two obstacles due to hydrodynamic instability and vortices. Through the comparative analysis of the flame and flow field in the duct during hydrogen explosion. It is found that the flame-obstacles-flow field coupling and its hydrodynamic phenomena determine the flame deformation and changes in propagation velocity. The result of the dimensional analysis shows that the drag coefficient can well depict the effects of the shape of the obstacles, and the dimensionless qualitative and quantitative model of flame propagation speed is given and verified.     

Effect of initial turbulence on vented explosion overpressures from lean hydrogen–air deflagrations

To examine the effect of initial turbulence on vented explosions, experiments were performed for lean hydrogen–air mixtures, with hydrogen concentrations ranging from 12 to 15% vol., at elevated initial turbulence. As expected, it was found that an increase in initial turbulence increased the overall flame propagation speed and this increased flame propagation speed translated into higher peak overpressures during the external explosion. The peak pressures generated by flame–acoustic interactions, however, did not vary significantly with initial turbulence. When flame speeds measurements were examined, it was found that the burning velocity increased with flame radius as a power function of radius with a relatively constant exponent over the range of weak initial turbulence studied and did not vary systematically with initial turbulence. Instead, the elevated initial turbulence increased the initial flame propagation velocities of the various mixtures. The initial turbulence thus appears to act primarily by generating higher initial flame wrinkling while having a minimal effect on the growth rate of the wrinkles. For practical purposes of modeling flame propagation and pressure generation in vented explosions, the increase in burning velocity due to turbulence is suggested to be approximated by a single constant factor that increases the effective burning velocity of the mixture. When this approach is applied to a previously developed vent sizing correlation, the correlation performs well for almost all of the peaks. It was found, however, that in certain situations, this approach significantly under predicts the flame–acoustic peak. This suggests that further research may be necessary to better understand the influence of initial turbulence on the development of flame–acoustic peaks in vented explosions.     

变工况下直接空冷机组最佳真空的分析

为了提高直接空冷机组运行经济性,以冷端系统的变工况模型为基础,通过计算空冷凝汽器风机送风量增大时空冷机组发电功率与对应风机耗功功率的增量,得到直接空冷机组凝汽器最佳真空的确定方法．根据相似定律确定迎面风速对风机耗功的影响,并通过冷端系统数学模型的分析和简化得到机组背压与发电功率的关系,最终导出了不同环境温度和排汽热负荷下迎面风速对应的最佳真空．根据模型对某330Mw机组在变工况下的最佳真空进行了计算,结果表明：随着排汽量的增加或环境温度的升高,最佳真空及对应的风量都增加;当环境温度高于20℃时,环境温度对最佳真空的影响更加突出．     

Characteristics of flame propagation in a vortex core: Validity of a model for flame propagation

Characteristics of flame propagation for methane-, propane-, and hydrogen-air mixtures and the validity of a model for flame propagation in a vortex core were investigated experimentally using a vortex ring generated by pulsing a quantity of the mixture through a circular nozzle. The experimental results show that the ratio of the flame speed to the maximum tangential velocity of the vortex core decreases as the maximum tangential velocity is increased. This experimental observation agrees qualitatively with predictions of a model that considers the shape of the flame tip in the vortex core. The flame speeds calculated by the proposed model are proportional to the square root of the ratio of the density of the unburned gas to that of the burned gas (ρu/ρb)^1/2, which agree with the experimental results. The present predictions better match the experimental results than those calculated using Chomiak's model. The constant of proportionality between flame speed and (ρu/ρb)^1/2 depends on the type of fuel used. This experimental observation is not predicted by Chomiak's model. The flame speed in the vortex core can be predicted by a model of flame propagation taking into account the shape of the flame tip. The calculated flame speeds, however, are still higher than those obtained in experiments. In order to predict the flame speed exactly, not only the shape of the flame tip in the vortex core, but also the effects of baroclinic torque, curvature of the vortex core, unsteadiness of the propagation velocity, and structure of the flame in the vortex core should be considered.     

A NEW ANALYTIC SOLUTION OF THE NAVIER-STOKES EQUATIONS FOR MICROCHANNEL FLOWS

In this article we present a new analytic solution of the Navier-Stokes equations for microchannel flows. The solution is based on the concept of the continuum approach using the Chapman-Enskog method, but built upon the proposal to introduce a hyperbolic tangent function of Kn number in the power series of the distribution function and slip boundary condition. The physics behind the mathematical modification are discussed. With the slip boundary condition accurate to O(tanh(Kn)), the solution of the Navier-Stokes equations is extended successfully to the transition flow regime. The analytic solutions are compared with results of DSMC in both slip flow and transition flow regimes. Satisfactory agreements on the velocity profiles and pressure distributions have been achieved. The extension of the upper Knudsen number limits of continuum approach is significant in molecular gas dynamics.     

The Particle Image Velocimetry Experimental Study of Flame Extinction in Standard Square Flammability Column Resulting From Stretching

The main goal of this work is to determine experimentally local stretching rate distribution along the limits of methane/air and propane/air flames, using particle image velocimetry (PIV). This method allows obtaining necessary moving flame velocity fields in a standard flammability column and also recognition of the flame structures. For this purpose each mixture was seeded with MgO particles (of known size) before entering the tube (column), using a special system. The amount of seeds in the mixture, their dispersion system, and the laser power producing a sheet of light penetrating the column were carefully chosen (so as not to disrupt the combustion or flame propagation in it). After a learning process, this finally it allowed us obtain good-quality velocity field images in the region of concern, images acceptable for further processing. The methodology developed for these experiments proved to be reliable and able to supply analyses with repeatable data. On the basis of performed experiments it was possible to derive the flame stretching rate that causes its extinction in both mixtures.     

Effects of High-Order Slip/Jump,Thermal Creep,and Variable Thermophysical Properties on Natural Convection in Microchannels With Constant Wall Heat Fluxes

Natural convection gaseous slip flows in open-ended vertical parallel-plate microchannels with symmetric wall heat fluxes are numerically investigated. A second-order model, including thermal creep effects, is considered for velocity slip and temperature jump boundary conditions with variable thermophysical properties. Simulations are performed for wide range of Rayleigh numbers from 5 × 10^{? 6} to 5 × 10^{? 3} in the continuum to slip flow regime. The developing and fully developed solutions are examined by solving the Navier–Stokes and energy equations using a control volume technique. It is found that the second-order effects reduce the temperature jump and the slip velocity, whereas thermal creep strongly increases the slip velocity in both developing and fully developed regions. Moreover, the rarefaction effects increase the flow and heat transfer rates considerably, while decreasing the maximum gas temperature and friction coefficient as compared to the continuum limit. It was also shown that the axial temperature variations of the gas layer adjacent to the wall in the modeling of the thermal creep are of paramount importance and neglecting these variations, which is common in literature, leads to unphysical velocity and temperature distributions.