Investigation on the flameholding mechanisms in supersonic flows: backward-facing step and cavity flameholder

Abstract  

As effective devices to extend the fuel residence time in supersonic flow and prolong the duration time for hypersonic vehicles cruising in the near-space with power, the backward-facing step and the cavity are widely employed in hypersonic airbreathing propulsive systems as flameholders. The two-dimensional coupled implicit RANS equations, the standard k-ε turbulence model, and the finite-rate/eddy-dissipation reaction model have been used to generate the flow field structures in the scramjet combustors with the backward-facing step and the cavity flameholders. The flameholding mechanism in the combustor has been investigated by comparing the flow field in the corner region of the backward-facing step with that around the cavity flameholder. The obtained results show that the numerical simulation results are in good agreement with the experimental data, and the different grid scales make only a slight difference to the numerical results. The vortices formed in the corner region of the backward-facing step, in the cavity and upstream of the fuel injector make a large difference to the enhancement of the mixing between the fuel and the free airstream, and they can prolong the residence time of the mixture and improve the combustion efficiency in the supersonic flow. The size of the recirculation zone in the scramjet combustor partially depends on the distance between the injection and the leading edge of the cavity. Further, the shock waves in the scramjet combustor with the cavity flameholder are much stronger than those that occur in the scramjet combustor with the backward-facing step, and this causes a large increase in the static pressure along the walls of the combustor.     

Oxidation process of polysulfides in charge process for lithium�Csulfur batteries

The oxidation of polysulfides to element sulfur in charge process was studied by solution thermodynamic analysis and means of cyclic voltammetry (CV), X-ray diffraction (XRD), and charge?Cdischarge test. Basing on the solution thermodynamic analysis, the oxidation process of polysulfides to element sulfur would arise only if the charge voltage exceeds 3.36 V in a lithium?Csulfur cell employing 1.0 M LiN(CF₃SO₂)₂ in 1,2-dimethoxy ethane. Furthermore, the minimum of charge voltage which can push the oxidation would fall down with the increasing solubility of elemental sulfur in electrolyte solution. These analyses were confirmed by practical measurements. One new anodic peak corresponding to the oxidation process of polysulfides to solid sulfur was observed by CV. Both XRD patterns and charge?Cdischarge test showed that the element sulfur appeared in the cathode after the battery was charged over 3.4 V. Hence, the lithium?Csulfur cell charged over 3.4 V exhibited an improved cycle life since the capacity degradation between the first cycle and the second was depressed. In order to improve the energy efficiency, carbon disulfide was added in the electrolyte solution of lithium?Csulfur cell to increase the solubility of sulfur.     

单目摄像机-激光测距传感器位姿测量系统

在单目视觉测量中,由于模型自身的限制,沿摄像机光轴方向上的位移测量精度一般远低于垂直光轴方向上的位移测量精度.首先,从数学模型上分析了单目视觉位姿测量在沿光轴方向上位移测量精度低的原因;然后,为了提高沿摄像机光轴方向上的位移测量精度,通过在沿摄像机光轴方向上加装一个高精度的激光测距传感器,设计了一种单目摄像机-激光测距...     

同轴电介质-金属-电介质结构表面等离激元色散研究

从平面电介质-金属薄膜-电介质对称结构中表面等离激元的色散方程出发,建立模型推导得到了同轴电介质-金属薄膜-电介质结构中表面等离激元的色散方程,在这种结构中各阶模式都发生了分裂,形成了高折射率高损耗和低折射率低损耗的两种模式,该模型为理解同轴电介质-金属薄膜-电介质结构表面等离激元模式的传播提供了直观的图像。用色散方程计算了0.5～1.5μm波长范围内各阶模式的等效折射率以及传输损耗,并将该结果与有限元方法数值模拟得到的结果进行对比分析,分析表明,两者吻合得很好。并分析了计算结果以及色散方程的适用范围。     

光纤激光阵列目标在回路自适应光学系统的数值模拟

研究了光纤激光阵列目标在回路自适应光学系统在实际大气环境中的传输.从光纤激光阵列、激光在大气中的传输、目标和优化算法四个方面,建立了光纤激光阵列目标在回路自适应光学系统的理论模型.在不同强度大气湍流下,对系统补偿大气湍流影响的效果进行了研究,并对系统控制参数优化进行了简单探讨.结果表明,目标在回路闭环控制结构能够有效补...     

An aspect ratio agglomeration multigrid for unstructured grids

A robust aspect ratio‐based agglomeration algorithm to generate high quality of coarse grids for unstructured and hybrid grids is proposed in this paper. The algorithm focuses on multigrid techniques for the numerical solution of Euler and Navier–Stokes equations, which conform to cell‐centered finite volume special discretization scheme, combines vertex‐based isotropic agglomeration and cell‐based directional agglomeration to yield large increases in convergence rates. Aspect ratio is used as fusing weight to capture the degree of cell convexity and give an indication of cell stretching. Agglomeration front queue is established to propagate inward from the boundaries, which stores isotropic vertex and also high‐stretched cell marked with different flag according to aspect ratio. We conduct the present method to solve Euler and Navier–Stokes equations on unstructured and hybrid grids and compare the results with single grid as well as MGridGen, which shows that the present method is efficient in reducing computational time for large‐scale system equations. Copyright © 2013 John Wiley & Sons, Ltd.     

The degree of the E-characteristic polynomial of an even order tensor

The E-characteristic polynomial of an even order supersymmetric tensor is a useful tool in determining the positive definiteness of an even degree multivariate form. In this paper, for an even order tensor, we first establish the formula of its E-characteristic polynomial by using the classical Macaulay formula of resultants, then give an upper bound for the degree of that E-characteristic polynomial. Examples illustrate that this bound is attainable in some low order and dimensional cases.     

基于空间结构内部锁相的光纤激光相干合成技术

介绍了本课题组近年来在基于空间结构内部锁相的光纤激光相干合成方面的研究工作,给出了空间结构内部锁相相干合成的基本原理,搭建了七路低功率光纤激光阵列实验系统,结果表明在内部相位噪声校正基础之上,可以稳定补偿外部相位差进而实现激光阵列同相位输出,验证了内部锁相方法的可行性。进一步介绍了空间结构内部锁相技术在目标在回路相干合成、阵列光束光场调控等方面的拓展应用,通过实验论证了空间结构内部锁相技术能够有效提升目标在回路相干合成系统的相位控制带宽,并在远场有效生成轨道角动量光束阵列,其拓扑荷数可从-1到+1切换。     

彩色光弹性干涉影像分析系统

自行开发的“彩色光弹性干涉影像分析系统”首先利用CCD成像和图像采集设备,将光弹图像以数字图像的形式存储到计算机,然后通过对存储的光弹图像进行处理,得到物体边界、等差线、等倾线等数据。最后根据这些数据,绘制出主应力迹线,并进行二维的和三维的应力分析。本文着重介绍了系统整体设计以及系统研制的难点问题(彩色光弹图像处理、主应力迹线的绘制等)。系统可以通过对彩色图像进行分解,应用目前已经比较成熟的灰度光弹图像处理技术,来完成彩色图像的处理;也可以直接应用彩色信息来确定条纹级数,进行相关处理。彩色图像能够比灰度图像提供更精确的图像信息,以满足高精度测量的要求。