提高微晶硅薄膜太阳电池效率的研究

采用甚高频等离子体增强化学气相沉积技术制备了系列微晶硅薄膜太阳电池，指出了气体总流量和背反射电极的类型对电池性能参数的影响.电池的I-V测试结果表明：随反应气体总流量的增加，对应电池的短路电流密度、开路电压和填充因子都有很大程度的提高，结果使得电池的光电转换效率得以提高.另外，ZnO/Ag/Al背反射电极能明显提高电池的短路电流密度，进而也提高了电池的光电转换效率.对气体总流量和背反射电极类型影响电池效率的原因进行了分析. 关键词： 微晶硅薄膜太阳电池 气体流量 ZnO/Ag/Al背反射电极     

含重叠路段交通系统中信息反馈策略的比较研究

智能交通系统可以实时反馈路况信息、诱导出行,进而缓解交通拥堵.在时间反馈和平均速度反馈两种信息服务策略下,考虑重叠路段的影响,利用元胞自动机模型对人们的路径选择行为进行了模拟.结果表明,在含有重叠路段的网络中,相对时间反馈策略而言,平均速度反馈策略可以更有效地降低人们的出行时间,其优势在重叠部分长度占路径长度的比例很小和较大时十分明显,且随路径长度的增加而上升.在提高路网的利用效率方面,平均速度反馈策略的优势随路径长度的增加而逐渐下降,随重叠部分长度的增加而先降后升. 关键词： 交通流 元胞自动机 智能交通系统 信息反馈策略     

微液滴动力学特性的耗散粒子动力学模拟

对传统的耗散粒子动力学方法进行了改进.改进的耗散粒子动力学方法采用了包含远程吸引力和近距排斥力的保守力势函数,从而使得用耗散粒子动力学方法模拟多相流动成为可能.应用改进的耗散粒子动力学方法,对微尺度下液滴的形成及液滴在微重力下的大幅度振荡变形进行了数值模拟.计算结果表明,改进的耗散粒子动力学(DPD)方法能够有效地描述微尺度下液滴的动力学特性,对研究复杂流体多相流动有着重要的意义. 关键词： 多相流 微液滴 耗散粒子动力学(DPD)方法 保守力势函数     

Combustion of bimodal nano/micron-sized aluminum particle dust in air

The combustion of bimodal nano/micron-sized aluminum particles with air is studied both analytically and experimentally in a well-characterized laminar particle-laden flow. Experimentally, an apparatus capable of producing Bunsen-type premixed flames was constructed to investigate the flame characteristics of bimodal-particle/air mixtures. The flame speed is positively affected by increasing the mass fraction of nano particles in the fuel formulation despite the lower flame luminosity and thicker flame zone. Theoretically, the flames are assumed to consist of several different regimes for fuel-lean mixture, including the preheat, flame, and post flame zones. The flame speed and temperature distribution are derived by solving the energy equation in each regime and matching the temperature and heat flux at the interfacial boundaries. The analysis allows for the investigation of the effects of particle composition and equivalence ratio on the burning characteristics of aluminum-particle/air mixtures. Reasonable agreement between theoretical results and experimental data was obtained in terms of flame speed. The flame structure of a bimodal particle dust cloud may display either an overlapping or a separated configuration, depending on the combustion properties of aluminum particles at different scales. At low percentages of nano particles in the fuel formulation, the flame exhibits a separated spatial structure with a wider flame regime. At higher nano-particle loadings, overlapping flame configurations are observed.     

Sub-iteration leads to accuracy and stability enhancements of semi-implicit schemes for the Navier–Stokes equations

We present an iterative semi-implicit scheme for the incompressible Navier–Stokes equations, which is stable at CFL numbers well above the nominal limit. We have implemented this scheme in conjunction with spectral discretizations, which suffer from serious time step limitations at very high resolution. However, the approach we present is general and can be adopted with finite element and finite difference discretizations as well. Specifically, at each time level, the nonlinear convective term and the pressure boundary condition – both of which are treated explicitly in time – are updated using fixed-point iteration and Aitken relaxation. Eigenvalue analysis shows that this scheme is unconditionally stable for Stokes flows while numerical results suggest that the same is true for steady Navier–Stokes flows as well. This finding is also supported by error analysis that leads to the proper value of the relaxation parameter as a function of the flow parameters. In unsteady flows, second- and third-order temporal accuracy is obtained for the velocity field at CFL number 5–14 using analytical solutions. Systematic accuracy, stability, and cost comparisons are presented against the standard semi-implicit method and a recently proposed fully-implicit scheme that does not require Newton’s iterations. In addition to its enhanced accuracy and stability, the proposed method requires the solution of symmetric only linear systems for which very effective preconditioners exist unlike the fully-implicit schemes.     

Anti-diffusion method for interface steepening in two-phase incompressible flow

In this paper, we present a method for obtaining sharp interfaces in two-phase incompressible flows by an anti-diffusion correction, that is applicable in a straight-forward fashion for the improvement of two-phase flow solution schemes typically employed in practical applications. The underlying discretization is based on the volume-of-fluid (VOF) interface-capturing method on unstructured meshes. The key idea is to steepen the interface, independently of the underlying volume-fraction transport equation, by solving a diffusion equation with reverse time, i.e. an anti-diffusion equation, after each advection time step of the volume fraction. As the solution of the anti-diffusion equation requires regularization, a limiter based on the directional derivative is developed for calculating the gradient of the volume fraction. This limiter ensures the boundedness of the volume fraction. In order to control the amount of anti-diffusion introduced by the correction algorithm we propose a suitable stopping criterion for interface steepening. The formulation of the limiter and the algorithm for solving the anti-diffusion equation are applicable to 3-dimensional unstructured meshes. Validation computations are performed for passive advection of an interface, for 2-dimensional and 3-dimensional rising-bubbles, and for a rising drop in a periodically constricted channel. The results demonstrate that sharp interfaces can be recovered reliably. They show that the accuracy is similar to or even better than that of level-set methods using comparable discretizations for the flow and the level-set evolution. Also, we observe a good agreement with experimental results for the rising drop where proper interface evolution requires accurate mass conservation.     

Discrete rearranging disordered patterns, part II: 2D plasticity, elasticity and flow of a foam

The plastic flow of a foam results from bubble rearrangements. We study their occurrence in experiments where a foam is forced to flow in 2D: around an obstacle; through a narrow hole; or sheared between rotating disks. We describe their orientation and frequency using a topological matrix defined in the companion paper (F. Graner, B. Dollet, C. Raufaste, and P. Marmottant, this issue, 25 (2008) DOI 10.1140/epje/i2007-10298-8), which links them with continuous plasticity at large scale. We then suggest a phenomenological equation to predict the plastic strain rate: its orientation is determined from the foam's local elastic strain; and its rate is determined from the foam's local elongation rate. We obtain a good agreement with statistical measurements. This enables us to describe the foam as a continuous medium with fluid, elastic and plastic properties. We derive its constitutive equation, then test several of its terms and predictions.     

多级轴流压气机全工况特性计算

本文使用三维粘性流动计算软件Fine／Numeca,对某十五级轴流压气机进行了内流流场和全工况特性的数值计算尝试。分析了该压气机在设计工况和非设计工况的性能,同时把整机计算结果和前七级叶片的计算结果进行了比较。计算结果表明,当计算的级数较少时,目前的软件和硬件平台可以比较合理地预测压气机的全工况特性;而当计算的级数较多时,准确的数值模拟仍需要更为准确的多级模型和数值方法。     

统一的对流扩散型可压缩流体力学方程与解法

流体力学的动量方程、能量方程、湍动能方程和耗散方程都具有对流扩散方程的形式,但连续方程却不是对流扩散型的。对于可压缩问题,本文通过合理的数学推导,不作任何近似、假定与简化,得到一个全新的连续方程形式．该连续方程以压力为未知变量,并具有对流扩散型形式,使得所有的流体动力学方程组都具有完全统一的方程形式,给出了这种三维对流扩散方程组的有限精确差分计算格式。对流体力学的进一步发展具有一定意义．     

动叶顶部间隙对1+1/2对转涡轮性能的影响

本文对1+1／2对转涡轮中动叶顶部间隙大小对涡轮性能的影响进行了详细的数值模拟。对涡轮总体性能参数、节距平均出口气流角、出口节距平均相对马赫数以及不同叶高的负荷进行了对照,发现间隙对1+1／2对转涡轮性能影响明显,尤其是当间隙较大时。当低压动叶间隙宽度达到高压动叶前缘叶高的4．5％时,低压动叶间隙中流动普遍超音,在设计1+1／2对转涡轮时需要加以重点考虑。