灯泡贯流式水轮机在小流量工况下的内流特性分析

为了研究灯泡贯流式水轮机在小流量工况下的内流特性,以某水电站贯流式水轮机为研究对象,利用RNG k-ε湍流模型对全流道进行数值计算,分析在小流量工况下尾水管恢复系数和全流道流动情况,并定量分析水轮机部分过流部件的压力脉动情况。结果表明：在小流量工况下当电站水头和导叶开度不匹配时,尾水管的恢复系数减小,当尾水管恢复系数减小2%~3%时,机组效率降低1%;受转轮主流区和尾水管死水区的相互作用,在尾水管边壁会形成漩涡结构,有明显的回流且在边壁存在高速流体;在小流量工况下,机组主要受低频压力脉动影响,转轮进口处的主频是由在转轮转动和导叶提供机组环量时产生的,在尾水管中主要受频率为0.2 Hz的低频压力脉动影响,压力脉动系数沿尾水管出口方向依次增大,最大为1.75%。     

基于低速预处理的风力机翼型外形优化设计

针对经典的S809翼型,耦合基于低速预处理的流场求解方法和序列二次规划方法,开展针对翼型升阻比的翼型气动外形优化设计研究。优化结果显示优化翼型具有较大的翼型前缘半径和较平坦的上表面。数值计算结果表明,优化翼型在设计点1的状态下升阻比提高43.3%,在设计点2的状态下升阻比提高48.9%。进一步数值验证表明,优化翼型在雷诺数为5.0×10⁵状态下的最大升力系数从S809翼型的1.140增大到1.297,在雷诺数为1.0×10⁶状态下的最大升力系数从1.236增大到1.418。在优化翼型的基础上,开展翼型气动外形人工修型研究,数值模拟表明修型翼型能更好地消除气流分离,从而进一步增大翼型升力系数、减小翼型阻力系数。     

白龙江中上游径流变化及其对气候和人类活动的响应

河川径流是水资源重要组成部分,径流变化归因分析对于流域水资源安全及管理具有重要意义。基于白龙江中上游1960~2015年水文气象资料,采用多种统计学方法、基于Budyko假设的弹性分析法研究径流变化特征及归因分析。结果表明,径流呈显著下降趋势,线性倾向变化率为-2.69×10⁸m³/10a,其临界突变发生在1986年,呈先上升后下降的趋势;径流量分布集中,6~10月占比63.59%,夏秋两季占比70.62%;径流减少受气候变化和人类活动影响,双累积曲线法影响率分别为33.02%、66.98%,累积量斜率变化率法影响率分别为44.17%、55.83%,弹性分析法影响率分别为35%~39%、61%~65%,可见人类活动为径流减少的主要影响因素,同时,弹性分析法计算结果介于其他两种方法计算结果之间,最为可靠。     

基于生态足迹法的天水市土地整治效果研究

土地整治是盘活存量土地、强化节约集约用地、适时补充耕地和提升土地产量的重要手段,分析土地整治效果对土地整治项目的评价具有重要意义。本文运用生态足迹模型对2007-2012年间土地整治项目中的新增耕地的生态足迹及生态承载力进行了测算与评价。结果表明：1）2007年天水市土地整治后的新增耕地人均生态承载力为0.000 6 hm^2,占当年总的人均生态承载力0.648 4 hm^2的0.09%,占当年人均生态赤字1.074 4 hm^2 的0.05%。2011年新增耕地的人均生态承载力为0.001 2 hm^2,占当年总的人均生态承载力0.734 7 hm^2中的0.16%,占当年人均生态赤字1.118 4 hm^2的0.10%,若没有进行土地整治,天水市2011年生态赤字会增加0.10%。2）通过2007-2011年天水市人均新增耕地面积与其生态承载力的动态变化,可以发现,随着新增耕地面积的增多,人均生态承载力也相应变化,两者的关系呈现为正相关,一定程度上缓解了天水市的生态环境压力。      

Astragali Radix-derived nitrogen-doped porous carbon: An efficient electrocatalyst for the oxygen reduction reaction

The development of biomass-derived nitrogen-doped porous carbons (NPCs) for the oxygen reduction reaction (ORR) is important for sustainable energy systems. Herein, NPCs derived from Astragali Radix (AR) via a cost-effective strategy are reported for the first time. The as-prepared AR-950-5 catalyst shows a stacked layer-like structure and porosity. Notably, the optimized AR-950-5 delivers catalytic activity comparable to that of commercial Pt/C (C-Pt/C), with high onset potential, positive half-wave potential and large limiting current density. It also displays superior long-term stability and methanol tolerance for ORR. This work will pave the way for a new approach in the development of highly active and low-cost NPCs for fuel cells.     

Transition-metal-doped NiSe2 nanosheets towards efficient hydrogen evolution reactions

Nano Research - Transition metal diselenides are promising electrocatalysts for hydrogen evolution and therefore different approaches have been proposed to enhance their catalytic activity. Herein,...     

Recovering CH4 from natural gas hydrate with CO2 in porous media below the freezing point

The recovery of methane from gas hydrate layers that have been detected in permafrost regions is a promising perspective in the future. In order to study the replacement characteristics of CO₂–CH₄ hydrate in permafrost environments, simulation experiment was carried out in this work. The results indicated that the replacement rate and efficiency increased with the increasing of injection pressure of CO₂ gas. And the replacement rate and efficiency reached up to 0.403?mmol/h and 13.20% during the experiment. Furthermore, the results also showed that the replacement rate of CO₂–CH₄ hydrate was slower below the freezing point.     

A novel approach for mobile malware classification and detection in Android systems

Multimedia Tools and Applications - With the increasing number of malicious attacks, the way how to detect malicious Apps has drawn attention in mobile technology market. In this paper, we proposed...     

Probabilistic load flow calculation with quasi-Monte Carlo and multiple linear regression

In this paper, quasi-Monte Carlo combined with multiple linear regression (QMC-MLR) is proposed to solve probabilistic load flow (PLF) calculation. A distinguishing feature of the paper is that PLF is approached by a low-dimensional problem with the concept of the effective dimension, and thus QMC based on low-discrepancy sequences is used to improve the sampling efficiency of the Monte Carlo simulation (MCS). Moreover, according to the relationship between linear correlation and linear regression, the MLR-based correlation control technique is developed to arrange the orders of samples in order to introduce prescribed dependences between variables. The proposed method is tested with the IEEE 118-bus system. Simulation results indicate that the MLR-based technique is robust and efficient in handling correlated non-normal variables and the proposed method shows better performances in PLF calculation compared with other MCS techniques, including simple random sampling (SRS), Latin hypercube sampling (LHS) and Latin supercube sampling (LSS).