Numerical investigation of pressure drop characteristics of gas channels and U and Z type manifolds of a PEM fuel cell stack

Fluid flow manifold plays a significant role in the performance of a fuel cell stack because it affects the pressure drop, reactants distribution uniformity and flow losses, significantly. In this study, the flow distribution and the pressure drop in the gas channels including the inlet and outlet manifolds, with U- and Z-type arrangements, of a 10-cell PEM fuel cell stack are analyzed at anode and cathode sides and the effects of inlet reactant stoichiometry and manifold hydraulic diameter on the pressure drop are investigated. Furthermore, the effect of relative humidity of oxidants on the pressure drop of cathode are investigated. The results indicate that increase of the manifold hydraulic diameter leads to decrease of the pressure drop and a more uniform flow distribution at the cathode side when air is used as oxidant while utilization of humidified oxidant results in increase of pressure drop. It is demonstrated that for the inlet stoichiometry of 2 and U type manifold arrangement when the relative humidity increases from 25% to 75%, the pressure drop increases by 60.12% and 116.14% for oxygen and air, respectively. It is concluded that there is not a significant difference in pressure drop of U- and Z-type arrangements when oxygen is used as oxidant. When air is used as oxidant, the effect of manifold type arrangement is more significant than other cases, and increase of the stoichiometry ratio from 1.25 to 2.5 leads to increase of pressure drop by 527.3%.     

A modified two-dimensional numerical method for prediction of outer wall temperature distribution of rectangular micro-combustors

Two-dimensional (2D) numerical models are frequently adopted to investigate combustion and thermal performances in rectangular micro-channels for micro-thermophotovoltaic and thermoelectric devices. However, large error may exist by applying a simple 2D model. In the present work, the outer wall temperature distributions predicted by 3D model and simple 2D model were compared. The results showed that the maximum relative error of the simple 2D model depends significantly on the aspect ratio (α) of the micro-channel. To be specific, the maximum relative error was >30% for α = 1 and > 10% for 2≤α ≤ 4. However, it was <5% for α ≥ 9. A new 2D model was proposed to modify the underestimated heat loss ratio. The new computational results demonstrated that the maximum relative error of α = 1 decreased to 8.07% and for micro-channels with α ≥ 2, all the maximum relative errors are <5%. In summary, the modified 2D numerical model can achieve a satisfactory prediction with low computation cost.     

PIC simulation of plasma properties in the discharge channel of a pulsed plasma thruster with flared electrodes

Plasma in the discharge channel of a pulsed plasma thruster (PPT) with flared electrodes is simulated by a self-developed two-dimensional code. The fully particle-in-cell method with Monte Carlo collision is employed to model the particle movement and collisions and investigate the plasma properties and acceleration process. Temporal and spatial variations of the electron density distribution and the ion velocity between electrodes are calculated and analyzed in detail. The computational results of the electron number density, which is in the order of 10²³ m⁻³, show good agreements with experimental results of a PPT named ADD SIMP-LEX. The ion velocity distributions along the center line of the channel lead to a comprehensive understanding of ions accelerated by electromagnetic field. The electron distributions of PPT with discharge voltages varying from 1300 to 2000 V are compared. The diffusion of electrons presents strong dependency on discharge voltage and implies higher degree of ionization for higher voltage.     

基于SIR模型的隐蔽信道数据安全检测仿真

隐蔽信道数据分布散乱,对数据检测造成阻碍。针对传统的隐蔽信道数据检测方法存在检测速度慢、有效性差等问题,提出一种基于SIR模型的隐蔽信道数据安全检测方法。构建SIR隐蔽信道模型,使用在线检测模型进行隐蔽信道数据编码处理,使用密度聚类算法对隐蔽信道编码数据进行搜索聚类,划分密度区域,通过判断各密度区域数据有效性,完成隐蔽信道数据的密度聚类。利用决策树对聚类完成的数据进行特征属性提取,引入特征属性获取新的信息递增率,通过数据间差异性计算完成隐蔽信道数据安全检测。实验结果表明,所提方法能有效完成隐蔽信道数据检测,精准度、效率和稳定性均优于传统方法,且检测耗时少,具有显著优势。     

关于硬件木马电路检测技术分析及发展趋势探讨

本文结合HTH电路的复杂性等特征分析与实际应用现状,研究了HTH电路的具体检测技术方法并对当前热点技术进行对比分析,探讨了HTH电路检测方法的技术发展趋势。     

A Strategy of Signal Detection for Performance Improvement in Clipping Based OFDM System

In this paper, the supervised Deep Neural Network (DNN) based signal detection is analyzed for combating with nonlinear distortions efficiently and improving error performances in clipping based Orthogonal Frequency Division Multiplexing (OFDM) ssystem. One of the main disadvantages for the OFDM is the high Peak to Average Power Ratio (PAPR). The clipping is a simple method for the PAPR reduction. However, an effect of the clipping is nonlinear distortion, and estimations for transmitting symbols are difficult despite a Maximum Likelihood (ML) detection at the receiver. The DNN based online signal detection uses the offline learning model where all weights and biases at fullyconnected layers are set to overcome nonlinear distortions by using training data sets. Thus, this paper introduces the required processes for the online signal detection and offline learning, and compares error performances with the ML detection in the clipping-based OFDM systems. In simulation results, the DNN based signal detection has better error performance than the conventional ML detection in multi-path fading wireless channel. The performance improvement is large as the complexity of system is increased such as huge Multiple Input Multiple Output (MIMO) system and high clipping rate.     

A star pattern recognition algorithm for cameras with large FOV

In most large field of view (FOV) observations, the distortion problem is inevitably and significantly more serious than in small FOV ones. In the circumstances, many traditional star identification approaches are not able to efficiently identify stars any more. In order to deal with this problem, we put forward a star identification method that is less sensitive to distortion. The method first processes stars in the central area of the image, using traditional identification logic, and then applies the region growing strategy to enlarge the identified regions iteratively until the entire image is covered. The performance of the new scheme is analysed in the presence of both simulated data and real data. The results show that the proposed algorithm has the advantage of speed, and the strategy of regional extension can efficiently identify stars in large FOV images compared with other existing algorithms.     

Research on the identification performance of UHF RFID system considering the correlation of cascaded channel

The identification rate of UHF RFID system was restricted by multipath propagation effects.The system identification performance was studied considering the correlation coefficient between forward and reverse channels.Based on the generalized Rician fading channel model,the analytical expression of identification rate was derived under independent,full correlation and correlation cases.Compared with the existing analysis,the proposed uniform calculation formula of identification rate was for any correlation coefficient and kinds of channel conditions.The numerical computation and Monte-carlo simulations show that the influences of different correlation coefficients,channel conditions,sensitivity and distance on the identification rate.     

基于消息队列的通信系统异步传送控制仿真

传统方法对通信系统异步传送控制效果不佳,存在信号干扰及传送时间间隔较长等问题。为此引入消息队列技术,提出一种新的传送控制方法来解决以上问题。对通信系统的空时信道进行空时编码设计,实现空时信道的编码增益与分集增益;利用空时编码实现通信系统中继节点的空间分集,根据空间分集结果制订异步空时传送方案;采用消息队列技术构建通信系统异步传送控制模型,实现通信系统的异步传送控制。为了验证所提方法异步传送控制效果,设计对比实验。实验结果表明,上述方法的传送时间间隔较短,抗干扰能力强,平均分集增益高,说明所提方法更适用于通信系统的异步传送控制。