PLC仍在全方位地发展(上)

以中心点法和验算点法为代表的一次二阶矩方法计算简便,但对非线性程度较高的结构功能函数,其计算结果与精确度相差过大。应用数学逼近中的拉普拉斯渐进方法将非线性功能函数在验算点处作二次展开来研究结构的可靠度问题,能较高精度的逼近精确结果。算例分析表明,当随机变量的数目较多时,由一次二阶矩方法计算的结果与精确解相差较大,而二次二阶矩方法的计算结果与精确解非常接近。     

Solution of moving boundary problems by coordinate transformations

The numerical solution of a two-dimensional moving boundary problem in which the solid changes its volumen in two directions is presented. A coordinate transformation that reduces the process to a one-dimensional variation is proposed. Numerical results by two different methods are obtained. The numerical results are compared satisfactorilly with available experimental data.     

Optimizing wind farm siting to reduce power system impacts of wind variability

We introduce and solve two variants of a biobjective optimization model to reduce the negative impact of wind variability on the power system by strategically locating wind farms. The first model variant considers average changes in wind power over time; the second captures extreme fluctuations in wind power. A complementary set of wind sites is selected with the aim of minimizing both residual demand and the variability in residual demand. Because exact optimization is computationally intensive, we develop two heuristics—forward and backward greedy algorithms—to find approximate solutions. The results are compared with the exact optimization results for a well‐selected subset of the data as well as to the results from selecting sites based on average wind alone. The two models are solved using demand data and potential wind sites for the Southwest Power Pool. Though both objectives can be improved by adding more sites, for a fixed number of sites, minimizing residual demand and variability in residual demand are competing objectives. We find an approximate efficient frontier to compare trade‐offs between the two objectives. We also vary the parameter in the heuristic that controls how the two objectives are prioritized. For the case study, the backward greedy algorithm is more effective at reducing the wind power variability than the forward greedy algorithm. Furthermore, using the backward algorithm for the full dataset is more effective than solving the exact optimization on a subset of the data when the results are evaluated using the full dataset.     

Evaluation of the amount of the atmospheric humidity condensed naturally

Two methods of the evaluation of the amount of the atmospheric humidity condensed naturally are proposed. The comparison of these two methods is done from climatological considerations. The results in a hot and humid climate are very close; but in a hot and dry climate there is gap of 4% between the two methods.Experiments were conducted in three different climates; the results are similar to those of the theory. But the dew collected represents about 40% of which must be obtained.This paper describes the study and presents the results.     

Numerical simulation of deflecting flow in a symmetric enlarged channel

Numerical results of three-dimensional separated flow and heat transfer in a rectangular channel with a suddenexpansion are presented in this paper.Numerical simulations of Navier-Stokes and energy equations are carriedout using the finite difference method.The results of three-dimensional calculations are compared with thetwo-dimensional ones,and effects of the aspect ratio of channel upon the flow are shown.The transition fromsymmetric to asymmetric flow appears at lower Reynolds number as increasing the aspect ratio.The details oflocal heat transfer characteristics in two different separated flow regions on two downstream walls are clarified.Two-dimensionality of the flow and heat transfer almost disappears for the aspect ratio considered.     

Natural convection between two floors of a building via a horizontal opening – Measurements in a one-half scale model

The paper describes the results of an experimental study of natural convection in a closed half-scale model of a building comprising two floors connected by a stairway, and a horizontal opening between the two floors. The driving force for the flow is a heat source in the lower floor. The results indicate a complex three-dimensional flow within the model. The rate of heat losses from each wall, wall temperatures, air speed and temperature in the horizontal opening for a range of heat inputs are presented and discussed. The results are then used to suggest boundary conditions for numerical simulation of the flow.     

Assessment of homotopy analysis method and homotopy perturbation method in non-linear heat transfer equation

Two new analytical methods to solve nonlinear heat transfer equations are homotopy perturbation method and homotopy analysis method. Here, homotopy analysis method, which gives us a vast freedom to choose the answer type, is applied to solve nonlinear heat transfer differential equations and analytical results are compared with those of HPM and the numerical results. In this study, the procedure of HAM is applied to two cases in different ways according to the physics of the target problem. Comparing the two methods, our attention is focused on the results accuracy; and applicability of different methods in many cases with different limitation is studied. In the two examples of this paper, the effect of small parameter increaser on the accuracy of the analytical results of two methods also has been studied. The first differential equation is the modeling equation of a cooling lumped system with combined convection and radiation. The second one is the modeling equation of heat transfer with conduction in a slab of thermal dependent conductivity.     

柴油机喷油泵传动轴防护罩断裂故障分析及优化

为解决某船用柴油机喷油泵传动轴防护罩断裂故障,提升零部件可靠性,运用Hypermesh、Abaqus以及Femfat等仿真软件,对该喷油泵传动轴防护罩断裂故障产生的原因进行仿真分析.结果 显示,防护罩断裂原因为一阶模态小于激振频率的1.2倍,易产生共振.在此基础上提出两种优化方案,对两种方案进行模态、强度及疲劳仿真计算...     

Forces on Conductor Segments and Magnetized Ferrous Cores using a Three Dimensional Finite Element Vector Potential Method

This paper presents two finite element based methods for the calculation of forces on conductor segments and magnetized ferrous cores in three dimensional magnetic fields. The formulation is based on the three dimensional magnetic vector potential and the curl curl equation. Results of use of the two methods in the calculation of forces in two examples are reported. The first example is that of two currentcarrying conductors (bus bars), while the second example is that of an iron cored armature (coil) and keeper. These two relatively simple examples were chosen for purposes of comparing the three dimensional finite element results with existing two dimensional methods and closed form approximations. The results of comparison of these forces are presented here. This comparison confirms the validity and desirability of the present 3D formulation of forces by highlighting some of the limitations of the known analytical and two dimensional methods in certain applications. The two methods presented here are applicable to a variety of problems of force calculations in various electrical devices and machinery.     

A three-dimensional two-phase flow model for a liquid-fed direct methanol fuel cell

A three-dimensional, two-phase, multi-component model has been developed for a liquid-fed DMFC. The modeling domain consists of the membrane, two catalyst layers, two diffusion layers, and two channels. Both liquid and gas phases are considered in the entire anode, including the channel, the diffusion layer and the catalyst layer; while at the cathode, two phases are considered in the gas diffusion layer and the catalyst layer but only single gas phase is considered in the channels. For electrochemical kinetics, the Tafel equation incorporating the effects of two phases is used at both the cathode and anode sides. At the anode side the presence of gas phase reduces the active catalyst areas, while at the cathode side the presence of liquid water reduces the active catalyst areas. The mixed potential effects due to methanol crossover are also included in the model. The results from the two-phase flow mode fit the experimental results better than those from the single-phase model. The modeling results show that the single-phase models over-predict methanol crossover. The modeling results also show that the porosity of the anode diffusion layer plays an important role in the DMFC performance. With low diffusion layer porosity, the produced carbon dioxide cannot be removed effectively from the catalyst layer, thus reducing the active catalyst area as well as blocking methanol from reaching the reaction zone. A similar effect exits in the cathode for the liquid water.     

Doubly-fed induction machine differential cascade. II. Verificationby test

In this paper, the doubly-fed differential cascade (DFDC) analysis, in the steady state, given in Part I, is verified by tests. The DFDC consists of two similar doubly-fed machines having the corresponding phases of the two rotors connected. Two sources are used to supply the DFDC stators. One is the mains and the other is a variable frequency converter. The DFDC is loaded with two DC machines acting as brakes. Operations similar to the ones required to provide propulsion to a vehicle are simulated. Variable speed and direction, when the vehicle is moving on a straight course, is simulated by varying the frequency of the frequency converter. Vehicle turn is simulated by distributing the load on the two machines. A nonmoving vehicle, held on a ramp by the DFDC developed torque, is simulated by applying the same frequency to both machines. The experimental results are compared with the results from the analysis     

燃烧室内自激励振荡燃烧的数值研究

采用数值方法研究了Langhorne型燃烧室内自激励燃烧驱动振荡的不稳定现象．应用非稳态雷诺平均法、雷诺应力紊流模型和涡团耗散燃烧模型,捕获了该类型燃烧室内两种当量比条件下的不稳定燃烧特性．数值模拟结果给出振荡燃烧发生时燃烧室内的压力振荡频率和幅值,并同实验结果进行了比较．证明了在两种当量比下分别存在着“强振荡”和“弱振荡”,并给出了两种当量比条件下不稳定燃烧时火焰的周期影像,详细分析了振荡发生时燃烧室内的火焰发展情况．     

Solar ICS systems with two cylindrical storage tanks

Two types of ICS solar water heaters designed, constructed and tested. The systems consist of two cylindrical storage tanks, which are connected in series and are horizontally incorporated in a stationary asymmetric CPC type mirror. The efficient operation of the system is due to the thermal losses suppression of the two inverted cylindrical surfaces and the effective use of the two tanks during sunshine period. Low cost and durable materials are used to construct the systems. The mean daily efficiency and the thermal performance of the hot water storage during night are calculated from outdoor experimental data. The results show that the proposed ICS systems are efficient and suitable for practical use as DHW systems.     

大涡模拟在船用燃气轮机进气流场中的应用

燃气轮机进气系统的结构和气动性能是燃气轮机装舰技术研究的一个重要组成部分。针对这方面的要求,进行燃气轮机进气系统的设计及数值模拟计算,为设计制造高性能的船用燃气轮机进气系统提供可靠的依据。对两种方案的进气系统作了数值计算和实验研究,其结果是相当令人满意的,对进气系统的实际使用具有非常重要的意义。     

两相闭式热虹吸管传热特性研究

本文通过实验研究在不同热流密度下,两相闭式热虹吸管传热特性,并通过计算比较了三个较为常用的换热准则系统,同时提出了改进意见,为热管换热器的设计提供了较为可靠的依据。     

Experimental and numerical studies of the pressure drop in ceramic foams for volumetric solar receiver applications

This paper presents experimental and numerical studies of the pressure drop in ceramic foams for solar air receiver applications. There are three main aims in this study. The first is to measure the pressure drop in the studied ceramic foams, and to build an empirical model based on the experimental results and a parametric numerical simulation. The second aim is to study flow field characteristics in the ceramic foams, especially in the vicinity of the interface. The third is to study the pressure drop characteristics of two modified structures (by manufacturing holes on the ceramic foams) that are expected to decrease the pressure drop in ceramic foams, but maintain good heat transfer properties. The experimental results from the samples, including two modified structures, along with the simulation results, show that the pressure drop in the ceramic foams follows a modified Darcy relationship. The experimental results also show that the two modified structures dramatically decrease the pressure drop (with pressure drop decreases up to 70% at a superficial velocity of 5 m/s). Based on both the experimental and the simulation results, a generalized model for predicting the pressure drop in ceramic foams was proposed.     

Uncertainty calculation applied to different regression methods in the quasi-dynamic collector test

This paper discusses and evaluates the quasi-dynamic test procedure for solar thermal collectors according to EN12975 in view of the uncorrelated regression uncertainty of the collector model parameters obtained from the test. In this discussion the results of two procedures for the regression coefficients identification, the least square (LS) and the weighted least square (WLS) regression methods are analyzed. The uncertainties of the both, the LS and WLS results are calculated and validated. The inter-comparison of the two methods shows comparable normalized efficiency curves, but superior performance of the WLS method with respect to the uncertainties.     

Investigations on the flow field in an unshrouded pump impeller geometry

Transient CFD simulations are performed to investigate the flow behaviour inside an unshrouded pump using the SAS method. Two blade designs are compared at two different tip clearances and the results are validated by measurements. The detected vortex structure is visualized by the normalized helicity, further discussed regarding its development and behaviour and finally implicated to the efficiency of the two different blade designs.