Digital image correlation based high-speed crack tip locatingmethod and its application

Based on a digital image correlation（DIC）method with the measurements of a high speed crack＇s displacement and strain fields,a technique to accurately and automatically locate its crack tip has been developed.The crack tip is identified by finding the abrupt jump on the opening（or dislocation）curve of a point on the trace of the crack propagation,while the opening is measured through a virtual extensometer technique and the abrupt jump is identified by finding the peak on the curve.The method was verified using a specially designed experiment and applied to measure the critical crack tip opening angle of a rock sample.Because the involvement of analytical models has been avoided and then the good performance could be ensured for low resolution speckle images,this technique is expected to be very useful in the quantitative study of high speed cracks in experiments using high speed cameras.

Turbulence flow and heat transfer of aluminum melt in tip cavity in process of thin-gauge high-speed casting

The uniformity of flow distribution of aluminum melt in tip cavity is a precondition to decide whether or not thin-gauge high-speed casting can be accomplished smoothly. The laws of aluminum melt flow and heat transfer in tip cavity can be found out through numerical simulation, which gives theoretical basis for solving the problem of the flow distribution of melt in tip cavity. A mathematical model with a low Reynolds number k—ε model for turbulence flow and heat transfer of aluminum melt in tip cavity was developed. The finite difference method was used to calculate the flow field and temperature field of aluminum melt in tip cavity. The phenomena and characteristics of turbulence flow and heat transfer were analyzed, including the characteristics of temperature distribution of turbulence similar to that of laminar flow. The simulation results are in good agreement with the experimental results for flow velocities and temperature at the exit of tip, which verifies the validity of the simulation results.

Optimal Feedback Control of Nonlinear Variable-Speed Marine Current Turbine Using a Two-Mass Model

This paper presents a contribution related to the control of nonlinear variable-speed marine current turbine(MCT)without pitch operating below the rated marine current speed.Given that the operation of the MCT can be divided into several operating zones on the basis of the marine current speed,the system control objectives are different for each zone.To deal with this issue,we develop a new control approach based on a linear quadratic regulator with variable generator torque.Our proposed approach enables the optimization of the rotational speed of the turbine,which maximizes the power extracted by the MCT and minimizes the transient loads on the drivetrain.The novelty of our study is the use of a real profile of marine current speed from the northern coasts of Morocco.The simulation results obtained using MATLAB Simulink indicate the effectiveness and robustness of the proposed control approach on the electrical and mechanical parameters with the variations of marine current speed.

CFD-Based Performance Analysis and Experimental Investigation of Design Factors of Vertical Axis Wind Turbines under Low Wind Speed Conditions in Thailand

This paper presents effects of design factors on mechanical performance of Vertical Axis Wind Turbines (VAWTs), and an experimental investigation of optimal VAWT performance under low wind speed conditions in Thailand. Design factors include types of wind turbines, number of blades, types of materials, height-to-radius ratios, and design modifications. Potential VAWT models with different design factors are numerically analyzed within a virtual wind tunnel at various wind speeds by utilizing XflowTM?Computational Fluid Dynamics (CFD) software. The performance curves of each VAWT are obtained as plots of power coefficients against tip speed ratios. It is found that the type of wind turbine, number of blades, and height-to-radius ratio have significant effects on mechanical performance whereas types of materials result in shifts of operating speeds of VAWTs. Accordingly, an optimal VAWT prototype is developed to operate under actual low speed wind conditions. The performance curve from experimental results agrees with the CFD results. The proposed methodology can be used in the computer design of VAWTs to improve mechanical performance before physical fabrication.

Increasing the Efficiency of Grid Tied Micro Wind Turbines in Low Wind Speed Regimes

Major problem with grid tied micro wind turbine is synchronization and wind variability. Due to this problem the stability of available grid gets reduced. The stability can be achieved by output power control of the turbine. Major part of many countries like India, the annual mean wind speed is not high. The rated wind speed of turbine remain around 11 m/s and cut in is around 3.5 m/s. Due to this problem we aimed to develop a sustainable wind energy system that can provide stable power supply even at the locations of low wind speed of 2 - 4 m/s. To address this issue, a momentary impulse or external torque to the rotor by external motor is one of the good options to maintain the momentum of blades and thus provide stability for sufficient time. Various theoretical calculations and experiments are conducted on the above method. This would increase the output power and also the efficiency of wind turbine. We show that Return-On-Investment will be high as compared with other grid connected turbines. Our proposed concept in the present study, if implemented properly, can help the installation of number of wind turbines even at domestic level. It also makes the consumers energy independent and promotes the use of wind as a source of energy and may enter as a rooftop energy supply system similar to solar.

叶顶间隙对低比转数小流量泵性能的影响

为建立叶顶间隙对低比转速小流量泵性能的影响关系,采用数值计算的方法,以叶顶间隙宽s和叶轮出口宽b的比值(s/b)表征间隙大小,对不同间隙下泵的性能展开对比分析。分析发现,随着间隙的增加,泵的扬程和效率均逐渐降低,最大和最小间隙下的扬程和效率差分别达15.4%,5.5%。当间隙较小即s/b<0.2时,扬程曲线在小流量工况会出现不稳定的“驼峰”现象,在s/b>0.3后,设计工况附近效率的降幅显著增大。从流动特性来看,随着叶顶间隙的增大,间隙泄漏流对叶轮内主流区影响加剧,会诱导严重的二次流和旋涡的产生,同时间隙内不稳定流动也会产生能量损失,造成扬程和效率下降。为低比转数小流量泵的设计与优化提供借鉴意义。

泡沫铜对密闭管道内合成气爆炸特性影响的实验研究

为研究泡沫铜孔隙密度和H2体积分数对合成气爆炸特性的影响,在封闭的管道中安装了孔隙密度为15、25和40 ppi的泡沫铜,实验分析了当量比为1的合成气-空气在不同H_(2)体积分数时的火焰结构、尖端速度和超压等参数变化规律。实验结果表明:火焰在泡沫铜上游的行为是受“郁金香”火焰形成过程的影响,泡沫铜对其没有影响。但是孔隙密度和H2体积分数的改变不仅会影响“郁金香”火焰的形成时间,还会影响变形“郁金香”火焰的形成。泡沫铜将火焰分割促使其从层流向湍流转化,对爆炸火焰传播起到加速作用。泡沫铜会引起管道内超压和火焰尖端速度的极大提升,且孔隙密度越小,H_(2)体积分数越大,火焰穿过泡沫铜后的最大火焰尖端速度越大,压力上升幅度越大,超压峰值越高。

基于RBF神经网络和模型在线更新的风速估计方法

基于NREL5MW风力发电机原模型和简化模型,进行流固耦合和计算流体力学仿真,研究结果表明风力发电机桨距角和叶尖速比影响风轮上游来流风速演化。结合来流风速演化和推力系数理论模型表明,通过推力更新诱导系数,进而在线更新风速演化模型,能提高风速估计精度。考虑推力时变性和非定常性,提出以桨距角和叶尖速比为输入,基于RBF神经网络的推力逼近方法,进而提出基于模型在线更新的风轮平面有效风速估计方法。最后通过激光雷达现场测风数据进行验证,结果表明,基于RBF神经网络和模型在线更新的风速估计方法较基于时间信息提取算法和来流风速演化模型的风速估计方法精度提高15.5%。

电动轮椅颠簸路面行驶状态研究

为提升电动轮椅在颠簸路面行驶的安全性,建立了电动轮椅在此行驶状态下的数学模型,利用轮子与地面接触时产生的振动作为外界激励,分析了电动轮椅行驶中常规线性模型与振动非线性模型行驶速度与重力加速度及纵向位移之间的关系。为进一步描述电动轮椅在颠簸路面行驶过程中的动态特性,分别以行驶速度3.3 m·s^(-1)和3.7 m·s^(-1)为例进行数值模拟,得出在颠簸路面行驶时电动轮椅的振动频率和幅值随着行驶速度的增加而增大,而这将导致电动轮椅发生倾倒的概率加大,威胁乘员的安全,所以合理设置电动轮椅行驶速度,优化底盘减振设计,能够降低冲击载荷,保障安全行驶。

密闭管道内非连续分布甲烷/空气爆炸特性

在长度900 mm、截面为100 mm×100 mm的矩形密闭管道内开展预混气体非连续分布对甲烷/空气爆炸影响的试验研究。通过分段通入甲烷体积分数为9.5%的预混气体,形成甲烷/空气-空气和甲烷/空气-空气-甲烷/空气两种非连续分布模式。研究发现,随着空气区域缩短,火焰传播距离、火焰尖端速度和爆炸超压均增大。特别是在甲烷/空气-空气-甲烷/空气分布中,当中间空气区域小于500 mm时,火焰会穿越并引发二次爆炸,且其超压随空气区域减小而增加。研究结果可为天然气爆炸事故防控提供理论支持。

试样缺口对悬臂梁缺口冲击强度的影响

选择聚碳酸酯(PC)、聚酰胺6(PA6)、聚丙烯(PP)、丙烯腈-丁二烯-苯乙烯(ABS)、聚苯硫醚(PPS)、聚甲醛(POM)、聚苯乙烯(PS)和高抗冲聚苯乙烯(HIPS)共计8种原料为研究对象,从注塑与加工缺口的间隔时间、缺口底部剩余宽度、铣刀速度3个方面,研究缺口对悬臂梁缺口冲击强度的影响。结果表明,PC、PP、POM、PS在注塑完成后1 h内加工缺口,其悬臂梁缺口冲击强度高于状态调节24和48 h后再加工缺口的值;PA6受吸潮性的影响,状态调节48 h后的悬臂梁缺口冲击强度,高于1 h内及状态调节24 h后再加工缺口的值;PC、PA6、PP、POM和PS的铣刀速度在21 m/min,缺口底部剩余宽度在8.0、8.2 mm时,悬臂梁缺口冲击强度高于铣刀速度为12和30 m/min、缺口底部剩余宽度在7.8 mm时的值;PPS的悬臂梁缺口冲击强度受树脂基体中添加的玻璃纤维填料的影响,结果变化无规律。注塑与加工缺口的间隔时间、缺口底部剩余宽度、铣刀速度的改变对ABS和HIPS悬臂梁缺口冲击强度结果没有影响。

风力发电最大风能追踪综述

基于风能的充分利用,从分析风力机运行特性出发,针对变速恒频风力发电系统的特点,综述了捕获最大风能的方法,如叶尖速比控制、功率信号反馈、爬山搜索法、最大功率小信号扰动法、三点比较法。考虑到实际应用中风速变化及测量困难,为避免直接检测风速,提出一种新的最大风能追踪方法。详细讨论了已有方法的优劣。分析与研究表明,新的最大风能追踪方法简单、实用、可行。

燃料电池用高转速空压机叶轮的设计与参数优化

本文设计了一种燃料电池用高转速空压机,采用CFD方法对其气动性能进行仿真计算,得出空压机性能曲线,通过研究不同叶轮参数下空压机的性能变化,进而实现对叶轮的优化。首先是研究单一参数分别对空压机性能的影响,参数包括叶片数量、叶轮出口宽度和叶顶间隙,然后考虑多参数之间的相互作用,采用正交实验法分析了多参数下空压机的性能变化。

垂直轴风力机动态特性及气动性能

垂直轴风力机流场属典型的非定常大分离流动,因其气动性能复杂,采用工程气动模型会有较大误差.为了研究垂直轴风力机动态失速与翼型附着涡的形成与发展,针对4种不同厚度的NACA对称翼型系列,基于Fluent软件的滑移网格技术,并选用S-A湍流模型和基于压力的Simple算法对H型垂直轴风轮流场进行瞬态CFD计算,得到了风轮旋转中动态失速相位角范围,较好地解释了小叶尖速比下翼型多处于动态失速区的流动机理.同时,提出了升阻系数计算方法,计算得到了该4种翼型系列的叶片扭转力矩、风力机功率和风能利用系数随叶尖速比变化规律.研究结果表明,风力机运行中翼型的动态与静态特性存在较大差别,翼型厚度对风力机扭转力矩、功率和风能利用率具有较大影响.故在进行垂直轴风力机设计时应综合考虑垂直轴风力机的翼型厚度等几何参数与旋转等动态参数对其气动特性的影响.

变速恒频风力发电机最大功率跟踪控制策略的研究

分析了风能的特性,对双馈电机数学模型进行了分析与研究,在试验的基础上,基于最佳叶尖速比控制(即风能最大利用率)和机械功率最大转化效率控制,提出了兆瓦级变速恒频双馈风力发电机组双闭环的一种控制策略方案,并用M A T LA B软件进行系统仿真,证明该方案是可行的。

基于液压传动的离网型风力机“变速恒频”控制研究

在自行研制的离网型风力机能量液压传递系统的基础上,提出通过电液调控实现风力机的风轮最大能量捕获和发电机最佳工作转速的"变速恒频"控制。通过建模仿真和试验,结果表明:当风速在额定值以下变化时,通过调节变量泵排量,可产生相应的风轮反力矩,使风轮转速与风速关系维持在最佳的叶尖速比,从而保证风轮最大能量捕获;而无论风轮转速如何变化,通过调节变量马达排量,就可实现发电机转速稳定在额定值附近。

风能致热系统研究

论述了风能致热系统的工作原理。通过对系统与负载匹配的研究，提出采用薄壁孔元件作为阻尼元件可使系统始终工作在最佳工作点有效地将风能转化为热能。

独立运行风电机组的最佳叶尖速比控制

介绍了采用爪极无刷自励发电机的５ｋＷ风电机组的性能特点。采用最佳叶尖速比控制和稳压控制相结合的控制方法，使风力机在额定风速以下及蓄电池没有充满时按最佳效率运行。当蓄电池接近充满时，控制风电系统稳压运行，保证蓄电池安全可靠充电。该风电机组及其控制已实际应用。

基于叶片弦长的小型H型垂直轴风机气动性能分析

针对不同弦长下的小型H型垂直轴风轮模型,建立风轮外流场CFD(Computational Fluid Dynamic)模型,采用移动网格技术,选用RNG两方程湍流模型和基于压力的隐式Couple算法进行瞬态计算,获得风轮气动性能曲线,进而分析叶片弦长对小型H型垂直轴风力机风轮气动性能的影响。

风力机叶片气动设计及偏载计算

风力发电机组叶片的气动特性直接影响到机组效率,考虑了风力机叶片气动损失,运用Schmitz方法对1MW定桨风力发电机组的叶片进行了气动设计。叶片从叶根到叶尖采用了不同翼型,以满足叶片强度和气动性能的要求。不同翼型之间采用了3次样条插值后的过渡翼型,同时叶根和叶尖也进行了处理,以满足工艺和实际的要求。在所设计的风力机叶片的基础上,计算了风力机的偏载特性,由于风力机起动与空载时,Schmitz方法误差较大,该文采用了Schmitz理论的扩展算法来计算风力机的偏载特性。设计结果表明,基于Schmitz理论设计的多翼型风力机叶片考虑了叶片的损失,方法简单,计算出来的叶片弦长和扭角符合实际叶片的规律。采用了扩展算法后的偏载特性曲线,考虑了各种损失,与实际风力机表现的特性规律相符。