Energy-based iteration scheme of the double-multiple streamtube model in vertical-axis wind turbines

Due to their simplicity, blade element momentum models, such as the double-multiple streamtube (DMS) model, are among the most common models to predict the performance of Darrieus vertical-axis wind turbines (VAWTs). A two-dimensional energy-based iteration scheme of the DMS model (EB-DMSM) is shown in the present work. Its purpose is to improve predictions of power performance and flow expansion. This new approach is compared with a momentum-based iteration scheme (MB-DMSM) and the results of a two-dimensional computational simulation of a 12-kW straight-bladed VAWT. The mathematical representation of streamlines used for modeling the flow expansion is in good agreement with the simulations. Convergence of both schemes is achieved for tip-speed ratios (TSRs) up to 4. Failure of the models to convergence at higher TSRs is attributed to their inability to adequately represent the aerodynamic forces acting on the blades, due to the simplicity of their formulation. Relative to computational simulations, the maximum differences in the peak power coefficient predictions are 16 and 32 % and in the flow expansion predictions are 53 and 5 % for the MB-DMSM and EB-DMSM, respectively. Corrections are required to improve predictions of power performance and flow expansion of turbines with different geometric and operational parameters.     

碳纳米纤维纸-玻纤/环氧复合材料对风力发电叶片的影响

针对风力发电叶片在多风沙环境下的固体粒子冲蚀磨损行为, 研究了风力发电叶片专用环氧树脂(EPIKOTE^TM RIM 135、EPIKURE^TM RIM H 137)、传统玻纤增强环氧复合材料和新型碳纳米纤维纸-玻纤/环氧复合材料的固体粒子冲蚀磨损行为, 并测试了不同材料的玻璃化转变温度, 进而对比分析了其对冲蚀磨损的影响; 针对风力发电叶片在寒冷环境下表面容易结冰的现象, 研究了上述三种材料表面的疏水性能, 并测试了它们对水的接触角大小。结果表明: 碳纳米纤维纸-玻纤/环氧复合材料具有良好的界面结合, 且碳纳米纤维纸的引入提高了碳纳米纤维纸-玻纤/环氧复合材料的玻璃化转变温度(从55 ℃提高到63 ℃), 从而改善了其耐固体粒子冲蚀磨损性能; 同时, 碳纳米纤维纸的加入改善了碳纳米纤维纸-玻纤/环氧复合材料的表面疏水性能(接触角从104°提高到131°)。     

Experimental and numerical studies on the performance and surface streamlines on the blades of a horizontal-axis wind turbine

Investigating laminar separation over the turbine blade of a horizontal-axis wind turbine (HAWT) has been considered an important task to improve the aerodynamic performance of a wind turbine. To better understand the laminar separation phenomena, in this study, the aerodynamic forces of a SD8000 airfoil (representing the sectional blade shape) in the steady-state conditions were first predicted using an incompressible Reynolds-averaged Navier–Stokes solver with the γ–Re _θt and k–k _L–ω transition models. By comparing simulation and experimental results, the k–k _L–ω transition model was chosen to simulate the laminar separation on three-dimensional (3D) turbine blade. Experimentally, a HAWT with three blades was then tested in a close-circuit wind tunnel between the tip speed ratios (TSRs) of 2 and 7 at the wind speed of 10 m/s. In addition, through computational fluid dynamics, the turbine performance and flow characteristics on the blade as blade is rotating were investigated. It is shown that 3D simulations agreed well with the experimental results with regard to the mechanical power of the HAWT at the testing TSRs. Moreover, the separation and reattachment lines on the suction surface of the turbine blade were also observed through the skin friction line, indicating that laminar separation moved toward the trailing edge with the increasing TSR at the blade tip region.     

Experimental-numerical investigation of the dynamic stability of flexural-torsional vibrations of compressor blades under conditions of attached and separated flow. part 3. mutual aerodynamic couplings

The paper considers the results of a study of mutual aerodynamic couplings which give rise to nonstationary aerodynamic interactions between the translational and angular components displacements of neighboring ring blades in the case of variation of attack angle, reduced vibration frequency and the geometric parameters of the blade cascade. The effect of mutual aerodynamic couplings between neighboring blades on the aerodynamic stability of flexural-torsional vibrations of the blades of the peripheral section of the shrouded fan blade ring of gas turbine engine for an attack angle of 15° has been estimated.     

基于转静干涉效应的压气机叶片气动载荷分析

转静叶排的相互作用会使压气机内部流场存在复杂的非定常性。为深入研究压气机叶片的气动载荷特性,以某型航空发动机压气机为研究对象,考虑叶排间的转静干涉效应,利用滑移网格技术对整个叶盘的三维流场展开模拟,求解干涉周期T_b内压气机转子内部的流动规律。同时对叶片气动载荷的非定常特性进行进一步分析,讨论了不同压比、转速对压气机叶片气动载荷的影响。结果表明叶片压力面和吸力面气动载荷波动峰值的主导频率皆为转静干涉频率f₀的倍频,其中一倍频（1×f₀）分量占主导地位。在干涉周期T_b内,叶片表面压力涡发生周期性的迁移与耗散,压力面和吸力面气动载荷的变化呈相反趋势。随着压比的增加,压气机叶片气动载荷逐渐增大,但其脉动幅值和频谱峰值基本不变。转速的升高使得转静干涉的频率增大,增强了压气机叶片气动载荷的非定常特性。研究结果能够应用于叶盘结构的气动优化设计,可为高性能航空发动机压气机的研制提供支持和参考。     

Numerical and experimental investigation of axial fan with trailing edge self-induced blowing

Axial fans often show adverse flow conditions at the fan hub and at the tip of the blades. In the present paper, a modification of conventional axial fan blades with numerical and experimental investigation is presented. Hollow blades were manufactured from the hub to the trailing edge at the tip of the blades. They enable the formation of self-induced internal flow through internal passages. The internal flow enters the internal radial flow passages of the hollow blades through the openings near the fan hub and exits through the trailing edge slots at the tip of the blade. The study of the influence of internal flow on the flow field of axial fan and the modifications of aerodynamic characteristics of the axial fan have been made. The numerical and experimental results show a comparison of integral and local characteristics of the axial fan with the internal flow, compared to characteristics of a geometrically equivalent fan without internal flow. The experimental results of local characteristics were performed with a five-hole probe and computer-aided visualization. A reduction of adverse flow conditions near the trailing edge at the tip of the blade was achieved, as well as boundary layer reduction on the blade suction side and the reduction of flow separation. The introduction of self-induced blowing led to the preservation of the direction of external flow, defined by blade geometry, and enabled maximal local energy conversion. The integral characteristic reached higher degree of efficiency.     

Response surface approach to aerodynamic optimization design of helicopter rotor blade

This paper describes a hovering rotor blade design through the suitable combination of flow analysis and optimization technique. It includes a parametric study concerned with the influence of design variables and different design conditions such as objective functions and constraints on the rotor performance. Navier–Stokes analysis is employed to compute the hovering rotor performance in subsonic and transonic operating conditions. Response surface method based on D‐optimal 3‐level factorial design and genetic algorithm are applied to obtain the optimum solution of a defined objective function including the penalty terms of constraints. The designs of the rotor airfoil geometry and the rotor tip shape are performed in subsonic and transonic conditions, and it is observed that the new rotor blades optimized by various objective functions and constraints have better aerodynamic characteristics than the baseline rotor blade. The influence of design variables and their mutual interactions on the rotor performance is also examined through the optimization process. Copyright © 2005 John Wiley & Sons, Ltd.     

Characterization of Damage Induced by Impacting Objects in Udimet-500 Alloy

Foreign object damage (FOD) is one of the common mechanisms in turbine blade failures which reduces high cycle fatigue life and contributes significantly to premature failure. Therefore, this study experimentally simulated FOD on the first stage blade of a gas turbine. Foreign objects with two parameters of object nose shape (spherical, conical, and flat) and impact angle (90° and 45° relative to the blade surface) were impacted on the surface of flat specimens of Udimet-500 at a temperature of 733°C with velocity of 300 m/s (same as working condition of blades). In order to evaluate the impact site morphologies, induced craters were first studied by scanning electron microscopy (SEM). Then samples were sectioned symmetrically, and by using image analysis tool software, the depth of each crater was measured. Finally, based on the results, it was found that maximum and minimum stress concentration factors and induced microcracks, respectively, are regarding to flat projectiles with impact angel of 45° and spherical at impact angle of 90°.     

多翼离心风机气动噪声计算与降噪设计研究

以船用空调通风系统中多翼离心式风机为对象,建立风机内部流体三维建模,采用CFD软件进行稳态与非稳态计算.将得到的风机内部流场结果导入LMS Virtual.Lab声学软件中进行噪声预估,同时与实验结果对比,验证风机气动噪声计算的准确性.根据分析得知,风机气动噪声主要噪声源位于叶轮处并且与其内部流场分布和自身结构密切相关...     

叶型改进对离心通风机内部气动激励影响机理的数值分析研究

叶片是离心通风机核心部件,为研究叶型改进对离心通风机振动的影响,对某型离心通风机进行气动激励的仿真计算;通过定常计算,发现叶型改进能够推迟流动分离,减小诱发振动的激励因素;通过非定常计算,发现叶型改进对离心通风机气动激励的影响主要体现在压力脉动的改善,尤其是叶频脉动的降低;用计算分析结果对通风机振动变化进行预估,样机试验结果表明,叶型改进能够降低叶频振动,同时也表明气动激励分析可为通风机低噪声改进效果提供判断依据。     

Robust design using Bayesian Monte Carlo

In this paper, we propose an efficient strategy for robust design based on Bayesian Monte Carlo simulation. Robust design is formulated as a multiobjective problem to allow explicit trade‐off between the mean performance and variability. The proposed method is applied to a compressor blade design in the presence of manufacturing uncertainty. Process capability data are utilized in conjunction with a parametric geometry model for manufacturing uncertainty quantification. High‐fidelity computational fluid dynamics simulations are used to evaluate the aerodynamic performance of the compressor blade. A probabilistic analysis for estimating the effect of manufacturing variations on the aerodynamic performance of the blade is performed and a case for the application of robust design is established. The proposed approach is applied to robust design of compressor blades and a selected design from the final Pareto set is compared with an optimal design obtained by minimizing the nominal performance. The selected robust blade has substantial improvement in robustness against manufacturing variations in comparison with the deterministic optimal blade. Significant savings in computational effort using the proposed method are also illustrated. Copyright © 2007 John Wiley & Sons, Ltd.     

Dynamic and fatigue compressor blade characteristics during fluid-structure interaction: Part I—Blade modelling and vibration analysis

Fatigue failure in blades resulting from alternating loads is a primary cause of aircraft engine failure. In this study, the dynamic and fatigue performance of a compressor blade, which is prone to fatigue failure in practical use, is verified through numerical analysis and tests to identify specific failure causes and ultimately prevent blade failure. Two companion papers describe this work. Part I—blade modelling and vibration analysis establishes a compressor blade model based on reverse engineering and investigates the blade's dynamic characteristics during aerodynamic and structural behaviour interactions under various flight conditions. To accurately describe the dynamic performance of a real blade, three blade models with different fitting precisions are constructed to support dynamic response analysis. In addition, the flow field of each model is constructed to simulate the aerodynamic loads that cause blade vibration. A numerically based analysis of the fluid-structure interaction is subsequently performed. A reasonable calculation model is selected by comparing the dynamic characteristics among these models. The Campbell diagram is analysed to determine resonance probability, and blade failure cause is ultimately verified based on the vibration analysis. The results show that different accuracies in the blade calculation model produce varying degrees of error; the calculation model's ability to reflect the dynamic and fatigue behaviours of a real blade is recommended to be used as a critical evaluation index in the numerical study. The first-order harmonic resonance occurring at critical speeds may contribute to blade failure.     

Aerodynamic optimization of turbomachinery blades using evolutionary methods and ANN-based surrogate models

A fast, flexible, and robust simulation-based optimization scheme using an ANN-surrogate model was developed, implemented, and validated. The optimization method uses Genetic Algorithm (GA), which is coupled with an Artificial Neural Network (ANN) that uses a back propagation algorithm. The developed optimization scheme was successfully applied to single-point aerodynamic optimization of a transonic turbine stator and multi-point optimization of a NACA65 subsonic compressor rotor in two-dimensional flow, both were represented by 2D linear cascades. High fidelity CFD flow simulations, which solve the Reynolds-Averaged Navier-Stokes equations, were used in generating the data base used in building the ANN low fidelity model. The optimization objective is a weighted sum of the performance objectives and is penalized with the constraints; it was constructed so as to achieve a better aerodynamic performance at the design point or over the full operating range by reshaping the blade profile. The latter is represented using NURBS functions, whose coefficients are used as the design variables. Parallelizing the CFD flow simulations reduced the turn-around computation time at close to 100% efficiency. The ANN model was able to approximate the objective function rather accurately and to reduce the optimization computing time by ten folds. The chosen objective function and optimization methodology result in a significant and consistent improvement in blade performance.     

风力机运行过程中气动载荷变化工程分析

为提高风力发电机组设计可靠性,需得到风力机在实际运行过程中叶片气动载荷的计算方法.从工程角度出发,对风力发电机使用的NACA专用翼型进行分析,结合薄翼理论推导出攻角变化特点,并依据风切变理论和塔影效应得到风力机实际运行时叶片在各个位置点的气动载荷.依据实际情况对风轮受力平面进行工程简化,得到风力机叶片气动载荷的计算方法.将此方法计算出的气动载荷与试验测量结果进行比对,吻合度较好.     

Aerodynamic design and analysis of a 10 kW horizontal-axis wind turbine for Tainan,Taiwan

The purpose of the present study is to develop a small-scale horizontal-axis wind turbine (HAWT) suitable for the local wind conditions of Tainan, Taiwan. The wind energy potential was first determined through the Weibull wind speed distribution and then was adapted to the design of the turbine blade. Two numerical approaches were adopted in the design and analysis of the HAWT turbine blades. The blade element momentum theory (BEMT) was used to lay out the shape of the turbine blades (S822 and S823 airfoils). The geometry of the root region of the turbine blade was then modified to facilitate integration with a pitch control system. A mathematical model for the prediction of aerodynamic performance of the S822 and S823 airfoils, in which the lift and drag coefficients are calculated using BEMT equations, was then developed. Finally, computational fluid dynamics (CFD) was used to examine the aerodynamic characteristics of the resulting turbine blades. The resulting aerodynamic performance curves obtained from CFD simulation are in agreement with those obtained using BEMT. It is also observed that separation flow occurred at the turbine blade root at the tip speed ratios of 5 and 7.     

Temperature-dependent dielectric and microwave absorption properties of silicon carbide fiber-reinforced oxide matrices composite

Simultaneous improvement of mechanical and microwave absorption properties of the composites at high temperatures still undergoes considerable challenges. We have investigated the high-temperature microwave absorbing properties of the silicon carbide fiber-reinforced oxide matrices (SiC_f/mullite–SiO₂) composite on the basis of our previous work. Results indicate that the complex permittivity increases from 8.19 ? j5.09 to 16.39 ? j9.83 at 10 GHz with the temperature rising from 200 to 600 °C. The SiC_f/mullite–SiO₂ composite has relatively high tanδ values indicating superior microwave attenuation ability. The reflection loss (RL) values of the composite increase with rising thickness. It can be noticed that the RL response curves of different thicknesses are basically consistent at 200 and 400 °C. In addition, the RL value of the composite is less than ??5 dB in the whole X band when the thickness is under 2.9 mm and the temperature is below 400 °C. The hybrid oxide matrices of mullite and SiO₂ are beneficial to improve the dielectric properties, especially high-temperature microwave absorption properties of the SiC fiber-reinforced ceramic matrix composite. The superior microwave absorption properties indicate that the SiC_f/mullite–SiO₂ composite is a promising candidate in aircraft engine nozzle and aerodynamic heating parts of aircrafts at high temperatures.     

Design and Validation of the MBW Standard Humidity Generators

MBW Calibration AG (MBW) is the Designated Institute (DI) for humidity appointed by the Federal Institute of Metrology, METAS. MBW currently offers calibration and measurement capabilities (CMC) for frost/dew-point hygrometers by comparison with precision chilled-mirror transfer standards that have been calibrated using the primary standards of leading European National Metrology Institutes or DI. The design, construction and validation of two standard humidity generators to be used as the Swiss national standards for the primary realization of frost/dew-point temperature in the range from ? 90 °C to + 95 °C are presented and discussed. The generators are operated as continuous flow “single-pressure” generators in the range from ? 80 °C to ? 10 °C with saturation over ice and from 0.5 °C to + 95 °C with saturation over water. Additionally, they are used in “two-pressure” mode for saturation over ice down to frost-point temperatures of ? 90 °C and down to ? 20 °C for saturation over water. The main saturators of both generators have been designed to fit in commercially available calibration baths with either ethanol or distilled water as the heat transfer fluid for saturator temperatures below and above 0 °C, respectively. Saturator temperature is measured using standard platinum resistance thermometers and a purpose-built precision thermometer. Pressure measurements are taken with gauge pressure transducers and a separate barometric sensor, to reduce the influence of the atmospheric pressure on the measurement of the pressure ratio and make full use of the correlation of pressure measurements and enhancement factors when operating in two-pressure mode. A totally automated pre-saturation and flow control system facilitates the calibration of state-of-the-art chilled-mirror transfer for standards without manual readjustment of the generated flowrate to ensure a constant volumetric flow at the conditions of the mirror. The uncertainty budget leading to the CMC for frost/dew-point temperature realization is presented in the context of the experimental validation performed. The results in the overlapping range of both generators are presented and used as further evidence of the saturation efficiency of both standards.     

高雷诺数下错列双圆柱气动干扰的机理研究

为了进一步澄清小间距错列双圆柱的气动干扰机理,该文采用大涡模拟方法,在高雷诺数下（Re=1.4×105）,研究了间距为2倍圆柱直径的错列双圆柱的气动性能和流场特性随风攻角的变化规律,分析了两个圆柱气动力系数相关性,探讨了下游圆柱气动力与流场结构的内在联系,对下游圆柱平均升力的流场机理提出了新的解释。研究表明,大涡模拟得到的结果与风洞试验值吻合良好;下游圆柱的气动性能、流场结构和两个圆柱气动力相关性均会随风攻角发生剧烈变化;风攻角在0°~10°时,下游圆柱受平均负阻力作用,其原因分别为两圆柱间的回流区和间隙流;风攻角在10°附近时,下游圆柱受很大平均升力作用,风压停滞点偏移、两圆柱间高速间隙流、下游圆柱间隙侧剪切层的提前分离和再附是平均升力出现的三个因素。     

Comparison of Fatigue Life Between C/SiC and SiC/SiC Ceramic-Matrix Composites at Room and Elevated Temperatures

In this paper, the comparison of fatigue life between C/SiC and SiC/SiC ceramic-matrix composites (CMCs) at room and elevated temperatures has been investigated. An effective coefficient of the fiber volume fraction along the loading direction (ECFL) was introduced to describe the fiber architecture of preforms. Under cyclic fatigue loading, the fibers broken fraction was determined by combining the interface wear model and fibers statistical failure model at room temperature, and interface/fibers oxidation model, interface wear model and fibers statistical failure model at elevated temperatures in the oxidative environments. When the broken fibers fraction approaches to the critical value, the composites fatigue fracture. The fatigue life S–N curves and fatigue limits of cross-ply, 2D and 3D C/SiC and SiC/SiC composites at room temperature, 550 °C in air, 750 °C in dry and humid condition, 800 °C in air, 1000 °C in argon and air, 1100 °C, 1300 °C and 1500 °C in vacuum, have been predicted. At room temperature, the fatigue limit of 2D C/SiC composite with ECFL of 20 % lies between 0.78 and 0.8 tensile strength; and the fatigue limit of 2D SiC/SiC composite with ECFL of 20 % lies between 0.75 and 0.85 tensile strength. The fatigue limit of 2D C/SiC composite increases to 0.83 tensile strength with ECFL increasing from 20 to 22.5 %, and the fatigue limit of 3D C/SiC composite is 0.85 tensile strength with ECFL of 37 %. The fatigue performance of 2D SiC/SiC composite is better than that of 2D C/SiC composite at elevated temperatures in oxidative environment.     

The effect of ZrO2 and TiO2 on solubility and strength of apatite–mullite glass–ceramics for dental applications

The effect of ZrO₂ and TiO₂ on the chemical and mechanical properties of apatite–mullite glass–ceramics was investigated after sample preparation according to the ISO (2768:2008) recommendations for dental ceramics. All materials were characterized using differential thermal analysis, X-ray diffraction, scanning electron microscopy and energy dispersive spectroscopy. X-ray fluorescence spectroscopy was used to determine the concentrations of elements present in all materials produced. The chemical solubility test and the biaxial flexural strength (BFS) test were then carried out on all the samples. The best solubility value of 242 ± 61 μg/cm² was obtained when HG1T was heat-treated for 1 h at the glass transition temperature plus 20 °C (T_g + 20 °C) followed by 5 h at 1200 °C. The highest BFS value of 174 ± 38 MPa was achieved when HG1Z and HG1Z+T were heat-treated for 1 h at the T_g + 20 °C followed by 7 h at 1200 °C. The present study has demonstrated that the addition of TiO₂ to the reference composition showed promise in both the glass and heat-treated samples. However, ZrO₂ is an effective agent for developing the solubility or the mechanical properties of an apatite–mullite glass–ceramic separately but does not improve the solubility and the BFS simultaneously.