叶片尾缘劈缝射流尾迹相干结构数值研究

为揭示射流速比对尾迹流场及相干结构的影响规律,以叶片尾缘劈缝射流尾迹为研究对象,在非定常数值模拟结果基础上,采用快速傅里叶变换(Fast Fourier Transform,FFT)和本征正交分解(Proper Orthogonal Decomposition,POD)得到不同射流速比时尾迹脉动速度的频谱特性和POD模态及模态系数的频谱特性,研究表明:采用SAS(Scale Adaptive Simulation)湍流模型得到的计算结果与PIV实验结果吻合较好;不同射流速比时,尾迹中大尺度相干结构特性发生变化,导致回流区形态和空间尺度发生改变;上、下板尾缘脱落涡的主导结构均为卡门涡,但是不同射流速比时其脱落频率发生较大改变;射流速比的变化导致上、下板某一侧的旋涡强度增强,向下游输运距离更远,上、下板尾缘卡门涡的强度及空间尺度呈不对称性。     

不同材质风力机叶片绕流流场特征变化PIV实验研究

考虑风力机叶片变形对绕流流场的影响,通过粒子图像测速（PIV）方法,采集两副翼型相同、材质分别玻璃聚酯（叶片Ⅰ）和玻璃聚醋内部填充泡沫（叶片Ⅱ）的水平轴风力机叶片绕流流场信息,对不同测试截面处流场数据进行分析。研究结果表明：相比于叶片Ⅰ,相同工况下,叶片Ⅱ的总体涡量、回流区的面积和时均速度更大,轴向雷诺应力更小;随着相对轴向距离的增加,时均速度在相对高度方向波动减小,材质对轴向雷诺应力影响逐渐减弱。实验工况下,叶片材质对绕流流场特征的影响大于叶尖速比对其的影响,并沿径向方向有增大的趋势。     

零质量射流对风力机翼型动态失速特性的影响

为了改善风力机大厚度翼型的气动性能,采用零质量射流对翼型附近的流动进行流动控制。采用非定常雷诺时均模拟方法(URANS)对动态失速状态下带零质量射流的DU97-W-300翼型的绕流场进行数值模拟,并对比控制前和控制后的翼型气动特性。结果表明,随着射流折合频率的增加,翼型失速攻角逐渐增大,升力系数曲线的波动次数逐渐减小。零质量射流可以有效抑制流动分离,其抑制动态失速的能力随翼型折合频率的增加而增强,随激励器动量系数的增加而增强。     

边界层吸气对圆柱绕流尾涡脱落结构及圆柱受力的影响

为抑制圆柱绕流时受到的涡激力、保护圆柱结构安全,通过施加边界层吸气控制圆柱在涡激效应下受到的周期力.采用数值模拟方法分析Re =200时吸气口位置和吸气强度对圆柱受力情况及尾涡脱落结构的影响.结果 表明:当吸气口布置在60°、90°和120°位置时,添加吸气条件会显著减小圆柱受到的涡激力;吸气条件的引入对圆柱尾涡的脱落...     

合成射流控制策略对垂直轴风力机性能影响研究

设计一种斜出口合成射流激励器并将其应用于垂直轴风力机控制其动态失速,建立不同射流孔数量的叶片并采用5种不同合成射流激励器控制策略,通过FLUENT15.0并采用Realizable k-ε湍流模型分析射流孔数量与控制策略对垂直轴风力机气动性能的影响,进一步研究垂直轴风力机涡量场结构。结果表明:当采用上开口抛物线控制策略、射流吹气系数为0.035,射流孔数量为2时,风能利用系数与平均力矩系数均提升15.2%,随着射流孔数量增多,气动性能降低;采用传统合成射流控制策略的垂直轴风力机承受近乎2倍的载荷波动,改进的控制策略可减小叶片在小攻角时的载荷波动,从而相对提升垂直轴风力机的运行稳定性;另外,合成射流技术可抑制叶片吸力面大涡的生成与发展并使叶片强尾涡削弱成多个小尾涡,减小多个叶片间的流动干扰并降低转轴尾涡强度,从而改善全局流场结构。     

Study on Flow Characteristics of Bluff-body Stabilized Turbulent Premixed Flames

采用粒子图像测速技术对钝体燃烧器出口下游流场进行了测量,比较分析了冷、热态流动结构以及速度对流场特性的影响.结果表明：钝体燃烧器冷、热态流动特性具有明显差异：冷态流场属于环流主导型,中心轴线上有前、后两个滞止点;而热态流场表现为射流主导型结构,并且随着中心射流流速的增大射流的主导作用增大,中心轴线上没有出现滞止点,并且中心轴线上速度最小值随着中心射流流速增大而减小且接近燃烧器出口,随着速度比的增大而增大且远离燃烧器出口;冷态条件下靠近燃烧器出口处湍动能较大,热态条件下燃烧器下游的湍动能较大,速度较高的工况具有较大的湍动能.     

基于吹吸结合射流的S809翼型增升减阻研究

为改善风力机翼型气动特性,提出吹吸结合射流(Suction-Blow Combined Jet,SBCJ)方式,以S809为基础翼型,研究在不同攻角、射流动量系数及开孔位置时SBCJ的控制效果,分析其控制机理和影响规律.结果 表明:SBCJ可移除翼型吸力面低动量流体并改变尾缘库塔条件,从而显著增大翼面两侧压差,最终提升翼型气动性能;当射流动量系数较小时,翼型升力显著增大、修正阻力减小且流动分离减弱;当射流动量系数为0.01、吸气孔距前缘0.15c、吹气孔距尾缘0.2c、攻角为10°时,翼型修正升阻比提升率最大.     

栅前端壁射流抑制二次流的数值研究

基于射流的主动流动控制方法,在透平叶栅端壁前缘位置开设射流孔.通过数值模拟分析了射流孔位置、倾斜角、俯仰角和动量系数等因素对前缘马蹄涡以及流动损失的影响.结果表明:在前缘马蹄涡流动分离线上布置射流孔以及倾斜角方向与射流孔所在位置的近端壁极限流线保持一致的情况下,射流孔俯仰角为4 0 °、动量系数为0.1％时总压损失系数...     

垂直浓淡煤粉燃烧方式下炉内拟序结构研究

采用ＰＤＡ测量系统,对四角切圆锅炉中垂直浓淡煤粉燃烧方式下影响一次风与主流场混合和扩散的炉内拟序结构进行了实验研究和湍流特性分析。结果表明,一次风射流由于与上游横向射流的相互作用,在背火侧形成尾迹涡,在向火侧形成剪切涡,其中尾迹涡和剪切涡中有较高的湍动能和剪切应力,对颗粒的扩散起着主控作用。并从理论上分析了垂直浓淡煤粉燃烧技术可能不利于煤粉颗粒的燃尽,而且是容易导致水冷壁结渣的原因。     

非对称翼型绕流流动机理研究

为研究翼型在均匀流情况下的绕流流动机理,采用粒子图像测速技术(PIV)对非对称翼型NACA64-418进行风洞试验,对不同攻角、不同风速下翼型绕流流场进行可视化分析,并引入涡旋强度物理量准确分析翼型表面涡脱落及再附现象,以更精确描绘流场涡结构。研究表明:当Re=1.02×10~5,攻角16°≤α23°时,翼型表面发生再附,分离涡消失;而当攻角大于23°时,再附现象消失,临界速度随攻角的增大而减小。该试验方案及分析模型方法可以准确预测翼型表面涡脱情况,对翼型剪切层内不稳定涡旋及再附研究有着重要意义。     

大尺度展向波形圆柱绕流流场结构的数值研究

针对大尺度展向波形圆柱绕流的减阻特性,通过大涡模拟（LES）研究波形振幅对圆柱体绕流流场结构的影响,获得波形圆柱体绕流气动性能曲线、尾迹时均流速分布和非定常涡量场分布,最后与直圆柱绕流的流场结构进行对比分析。结果表明,波形圆柱绕流的平均阻力系数小于直圆柱体绕流,流向涡的形成改变了圆柱近尾迹区的流场结构,因此,波形圆柱体尾迹涡系表现得更为紧凑,尾迹涡流得到拉伸与破裂。在亚临界雷诺数为3000时,最大阻力系数减少18.3%,最优振幅比为0.152;且波形圆柱体的升力波动大大减少,甚至得到抑制。由于波形表面会形成更稳定的三维自由剪切层,这样的自由剪切层在下游位置卷起漩涡,大大地改变了圆柱周围的流场结构。研究表明振幅比在确定波形圆柱后面的三维涡旋结构中起着至关重要的作用,并对升力波动和流动阻力的降低有着显著的影响。     

Numerical simulation of vortex structures and heat transfer behind a hill in a laminar boundary layer

The present study examines a three‐dimensional numerical simulation of vortex structures and heat transfer behind a hill mounted in a laminar boundary layer. A vortex pair is formed symmetrically in the separation bubble behind the hill, and a hairpin vortex is periodically shed in the wake. The hairpin vortex moves downstream with time, and the gradient of the head of the hairpin vortex increases. Further downstream, the hairpin vortex is deformed to an Ω‐shaped structure. In the growth process of the hairpin vortex, horn‐shaped secondary vortices grow near the wall. The dissipation rate of the temperature fluctuation around the hairpin vortex increases because the heated fluid near the wall is removed to the free stream by Q2 ejection. Heat transfer increases due to the legs of the hairpin vortex and secondary vortices. These vortices generate high turbulence in the flow field and also contribute to an increase in Reynolds shear stress and turbulent heat flux. © 2008 Wiley Periodicals, Inc. Heat Trans Asian Res, 37(7): 398–411, 2008; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20217     

Lattice-Boltzmann simulations of flow past stationary particles in a channel

Direct numerical simulations of turbulent flow past stationary particles in a channel are carried out to determine the influence of the particle on the flow structure. The position of the particles in the channel, the particle size, the Reynolds number, and the number of particles are varied. The studies are carried out using the lattice-Boltzmann method. The details of the flow field are analyzed to provide insight into the factors that control the distance of influence. In the case of a single particle, when the particle is close to the wall, the mean wake moves away from the wall. Moreover, there is only one dominant vortex in the wake instead of two symmetric vortices. In the case of multiple particles, the vortices shed from the particles interact with each other. In the single particle cases considered, the effect of the particle is felt for about 20 diameters downstream. When multiple particles are present, interaction between the vortices shed by the particles lengthens the distance of influence.     

An Experimental Study on 3—D Flow in an Annular Cascade of High Turning Angle Turbine Blades

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Mist/steam cooling by a row of impinging jets

Mist/steam cooling has been studied to augment internal steam-only cooling for advanced turbine systems. Water droplets generally less than 10 μm are added to 1.3 bar steam and injected through a row of four round jets onto a heated surface. The Reynolds number is varied from 7500 to 22,500 and the heat flux varied from 3.3 to 13.4 kW/m². The mist enhances the heat transfer along the stagnation line and downstream wanes in about 3 jet diameters. The heat transfer coefficient improves by 50-700% at the stagnation line for mist concentrations 0.75-3.5% by weight. Off-axis maximum cooling occurs in most of the mist/steam flow but not in the steam-only flow. CFD simulation indicates that this off-axis cooling peak is caused by droplets’ interaction with the target walls.     

Simulation of free surface MHD‐flow and heat transfer around a cylinder

Magnetohydrodynamic (MHD) free‐surface flow and heat transfer of liquid metal around a cylinder under different Reynolds numbers were simulated numerically. The effects of the application of a magnetic field on wake and vortex shedding were analyzed. The characteristics of flow fields and temperature as well as Lorentz forces under two different Reynolds numbers were presented. The results showed that magnetic field could not only change substantially the flow pattern, but also suppress turbulent viscosity and surface renewal, which degraded heat transfer. Under the same Hartmann numbers, compared with the MHD‐flow and heat transfer of lower Reynolds numbers, the turbulence intensity and interaction between free surface and wake were still stronger for higher Reynolds numbers; consequently, the heat transfer was still high. © 2007 Wiley Periodicals, Inc. Heat Trans Asian Res, 37(1): 11–19, 2008; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20189     

Heat transfer in channel flow around a rectangular cylinder

Heat transfer and flow visualization experiments have been made in a channel with a rectangular cylindrical section having various width-to-height ratios. Vortices were observed to shed periodically from the cylinder and then reattach to the channel wall. This reattachment of the vortices induces a periodic fluctuation in heat flux at the wall and enhances the heat transfer in the downstream region of the cylinder. The streamwise position of the maximum Nusselt number moves downstream with decreasing width-to-height ratio, b/h, of the cylinder. When b/h = 2.0, however, the heat flux periodicity disappears because the wake narrows intermittently owing to reattachment of the separated flows to the upper and lower surfaces of the cylinder. © 1998 Scripta Technica. Heat Trans Jpn Res, 27(1): 84–97, 1998     

Flow and surface heat transfer analysis of a square cylinder in turbulent cross-flow

Flow passing a heated square cylinder is investigated using a hybrid LES-RANS approach on unstructured grids at a moderate Reynolds number of 22, 050. The effects of inflow turbulence on the flow field as well as surface convective heat transfer are studied by adopting a grid-based random-number method (GRM). Validation of the GRM method is carried out by generating and simulating a decaying homogeneous isotropic turbulent flow. Analysis of the turbulent quantities and comparisons with the Synthetic Coherent Eddy Method (SCEM) suggest that the cheaper GRM is able to generate good quality inflow turbulence, despite a longer transition region is required. Studies of the heated square cylinder in crossflow show that the inflow turbulence results in an early breakup of the shear layer, which leads to further effects on the vortex shedding and surface heat transfer. The surface convective heat transfer is increased with inflow turbulence, especially on the front, top and bottom surfaces, while limited influence is found on the rear surface. It is shown by the spectra that the effects of the inflow turbulence mainly focus on the turbulent shear layers, as well as heat transfer of the corresponding surfaces.     

An Experimental Study of the Drag Force on a Cylinder Exposed to an Argon Thermal Plasma Cross Flow

Experimental data are presented concerning the drag force on a cylinder exposed to an argon plasma cross flow with temperatures about 10~4 K and velocities about 10~2m/s. Using a method of sweeping a cylindrical probe across an argon plasma jet, the total drag force on the cylinder can be measured as a function of the lateral distance of cylindrical probe with respect to the plasma-jet axis. Through the Abel inversion, the drag force for per unit of cylinder length and thus the drag coefficient of cylinder have been measured under plasma conditions and compared with the values obtained from the standard drag curve of the cylinder in an isothermal flow. Experimental results show that the measured drag forces are always less than their counterparts read from the standard drag curve with the same Reynolds numbers based on the oncoming plasma properties. The drag force on the cylinder exoposed to a thermal plasma flow is shown to be approximately proportional to the square root of cylinder diameter in the present experiment and it increases slightly with increasing surface temperature of the cylinder. It is also shown that applying a voltage between the drag probe and the anode of the plasma jet generator has little effect on the drag force of cylinder under the experimental conditions. The drag force on a cylinder with finite length exposed to an argon plasma with the axis parallel to the plasma jet is independent of ratio of cylinder length to its dismeter L/d for the cases when L/d≤1.