基于时域激光诱导辐射确定湍流火焰烟黑粒径

对非稳态丙烷湍流火焰中的烟黑粒径进行在线测量.高能脉冲激光直接照射火焰中的烟黑微粒,其受热辐射光谱信号由光电倍增管检测.为降低信号噪音,采用分段平均方法拟合获得烟黑等亚微米量级固体颗粒粒径.所得结果与透射电镜测量结果吻合.通过统计多个激光脉冲下颗粒各粒径值,获得了不同空燃比下丙烷湍流火焰中烟黑粒径分布的变化规律.本方法为非稳态燃烧过程中可吸入颗粒物尺寸分布提供了一种有效的在线监测手段．     

方形气固流化床从鼓泡到湍动流型转变的实验研究

在368mm×368 mm方形气固床中对FCC颗粒进行流态化实验,研究了从鼓泡到湍动的流型转变行为及相关问题.实验测试了多个床层截面的压差波动,由波动标准差随表观气速变化的最大值获得了流型转变速度Uc.结果表明,方形床中的流态化平均行为在床层截面纵横两个方向一致,沿床层轴向呈现不均匀性,底部存在颗粒密相区;不同床层截面转变速度Uc不同,由床层上部向下扩展,Uc呈线性增加关系,说明流型转变至上而下进行;静床高度对Uc影响显著,静床高度增加,床层轴向各截面Uc相应增加;以上结果表明了目前仅与气固物性参数相关联的Uc预测模型有待改进.     

壁湍流周期性吹吸减阻的实验研究

为了研究局部周期性吹吸小扰动对湍流边界层的影响和湍流相干结构减阻控制机理,用实验方法研究了局部周期性吹吸小扰动与湍流边界层的相互作用.采用热线测速技术精细测量了施加吹扰动前后平板湍流边界层不同法向位置的流向、法向和展向速度分量的时间序列信号;通过测量扰动源下游近壁区域平均速度剖面的变化,发现在扰动下游一定空间范围内,粘性底层增厚和壁面减阻;通过对湍流度变化的分析,发现扰动影响猝发的发生;此外通过对展向平均涡量变化的分析发现,在扰动下游形成了2个定常的、强度相似的、大尺度的展向涡,它们起到流体滚动轴承的作用,减小了壁面摩擦阻力.     

交叉缩放椭圆强化换热管内流动和传热性能的数值研究

以交叉缩放椭圆强化换热管为几何模型,对管内的传热和流动进行数值模拟,计算结果表明,过渡段流场不同于直管段,纵向涡流较大,湍流强度和湍动能都较大,过渡段强化传热效果明显好于直管段,但阻力也增加了;并且发现随着椭圆长轴与短轴之比的增加,换热增强,为交叉缩放椭圆强化换热管在换热领域的实际应用提供了理论依据.     

Experimental research on drag reduction by polymer additives in a turbulent pipe flow

In order to investigate the effects of injection position on drag reduction as well as further the effects of polymer additives on turbulent structures, LDA measurements of turbulent pipe flows were conducted. The results show that the amount of drag reduction grows with the increase of the Reynolds number, and injecting the polymer at the centre of pipe is more effective than at the wall. Due to the addition of polymer solution, the axial, radial r.m.s. velocity fluctuations and Reynolds stress decrease over the entire pipe cross‐section, the time auto‐correlation coefficients of axial and radial velocity fluctuation at the centre of pipe decay more slowly, the number of spectrum peaks is decreased, and the peak shifts towards lower wave numbers. The results also reveal that, due to the addition of polymer solution, the large‐scale vortices are enhanced and small‐scale vortices are suppressed.     

矩形断面明渠中恒定均匀流的机械能损失

明渠流中机械能损失的确定是水力学乃至工程流体力学中的重要内容.以构建的明渠恒定流新的总流能量方程及矩形断面明渠三维层流运动的理论解为基础,通过理论分析与三维数值计算,就矩形断面明渠中恒定均匀流的机械能损失系数随雷诺数及宽深比的变化进行了深化研究,得到如下成果:1)在层流条件下,机械能损失系数一般均与雷诺数和宽深比有关,只有对宽深比大于6.5的宽浅明渠,其相应流动的机械能损失系数才仅与雷诺数有关,且在雷诺数相同的条件下,一般矩形断面明渠流的机械能损失系数要大于宽浅明渠流的机械能损失系数;2)在紊流条件下,机械能损失系数也一般均随雷诺数与宽深比而变,但对给定的宽深比,存在相应的特征雷诺数,当流动的实际雷诺数大于该特征雷诺数后,机械能损失系数则不再随宽深比而变化,且在宽深比一定的条件下,雷诺数越小,其机械能损失系数的修正值越大.     

展向行波电磁力控制下的槽道湍流雷诺应力

为研究槽道湍流的电磁力减阻控制问题,采用直接数值模拟(DNS)方法对槽道湍流的展向行波电磁力控制问题进行数值研究.讨论了由展向行波诱导的诱导流场对槽道湍流流场的调制过程,分析了控制前后雷诺应力离散点分布、平均雷诺应力分布以及雷诺应力波谱分布的规律.研究表明,在合适参数条件控制下,展向行波电磁力的控制不仅使近壁条带结构基本消失,形成加宽的低速条带,而且能使控制后的流场中涡结构几乎消失,而仅剩下由展向行波电磁力诱导产生的带状负涡结构,同时展向行波电磁力在一定程度上抑制了湍流的脉动,减小了雷诺应力,最终使壁面阻力下降.     

Drag reduction of aqueous suspensions of fine solid matter in pipe flows

It is well known that drag reduction properties of polymer solutions in pipe flows are classified into two categories, based on the relationship between the friction factor and the Reynolds number. The degree of drag reduction either increases with an increase in the Reynolds number or is independent of the Reynolds number. In the latter case, the drag reduction behavior is referred to as type B behavior. In this study, the pressure losses of aqueous suspensions of biofibers and graphene oxide flat particles in pipe flows were measured to clarify the effect of the shape of suspended fine solid matter on the friction factor. The experimental results demonstrated that the behavior of the friction factor corresponded with the type B behavior of drag reduction. The mean velocity profile in the range of the turbulent flow was also estimated from the friction factor data.     

On the gas dynamics of HVOF thermal sprays

An experimental study of the gas-dynamic aspects of the high-velocity oxyfuel (HVOF) thermal spray process has been performed using commercially available HVOF equipment (Hobart-Tafa JP-5000, Ho-bart-Tafa Technologies, Inc., Concord, NH). Optical diagnostic techniques, including microsecond-expo-sure schlieren and shadowgraph imaging, were applied to visualize the hot supersonic jet produced by this equipment without particle injection. Rapid turbulent mixing of the jet with the surrounding atmos-phere was observed, which is an issue of concern in coating quality due to the possibility of oxidation of sprayed particles. This mixing appears to be a function of the ratio of densities of the hot jet and the cold atmosphere as well as a function of the velocity of the jet, rather than one of combustion-chamber pres-sure or barrel length. The supersonic core of the HVOF jet dissipates rapidly due to the mixing, so that the jet is no longer supersonic when it impinges on the target surface being sprayed. Secondary issues also observed in this study include strong jet-noise radiation from the HVOF plume and the entrainment and induced bulk motion of the surrounding air.     

异形坯连铸结晶器内三维流场的数值模拟

根据湍流理论及异形坯连铸的特点, 建立了异形坯结晶器三维流场数学模型, 对不同条件下的流场进行了数值模拟, 分析了水口结构和工艺参数变化时异形坯结晶器内涡心深度的变化规律及液面湍动能的分布状态. 结果表明 采用上倾式水口, 水口夹角为120°, 拉速为0.9m/s时, 结晶器内流场分布较为合理, 液面较稳定. 模拟结果与水模实验结果吻合较好. 该模型可对连铸过程进行离线分析, 确定最佳参数, 并可作为在线控制模型的基础.