不对称撞击流的实验研究与数值模拟

采用恒温式热线风速仪(CTA)测量不对称撞击流流场.用CFD软件对流场进行数值模拟,得到了和实验相吻合的结果.结果表明,不对称撞击流轴线上轴向速度除撞击面附近外,基本符合圆射流衰减规律.当喷嘴间距一定时,轴线撞击面驻点偏移量随气速比和喷嘴直径的增大而增大.当气速比一定时,轴线撞击面驻点偏移量随喷嘴间距增大而增大.     

浸没对置撞击流的液相混合行为研究

利用无干扰流场测试手段——平面激光诱导荧光技术(PLIF),对浸没对置撞击流混合反应器在等动量撞击下的浓度场进行分析,并分析比较对称撞击和不对称撞击下的液相混合行为。利用混合均匀度U和均匀混合时间τ95定量描述测量平面的示踪剂浓度分布,得出其在不同内径比、不同喷嘴间距下的变化规律,并进一步分析对称撞击时,两喷嘴出口流体的不同动量比对混合的影响。研究发现:在小喷嘴间距L10D(L为喷嘴间距;D为两对置喷嘴内径和的平均值)下,等动量撞击时,对称撞击的混合效果要优于不对称撞击;对称撞击和不对称撞击的最优混合间距均为L=3D。对称撞击时,两喷嘴出口流体的动量比越接近于1,越有利于液相混合。     

两喷嘴对置撞击流驻点偏移规律

运用烟线法和热线风速仪对两喷嘴撞击流流场进行了实验研究,考察了不同喷嘴间距、不同气速比下撞击面的稳定性和撞击面驻点的偏移规律。结果发现,当2D≤L≤4D（L为喷嘴间距,D为喷嘴直径）时,若两喷嘴气速相等,撞击面不稳定,在轴线上发生振荡;当两喷嘴气速有小的差异时,撞击面驻点将发生大幅度的偏移,此时流场变得相对比较稳定。当2D≤L≤8D时,撞击面驻点位置对气速比变化很敏感;当L<2D和L>8D时,撞击面驻点位置对两喷嘴气速比的变化逐渐变得不敏感。     

两喷嘴对置撞击流径向射流流动特征

对喷嘴间距与喷嘴直径比为0.5～100范围内两喷嘴对置撞击流径向射流的湍流脉动特征、速度分布和扩展率等进行了实验研究和数值模拟。研究结果表明,撞击流径向射流明显较自由射流湍动强烈;从驻点开始径向射流速度逐渐增大到最大值后开始衰减,射流呈现自相似性;随着喷嘴间距增大,撞击流径向射流的扩展率呈现增大的趋势,大约为自由圆射流的1.5～3倍。采用CFD软件对撞击流径向射流的速度分布特征进行了数值模拟,与实验结果相比,两方程湍流模型预报的撞击流径向射流的扩展率明显偏小,雷诺应力模型的预报精度有较大改进。     

小间距两喷嘴对置撞击流流场的数值模拟与实验研究

运用热线风速仪和CFD软件对小喷嘴间距下两喷嘴对置撞击流时均流场进行了实验研究和数值模拟,并和文献中的实验结果和近似解析式进行了比较。研究结果表明：由于边界层存在,单股喷嘴出口速度分布为“礼帽”形状分布;在L<2D(L为喷嘴间距,D为喷嘴直径)时,喷嘴出口速度剖面出现中间低、两边高的“双峰”形状, L=2D时,“双峰”形状消失。随着喷嘴间距的增大,相同气速比导致的撞击面驻点的偏移量增大。相同气速比下,喷嘴出口为“礼帽”分布时驻点的偏移量比均匀分布时大。文献中的撞击流流场的近似解析式对喷嘴出口速度分布为均匀分布有很好的精度,当喷嘴出口速度为“礼帽”分布时,文献中近似解析式的预报精度变差。     

双组分层不对称撞击流流场能量的研究

采用二维高速粒子图像测试技术(TR-PIV)和平面激光诱导荧光技术(PLIF)测量了双组分层不对称撞击流流场,利用POD分析方法提取流场含能大尺度结构,分解瞬时流场得出主要模态能量,考察了双层撞击流在不同射流Re、不同间距及不同直径比下不对称流场能量的变化规律。主要结论如下:同喷嘴直径下,不对称流场能量总是大于对称流场,在上下撞击驻点形成交叉时,流场整体能量较高,同时流场能量随喷嘴间距的增大而减小;不同喷嘴直径下,流场能量随直径比K增大而增大,直径比K1时,不对称流场能量也总是大于对称流场。综上所述,流场的这种不对称性有利于整体提高流场能量及流动特征。     

水平对置撞击流的POD分析及混合特性

利用平面激光诱导荧光技术和二维高速粒子图像测速技术对浸没对置撞击流的湍流场进行了测量,利用本征正交分解方法对所测两喷嘴出口流体不同动量比(M)和不同喷嘴间距(L/d)下的流场进行分解,提取流场中含能大尺度结构. 结果表明,撞击流瞬时流场的能量基本集中在1阶模态,M=1工况下,1阶模态的能量占总湍动能的比例最高,约为50%,2阶模态约占总湍动能的2%~5%,3阶模态约占2%,与4阶模态差异甚微;在L/d=3时前4阶模态能量占比最大,约为38.6%;低阶本征模态中存在明显的相干结构,主要位于非稳定径向射流附近区域,且大尺度的流动结构与撞击流的液相混合行为直接相关,流场中相干结构尺度越大,能量越大,越有利于快速均匀混合.     

撞击流反应器撞击面稳定性研究进展

撞击流技术因其良好的混合特性近些年用于强化制备超细粉体反应中的混合过程。撞击面的稳定影响反应器内的混合效果,所以本文对撞击面稳定性的研究进行了综述。撞击流反应器不同结构形式包括平面撞击流、轴对称撞击流和微型撞击流等。文中简述了撞击流稳定性的实验研究手段,分析轴对称撞击流反应器的径向偏转振荡的起止条件和不同喷嘴间距下的轴向偏移振荡规律,并且分析平面撞击流反应器的撞击面偏转周期以及偏转振荡的起止条件。得出轴对称撞击流与平面撞击流撞击面驻点的振荡对混合都有促进作用,并且偏移振荡周期不定,轴对称撞击面偏移振幅与喷嘴间距和雷诺数相关。平面撞击流的偏转振荡周期与进口流速成反比,反应器结构参数是撞击流稳定性的影响因素之一。根据轴对称撞击流偏移振荡对混合的促进作用,本文提出一种新型的预设流量波形双组撞击流反应器。新型撞击流反应器的独特结构克服了物料反应通道单一缺点,通过预设波形控制其进口流量,增大其撞击面偏移振幅,消除撞击面无序振荡,使流动轨迹扩展,扩大混合区域,并设计实验装置与方法讨论动态流量撞击流反应器撞击面稳定性对混合效果的影响。最后,本文对轴对称撞击流反应器的混合性能研究前景进行展望。     

同轴对称撞击流中的颗粒运动行为孙志刚

采用高速相机对同轴对称撞击流中颗粒运动行为进行了实验研究,采用图像处理软件对获得的图片进行分析处理,得到了颗粒在同轴对称撞击流中的运动轨迹、运动速度、最大渗入距离和旋转速度等参数.考察了喷嘴出口气速、颗粒大小、颗粒出口速度以及喷嘴间距对颗粒运动行为的影响.结果表明:颗粒在同轴对称撞击流中的运动有直接射出模式和振荡模式,其中振荡模式占70%;出口气速的增加使颗粒轴线速度和旋转速度变大;随着颗粒相对出口速度、颗粒Stokes数和喷嘴间距的增加,颗粒的最大渗入距离变大.     

平面气固撞击流周期振荡的模拟分析

由于喷嘴截面的高宽比远小于1,平面狭缝喷嘴撞击流可看作二维撞击流。采用欧拉双流体模型对二维气固撞击流进行数值模拟。模拟结果表明,当固相负载率m≤8.2,颗粒粒径为60 μm≤d_p≤175 μm时,大间距的平面气固撞击流也出现了周期振荡。通过分析射流轴线上不同位置的压力和速度的瞬时值和平均值,认为周期振荡是由于撞击面上压力释放和持续射流的共同作用导致。讨论了不同条件对振荡周期的影响:振荡周期随喷嘴间距或颗粒粒径的增加而增加;而随射流Reynolds数的增加或者固相负载率的增加而减小。     

喷口间距对双矩形平行射流流场的影响

用激光粒子测试仪（PIV）测量了双矩形喷口平行射流的流场特性,研究了不同喷口速度、不同喷口间距下双射流的混合特性。结果表明,喷口速度增大,双射流对称线上速度绝对值增大,但速度最大值出现的位置基本不变。喷口间距增大后,双射流的混合推迟,合并点后移,传递到对称线上的动量减弱,在合并点上的最大速度值减小。间距比与合并点的关系呈线性;但与大间距比相比,小间距比对合并点的影响更为敏感,关联式斜率更大。对湍流特性分析发现,双射流的主要动量传递发生在混合区,喷口间距增大,混合区与喷口的距离增加。     

Study on factors influencing stagnation point offset of turbulent opposed jets

Turbulent opposed jets were experimentally studied by the hot‐wire anemometer measurement, the smoke‐wire flow visualization, and the CFD simulation at L = 1?20D (where L is the nozzle separation and D is the nozzle diameter) and Re > 4500. The instability pattern of turbulent opposed jets was identified by investigating the smoke‐wire photos recorded by a high‐speed camera. The factors affecting stagnation point offset, such as the bulk velocity, the velocity profile, and the turbulence intensity at the nozzle exits were investigated. Results show that the stagnation point offset is the main instability regime of turbulent opposed jets. Uniform exit velocity profile and increasing exit turbulence intensity will decrease the stagnation point offset of turbulent opposed jets. © 2010 American Institute of Chemical Engineers AIChE J, 2010     

加齿被动控制自由湍射流多尺度涡结构的实验研究

1 INTRODUCTION Noncircular jets were identified as an efficient tech- nique of passive control that allows significant im- provements of performance on various practical sys- tems at a relatively low cost because noncircular jets rely solely on the change in the geometry of the noz- zle. In both aerospace industry and chemical industry noncircular jets are widely applied to promote frag- mentation of the large eddy structures in the flow, enhance mixing of various fluid components, speed u…     

Investigation of a two impinging-jets contacting device for liquid-liquid extraction processes

A two impinging-jets contacting device (TIJCD) for liquid-liquid extraction processes was proposed and tested through the standard test system recommended by the European Federation of Chemical Engineering (EFCE), toluene-acetone-water, as a typical example of liquid-liquid extraction processes. The results obtained for the overall volumetric mass transfer coefficient (capacity coefficient) were more than three times higher than those reported in the conventional contactors and in an air-driven two impinging-streams contactor. These experimental results clearly indicate the greater performance capability of TIJCD relative to those of conventional contactors. To have a perfect study on the pertinent parameters of the TIJCD, the influence of jets momentum, nozzles diameter and the inter nozzle distance have been investigated. In addition, the enhancing effect of the impingement zone has been studied using a non-impinging-jets contacting device. Based on the experimental data obtained, an increase in the liquid flow rates results in an increase in the extraction efficiency and overall volumetric mass-transfer coefficient, while increasing the distance between nozzles decreases the extraction efficiency. In addition, it was found that the effect of nozzle diameter on the extraction efficiency could be dependent upon the jets velocity.     

Influence of electric field interference on double nozzles electrospinning

The low production rate of electrospinning process may limit the industrial use of single needle system. To meet high yield requirement and uniform fibers, a bottom‐up multiple jets electrospinning nozzle was designed, each nozzle can emit 6–18 jets. The influence of electric field interference on jet path, membrane shape, and fiber morphology were investigated. Experiment finds that electrical field strength in the closer part of two nozzles is weakened because of electric field interference when the distance between two nozzles is 30 mm, making the jet hard to emit in this section, and closer part of electrospun fiber webs has fewer fibers. The spinning in far side part of two nozzles is similar to that of single nozzle. While in middle part of one nozzle, the jet path is short, elongation of jets smaller, the formed fibers thicker, solvent evaporation less sufficient. When the distance of two nozzles is increased to 50 mm, influence of electric field interference is weaker, the electrospun fiber web and average diameter of fibers are almost the same as that of single nozzle electrospinning. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

Designing a Jet Mill Nozzle of Maximum Efficiency

A method of calculation for an accelerating jet mill nozzle profile which provides a maximum kinetic energy for solid particles at the nozzle outlet, is developed. The gas‐solid flow in the nozzle is described by a well‐known one‐dimensional model. The effect of both solid loading and particle size on the optimal nozzle profile, as well as on the main flow parameters is studied. Distribution of particle velocities in the computed optimal nozzle is compared to that in a conical convergent‐divergent nozzle of the same length. Computations demonstrated that optimized nozzles are profitable for fairly low solid loadings. The kinetic energy of solids at the optimized nozzle outlet is about 50% higher than that in a conical nozzle if the solid/gas mass flow ratio equals unity.     

陶瓷过滤器脉冲反吹用喷嘴优化设计

The experimental study was carried out to optimize the nozzle shape and dimension for the pulse cleaning of a ceramic filter candle. A bench scale unit of ceramic filter consisting of four commercial filter elements was used to measure the traces of the transient pressure around the nozzle and the overpressure in the filter cavity during the pulse-jet injection of pulse gas. Overpressure in the filter cavity is related to the pulse cleaning force. Nozzle design is concerned to increase the overpressure at the open end of filter element of pulse cleaning inlet, as well as to minimize the consumption of pulse gas. Convergent nozzle induces more secondary flow and generates higher pulse cleaning effect than straight nozzle. Nozzles of different convergent ratio （ratio of outlet to inlet diameter of nozzle） by changing the convergent angle and height were tested. The outlet diameter of convergent nozzle seriously influences the cleaning effect. The optimum convergent ratio increases with the increase of pulse gas pressure The nozzle position （distance of nozzle tip from the open end of filter inlet） is also important to decide the nozzle dimension. Nozzle of large outlet diameter accepts high pressure of pulse gas to provide large overpressure in the filter cavity of top position by applying long distance.