Calculation and verification of formula for the range of sprinklers based on jet breakup length

Jet breakup length is an important parameter which reflects the length of sprinkler range.Based on the linear instability theory,the dispersion equation of cylindrical jet was established and the theoretical value of jet breakup length was calculated.The jet breakup length and initial amplitude of surface wave were measured by applying the high-speed photography technology.Meanwhile,the numerical simulation was conducted by combining Level Set-VOF method for describing the jet breakup length to verify the theoretical and experimental results.Within the jet velocity and working pressure range of discussion,the results of comparison showed that the theoretical analysis gave a reasonable explanation to the influence of jet velocity,nozzle diameter and nozzle cone angle on jet breakup length.Comparing the theoretical value of jet breakup length with the experimental and simulated values,the three results accorded one another.The experimental jet breakup lengths were the lowest and the simulation values were the largest,and the relative error was less than 10%,especially the theoretical value was closer to the average value.For choosing the theoretical calculation of jet breakup length,a semi-empirical and semi-theoretical formula of range for the rotating sprinkler was concluded by the curve fitting method and the fitting formula was verified.The results showed the high accuracy of the ranges determined by this formula and the average relative error was less than 2.5%.The new formula was in good agreement with the data of different types of sprinklers comparing with other empirical formulas,and the error was only 5%.Meanwhile,the possibility of using this formula widely to determine the ranges of same series of sprinkler was confirmed.

Study on the breakup length of circular impinging jet

Circular impinging jet, which is widely used in accelerated control cooling （ACC） equipment to accelerate the cooling of hot rolled plates, is subject to breakup, and may result in undesirable cooling effect. Therefore, the jet breakup should be avoided as possible in industrial production. The objective of this study is to find the relation of the processing parameters of the ACC equipment versus the breakup length of jet with weaker turbulence. To obtain quantitative findings, not only relative experimental study but also numerical simulation was carded out. For a weaker turbulent water jet, the breakup length increases with the increase of jet diameter, as well as with the jet velocity; jet diameter has a significant effect on the breakup length for a certain flow rate when compared with jet velocity; finally a suggested correlation of the jet breakup length versus jet Weber number is presented in this study.

毫米级低速圆管自由射流的空中形态分析

自由射流是各类机械设备和管路设施中的常见现象,对毫米级圆管低速射流空中形态的研究,可以为射流距离、水流入水姿态和相应消能及噪声的分析提供依据。为探索毫米级圆管低速射流空中形态,分别以4,5,6 mm圆管射流为对象,研究不同低速、圆管直径对射流连续及破碎的影响。采用高速摄像机捕捉射流碎破点和破碎水滴变化,测量并分析流速、连续长度和破碎水滴直径的特征以及与无量纲雷诺数Re和韦伯数We的关系。结果表明:在管径相同时,射流连续长度随出口流速的增大而增长;随着管径变大,连续长度增加的幅度变小;在流速相同时,连续长度随着喷嘴内径增大而减小。射流连续长度随雷诺数Re、韦伯数We的增大而减小,在We=3 000~3 500附近由曲线关系变为直线关系,拟合出射流连续长度与We的关系式。具体描述破碎水滴的聚合过程,分析破碎后的主水滴与卫星水滴的大小、聚合方式及振动特点,其中,水滴大小与喷嘴内径存在线性关系,均随着喷嘴内径的增加而增大,卫星水滴的大小基本符合Rayleigh线性不稳定理论预测产生液滴尺寸,不同的聚合方式会导致射流掺气量的不同,为不同条件下的水流入水姿态及相应消能与降噪提供参考,并得到水滴平均振动周期为25.67 ms。

Dominant Factors of Metal Jet Breakup in Micro Droplet Deposition Manufacturing Technique

For the solutions of random variations of metal jet breakup and difficulties in controlling and predicting the process parameters （e.g. jet length） in micro droplet deposition manufacturing technique, experimental methods combining with theoretical analyses have been developed. The jet formation, jet length and their dominant factors （oxygen concentration and disturbance frequency, etc.） are discussed. The statistical law of jet length is found that the probability density function （PDF） of jet length is a log-normal distribution. The results show that the formation and size accuracy of metal jet breakup are improved by adjusting the gas pressure and optimizing the disturbance frequency. Under this circumstance, the jet length and morphological deviation can be minimized, which provides a stable droplet stream for the subsequent manufacturing process.

撞击式射流破碎特性的实验研究

基于自行搭建的撞击式射流系统,采用高速摄影技术,研究了对称射流撞击和非对称撞击射流液膜的破碎特性.分析研究了射流撞击夹角及喷嘴内径对射流撞击破碎的破碎模式和破碎特征(破碎长度、液膜长度、液膜宽度、液膜长宽比)的影响规律.对比研究了3种不同流体——水、甘油、卡波姆凝胶(非牛顿流体)的破碎特性.在实验的韦伯数范围内,喷射模式可分为3种破碎模式,即封闭液膜模式、开边界模式、液线液滴模式;破碎长度随韦伯数的增大,呈先增大后减小的趋势;减小撞击夹角可以减小液膜的长宽比,喷嘴内径的大小不会改变液膜长宽比的值;与对称撞击相比,非对称撞击更能加剧液膜的破碎.

荷电液体射流破碎长度理论与实验

基于表面波线性不稳定性理论建立了荷电液体射流色散方程并进行数值计算,应用高速摄影技术测量荷电液体射流破碎长度,对理论分析结果进行实验验证。在讨论的射流速度、荷电电压范围内,随着射流速度或荷电电压的增加,0阶、1阶表面波最大增长率不断提高,但0阶表面波最大增长率始终大于1阶表面波,主导射流破碎过程;提高射流速度,射流破碎长度先增加至一个极值之后逐渐减小,荷电电压总是促进射流破碎,减小射流破碎长度;提高液体电导率,射流破碎长度增大。实验结果表明,线性不稳定性理论能很好地解释射流速度、荷电电压和液体电导率对射流破碎长度的影响,但在预测射流破碎长度极值对应的临界射流速度时存在一定的偏差。

幂律流体撞击式射流破碎特征试验

基于自行搭建的撞击式射流系统,采用高速摄影技术研究了非对称条件下幂律流体黏度对撞击式射流破碎特征的影响规律.结果表明:随着幂律流体黏度的增加,射流破碎模式种类增加;除弓形液线模式外,其余模式开始出现的速度与流体黏度成正比;在射流速度v较低时,破碎长度Lbu与流体黏度成反比;两股不同黏度流体撞击时,Lbu随v呈'双峰模式'且在较高v时与流体黏度成正比;相同黏度撞击时,则呈'单峰模式'且在较高v时Lbu最大;喷雾锥角α随v增加而增大,最后趋于的稳定值与流体黏度成反比;v较高时,α与流体黏度成反比.

射流式O2（^1△）发生器射流破断长度测量

实验研究了甘油 -水溶液通过用不同加工方法和不同喷管长度 /直径比的喷管形成射流的破断长度。射流的行为与喷管内是层流还是湍流的流动状态关系极大。当射流速度比较大时 ,喷管的缺陷如进口出口的毛刺和流道壁的粗糙度是诱发湍流使射流破断的主要原因。这些结果对化学氧碘激光的射流式 O2 ( 1Δ)发生器的喷管设计和制造提供了有价值的参考。

低压射流不同喷嘴参数及压力下破碎过程实验

研究了不同低压下喷头喷嘴直径和喷嘴锥角对射流破碎的影响。采用高速摄像仪对低压圆柱射流的射流核心长度和射流破碎长度进行实验,测量了不同喷嘴结构的流量、射程和末端水滴直径。结果表明:同一压力下,当喷嘴锥角不变时,随着喷嘴直径的增大,喷头流量、射程和喷头末端水滴直径都变大,射流核心长度和破碎长度均增大;当喷嘴直径不变时,随着喷嘴锥角的增大,喷头流量逐渐减小,而喷头射程呈先增大后减小趋势,喷头末端水滴直径也变大,射流核心长度逐渐减小,射流破碎长度先减小后增大。综合考虑射程和雾化效果,直径为5 mm、锥角为45°的喷嘴为最优选择。同时通过对不同Re数和We数的实验和分析,给出了适合低压喷嘴的两种射流特征长度的拟合关联式。

异形喷嘴自由射流破碎长度及射程的实验研究

依据面积相同原则,设计了2种形状的异形喷嘴,测量了异形喷嘴的流量、射程和末端水滴直径,并与圆形喷嘴对比分析,同时在实验基础上分析了喷嘴出口形状、压力及锥角的改变对射流破碎长度的影响。结果表明,工作压力相同时,射程和末端水滴直径均以圆形喷嘴最大,三角形喷嘴最小;同一压力下,三角形喷嘴的射流破碎段最短,而四边形喷嘴的射流破碎段最长;随着喷嘴锥角的增大,异形喷嘴在锥角为55°时射流射流破碎长度段最短,而圆形喷嘴在锥角为45°时破碎段最短,说明圆形喷嘴选择锥角45°时射程和雾化效果均为最优。

喷射压力对喷雾的影响

采用闪光摄影技术,在射流试验台上进行了较低喷射压力范围内瑞利模式液膜射流碎裂过程试验,在喷雾特性试验台上进行了较高喷射压力范围内柴油雾化质量的试验,对2种试验结果进行了比较分析。结果表明:由小宽厚比喷嘴喷射出的瑞利模式低速液膜射流碎裂长度随喷射流速的增大几乎呈直线增加,与喷射压力没有直接关系,喷射压力只有通过喷射流速才对射流碎裂长度产生影响,喷射流速是射流碎裂过程中的稳定因素;雾化模式的喷雾质量随喷射压力的增大显著改善,喷射压力和喷射流速转变成为液体碎裂的不稳定因素。

Influences of nozzle parameters and low-pressure on jet breakup and droplet characteristics

Experiments were carried out to investigate the influences of nozzle geometric parameters and injection pressure on jet breakup characteristics using a high-speed photography(HSP)technique.The flow rates and spraying ranges of sprinkler with different nozzles were measured.In this research,HSP technique was also used to photograph the drops emitted by sprinkler with different nozzles at different pressures,photographs were taken at different horizontal distances from the sprinkler and the equivalent circle diameter was used to represent the particle sizes.Based on HSP technology,the effects of flow velocity and nozzle geometric parameters on jet breakup length were studied,and the droplet diameters with different nozzle types were obtained.The result showed that for the sprinkler with different nozzles,the breakup length decreased with the increases of pressures.At the nearby(3-9 m)region and distant(12-18 m)region of sprinkler,the droplet diameters of sprinkler with type B nozzle were the largest,which meant the sprinkler with type B nozzle was the optimal choice by synthesizing the droplet diameter distribution.The fitting relationship of jet breakup length with Reynolds number and Weber number(Re and We),and the regression equation of the end droplet diameters were deduced with errors of less than 5%and 4%respectively.

微细电解射流喷射装置研制与工艺试验

为提高微细电解射流加工效率及加工质量,研制了专用喷射装置,并利用该装置进行了初步工艺试验。结果证明该装置保证了电解液在其腔体中能够得到充分"负极化",且能获得满足加工要求的稳定破碎长度,满足了电解射流加工试验要求。

液体圆柱射流破碎过程的实验

采用高速摄像仪对圆柱射流的表面剥离现象和射流破碎长度进行了实验,在低速射流下,分析了Rayleigh模式下主液滴和卫星液滴形成过程及运动规律.随着射流速度的增大,射流表面开始出现了表面剥离小液滴.并发现存在一个临界雷诺数,当超过临界雷诺数时,射流表面出现表面剥离现象.通过对射流中表面剥离现象的实验和分析,给出了两种射流破碎长度随雷诺数变化规律和破碎长度拟合关系式.

压力旋流喷嘴射流稳定性的实验研究

压力旋流喷嘴射流稳定性对射流的破碎雾化具有重要作用。搭建了压力旋流喷嘴射流稳定性实验平台,在此基础上进行了压力旋流喷嘴射流稳定性实验研究。通过激光衍射方法获取射流振荡频率信息,并以射流振荡频率表征射流的稳定性;利用高速摄影方法采集射流图像并分析射流破碎长度、锥角等形态参数的变化规律。研究结果表明,射流形态与射流流量、射流液体物性以及射流喷嘴内径密切相关;射流振荡频率分别与射流流量和射流液体黏度成近似线性关系;喷嘴出口内径与射流振荡频率之间的关系则比较复杂,相同流量时出口内径较小喷嘴的射流振荡频率大于出口内径较大喷嘴,流量超过一定值后,不同出口内径喷嘴的射流振荡频率趋于一致。

数值分析喷水孔结构对水针断裂长度的影响

根据水刺加工工艺要求设计了圆锥圆柱型、流线型、圆柱圆锥型3种喷水孔结构。采用大型流体计算软件(CFD)模拟水流流过喷水孔内部的流体流动通道,在大气中形成动力水针过程,并依据数值模拟结果分析喷水孔结构对水针断裂长度的影响。经分析认为圆锥圆柱型喷水孔结构使水针表面易于发散形成水雾,水针断裂长度短;流线型喷水孔结构降低了流体流动通道对水流流动的扰动程度,水针断裂长度有所提高;而圆柱圆锥型喷水孔结构最利于水针保持柱状结构状态,水针断裂长度最长。

摇臂式喷头圆射流初级碎裂数值模拟与实验

为探明摇臂式喷头射流碎裂机理,基于VOF多相流模型理论,采用计算流体动力学（CFD）分析软件Fluent,在200~600 k Pa的中低压条件下,使用几何重建（Geometry reconstruction）方法进行界面跟踪,用瞬态PISO方案求解控制方程,对摇臂式喷头圆射流初级碎裂进行数值模拟,获得了初级碎裂液滴直径和射流碎裂长度。采用高速摄影技术进行实验测量,分析了初级碎裂液滴直径和射流碎裂长度模拟值和实测值的相对误差,讨论了初级碎裂液滴直径和射流碎裂长度随喷嘴直径和工作压力的变化情况。结果表明,摇臂式喷头圆射流初级碎裂包括连续段、过渡段和碎裂段3个典型形态,喷嘴直径和入口压力是影响射流碎裂长度和射流初级碎裂液滴直径的主要因素,射流初级碎裂液滴直径D与喷嘴直径d（d〉5 mm）有较好的相关性（D=1.634d,R2=0.912）,初级碎裂液滴直径的模拟值与实测值相对误差为23.92%,拟合精度良好。给出了射流碎裂长度L与韦伯数We的拟合模型,该模型能较好预测摇臂式喷头在低压条件下射流碎裂长度。

电磁激励谐振对射流碎裂过程的促进作用

根据量纲一色散关系式数值计算得到的射流碎裂点最不稳定波数,在射流试验台上对平面液膜射流进行电磁激励谐振,采用闪光摄影技术对有无电磁激励谐振的液膜射流碎裂过程进行比较试验研究。结果表明:电磁激励谐振对于射流的碎裂过程影响很大,按照量纲一色散关系式计算所得的表面波最不稳定波数设置的电磁激励谐振能够大幅度缩短射流的碎裂长度,静止空气环境中射流的碎裂长度缩短了17%～60%,空气助力环境中射流的碎裂长度缩短了13%～52%,明显改善了一级雾化的效果。

不同环境压力下幂律流体撞击射流破碎特性

基于自行搭建的射流系统和定容弹系统,采用高速摄影技术,获得了高环境压力下幂律流体对称撞击式射流的喷雾形貌,提取了射流特征参数破碎长度L与表面波长λ,研究了环境参数(环境压力)、射流参数(韦伯数We)、结构参数(喷孔直径)与物性参数(流体黏度)对射流破碎的影响.结果表明:幂律流体对称撞击式射流共有封闭边界模式、开边界模式、无边界模式、弓形液线模式和完全发展模式共5种破碎模式;在大气环境压力下,L随着We的增加呈"双峰"模式变化;而在有环境压力的情况下,L与We则呈"单峰"模式变化;与大气环境压力相比,高环境压力更有利于幂律流体对称撞击式射流破碎;而喷孔直径与流体黏度的增大均不利于撞击式射流破碎.提出了预测幂律流体对称撞击射流破碎表面波长的破碎模型.

非淹没性低速圆射流特性研究

介绍了射流的基本概念、射流的分类和圆射流断裂理论。结合韦伯和列维奇半经验计算公式,在不同射流速度下,对两种孔径的孔口(d0=2mm和3.4mm)进行了非淹没性圆射流特性研究,并做了射流断裂长度的测定。通过实验值和半经验计算值的相互比较,结果显示,在较慢的射流速度下,用韦伯给出的半经验公式计算出的射流断裂长度值和实验值较接近。而在速度稍大一些时,用列维奇给出的公式计算出的射流长度值和实测值吻合得较好。