自由磨料射流轴心磨料速度模型及分析

为了研究自由磨料射流轴心磨料速度,以自由磨料射流轴心磨料为研究对象,利用自由磨料射流轴心流体速度方程及磨料运动方程,建立了自由磨料射流轴心磨料速度模型,并基于等分法和迭代算法的数值求解方法,求解了该速度模型,得出了自由磨料射流轴心磨料速度变化规律,最后利用PIV实验对该速度模型进行了验证。结果表明:(1)自由磨料射流轴心磨料速度随着磨料运动距离的增加先增大后减小;在初始段末端磨料速度未达到速度最大值,磨料速度最大值出现在基本段内。(2)自由磨料射流轴心磨料速度PIV实测值与理论值的变化趋势基本一致;实测值与理论值的平均百分比误差和标准偏差分别小于3%和2 m/s,吻合较好,验证了模型的正确性和准确性。     

磨料水射流对脆性材料的冲蚀研究

磨料水射流技术作为一种特种加工技术，具有无刀具接触、无热影响区和加工范围广等优势，在众多领域得到应用。为了探究磨料水射流对脆性材料的冲蚀效果，构建和设计了磨料水射流外流场冲蚀仿真模型与磨料水射流冲蚀实验。以30 mm×50 mm的喷嘴外流场域为计算域，建立磨料水射流冲蚀仿真模型，并分析射流冲蚀过程中压力分布、水与磨料的速度分布及它们在射流中心线上的衰减规律。通过对氧化铝陶瓷材料的冲蚀实验，分析工艺参数对冲蚀孔径的影响，并结合仿真结果对比分析了射流束宽度与冲蚀孔径的关系。结果表明：水的速度随着喷嘴距离的增大而减小且分布范围变宽，射流宽度呈线性增大，磨料速度随喷嘴距离的增大而减小且分布范围基本不变；射流中心线上水的速度与磨料速度呈三段式衰减，水的第1段速度衰减段长度比磨料的长，但水的第2段速度衰减段长度比磨料的短；射流束能量的有效利用部分逐渐减小，但在15~25 mm的靶距范围内其有效利用部分较稳定，为40%；冲蚀孔径随喷嘴距离增大呈线性增大。研究结果为磨料水射流切割、铣削及抛光加工的参数选择提供实验依据，同时为磨料水射流加工过程仿真提供参考。     

前混合式磨料水射流磨料粒子加速过程数值模拟

针对前混合式磨料射流磨料加速过程运动复杂、实验研究困难及有限元处理超大变形存在网格畸变等问题,基于光滑粒子(SPH)耦合有限元(FEM)方法模拟前混合式磨料射流喷嘴不同阶段磨料粒子加速特征及磨料射流破碎靶体全过程。其中水介质用SPH建模,磨料粒子、喷嘴、靶体等用FEM建模。揭示磨料粒子群在喷嘴中的运动轨迹及喷嘴结构对磨料粒子加速影响规律。研究表明,磨料粒子进入喷嘴收敛段之前已基本达到与水介质相同速度,进入收敛段后因与水介质存在速度差使其获得加速,但与水介质速度差逐渐增大;进入直线段后水介质与磨料粒子一直加速,且水介质速度在直线段末端趋于稳定,在离开喷嘴的短距离范围内水介质速度趋于稳定,磨料粒子在核心段水流作用下继续加速,最终趋于稳定;磨料粒子群在喷嘴收敛段相互碰撞运动剧烈,进入直线段后相对平缓;流量一定下磨料粒子速度随喷嘴收敛段延长而增加,但增加有限;随直线段延长而增加,增加显著。数值模拟结果与相关文献吻合较好。     

基于SPH-FEM耦合算法的后混合磨料水射流冲击破岩数值模拟研究

采用SPH-FEM(smoothed particle hydrodynamics with finite element method)模拟了后混合磨料水射流在喷嘴中的混合过程,并研究了射流速度、磨料浓度以及岩石围压等因素对后混合磨料水射流破岩效果的影响规律。研究结果表明:在柱塞推动下,水与磨料在喷嘴的混合段、收敛段与直线段分别获得加速,最终磨料的速度可增加至纯水速度的80%;岩石破碎深度随射流速度呈近似线性增加,而破碎宽度随射流速度变化不大;磨料射流较纯水射流的破岩损伤更加明显,岩石的损伤随磨料浓度呈先增大后减小的趋势;岩石的破碎深度随着围压的增加呈近似线性减小的趋势。数值模拟结果与破岩实验现象基本吻合,该研究结果可为磨料水射流破岩的应用提供一定的理论支撑。     

微磨料浆体射流形成过程的能量传输

为了研究微磨料浆体射流形成的能量传输与转换,用碳化硅磨料对40CrMnMo7钢进行冲击,建立数学模型描述冲击力与射流压力、喷嘴效率、射流流量、水力功率等参数的相互影响,通过测量射冲击力、射流流量来预测射流系统的单位水力功率的出力状况。结果表明,该数学模型可以预测射流冲击力与水力功率之比,由系统压力模型计算的冲击力比实验结果小20%;在射流的滞止区内,压力与冲击力不呈线性关系,在相同压力下,磨料粒子具有更大的动能,微磨料浆体射流冲击力比纯水射流的大20%,单位水力功率提供的射流冲击力随着射流压力的增大而减小;射流冲击力随着喷嘴直径、射流压力的增大而增大,在靶距较小的情况下,冲击力随着靶距的增大而增大,当靶距达到一定值时,冲击力随着靶距的增大而减小。     

淹没磨料射流涡旋特性大涡模拟及研究

基于涡运动理论,采用固-液两相大涡模拟方法模拟了淹没磨料射流的涡量场,分析了涡旋扩散机理与卷吸特性以及磨料随涡运动在射流中的混合分布规律。流场剪切层中涡旋呈对称分布,沿轴线方向涡量呈指数衰减,衰减到最小值两侧涡旋混合成紊流混合区,而等速核内涡量几乎为零。相比于纯水射流,磨料的存在使得射流涡运动减弱,涡旋扩散角减小约50%,等速核增长约30%,减少了能量耗散。磨料在射流束内部时,受涡旋影响,趋向分布于高应变率、低涡量区,在涡旋下游侧磨料浓度最高。同时模拟研究了磨料参数对涡量场的影响规律,结果表明:磨料参数基本不影响扩散角(14.1°~15.1°)、等速核长度,卷吸能力随磨料浓度、粒径、密度的升高均呈现小幅度地降低,磨料密度对其影响程度最大,磨料粒径的影响最不明显。     

K9光学元件的锥形约束射流抛光流场与加工实验研究

提出了锥形约束射流抛光的加工方法,将原本垂直的细小射流约束成几乎平行于工件表面的环形射流。使用Fluent软件对抛光流场进行数值分析,根据Preston方程建立了材料去除函数模型;并以K9玻璃为加工对象,利用自主设计的锥形约束射流抛光平台进行加工实验。结果表明,锥形约束射流抛光方法法向最大速度比切向最大速度小63.9%,从而减少射流在垂直于工件表面方向的冲击损伤;锥形约束射流抛光后,K9玻璃表面粗糙度在加工区域内呈“V”型分布,其算数平均粗糙度(Ra)值从95.40 nm降至14.52 nm,表面质量得到明显提高。锥形约束射流抛光主要依靠射流沿工件表面的剪切力,不仅有效减小了射流法向冲击力,对射流的约束还减小了射流束的发散;确定的表面去除函数表明该方法有望实现确定性抛光;另外,环形的射流出口提高了射流抛光的效率。     

结构参数对液态甲烷-丙烷喷射混合器混合特性的影响

为了优化液化天然气气质,探究结构参数对喷射混合器内部流场的影响,优化其结构,基于不同沸点低温液体互溶混合机理,以液态甲烷、液态丙烷为工质,分析了不同引射角度和喷嘴直径喷射混合器内两股流体的流动特性和混合效果。分析结果表明:液态甲烷和丙烷可互溶,引射角度发生变化时,速度核心发生了不同程度的偏移,随着引射角度的增加速度核心偏移程度逐渐得到改善。压降随着引射角度的增加呈现先减小后增大的变化趋势,混合流体出口压力呈增大的趋势,两股流体混合后引射流体质量分数沿中心轴线方向呈逐渐增加的趋势,当引射角度为90度时,两股流体混合后流场分布相对其它引射角度均匀,速度核心偏移现象较不明显,且压力降、混合流体出口压力及引射流体的质量分数最大。喷嘴直径过大时,喷射混合器出口位置出现了回流现象。随着喷嘴直径的增加喷射混合器内两股流体达到均匀混合所需的距离增大,压力降、混合流体出口压力及混合后引射流体质量分数逐渐减小,当喷嘴直径为13 mm时,压力降、混合流体出口压力和两股流体混合后引射流体的质量分数最大。     

基于SPH-FEM耦合算法的磨料水射流破岩数值模拟

磨料水射流破岩是一个涉及诸多因素的非线性冲击动力学问题。针对磨料水射流破岩过程的复杂性以及有限元分析法在处理超大变形问题时存在的网格畸变问题,基于光滑粒子(SPH)耦合有限元(FEM)的方法模拟了磨料水射流破岩过程,并结合模拟结果分析了在磨料浓度30%不同速度磨料水射流作用下岩石的损伤范围。其中磨料水射流采用SPH算法模拟,并通过修改关键字文件实现水与磨料两种不同组分,岩石采用H-J-C累计损伤模型。研究表明:岩石冲蚀坑首先成漏斗状,随着冲蚀坑不断加深,最终形成"V"形剖面和圆形截面组成的"子弹"体;损伤值由冲蚀坑沿径向方向向外急剧减少,岩石的损伤半径与冲蚀坑半径随着射流速度减小而减小,两者之比在1.8-2.2之间。计算结果与相关文献基本吻合,为研究磨料水射流破岩提供一种研究的方法。     

前混合磨料水射流对HTPB推进剂的切割 工艺参数优化研究

利用前混合磨料水射流，对高、低两种燃速的HTPB推进剂开展切割试验，着重研究不同切割条件下工艺参数对切割效率的影响，进而为工艺参数优化提供理论依据。选取切割速度（v）、出口压力（p）、磨料浓度比（T）和靶距（L）4个工艺参数为主要影响因素，以最大切割深度（H）作为切割效率的衡量指标，分别通过单因素试验和正交试验进行分析，进而完成工艺参数的优化。试验结果表明，最大切割深度随切割速度的增加而减小，且单位时间内的切割面积存在最佳值；随着出口压力的增加，最大切割深度在特定范围内近似线性增加，并逐渐趋于平缓；磨料浓度比与靶距和最大切割深度均存在最佳对应关系。正交试验结果表明，切割速度对指标影响较为显著，靶距等3个工艺参数的影响相对较小。该研究可为前混合磨料水射流作为HTPB推进剂的工程化处废技术提供理论支持。     

工业喷嘴喷注噪声的抑制技术研究

对于喷注噪声的控制,有效的方法是从声源上来降低或控制.根据喷注噪声产生原理,理论分析了适当改变喷口形状,使沿喷注轴向流速在垂直方向的梯度减小,可使湍流发声的声源强度减弱,从而使喷注噪声降低.数值计算结果表明改进后的喷口可降低喷注流速.实验证实改变喷注的流场可以有效的控制喷气噪声,可使噪声A声级降低3～4个分贝,且喷射压力没有显著降低.     

Research on the Mechanism of Abrasive Particles Accelerated and a Cutting System of a Premixed Abrasive Jet

Forces acting on abrasive in the process of speeding up have been analyzed. Motion differential equation of abrasive in a pipeline and nozzle has been given,respectively. Mechanisms of abrasive particles accelerated in a premixed abrasive jet has been analyzed. The study shows that driven by high-pressure water,velocity of an abrasive is near to velocity of water in pipeline through the acceleration distance. In the taper section of a nozzle,water and abrasive particles are greatly accelerated at the same time. But velocity of an abrasive always lags behind velocity of water. A premixed abrasive jet cutting system has been introduced. The structure and working principles of the system have been given. The system is an assembly of abrasive screening and filling. By use of the premixed abrasive jet cutting system established,cutting experiments have been made to test the main parameters which influence the cutting performances such as working pressure,standoff and traverse velocity,and the nozzle diameter affecting cutting chink width.     

Parametric optimization of abrasive water-jet machining processes using grey wolf optimizer

Abrasive water-jet machining (AWJM) is a hybrid advanced machining process, which can be economically applied to machine almost any kind of material. It employs a high velocity waterjet to propel abrasive particles through a nozzle on the workpiece surface for material removal. The machining performance of AWJM process naturally depends on its several control (input) parameters, like water pressure, nozzle diameter, jet velocity, abrasive concentration, nozzle tip distance etc., which have also predominant effects on its responses, i.e., material removal rate, surface roughness, overcut, taper etc. In this paper, a new evolutionary algorithm, i.e., grey wolf optimizer (GWO), a technique based on the hunting behavior of grey wolves, is applied for finding out the optimal parametric combinations of AWJM processes. The main advantage of this algorithm is that it does not accumulate towards some local optima, and the presence of a social hierarchy helps it in storing the best possible solutions obtained so far. The derived results using GWO exhibit a significant improvement in the response values as compared to the previous attempts for parametric optimization of AWJM processes while applying other algorithms.     

A novel abrasive blasting process: Abrasive medium classification and CFD simulations

This study concerns the determination of the significant factors for an innovative deburring process: low pressure abrasive water‐jet blasting. The abrasive medium aluminum oxide (Al₂O₃) is classified according to the individual characteristics of different grain sizes. Then, the particle behavior in the air jet is analyzed with an optical measuring method, Particle Image Velocimetry (PIV); the velocity profile and the particle distribution of the dispersed system are obtained. Computational Fluid Dynamics (CFD) simulations were verified by comparing the experimental and numerical results, and the velocity range for the abrasive particles has been specified.     

Domain-adaptive finite difference methods for collapsing annular liquid jets

A domain-adaptive technique which maps a time-dependent, curvilinear geometry into a unit square is used to determine the steady state mass absorption rate and the collapse of annular liquid jets. A method of lines is used to solve the one-dimensional fluid dynamics equations written in weak conservation-law form, and upwind differences are employed to evaluate the axial convective fluxes. The unknown, time-dependent, axial location of the downstream boundary is determined from the solution of an ordinary differential equation which is nonlinearly coupled to the fluid dynamics and gas concentration equations. The equation for the gas concentration in the annular liquid jet is written in strong conservation-law form and solved by means of a method of lines at high Peclet numbers and a line Gauss-Seidel method at low Peclet numbers. The effects of the number of grid points along and across the annular jet, time step, and discretization of the radial convective fluxes on both the steady state mass absorption rate and the jet's collapse rate have been analyzed on staggered and non-staggered grids. The steady state mass absorption rate and the collapse of annular liquid jets are determined as a function of the Froude, Peclet and Weber numbers, annular jet's thickness-to-radius ratio at the nozzle exit, initial pressure difference across the annular jet, nozzle exit angle, temperature of the gas enclosed by the annular jet, pressure of the gas surrounding the jet, solubilities at the inner and outer interfaces of the annular jet, and gas concentration at the nozzle exit. It is shown that the steady state mass absorption rate is proportional to the inverse square root of the Peclet number except for low values of this parameter, and that the possible mathematical incompatibilities in the concentration field at the nozzle exit exert a great influence on the steady state mass absorption rate and on the jet collapse. It is also shown that the steady state mass absorption rate increases as the Weber number, nozzle exit angle, gas concentration at the nozzle exit, and temperature of the gases enclosed by the annular liquid jet are increased, but it decreases as the Froude and Peclet numbers, and annular liquid jet's thickness-to-radius ratio at the nozzle exit are increased. It is also shown that the annular liquid jet's collapse rate increases as the Weber number, nozzle exit angle, temperature of the gases enclosed by the annular liquid jet, and pressure of the gases which surround the jet are increased, but decreases as the Froude and Peclet numbers, and annular liquid jet's thickness-toradius ratio at the nozzle exit are increased. It is also shown that both the ratio of the initial pressure of the gas enclosed by the jet to the pressure of the gas surrounding the jet and the ratio of solubilities at the annular liquid jet's inner and outer interfaces play an important role on both the steady state mass absorption rate and the jet collapse. If the product of these ratios is greater or less than one, both the pressure and the mass of the gas enclosed by the annular liquid jet decrease or increase, respectively, with time. It is also shown that the numerical results obtained with the conservative, domain-adaptive method of lines technique presented in this paper are in excellent agreement with those of a domain-adaptive, iterative, non-conservative, block-bidiagonal, finite difference method which uncouples the solution of the fluid dynamics equations from that of the convergence length.     

旋进射流喷嘴的频率及流场特性研究

在圆筒套管内设置一块孔板构成旋进射流喷嘴，由此获得持续稳定的旋进射流，研究了喷嘴最优的结构参数。用热线风速仪测量出口射流的速度并进行功率谱分析，得到旋进射流的频率及流场特性。研究表明，旋进射流具有较高的紊流强度，速度场沿径向的分布趋向平缓，其进动频率随流量增大而加快，喷嘴的尺寸和结构对射流进动有影响。     

Modeling of the supersonic argon plasma jet at low gas pressure environment

Numerical simulation results are presented concerning the heat transfer and fluid flow within the supersonic argon plasma jet encountered in low pressure (or soft vacuum) plasma spraying (LPPS). The plasma parameters at the inlet section of the plasma jet are taken from our modeling results of the subsonic-to-supersonic d.c. arc plasma torch. The mach number, temperature and static pressure at the center of the plasma jet on the torch exit section are 2.8, 13 200 K and 6000 Pa, respectively, whereas the environment (i.e. vacuum chamber) pressure is 0.1 atm. Those parameters are typical for LPPS. The plasma jet is assumed to be axi-symmetrical and in local thermodynamic equilibrium state. The All-Speed SIMPLE algorithm is coupled with the FAST-2D program to simulate the whole plasma jet containing both the supersonic and subsonic flow regions. Modeling results clearly show that there exist several successive temperature, velocity and static wave crests and troughs. The fluctuation magnitudes of those parameters reduce rapidly in the flow direction, along with the flow transformation from the supersonic flow regime into the subsonic flow regime. The existence of a series of compression and expansion waves in the region near the torch nozzle exit shows clearly the over-expanded characteristics of the supersonic plasma flow.     

Theoretical analysis of piercing delicate materials with abrasive water‐jet

Abstract

An abrasive water‐jet cutting process is a process in which water is pressurized up to a very high value and forced through a very small orifice to form a very thin high speed jet beam. This thin jet beam is then directed through a chamber and fed to a secondary nozzle, which is called mixing tube. During this process, a vacuum is generated in the chamber, and abrasives and air are pulled into the chamber, through an abrasive feed tube. Usually, it takes a short time for abrasives to travel through the length of the tube and to be entrained into the jet. Before that, the jet impinging onto the solid surface is void of abrasives. During that short moment, the target material is exposed to a very high dynamic load. Although this high dynamic load only acts on target material for a very short period of time, its high value may crack target material quickly. This paper explores the theory behind the cracking phenomenon. Based on this exploration, understanding the piercing process completely becomes feasible.     

Application of particle image velocimetry for measuring He II thermal counterflow jets

The particle image velocimetry (PIV) technique was successfully applied for measuring the velocity of a He II thermal counterflow jet. Neutrally buoyant hydrogen-deuterium solid particles were used as tracer particles for PIV measurement. In the application, the normal component velocity was measured. The jet velocity profile and spatial decay of the jet velocity were compared with those of turbulent round jets of ordinary viscous fluids. The velocity measured near the jet nozzle exit was compared with the theoretical prediction for the normal component flow velocity.