Characteristic analysis of supersonic impinging jet in laser machining

The interaction of a gas jet with a workpiece in laser machining is investigated by studying the influence of the processing parameters on the dynamic characteristic of the gas flow in the hole, mainly including the mass flow rate and the thrust. The modeling of a supersonic turbulent jet impinging on a plate with a hole concentric with the jet is presented. Numerical simulations are carried out using an explicit, coupled solution algorithm with solution-based mesh adaptation. The model is able to make quantitative predictions of the effect of the standoff distance on the mass flow rate and the axial thrust. It is revealed that the shape of the hole has weak effects on shock structure, but it can improve the dynamic characteristic slightly. The standoff distance has great effect not only on the shock structure, but also on the jump and fluctuation of the thrust for the different exit Mach number.     

改进型撞击流反应器的颗粒混合特性

撞击流技术是现代社会强化过程的一种手段,具有巨大的发展潜力和应用前景。然而,在传统撞击流反应器中,颗粒混合效果不佳,导致相关反应进行不充分。为此一种入口速度动态变化的撞击流反应器应运而生,反应器的流场同样分为入口区、撞击区、出口区和漩涡区,但撞击面在几何中心处波动,从而使颗粒的混合区域扩展到整个反应器,并采用流体计算动力学对该改进型的撞击流反应器进行数值模拟。在保证平均流量相同的情况下,采用三角直线型的流速进入撞击流反应器,并与传统撞击流反应器进行分析对比。分析结果表明,颗粒的活动范围得到了有效地加强,与传统撞击流反应器有明显的优势,可以实现颗粒理想混合和相关反应充分进行。     

Classification of impinging jet flames using convolutional neural network with transfer learning

Journal of Mechanical Science and Technology - Depending on the equivalence ratio and the Reynolds number, impinging jet flames exhibit several modes of thermoacoustic oscillation. In this study,...     

Turbulent flow and heat transfer characteristics of a two-dimensional oblique plate impinging jet

Turbulent flow and heat transfer characteristics of a two-dimensional oblique plate impinging jet (OPIJ) were experimentally investigated. The local heat transfer coefficients were measured using thermochromic liquid crystals. The jet mean velocity and turbulent intensity profiles were also measured along the plate. The jet Reynolds number (Re, based on the nozzle width) ranged from 10, 000 to 35,000, the nozzle-to-plate distance (H/B) from 2 to 16, and the oblique angle (α) from 60 to 90 degree. It has been found that the stagnation point shifted toward the minor flow region as the oblique angle decreased and the position of the stagnation point nearly coincided with that of the maximum turbulent intensity. It has also been observed that the local Nusselt numbers in the minor flow region were larger than those in the major flow region for the same distance along the plate mainly due to the higher levels in turbulent intensity caused by more active mixing of the jet flow.     

Determining jet mixing characteristics using electrical resistance tomography

Coaxial jet and side entry mixers are used in a wide range of industries for a variety of processes including precipitation, polymerization and neutralization duties [Wei H, Garside J. Application of CFD modelling to precipitation systems. Chem Eng Res Des 1995; 75 (2): 219–27; Kolhapure NH, Tilton JN, Pereira CJ. Integration of CFD and condensation polymerization chemistry for a commercial multijet tubular reactor. Chem Eng Sci 2004; 59: 5177–84; Pipino M, Fox RO. Reactive mixing in a tubular jet reactor: A comparison of PDF simulations with experimental data. Chem Eng Sci 1994; 49 (24B): 5229–41]. Jet mixers are characterized by short contact times between the fluids and can be operated in continuous or semi-batch modes. Coaxial and side entry jets can be designed in order to deliver rapid turbulent mixing using short sections of pipeline. As the energy required for mixing is provided by the addition stream, the process-side pressure drop required for homogeneity is very low. A key design parameter for jet mixers is the mixing length, i.e. the length of pipe downstream of the injection point required to achieve a given degree of homogeneity. The mixing length can be affected by the addition geometry (for example, coaxial or side entry), orifice size and shape, operating conditions and material properties.

This paper presents the use of electrical resistance tomography (ERT) to monitor jet mixing via the addition of a conductivity tracer through coaxial and side entry jets. Multiple ERT sensors are fitted along the pipe downstream of the jet addition point. The ERT sensors enable real time, noninvasive measurement of conductivity within the pipe, utilizing a 3D finite element mesh consisting of 2500 tetrahedral elements. The effect of secondary (main pipe) flow rate and jet configuration on the nature of the tracer plume evolution and axial mixing is determined using this technique.     

An experimental study on the flow and heat transfer characteristics of an impinging jet

The flow and heat transfer characteristics of an impinging jet is investigated in two major stages. The first stage is about the investigation of the three dimensional mean flow and the turbulent flow quantities in free jet, stagnation and wall jet region. After a complete documentation of the flow field, the convective heat transfer coefficient distributions on the impingement plate are presented, during the second stage of the study. Heat transfer experiments using the new hue-capturing technique result in high resolution wall heating rate distributions. The technique is fully automated using a true color image processing system. The present heat transfer results are discussed in detail in terms of the flow characteristics. The measurements from the new method are compared with conventional heat flux sensors located on the same model. These heat transfer distributions are also compared with other studies available from the literature. The new non-intrusive heat transfer method is highly effective in obtaining high resolution heat transfer maps with good accuracy.     

On the mixing flow structure of a turbulent cross jet

An experimental study on the structure of a turbulent cross jet mixing flow is presented. Diffusion rates, two and three dimensional flow structures, mean velocities turbulence intensities and turbulent shear stresses of the mixing flow were measured as varying the velocity ratio. Self-similar forms for the dimensionless mean velocity and turbulent shear stresses was obtained by correlating the measurement data resulting a remarkable agreement. It was found that the deviation gradient(a) is linearly correlated with the velocity ratio(R), and the cross section of the mixing flow is an elliptic form.     

Morphology of ductile metals eroded by a jet of spherical particles impinging at normal incidence

A detailed morphological study of the erosion of two metals (an aluminum alloy and copper) by normal impact of spherical glass erodent particles was conducted. Currently there is no universally accepted theory which describes erosion observations at normal impact. In this study damage patterns were studied at several stages of erosion. Scanning electron microscopy and energy-dispersive X-ray spectroscopy were used and surface profile measurements were made. Transformations induced by the erosion process were also studied and the dynamics of the erosion are discussed.The morphology of the damage pattern is a manifestation of the flow pattern of erodent particles and it provides insight into possible mechanisms active at different stages of erosion. The simultaneous appearance of both radial tracks as well as concentric rings is believed to be reported here for the first time. Wave crests are observed which contain an accumulation of metallic flakes. These flakes are also observed in all regions of the tested specimens during all phases of erosion. From morphological comparisons of the pits over a wide variation in driving gas pressure and time, a well-defined damage and erosion process is evident and preliminary analysis is advanced to explain the formation of the various damage patterns.     

旋转射流搅拌器全场数值模拟分析

目前应用于大型工程流体储罐最安全、最有效的搅拌系统是旋转射流搅拌(RJM)系统.在RJM系统的设计开发过程中,工业上对该系统的要求是不仅要射程远,能够使储罐流体横向有运动,而且要求四周的扩散效果要更好.针对以高粘度流体为工作介质,采用标准k-ε湍流模型,对储罐内射流搅拌的全三维流场进行了数值模拟计算,估算了射流的流程、影响范围及搅拌效果,采用数值计算方法考察了搅拌效果.研究结果显示,该方法可以克服实验中经费投入和测量精度等方面的限制,对工程应用具有很重要的意义.     

Large eddy simulation of flow characteristics in an unconfined slot impinging jet with various nozzle-to-plate distances

To study the detailed flow structure of the unconfined plane impinging jet, a 3-dimensional flow analysis is carried out focused on the flow mechanism with the non-dimensional distance, L/d, as flow parameter by using the LES turbulent technique. The symbols d and L represent the nozzle width and the nozzle-to-plate distance, respectively. Then, the flow structures along both the stream-wise and spanwise direction are investigated. For these purposes, the plane impinging jet with Reynolds number of 11,000 is analyzed. The nozzle width was fixed at 1.5mm, but the nozzle-to-plate distance was varied between 4mm and 24mm. As a result, the plane impinging jet shows different flow patterns with L/d. In conclusion, the plane impinging jet is classified into three types with the non-dimensional variable, L/d. L/d≤4: Stable impinging jet, 4<L/d<11: periodically-stable impinging jet and L/d≥11: Unstable impinging jet.     

实现微米级灭菌范围控制的微细等离子体射流装置

为了实现微细等离子体的精准灭菌,设计了新型微细等离子体射流装置,并对该装置产生的含氧活性粒子(Reac-tive Oxygen Species,ROS)和含氮活性粒子(Reactive Nitrogen Species,RNS)分布范围及其灭菌范围进行研究.淀粉碘化钾混合溶液里的碘离子可以被微细等离子体射流产生的ROS...     

Effect of cross-flow velocity on flow and heat transfer characteristics of impinging jet with low jet-to-plate distance

An effect of cross-flow velocity on flow and heat transfer characteristics of impinging jet in the case of low jet-to-plate distance at H = 2D was experimentally and numerically investigated. In the experiments, the air jet from orifice impingement on the wall of wind tunnel while a cross-flow was simultaneously induced normal to the jet flow. The jet velocity was fixed while the cross-flow velocity was varied corresponding to velocity ratios (jet velocity/cross-flow velocity) VR = 3, 5 and 7. The temperature distribution on an impinged surface was visualized by using thermochromic liquid crystal sheet (TLCs), and Nusselt number distribution was evaluated by using image processing method. The flow pattern on impingement surface was visualized by using oil film technique. The numerical simulation was carried out for a better understanding of the jet flow in the cross-flow. The results show that Nusselt number peak shifts downstream and the Nusselt number peak increases with increasing cross-flow velocity.     

An experimental study on the distribution of the drop size and velocity in asymmetric impinging jet sprays

Distributions of the drop size and velocity in an asymmetric impinging jet are investigated by injecting water and a sodium carbonate (Na₂CO₃) solution, which simulates the mixing process in impinging jet sprays of liquid oxidizer and liquid fuel for liquid propellants. The liquid sheet formed from the impinging jet is visualized and the drop size distributions are obtained by using image processing for the visualized images. The drop size distribution of the asymmetric impinging jets is fitted to the Rosin-Rammler distribution function. The obtained drop size distributions according to the azimuth angle in the impinging jet are compared with the theoretical predictions of previous research. The experimental results of the drop size distributions are located between the two curves obtained from the theoretical predictions by treating each jet in the asymmetric impinging jets as an independent wall-impinging jet. PIV images using a double-exposure method were processed to obtain the drop velocity vector in the impinging jets. Whether the drops shedding from the edge of the asymmetric impinging jets occurs radially or tangentially is also investigated from the PIV results.     

Heat transfer in a turbulent jet impinging on a moving plate considering high plate-to-jet velocity ratios

In this paper, heat transfer characteristics of a turbulent slot jet impinging orthogonally on an isothermal moving hot plate is studied numerically. The governing equations were discretized using the finite volume method and the υ ² — f turbulence model was employed for turbulence modeling. The effect of the jet Reynolds number and the plate-to-jet velocity ratio (R) on the Nusselt were investigated. Despite of most previous studies, which have been restricted to R≤2, in the present research higher values of R, also were considered (0≤R≤6). Range of studied jet Reynolds number was between 3000 and 60000. The results indicate that at a fixed plate-to-jet velocity ratio increment of the Reynolds number leads to the enhancement of the average Nusselt number. For each Reynolds number, the average Nusselt number reduces with increasing the plate-to-jet velocity ratio until it becomes minimum at R = 1.25. For R>1.25 trend changes so that these parameters increase. In addition, it was found that only for R>2.5 the average Nusselt number is improved due to the plate motion in comparison with the stationary jet. The results are validated against available experimental data, showing good agreement.     

Performance of corrugated actuator-tabs of aspect ratio 2.0 on supersonic jet mixing enhancement

Journal of Mechanical Science and Technology - The present experimental investigation explores the ability of the corrugated actuator-tabs (or simply, the corrugated actuators) of aspect ratio 2 in...     

Abrasive jet machining for the microprofile control patterning of herringbone grooves

Fluid bearings have features of high speed and high rotation accuracy, and therefore, they are used in spindle motors of hard disk drives, cooling fans of central processing units, and other devices. Further, these bearings have microherringbone grooves on the shaft or sleeve inner surface that help generate dynamic pressure in the lubricant fluid around the shaft. Although the depth of the groove is constant, dynamic pressure can be increased by decreasing the depth from both ends to the central corner of each groove on a micron scale. This study aims to verify the effect of sloped herringbone grooves using computational fluid dynamics (CFD) analysis and to develop a new microfabrication method for manufacturing microsloped herringbone grooves on the shaft surface using abrasive jet machining. The generated dynamic pressure is analyzed using CFD; the results indicated that the sloped herringbone grooves result in an increase in the dynamic pressure at the groove tips and cause a decrease in the fluctuations in dynamic pressure in the circumferential direction.     

Active surge control for variable speed axial compressors

This paper discusses active surge control in variable speed axial compressors. A compression system equipped with a variable area throttle is investigated. Based on a given compressor model, a fuzzy logic controller is designed for surge control and a proportional speed controller is used for speed control. The fuzzy controller uses measurements of the change of pressure rise as well as the change of mass flow to determine the throttle opening. The presented approach does not require the knowledge of system equilibrium or the surge line. Numerical simulations show promising results. The proposed fuzzy logic controller performs better than a backstepping controller and is capable to suppress surge at different operating points.     

数控加工误差主动补偿方法

为提高零件的加工精度,提出了基于公差的局部误差补偿法,并通过修正数控程序主动补偿加工误差.分析零件加工表面误差的特点,根据实际公差要求找出超出公差范围的变形关键区域,修正其切削深度以实现误差的局部补偿.得到刀位控制点修正的切深后,重新规划带有误差补偿值的刀具轨迹.结合实际加工精度确定走刀步距和行距,经过后置处理生成零件修正的数控代码.通过实例验证了上述方法的可行性.