Numerical analysis for the coating thickness prediction in continuous hot-dip galvanizing

Gas wiping is a decisive operation in the hot-dip galvanizing process. Especially, it has a crucial influence on the thickness and uniformity of coating film, but may be subsequently responsible for splashing. To date, the number of fundamental studies on the jet structure impinging on a vertical moving strip for various nozzle systems has not been sufficient to draw any meaningful conclusion. In this connection, at first, to confirm the validation of numerical analysis, the impinging jet pressure on the surface of a vertical strip by experiment is compared with the results by numerical analysis. Next, after confirming for the superiority of a constant expansion rate nozzle in splashing, the relationship between the stagnation pressure and the impinging jet pressure distribution issuing from the nozzle system of constant expansion rate is investigated. Finally, by using the calculated wall shear stress, the relationships among the coating thickness, strip speed and nozzle stagnation pressures are clarified. It is found that the impinging wall pressure for the case of constant expansion rate nozzle is more favorable for the problem of splashing to the case of the conventional one. Furthermore, from the point of view of energy conservation, it is advisable to use a constant expansion rate nozzle as a gas-wiping nozzle.     

电弧喷枪流场的数值模拟研究

电弧喷枪流场特性是影响电弧喷涂质量的关键因素之一。运用商用流体计算软件Fluent，对手持式高速电弧喷枪头部及枪外射流进行了模拟计算，通过分析可以看出，雾化射流具有不对称、湍流等特点，其射流速度分布有明显的先递增后衰减的规律，在流场推动下，雾化的金属粒子首先被流场加速，到达最大值后速度开始衰减，随着喷涂距离的增加，粒子的速度逐渐下降，特别是靠近流场外部的粒子，速度的下降更快，最终失去了与基底的结合动能，成为灰尘，飞散到环境之中，这既浪费了材料，又污染了环境。为了提高沉积率，保证涂层质量，喷涂距离的控制显得非常重要。     

Experiment and Numerical Simulation of Free Water Jet by a Central-body Nozzle

The recent research about cavitation jet mainly focuses on the organ-pipe nozzle and triangular nozzle.The research content mainly includes the optimized design about the structure of nozzles,the observation and flow analysis about the cavitation jet in the water,and the theory of rock attacked by the cavitation jet,while the energy characteristic of the free jet is not studied yet.In China,the research about the central-body nozzle is almost empty.For the purpose of studying the energy characteristic and the structure of free water jet discharged from central-body nozzle,an experiment with phase Doppler particle anemometry(PDPA) technology is carried out to measure the free water jet flow,which is produced by a central-body nozzle under the jet pressure of 15 MPa.While five sections with different axial distances from the nozzle outlet are selected for data process and analysis,the axial and radial velocity and the droplets of the particle size are studied.Meanwhile,numerical calculation of corresponding flow field is conducted by using volume of fluid(VOF) multiphase model,and the jet flow feature is discussed.The experimental and calculating results show that the axial velocity of high speed jet flow dissipates slowly in the air,and the core area and diffused area are discovered.The diameter of droplet in the core area is small,and jet energy is concentrated,while in the diffusion area,water is mingled with ambient air and radial velocity is relatively large.Obvious low-pressure area exists behind the central body and potential cavitation may occur in that area.The proposed research reveals the energy characteristic of free jet discharged from central-body nozzle,provides the theoretical basis for preestimating erosion feature of the central-body nozzle and also the theoretical foundation for revealing the mechanism of erosion.     

气枪喷嘴除尘除水效率的实验研究

利用图像处理的方法对小尺度气枪喷嘴冲击射流的除尘除水效率进行研究,实现了对喷嘴除尘除水效率的定量测量。利用该方法对影响气枪喷除尘除水效率的各种因素进行研究,并将实验结果与用热线风速仪测量的结果进行了比较,得到了关于喷嘴工作最佳工况的一些结论。这些结论可对了解喷嘴的除尘除水工作原理及对气枪喷嘴进一步改进和优化提供帮助。     

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.     

An experimental study of the nozzle lip thickness effect on supersonic jet screech tones

It is well known that screech tones of supersonic jet are generated by a feedback loop driven by the instability waves. Near the nozzle lip where the supersonic jet mixing layer is receptive to external excitation, acoustic disturbances impinging on this area excite the instability waves. This fact implies that the nozzle lip thickness can influence the screech tones of supersonic jet. The objective of the present study is to experimentally investigate the effect of nozzle-lip thickness on screech tones of supersonic jets issuing from a convergent-divergent nozzle. A baffle plate was installed at the nozzle exit to change the nozzle-lip thickness. Detailed acoustic measurement and flow visualization were made to specify the screech tones. The results obtained obviously show that nozzle-lip thickness significantly affects the screech tones of supersonic jet, strongly depending on whether the jet at the nozzle exit is over-expanded or under-expanded.     

高压水射流数值模拟及混凝土路面切割试验研究

探讨了高压水射流的切割机理,并对圆形喷嘴高压水射流垂直冲击水泥混凝土建立了流体动力学模型,进行了数值模拟。通过对模拟结果的分析,得到射流喷嘴最大出口压力和量纲一冲击高度对水泥混凝土在滞止点及其附近压力值的影响。根据结果选取主要参数,设计制造出高压水切割设备并且完成水泥混凝土试块的切割试验。试验达到预期效果,证明此高压水切割设备工作的有效性。     

Numerical simulations and experimental studies on a target fluidic flowmeter

The aims of this work were to determine the flow characteristics of a target fluidic flowmeter by both experimental studies and numerical simulations, and thence to understand the flow phenomena in such a flowmeter. Experimental studies and numerical visualizations on the target flowmeter for water measurements are reported in this paper. Computational fluid dynamics using the software FLUENT were used in the numerical simulations and also to determine the best position to place the hot-film sensor for the detection of the maximum velocity oscillation signals in the flowmeter. The effect of the distance from the inlet nozzle to the target was studied. The flowmeter exhibited the least variations in the Strouhal number versus Reynolds number relationship when the ratio of the distance between the nozzle and target to the nozzle width (x₁/d) was 25.5 for Reynolds number greater than 500.     

冲击射流及其强化换热的研究进展

介绍了冲击射流的特点,对国内外冲击射流换热的实验研究和数值模拟研究的发展和现状进行了较为详细的综合论述。     

A suggestion of a new method for the calculation of the coating thickness in continuous hot-dip galvanizing

It is known that the distributions of the impinging pressure gradient and the shear stress at the strip surface play a decisive key role in the decision of the coating thickness in hot-dip galvanizing. So, to predict the exact coating thickness, it is essential that the distributions of the impinging wall jet pressure and the shear stress acting between the liquid film and jet stream are measured (or calculated) exactly for each specific coating condition. So far, to obtain the impinging wall jet pressure, it was assumed that the jet issuing from an air-knife is similar to the Hiemenz plane stagnation flow, and the wall shear stress could be predicted by an equation using the assumption of a non-negative Gaussian profile in impinging wall jet pressure in general, so that it cannot be reliable for some impinging wall jet regions and nozzle systems intrinsically. Nevertheless, one cannot find a suitable method to cope with the difficulties in measuring/calculating of the shear stress and the impinging wall jet pressure. Such a difficulty which will cause an inaccuracy in the coating thickness prediction. With these connections, in the present study, we suggest a new method named as a two-step calculation method to calculate the final coating thickness, which consists of the air jet analysis and coating thickness calculation. And, from the comparison of the results one may confirm the validation of the new suggested method.     

水射流穿丝喷嘴设计

本文针对慢走丝线切割机床的自动穿丝系统设计了一款新型喷嘴。利用Fluent软件对该喷嘴进行了仿真,确定了会聚角度对射流流场的影响。对喷嘴进行喷水试验,确定不同速度下射流的形态以及穿丝时射流的形态,验证了该喷嘴的可行性,为线切割自动穿丝喷嘴设计提供了一条新的思路。     

瞬态激振影响下微油滴撞壁行为的数值模拟研究

在油气微量润滑系统工作过程中,有效润滑是保障零件稳定运行的关键。但工作零件的振动会影响油滴的铺展,扰动润滑油膜的形成,从而影响零件的润滑效果。基于CLSVOF方法,建立油滴撞击振动壁面的模型,考察油滴撞击瞬时激振壁面过程中的油液形态演化规律。研究发现：油滴撞击激振壁面后产生飞溅和颈部射流现象;在不同振动频率下,油滴冲击产生二次飞溅的时间均整个周期的1/5附近;壁面振动减小了油滴的最大铺展因子,延长了油滴的铺展时间;在回缩阶段,油滴铺展因子呈现出典型的振荡衰减态势。     

Experimental investigations on impaction pin nozzles for inlet fogging system

Increasing power demands have necessitated the development of energy efficient systems in the industrial sector. At present, about 10% of the overall electric power used by large industrial plants is consumed by high-capacity compressors supplying compressed air. Likewise, in a gas turbine power plant, nearly half the generated power is used for driving the compressor. The work of compression is proportional to inlet air temperature, and cooling the inlet air can save considerable amount of power in large turbo machines during hot summer months. Inlet fogging is a popular means of inlet air cooling, and fog nozzles are the most critical components in an inlet fogging installation. Majority of these installations employ impaction pin nozzles. In the present work, experiments are conducted over a wide range of operating parameters in variable length wind tunnels of different cross sections in order to investigate the performance of impaction pin nozzle in inlet fogging. Flow visualization and measurements are carried out to analyze the fog behavior and identify suitable nozzle locations in typical air ducts. The results show that impaction pin nozzles are suitable for inlet fogging applications.     

高压水射流靶物探测用喷嘴结构参数优化

高压水射流靶物探测技术是一项结合了高压水射流技术和声音信号分类识别技术的新的技术和研究方向,要实现高压水射流对靶物的有效探测,就必须得到具有合理有效的特征值的信号。其中喷嘴结构是影响特征值信号的一个重要因素,在选择工程上常见的、结构较为合理的4种喷嘴的基础上,利用计算流体力学软件Fluent对喷嘴内部流场做了数值模拟分析,并从反射声信号的角度给出试验判定。试验结果表明,出口直径为0.8mm的圆锥长直线型喷嘴产生的射流性能及射流束的聚集性都较好,有利于产生较好反射声音信号的特征值。     

The self-induced oscillations of the under expanded jets impinging upon a cylindrical body

The present study addresses the flow characteristics involved in the self-induced oscillations of the underexpanded jet impinging upon a cylindrical body. Both experiment and computational analysis are carried out to elucidate the shock motions of the self-induced oscillations and to find the associated major flow factors. The underexpanded sonic jet is made from a nozzle and a cylindrical body is placed downstream to simulate the impinging jet upon an obstacle. The computational analysis using TVD scheme is applied to solve the axisymmetric, unsteady, inviscid governing equations. A Schlieren system is employed to visualize the self-induced oscillations generated in flow field. The data of the shock motions are obtained from a high-speed video system. The detailed characteristics of the Mach disk oscillations and the resulting pressure variations are expatiated using the time dependent data of the Mach disk positions. The mechanisms of the self-induced oscillations are discussed in details based upon the experimental and computational results.     

三维旋转喷射枪喷嘴结构的优化

基于工业油罐清洗对水射流射程远、打击力大和清洗面积大的三大要求,采用计算流体力学数值分析的方法,进行三维旋转喷射枪喷嘴结构的优化。结果表明：收缩角在20°~30°区间段内、长径比在20~3.0区间段内,喷嘴具有最佳的喷射性能。     

Numerical simulation of fluid flow and heat transfer in a plasma spray gun

A computational fluid dynamics (CFD) simulation for analyzing fluid flow patterns in a plasma spray gun is presented in this study. It is coupled with a heat transfer simulation of the plasma spray gun. Based on CFD and heat transfer theory, the numerical model of the nozzle in the plasma spray gun is developed, and the coupled simulation of the flow fluid and heat transfer is carried out with the semi-implicit method for pressure-linked equations (SIMPLE) method. Local turbulence, which will lead to appearance of a static-water region, is found at the front corner of the cooling channel in the nozzle. The locations insufficiently cooled are found in the wall near the heat source and in the gasket in the rear of the nozzle. Then, cooling processes with different parameters of cooling water are analyzed. The optimal velocity and direction of cooling water, which efficiently cool the nozzle and improve the service life of the plasma jet, are obtained .     

Numerical simulation of fluid flow and heat transfer in a plasma spray gun

A computational fluid dynamics (CFD) simulation for analyzing fluid flow patterns in a plasma spray gun is presented in this study. It is coupled with a heat transfer simulation of the plasma spray gun. Based on CFD and heat transfer theory, the numerical model of the nozzle in the plasma spray gun is developed, and the coupled simulation of the flow fluid and heat transfer is carried out with the semi-implicit method for pressure-linked equations (SIMPLE) method. Local turbulence, which will lead to appearance of a static-water region, is found at the front corner of the cooling channel in the nozzle. The locations insufficiently cooled are found in the wall near the heat source and in the gasket in the rear of the nozzle. Then, cooling processes with different parameters of cooling water are analyzed. The optimal velocity and direction of cooling water, which efficiently cool the nozzle and improve the service life of the plasma jet, are obtained .     

Effects of the geometric orientations of the nozzle exit on the breakup of free liquid jet

Free liquid jets are produced through various geometric orientations of the nozzle exit. The breakup lengths of liquid jets under various geometric orientations of the nozzle exit were studied. Images of jets were captured using a high-speed camera with a maximum frame rate of 1000 frames per second and were analyzed to determine the dynamics between jets and breakup lengths. The breakup length of jets changes with the cut angle of the nozzle exit. In addition, adding polymer reduces the effect of the cut angle of the nozzle exit on the breakup length for an entire range of velocities. The effect of the cut angle on breakup length is predominant for aqueous solutions with surfactants. This work provides motivation for further computational research to study jet dynamics in a partially covered nozzle exit, such as the case in which the boundary conditions near the nozzle opening is more complex with the cut angle and its vertex position, which directly reflects liquid jet dynamics.     

A numerical study on the spray-to-spray impingement system

The present article aims to perform numerical calculations for inter-spray impingement of two diesel sprays under a high injection pressure and to propose a new hybrid model for droplet collision on the basis of literature findings. The hybrid model is compared with the original 0’ Rourke’s model, which has been widely used for spray calculations. The main difference between the hybrid model and the O’Rourke’s model is mainly in determination of the collision threshold condition, in which the preferred directional effect of droplets and a critical collision radius are included. The Wave model involving the cavitation effect inside a nozzle is used for predictions of atomization processes. Numerical results are reported for different impingement angles of 60° and 90° in order to show the influence of the impinging angle on spray characteristics and also compared with experimental data. It is found that the hybrid model shows slightly better agreement with experimental data than the O’Rourke’s model.