Experimental investigation of the impaction of water droplets on cylindrical objects

The impaction of water droplets on isothermal cylindrical wires has been investigated experimentally in the present study. Mono-size droplets of 110, 350 and 680 μm in diameter were generated using piezoelectric droplet generators. The effects of droplet velocity and wire size were varied parametrically to reveal the impacting phenomena. Typical modes of the impaction outcome are disintegration and dripping. For droplets impacting on small wires, finer drops are disintegrated if the impacting droplet velocity is high, and larger dripping drops are observed if the velocity is low. For droplets impacting on large wires, bigger pendent drops are gradually formed which would eventually detach from the wires under the influence of gravity. In addition, droplets impacting on wires with waxy surface generate smaller dripping drops than that of the non-waxed wires. A non-dimensional regime map and new formulations in terms of the droplet Weber number, the wire Bond number and the size ratio of the wire diameter to incoming droplet diameter have been established to identify the regime for each mode of outcome and to predict the size of the dripping drops within the experimental limits.     

The impact of a single drop on a wetted solid surface

 The impact of single drops on a thin liquid film was studied to understand the mechanism of secondary atomisation of sprays colliding on a wetted, cold, solid surface. To span a wide range of conditions various mixtures of water and glycerol were used. The use of Weber number, Ohnesorge number and non-dimensional film thickness to describe the peculiarities of the phenomenon allowed to carry out the experiments under appropriate similarity conditions. The impact of millimetric drops was analysed in detail by photographic means, using both still photography to study impact morphology, and laser sheet visualisation to investigate secondary droplet formation. Two mechanisms of splash were identified, depending essentially on the liquid viscosity (Ohnesorge number), a parameter which appears to play an important role also in defining the splash morphology. A photographic documentation is annexed. The characteristic times of the crown formation, the non-linear evolution of cusps (jet formation) and the surface roughness influence are further discussed. The experimental results allow to propose an empirical correlation for the splashing/deposition limit, for a wide range of conditions, and a comparison to available previous works is presented. The influence of the film thickness and liquid viscosity on the splash is confirmed and quantified. Received: 1 March 1996/Accepted: 12 November 1996     

An experimental study of a water droplet impinging on a liquid surface

An experimental study is presented for water droplet impingement on a liquid surface. The impaction process was recorded using a high-speed digital camera at 1,000 frames/s. The initial droplet diameter was fixed at 3.1 mm ± 0.1 mm, and all experiments were performed in atmospheric air. The impact velocity was varied from 0.36 m/s to 2.2 m/s thus varying the impact Weber number from 5.5 to 206. The impacted liquid surface consisted of two fluids, namely water and methoxy-nonafluorobutane, C₄F₉OCH₃ (HFE7100). The depth of the water and HFE7100 pool was varied from 2 mm to 25 mm. The collision dynamics of water in the HFE7100 pool was observed to be drastically different from that observed for the water droplet impingement on a water pool. The critical impact Weber number for jet breakup was found to be independent of liquid depth. Water–HFE7100 impact resulted in no jet breakup over the range of velocities studied. Therefore, no critical impact Weber number can be defined for water–HFE7100 impact. Received: 27 June 2001/Accepted: 29 November 2001     

Production of secondary drops during the single water drop impact onto a plane water surface

The normal impact of single water drops onto a plane water surface was studied experimentally to reveal the amount of secondary drops produced from the rim of crown-like interfacial structure. Within the experimental ranges tested, the ratio of the total mass of secondary drops to the mass of primary drop was approximately within 0–1 and correlated well as the function of dimensionless parameter K that consisted of the impact Weber number We and the Ohnesorge number Oh (K = We Oh ^−0.4). The dependences of the number and the mean diameter of secondary drops on K and dimensionless film thickness were also investigated.     

Normal droplet impact on horizontal moving films: an investigation of impact behaviour and regimes

Investigations of the impact regimes for a liquid droplet impacting onto horizontal moving liquid films are presented here for the first time. The aim of this study is to show the main impact outcomes and to highlight the effect the movement of films has upon these outcomes. The droplet diameters used in this experiment were 2.47 and 3.86 mm with impact Weber number of between 0.6 and 460. The Reynolds number of the moving film was between 351 and 1,818 depending on the flow rate (1–5.5 L/min) with changes in the film height of between 4.3 and 9.4 mm. The results showed that the impact outcomes exhibit the same regimes established for the impact on static liquids but with different transition boundaries. Moreover, the shape of the impact outcomes is unsymmetrical similar to the inclined impact on static liquids. The impact outcomes are shown to undergo a transition as the Reynolds number of the film increases, this occurs at a threshold value close to the expected transition between laminar and turbulent flow.     

Drop impingement on a deep liquid surface: study of a crater's sinking dynamicsImpact d'une goutte sur une surface liquide profonde : étude de la dynamique d'enfoncement du cratère

When there is a drop impact on a liquid surface, two phenomena can appear depending on the impact Weber number: either vortex generation or jet formation; in this paper the second behavior is dealt with. Based on the comparison of experimental and theoretical results, the dynamic of splashing drops on deep liquid surfaces is analyzed; this work focuses on the crater's evolution and its maximum. The liquids used are water and ethyl-alcohol. Drop impacts are made with various impact velocities by creating drops from several heights above the liquid surface. A straightforward model to describe and predict the crater's sinking evolution is proposed and agrees well with the experimental results over a range of Weber numbers from 50 to 1500. To cite this article: D. Brutin, C. R. Mecanique 331 (2003).     

Effects of tangential speed on low-normal-speed liquid drop impact on a non-wettable solid surface

A stream of water drops colliding on a rotating cylindrical Teflon surface was observed. The collision resulted in partial rebound, deposition, and split deposition. The collision outcome was found to depend on the normal Weber number and the tangential Weber number, which represent the collision momentum in the normal and the tangential direction, respectively. The extent of influence of the centrifugal acceleration induced by the rotation was kept negligibly under 2% compared with the impact momentum change. Through careful measurements on the elongation of the liquid drop after impact, the impact regime boundaries are seen to be governed by the long and short axes of the elliptical-disk shape at the maximal spread. Partial rebound first changes to deposition when the long axis is 1.1 times the short axis; and then changes to split deposition when the long axis is 1.46 times the short axis. Furthermore, the dimensionless excess spread area was found to be directly proportional to the tangential Weber number.     

用格子Boltzmann方法模拟液滴撞击固壁动力学行为

首次用格子Boltzmann方法中的伪势模型对液滴撞击固壁的动力学行为进行了数值模拟.详细研究了液滴在壁面上的流动状态以及各种因素对撞击过程的影响.通过数值模拟得到:壁面的可润湿性越小,液滴越容易发生反弹,液滴的回缩速度越快;液滴的撞击速度越大,所得到的相对直径越大,回缩速度越快;液滴的粘性越小,所得到的相对直径越大;液滴的表面张力越大,液滴越容易发生反弹现象.另外,液滴的最大相对直径与We数满足一定的线性关系,这些结果与前人的理论预测和实验结果完全吻合.     

液滴撞击结构液靶成坑特点分析

本文介绍了液滴对多层液靶的撞击实验研究,实验中液体从不同高度滴落,同时改变液靶的液层厚度,通过高速摄影机记录撞击成坑的过程,将胶片扫描数据输入计算机,并通过图像软件处理获得最终成坑数据。由实验结果分析了液滴撞击多层液靶成坑的特点,在一定的撞击能量情况下,撞击成坑现象发生于第一层液靶内,界面能在多层液靶不互溶时成为影响成坑状态的一个重要参数。根据能量守恒定律,并通过对撞击能量转化过程的分析,得出了撞击成坑最大坑径公式。     

Experimental investigation on splashing and nonlinear fingerlike instability of large water drops

The fluid physics of the splashing and spreading of a large-scale water drop is experimentally observed and investigated. New phenomena of drop impact that differ from the conventional Rayleigh–Taylor instability theory are reported. Our experimental data shows good agreement with previous work at low Weber number but the number of fingers or instabilities begins to deviate from the R–T equation of Allen at high Weber numbers. Also observed were multiple waves (or rings) on the spreading liquid surface induced from pressure bouncing (or pulsation) within the impacting liquid. The first ring is transformed into a radially ejecting spray whose initial speed is accelerated to a velocity of 4–5 times that of the impacting drop. This first ring is said to be “splashing,” and its structure is somewhat chaotic and turbulent, similar to a columnar liquid jet surrounded by neighboring gas jets at relatively high impact speed. At lower impact speeds, splashing occurs as a crown-shaped cylindrical sheet. A second spreading ring is observed that transforms into fingers in the circumferential direction during spreading. At higher Weber number, the spreading of a third ring follows that of the second. This third ring, induced by the pressure pulsation, overruns and has fewer fingers than the second, which is still in a transitional spreading stage. Several important relationships between the drop impact speed, the spray ejection speed of the first ring, and the number of fingers of the second and third rings are presented, based on data acquired during a set of drop impact experiments. Issues related to the traditional use of the R–T instability are also addressed.     

Collision between immiscible drops with large surface tension difference: diesel oil and water

The collision outcomes of immiscible drops with large surface tension difference, namely, a water drop and a diesel oil drop, were observed experimentally. In a near head-on collision between immiscible drops with large surface tension difference, an “overlaying” action for the drop of the smaller surface tension, i.e., the diesel oil drop, to go around the surface of the drop of the larger surface tension, i.e., the water drop, occurs during the collision. This overlaying action reduces the reflex energy for head-on collisions, making reflex separation more difficult to occur. At the same time, due to the immiscibility, the liquid bridge during stretching separation becomes narrower, which makes stretching separation easier to happen. No coalescence could be observed for a collision of Weber number greater than 60. In addition, compound drops are produced frequently.     

Spatial instability of a viscous liquid sheet

In the present study, the spatial instability for a two‐dimensional viscous liquid sheet, which is thinning with time, has been analysed. The study includes the derivation of a spatial dispersion equation, numerical solutions for the growth rate of sinuous disturbances, and parameter sensitivity studies. For a given wave number, the growth rate of the disturbance is essentially a function of Weber number, Reynolds number, and gas/liquid density ratio. The analysis indicates that the cut‐off wave number of the disturbance becomes larger with an increase in Weber number or gas/liquid density ratio. Thus, the liquid sheet should produce finer drops. When the Reynolds number decreases, the higher viscosity has a greater damping effect on shorter waves than longer waves. This could explain that only large drops and ligaments were observed in past measurements for the disintegration of a very viscous sheet. The spatial instability results of the present study were also compared with the temporal theory. The importance of spatial analysis was found and demonstrated for the cases of low Weber numbers. The temporal theory underestimates growth rates when the Weber number is less than 100. The discrepancy between the two theories increases as the Weber number further decreases. Copyright © 2005 John Wiley & Sons, Ltd.     

Effect of solidification on drop fragmentation in liquid–liquid media

 This paper presents results of experimental and analytical investigation on molten alloy drop fragmentation in water pool. Emphasis is directed towards delineating the roles which melt to coolant heat transfer and melt solidification play in the fragmentation process. The strong impact of coolant temperature upon fragmentation process is addressed. A set of 23 drop fragmentation experiments were performed, in which 8 experiments employed a low melting point alloy, cerrobend-70 and 15 experiments using Pb–Bi eutectic alloy as drop fluid. The results show strong impact of coolant temperature on particle size distribution of the fragmented drops. A linear stability analysis of the interface between the two liquid fluids with thin crust growing between them, is performed. A modified dimensionless Aeroelastic number, for Kelvin–Helmholtz instability, is obtained and used as a criteria for fragmentation of molten drops penetrating into another liquid coolant media with lower temperature. The nondimensionalized mean diameter of the fragmented particles is correlated with the Aeroelastic number. Received on 26 March 2000     

网纹型表面微结构对Ti-6Al-4V水润滑摩擦学特性的影响

研究了Ti-6Al-4V合金表面网纹型微结构与Si3N4小球对摩时的水润滑摩擦学性能.利用电火花加工技术在Ti-6Al-4V合金表面加工出不同尺寸的网纹结构,运用正交试验设计方法分析了网纹宽度、深度、间宽比和网纹角度对Ti-6Al-4V合金水润滑摩擦学性能的影响.结果表明:具有合适几何参数的网纹结构能够降低摩擦副在水润滑条件下的摩擦系数和磨损量.当网纹角度在45°时,摩擦副的摩擦系数和磨损量能同时降低.网纹宽度对稳态摩擦系数的影响最大,而网纹深度和夹角对摩擦副材料的磨损影响最大.     

Drag characteristics of a turbulent boundary layer over a flat plate with transverse square grooves

 The effect of transverse square grooves on the drag characteristics of a flat plate has been investigated using direct drag measurements in a tow tank. Two flat plates with 2.5 and 5 mm transverse square grooves, with groove spacing (s) to groove width (w) ratio, s/w=10, 20 and 40, were tested at plate length Reynolds numbers in the range 2–10×10⁶. There is an increase in drag over the “smooth” plate for all s/w configurations. Received: 31 December 1997/Accepted: 4 April 1998     

Study of airborne particles produced by normal impact of millimetric droplets onto a liquid film

The aim of this work is to carry out an experimental investigation into the generation of airborne microparticles when millimetric droplets of aqueous solutions impact onto a liquid film. Impact experiments using 3.9 mm diameter droplets were carried out for Weber numbers between 159 and 808, with a fixed Ohnesorge number of 2 × 10⁻³ and film parameters S _f (the ratio between the thickness of the liquid film h _film and the diameter of the impacting droplet d _i) between 0.3 and 1. Observed results show that the deposition/splashing threshold is independent of the parameter S _f in agreement with the data in the literature. The aerosol measurement results demonstrate the production of solid particles from the evaporation of secondary microdroplets with diameters less than 30 μm formed when splash occurs. The median diameter of these microdroplets is around 20 μm, corresponding to a value of d ₅₀/d _i = 5 × 10⁻³. Taken together, the results show that the mass and the number of particles emitted increase as the Weber number increases. Moreover, at a Weber number of 808, the results show that the mass and number of particles emitted increases as the parameter S _f decreases. In this case, the mean number of microdroplets emitted per impact is equal to 14 for S _f = 1 and equal to 76 for S _f = 0.3.     

两种螺旋角干气密封环端面摩擦特性试验研究

干气密封环端面的接触摩擦不容忽视,在槽型设计与优化时也应该考虑端面的摩擦特性.利用端面摩擦磨损试验机,分别对16°和18°两种螺旋角的螺旋槽干气密封环进行测试,通过对比分析磨损量、摩擦温度、摩擦系数以及表面粗糙度来讨论密封环的摩擦特性和两种螺旋角对端面摩擦性能的影响.结果表明:18°螺旋角试验的磨损量、摩擦温升和摩擦系数均稍大于16°螺旋角试验,说明18°螺旋角试件的磨损比16°螺旋角的更加剧烈;石墨环被螺旋槽覆盖的外圈粗糙度的变化有较大的差异,内圈粗糙度变化相似,说明槽型参数的改变对石墨环磨损的影响主要体现在外圈.试验结果为今后端面摩擦学性能的研究及槽型优化提供了依据.     

Hypersonic perfect-gas flow past an axisymmetric blunt body with a narrow groove in its frontal surface

The features of the flowfield and heat transfer on the surface of an axisymmetric body with a narrow groove in its nose immersed in a hypersonic perfect-gas stream (M_∞=6.1) are studied on the basis of a numerical analysis of the two-dimensional Navier-Stokes equations. The calculations are performed for both thermally insulated and isothermal surfaces having grooves of different width. The calculated results are compared with experimental data. Moscow. Translated from Izvestiya Rossiiskoi Akademii Nauk, Mekhanika Zhidkosti i Gaza, No. 1, pp. 117–124, January–February, 2000. The study was carried out with the support of the International Scientific and Technical Center (project No. 036).     

Newtonian liquid jet impaction on a high-speed moving surface

In the railroad industry a friction modifying agent may be applied to the rail or wheel in the form of a liquid jet. In this mode of application the interaction between the high-speed liquid jet and a fast moving surface is important. Seven different Newtonian liquids with widely varying shear viscosities were tested to isolate the effect of viscosity from other fluid properties. Tests were also done on five surfaces of different roughness heights to investigate the effects of surface roughness. High-speed video imaging was employed to scrutinize the interaction between the impacting jet and the moving surface. For all surfaces, decreasing the Reynolds number reduced the incidence of splash and consequently enhanced the transfer efficiency. At the elevated Weber numbers of the testing, the Weber number had a much smaller impact on splash than the Reynolds number. The ratio of the surface velocity to the jet velocity has only a small effect on the splash, whereas increasing the roughness-height-to-jet-diameter ratio substantially decreased the splash threshold.     

Shape and stability of a liquid drop moving at low Weber numbers

The equilibrium cross sectional shape and stability of a liquid drop moving in a gaseous medium is studied analytically. Such liquid drops appear as the final product in numerous industrial spraying and atomisation processes. Raindrops falling at their terminal speed can also be described by the present model. The equilibrium shape is formed by the interaction of two main factors; the dynamic pressure distribution in the gaseous medium which tends to deform the liquid drop into an oblate shape and the surface tension which tends to restore the spherical shape. The meridional shape of the liquid drop is obtained as a power series in the Weber number. The linear stability of the deformed shapes described above to small surface disturbances is studied. The stability analysis shows the effect of the surrounding gas flow on the natural frequencies of oscillation (vibration) of the liquid drop. The liquid drop is found to be stable in the region of low Weber numbers studied with a decrease in oscillation frequency proportional to the Weber number. This is in agreement with existing experimental data. Extrapolation of the results here lead to a Weber number of W=5.33 for breakup, again in agreement with experimental correlations.